# GR, USRP, and GPIB measurements **URL:** \ **Category:** GNU Radio\ **Created:** [August 3, 2014, 6:49am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321 "2014-08-03T06:49:14Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [August 3, 2014, 6:49am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/1 "2014-08-03T06:49:14Z")
There have been a few messages on both GR and USRP lists about choosing proper hardware gain settings. I figured I’d write some code to sweep a B200 over frequency and gain, and generate some plots that are handy for choosing proper TX gain. It also demonstrates controlling test equipment via Linux GPIB while controlling the GR flow, and plotting the results, which makes a handy piece of test gear on the bench. I plan on doing the RX side next, where results must be pulled from the GR flow via messages or probes. > **[GitHub - madengr/usrp\_rf\_tests: Automated radio frequency test routines for...](https://github.com/madengr/usrp_rf_tests)** > > Automated radio frequency test routines for the Ettus USRP using GNU Radio and Linux GPIB - GitHub - madengr/usrp\_rf\_tests: Automated radio frequency test routines for the Ettus USRP using GNU Rad... Thanks, Lou KD4HSO – View this message in context: [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 4, 2014, 11:23am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/2 "2014-08-04T11:23:22Z")
1). It looks like the amplitude in UHD\_SIGGEN\_GUI is normalized to the peak amplitude of the combined two tones, thus it is safe to set to 1.0. Notice if you switch to a single tone, the amplitude increases when viewed on the spectrum analyzer. You must measure your output with a power meter or spectrum analyzer to get the power in dBm, then adjust the TX\_gain to change the level. That was really the premise behind my experiment, to figure out what is the output power for various gains and frequencies. I beleive max limit on the WBX is 31.0 since it has two 5-bit attenuators. I suppose I could have looked in the data sheet for the B200 transceiver chip. 1. I just choose 60 dB as I had to use something when initialized the usrp in the code. It was really just arbitrary. No, with the WBX you will be limited to 0-31 dB for the TX gain setting as the B200 is different hardware than the WBX. I actually have an N210+WBX also, but I have not gotten around to measuring it yet. 2. The one tone test uses a tone at 100 kHz. The two tone test uses ones at 50 kHz and 75 kH, hence 25kHz tone spacing at 62.5 kHz offset. All fall within the 250 kHz sampling rate. It does not matter if the intermod products fall outside the sampling rate as those are generated after the (ideal) DAC. 3. You must always stay below DAC full scale (+/- 1 signal). Even a sinusoid of amplitude 1.0 is not good since the DSP within the FPGA tend to periodically overflow; at least Im seeing that behavior within the B200. 3 dB below full scale (1/sqrt(2)) is sort of an arbitrary choice I made. The two tones have a 3 dB crest factor; the power crests up to 3 dB above average power. Hence I wanted the power crest to still be 3 dB below full scale. The digital modulations and multi-carrier can have much higher crest factors, thus you must back off to prevent DAC overflow. 4. 89 dB is the max setting for the B200. I just choose the 70 dB as that provides 0 dBm output power at 100 MHz. 5. If the Tektronix analyzer has a serial or ethernet maybe you could control it through that. I used Linux GPIB. I planned on using PyVISA but the Linux GPIB already had a python module. I may update it to use PyVISA, as VISA provides a hardware abstraction so you can use GPIB, serial, ethernet, etc. without having to change the code. You will need to create a new class within instruments.py for the tektronix analyzer, duplicating the methods of the 8566B. Thanks, Lou KD4HSO ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 5, 2014, 7:46pm UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/3 "2014-08-05T19:46:40Z")
Hi. Thank you for your detailed response. it helps makes things clear. Could you also clarify the below questions 1. While you increased the gain were you also looking for 1 dBCP in one tone and IIP3 or OPI3 point in the two tone test. If so, what values did you get for the B200 USRP? I would like to get a reference for USRPN210+WBX test. Usually the Ettus web specifies values only for the daughterboard like WBX is said to have 5 to 10 dBm on IIP3. So we don’t know for the entire device set. 1. Also in you notes, you have said " The one tone TX test consists of a 0.707 amplitude tone at 100 kHz offset from the center frequency. The tone amplitude is -3 dBFS, which is half power from full scale of the DAC." “The two tone TX test consists of 0.304 amplitude tones with 25 kHz spacing, offset 62.5 kHz from the center frequency. Each tone amplitude is -9 dBFS, thus a combined average power at -6 dBFS, and peak instantaneous power at -3 dBFS.” Can you explain this a little more. How did you map 0.707 amplitude to -3 dBFS? You say it s half power of full scale DAC so what is the full scale DAC power? For a USRP2 +WBX I found the following measures on spectrum Analyser, while sending a single tone with UHD\_SIGGEN.py Ampl Gain Attenuator Pout on spec Analyser Actual power (Pout +60) 0.707 25 60 -42.48 dBm 17.52 dBm 0.707 20 60 -48.53 dBm 11.47 dBm 0.707 15 60 -56.94 dBm 3.06 dBm 0.707 10 60 -64.75 dBm -4.75 dBm 0.707 5 60 -74.36 dBm -14.36 dBm however I don’t know how to relate it in terms of DAC power scale. Similarly for two tone you have said “The two tone TX test consists of 0.304 amplitude tones with 25 kHz spacing, offset 62.5 kHz from the center frequency. Each tone amplitude is -9 dBFS, thus a combined average power at -6 dBFS, and peak instantaneous power at -3 dBFS.” I get the relation like : 1/sqrt(2) = 0.707 ==\> -3 dBFS 1/(2\*sqrt(2)) = 0.304 ==\> -3 dbFS + 20log (1/2) == -3 dBFS - 6.02 = -6 dBFS Is my understanding correct. Could you clarify the calculation of average power as -6 dBFS, and instantaneous power as -3 dBFS too? look forward to your response. Thanks Gayathri ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 6, 2014, 8:13pm UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/4 "2014-08-06T20:13:48Z")
1. I have not calculated P1dB or IP3, yet. I will have to do that. I find it’s easier to look at a graph of IMD3 levels since it is more intuitive that working from the IP3, which is more useful for calculations. 2. 0 dBFS would be the DAC swinging full scale; i.e. -1 to +1 float value (DAC voltage or current if you want to think of it that way) from GNU Radio. That is the maximum power you can get from the DAC. So to drop the power by 1/2 (i.e. -3 dB), the amplitude must drop to 1/sqrt(2)=0.707. You always need to stay backed off from full scale, so -3 dB is a good point. You don’t care about the power from the DAC, only the power from the DAC + amplifier. The DAC should be very linear compared to the amplifier, and it is since the IMD really drops off with amplifier gain setting. Your measurements should be dropping in 5 dB steps; they are not. You can either decrease your video bandwidth, or better yet remove that 60 dB attenuator. Most spectrum analyzers can handle 30 dBm with the input attenuator at maximum. 60 dB is too much, not to mention it’s probably not exactly 60 dB. Let’s say you have two tone, each is 0 dBm, at 100 Hz and 101 Hz. The the total (mean square) power is 3 dBm; i.e. measure each tone individually and sum the power. At the beat frequency of 1 Hz the amplitudes add constructively and destructively; ie. doubling and going to nothing. That amplitude doubling gives an instantaneous (peak) power of 6 dBm, then it disappears to nothing, hence the average is still 3 dBm. So for two tones the peak power is 3 dB over average power, or 6 dB over that of a single tone. That’s why I set each tone power to -9 dBFS, so the peak power is -3 dBFS. Thanks, Lou KD4HSO Gayathri Ramasubramanian wrote > web specifies values only for the daughterboard like WBX is said to have 5 > Can you explain this a little more. How did you map 0.707 amplitude to -3 > > ``` > 1/(2*sqrt(2)) = 0.304 ==> -3 dbFS + 20log > > ``` > > Discuss-gnuradio mailing list > Discuss-gnuradio@ > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) – View this message in context: [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49821.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49821.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 10, 2014, 3:47am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/5 "2014-08-10T03:47:28Z")
I updated the repo with results for the N210+WBX: > **[GitHub - madengr/usrp\_rf\_tests: Automated radio frequency test routines for the...](https://github.com/madengr/usrp_rf_tests)** > > Automated radio frequency test routines for the Ettus USRP using GNU Radio and Linux GPIB Our single tone sweep results look very similar. Your IMD plot should be in dBc; plot the difference in power between one of the test tones and one of the IMD3 products. Mine don’t do below -50 dBc since I have the input attenuator on the analyzer set for the higher power level. I need to reduce it for the lower power levels. 1. Yes, that would be a calibration factor. If you measure 0 dBm on the spectrum analyzer, and -20 dBFS in the FFT, then 20 dB is the calibration factor, so -20 dBFS + 20 dB = 0 dBm. Note when you change the RX gain, then the calibration factor changes. If you increase gain 10 dB, then you must decrease the calibration factor 10 dB. 2. Using the FFT to manually measure level can be a problem if the tone is split between bins, so it’s better to use a coarse FFT. Measuring all the power in the channel at once may be preferable, and is like using a power meter. Use the complex\_to\_mag\_squared, followed by inegrate\_with\_decimation, then 10\*log() +K, where K is your calibration factor. If you decimate down to 1 Hz, i.e. decimation\_rate=sample\_rate, you can get very precise power readings. The accuracy drops off with tone power due to the wideband noise; just like a power meter. You would have to look at the USRP FPGA block diagram to find out exactly what is going on between the input and the FFT, but it is essentially fine tuning with an NCO then many stages of filtering and decimation. I’m sure it affects the amplitude slightly as different filters are used for different decimations, and the odd vs. even decimation. 1. Compression is the drop in gain (not power). For example look at this table where the USRP TX gain is stepped in 1 dB increments and the output power is measured: USRP\_TX\_Gain\_dB, Pout\_dBm, Gain = Pout\_dBm - USRP\_RX\_Gain\_dB 3.0, 8.0, 5.0 4.0, 9.0, 5.0 5.0, 9.9, 4.9 6.0, 10.7, 4.7 7.0, 11.3, 4.3 8.0, 12.0, 4.0 \<\<-- This is the P1dB 9.0, 12.6, 3.6 The P1dB is where the gain has dropped from 5.0 to 4.0. The P1dB referenced to the output is 12.0 dBm. The P1dB referenced to the USRP TX gain setting is 8.0 dB. Notice it is also where the digit after the decimal point repeats itself if the input is stepped in 1.0 dB increments; i.e it when from 5.zero to 4.zero. This is the quick n’ dirty method of finding P1dB. If the input moves in 1 dB steps, all you need to monitor is the most significant digit after the decimal point. 1. I have not done RX testing. My signal generators are non-synthesized and have no digital interface. I bought a new one on eBay and it should be here next week, but I still need another for a two tone test, not to mention the components to achieve proper isolation between the two. Thanks, Lou KD4HSO Gayathri Ramasubramanian wrote > 1. > > This changed to ~ 31 dBm at 900 MHz and ~ 23 dBm at 1.8 GHz. > So can we use the difference to be the calibration factor in the linear > daughterboards provide various levels of amplification in their analog > real > subtracted from the values got from UHD\_FFT and is ~ 25 dB on IIP3 and 50 > > * * * > > Discuss-gnuradio mailing list > Discuss-gnuradio@ > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) > > Plots for the Questions.docx (47K) \<[http://gnuradio.4.n7.nabble.com/attachment/49883/0/Plots%20for%20the%20Questions.docx>](http://gnuradio.4.n7.nabble.com/attachment/49883/0/Plots%20for%20the%20Questions.docx>); – View this message in context: [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49890.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49890.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 9, 2014, 10:02am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/6 "2014-08-09T10:02:05Z")
Hi Thanks again fro your explanations. You were correct about the measurements I had sent in earlier. I checked with the power meter and the O/p power at 450MHz @ ampl 0.707 and TX Gain of 25 gives 11.5 dBm. I have done the single tone and two-tone tests for USRPN210 +WBX board at 3 frequencies of 400 MHz, 900 MHz, 1800 MHz. The settings are similar to yours :AMPL 0.707, TX gain : varied from 0 to 31. Could you please check them and let me know if they are fine. Also I am trying to find the RX side linearity range. For this I connected signal generator to the RX port (RF2) of the USRPN210 and viewed the received spectrum on UHD\_FFT. I have plotted the readings that I got. I could see that there is a difference of around 35 dBm between the reading on UHD\_FFT and that on spectrum Analyser when the same input from signal geenrator was given to both (using a power splitter ) @ 400 MHz. This changed to ~ 31 dBm at 900 MHz and ~ 23 dBm at 1.8 GHz. Could you kindly clarify the below points regd this: 1. So it is good to assume \*35 dBm as a calibration factor @ 400 MHz \*for the USRPN210 +WBX device. i.e in case we use USRPN210 +WBX device test as Receiver for testing and use UHD\_FFT to plot the spectrum and take the reading of amplitude in dB and get a value of _25 dB_ , would it to fine to say that 25 - 35 = -10 dBm is the actual power received by USRP to an extent ( not only on the RX port). So can we use the difference to be the calibration factor in the linear range? 2. The link [Ettus Research - The leader in Software Defined Radio (SDR) | Ettus Research, a National Instruments Brand | The leader in Software Defined Radio (SDR)](http://www.ettusresearch.com/content/files/kb/application_note_uhd_examples.pdf) says that “When the FFT(default) view is used, the x-scale is the frequency, and the y-scale is amplitude.The y-scale shows the amplitude with “counts,” and the values do not typically correlate to a specific, absolute power input. The amplitude read on the display is useful for approximate comparisons. The level for a given input amplitude will vary a few dB across frequency and from unit to unit. Also, receiver daughterboards provide various levels of amplification in their analog chains,which will affect the amplitude result in the FFT”. Could you clarify what type of processing is done on the received signal from the point of reception till the display on UHD\_FFT briefly in terms of scaling/ normalization. The basic idea is to find a relation between the amplitude value shown on UHD\_FTT plot and basic power scale in dBm.( a factor to be summed/subtracted or multiplied or divided from UHD\_fft reading to get real power value in dBm) 1. On further increase of input power in steps of 1 dBm or 0.1 dBm, there was a 1 dB drop of power. We assumed this was the point of 1 dB Compression for the receiver side of USRPN210. Is this understanding correct? This comes around _-8 dBm @ 400 MHz, -4 dBm @ 900 MHz and ~ +2 dBm @ 1.8 GHz_. 2. I have also tried two tone test on the USRPN210 +WBX device. The plots shown under Q4. The IIP3 point comes ~ 4 dBm when the factor of 35 is subtracted from the values got from UHD\_FFT and is ~ 25 dB on IIP3 and 50 dBm on OIP3 ( which was told to me by my advisor as not the normal values to expect) . You said you plan to do the one tone and two tone test for RX side for B200. Do you find any similarity in results. Kindly let me know if I seem to be going wrong any where. Kindly Clarify the above points. I eagerly look forward to your responses. ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 11, 2014, 3:31pm UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/7 "2014-08-11T15:31:40Z")
You need to be careful with the WBX as input power above -20 dBm could damage the 2nd stage RX amplifier. Per the data sheets, the absolute maximum input power of the 2nd stage amp is below the saturated output power of the 1st stage amp. 1a) Yes, only valid for linear range. 1b) The device is always going to generate IMD, so it really depends on how much error you can tolerate. The IMD3 grow at 3x the rate of the main tones, so for a slow gain compression, the error added to the IMD levels won’t be much compared with how fast they grow. On my plots, -30 dBc is where the single tone power (visually) starts to deviate from linear. 1. Negative of the difference. The IMD are always going to be negative dBc, where the “c” means referenced to the carrier. 2. Optional, and tied to the daughterboard, specifically analog imparments in the I/Q sections. I don’t know the specifics; maybe ask on the USRP list. I think each correction file is serialized by the daughterboard, which make sense if you are running several USRP from one host. Thanks, Lou KD4HSO Gayathri Ramasubramanian wrote > linear > fundamental tone and the IMD product power @ harmonc freq. i.e [power @ > > - uhd\_cal\_rx\_iq\_balance > that it more tending towards the WBX board operation. Also the factors get > Kindly clarify the above points. > > Thanks again > > Regards > > Gayathri > > * * * > > Discuss-gnuradio mailing list > Discuss-gnuradio@ > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) – View this message in context: [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49901.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49901.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 11, 2014, 7:31am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/8 "2014-08-11T07:31:59Z")
Thanks for doing the TX side one tone and two tone tests for USRPN210. It helps me get a reference. 1. When I did the test for finding out the calibration factor, I found it was a constant value ntil a certain range and after that it dipped (e.g ard -8 dBm input power @ 400 Mhz). I image this is point near the saturation /compression region of the device. This was observed on 5 USRPN210+WBX devices that I tested. a) So I understand that this calibration factor is valid only in the linear range. Is it correct? b) if so, I should not use the calibration factor to turn the IMD products’ value on FFT to dBm using this factor as they occur only when the device is operating in the non linear range. Please let me know if my understanding is right. 1. You told me to plot the IMD plot with difference of power between fundamental tone and the IMD product power @ harmonc freq. i.e [power @ F1 - power @ (2F1-F2)] . But this is coming as a +ve value for me while your plot shows the Y axis on a negative scale. Would this also be might due to the attenuator you are using? or should I consider the negative of the difference? 1. Also you have told to run the UHD calibration routines should be run for the N210+WBX: Are these mandatory or optional? Do they affect the daughter board or mother board of the device? The devices I use are for common use in a lab and hence it is preferable not to change any of the configurations for specific use for long term. Hence my question. As per the information given at [USRP Hardware Driver and USRP Manual: Device Calibration and Frontend Correction](http://files.ettus.com/manual/page_calibration.html#calibration_) it seems that it more tending towards the WBX board operation. Also the factors get stored in the machine they are connected to. So in case we want to run the USRPN210 device with old configuration, it would be ok to just delete the files or run it from some other machine? Is this correct? Can any random number be specified for a serial number if not detected from the device or any specific format is to be used ( i.e min number of digits & alphabets etc) Kindly clarify the above points. Thanks again Regards Gayathri [http://files.ettus.com/uhd\_docs/manual/html/calibration.html](http://files.ettus.com/uhd_docs/manual/html/calibration.html) ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 16, 2014, 6:42am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/9 "2014-08-16T06:42:53Z")
You don’t need to take the FFT to get the power. See my attached example (in GR 3.7). Just take the output from the USRP, complex\_to\_mag\_squared, integration with decimation to 1 Hz, 10\*log10() + k, and probe at 1 Hz rate. I’m using an N210 + WBX with gain=15 dB. That gives me a calibration factor of k=-34.8 dB. I can vary power of my signal generator from -50 dBm to -10 dBm with less that 0.1 dB error in the calculated power. [https://dl.dropboxusercontent.com/u/49570443/power\_calc.grc](https://dl.dropboxusercontent.com/u/49570443/power_calc.grc) [https://dl.dropboxusercontent.com/u/49570443/power\_calc.grc.png](https://dl.dropboxusercontent.com/u/49570443/power_calc.grc.png) Thanks, Lou KD4HSO [http://gnuradio.4.n7.nabble.com/file/n49981/power\_calc.png](http://gnuradio.4.n7.nabble.com/file/n49981/power_calc.png) Gayathri Ramasubramanian wrote > “complex to Mag^2” block → “integrate and decimate” block → "10 log > > Gayathri > – > View this message in context: > [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49981.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49981.html) > Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 22, 2014, 9:52am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/10 "2014-08-22T09:52:08Z")
Hi Thank you for previous note. I removed the FFT block and made my flow graph similar to yours. It works now 🙂 It is the first time I am working with the QT GUI widgets so kindly help clear the foll doubts. 1. I do not have the QT\_FREQUENCY\_GUI\_SINK BLOCK you have used only the QT\_GUI\_SINK which gives the frequency spectrum. This is because of the lower version ( 3.6.4.1) I suppose. For a USRPN210+WBX board I got the calibration factor to be as follows: @ 400 MHZ, K = - 62.8 @ 900 MHz ; K = -58.8 @ 1.8 GHz ; K = --52.5 I tested for 3 devices (USRPN210+WBX) and got around same result . The Gain was set to 0. This does not match the value you had obtained with your test. Is there a major difference in the working b/w QT\_Frequency\_SINK and QT\_GUI\_SINK that would affect the final result?If so, Can you suggest a work around. . 1. The channel gain value seems to add to the transmitted power from signal generator. So the calibration factor also needs to have channel gain added to its value for getting same result. So it seems like the channel gain affects the signal power like a LNA and hence increases the strength of the signal wrt to the noise. Then how is it different from RX gain.? Im a little confused on where the gain parameters actually impact the value of the signal. 1. The QT block also seems to do an internal FFT default size of 256 and send out a signal. However this value does not match the value seen on WX FFT GUI ( the CUrsor on QT SINK actually gives a vlue very close to actual signal generator reading). is there major difference in signal processing of QT widgets as compared to WX widgets. I found some previous threads stating whats the difference as per the software processing level in GRC not about the way it processed the received signal. 2. Why is there a difference in the channel\_power\_dBm value and the value seen on the spectrum using the cursor. (refer the picture). The cursor shows -81.42 dB @ 0.0 Khz offset from centre freq, while the channel\_power\_dBm shows a value of -17.5 dBm 3. Since This gives the channel power, I suppose this cant be used to find the intermodulation’s power with two tone test. Is my understanding correct?or is there a work around for it too? Kindly clarify the above doubts. Look forward to your response. Thanks Gayathri ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 16, 2014, 3:21am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/11 "2014-08-16T03:21:23Z")
Hi Thank you for your note. I am trying to get the power of received signal using the blocks you had suggested previously. I have attached a flow graph snapshot ('uhd\_fft\_DK.grc.png"). I am using the UHD:USRP Source to receive the signal which in turn sends the signal in the foll path “complex to Mag^2” block --\> “integrate and decimate” block --\> “10 log +k” block—\> “signal probe” block. However there are errors mainly referring to the in-compatible I/O sizes of source (complex to Mag ^2 block) and sink (Integrate and Decimate block). The I/O size of “Complex\_Mag^2” block is said to be 4096, while the I/p size of i"ntegrate and decimate" block is 4. ==\> error as not compatible. Should I change the Vector length? I am not quite sure? The error propagates to other blocks if I change anything. Kindly help solve this. I have also attached another flowgraph (flowgraph screenshot.png) which works but it doesn’t have the “integrate and Decimate” block or the Log10+K block but it works and receives the signal (developed by someone else previously in our lab). Now since the “Log10+K” block is missing, does it mean the output is given in mwatts instead of dBm? I cant seem to use this version on as the “probe\_signal\_vector” block does not seem to be supported in GRC 3.6.4.1 which is the version in my system. I tried using simple probe\_signal block instead but again faced the I/o size incompatibility error? Is there a work around for this “probe signal vector” block? Also could you explain briefly how to approach and solve there I/o size errors. Look forward to your response Thanks Gayathri ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 24, 2014, 2:40am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/12 "2014-08-24T02:40:04Z")
1. I set the gain of the WBX to 15 dB, which is why our values do not match. If I use 0 dB WBX gain I get the following for a -40 dBm tone: 400 MHz, k=-30.6 900 MHz, k=-25.5 1.8 GHz, k=-19.2 There should be no computational differences between the GUI and frequency sinks. 1. The channel gain is the RX gain; it’s the same thing. The WBX receive side has two fixed gain stages followed by 0 - 31.5 dB of digital attenuation. That’s what you are setting for the gain in the UHD; a gain of 31.5 dB equates to 0 dB attenuation setting. Yes, you have to offset the computed power by RX gain if you want the reading to stay constant. Note the n\*log10()+k block does not allow k to me changed in real-time, so you have to multiply each sample by a constant. You must do the same if you want to calibrate the spectrum. See example here: [https://dl.dropboxusercontent.com/u/49570443/power\_calc\_2.grc](https://dl.dropboxusercontent.com/u/49570443/power_calc_2.grc) I have no idea about the differences between the WX and QT frequency sinks. They should give identical results with identical settings. 1. The channel power and spectrum are totally different computations, so they must have different calibration factors. See my linked example. 2. For finding intermod levels, you use the same math for the channel power calculation, but narrow the channel with a frequency translating filter and decimation. Either tune to each intermod product or have multiple channelizers in parallel. This is where it may be more efficient to do the FFT and find the peaks, but it’s harder to program that. Thanks, Lou KD4HSO Gayathri Ramasubramanian wrote > the QT\_GUI\_SINK which gives the frequency spectrum. This is because of > I tested for 3 devices (USRPN210+WBX) and got around same result . The > > stating whats the difference as per the software processing level in GRC > > Kindly clarify the above doubts. > Look forward to your response. > > Thanks > Gayathri – View this message in context: [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50066.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50066.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 25, 2014, 10:48am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/13 "2014-08-25T10:48:30Z")
Hello Gayathri, On 25.08.2014 10:17, Gayathri Ramasubramanian wrote: > Hi > Thank you for your note. > My questions here are just based on my previous measurements and your last > mail. kindly clarify the same. > > 1. Is it possible that different USRPN210 devices with WBX boards have the > different calibration factors. I set the channel gain to 0 and still get : > […] > Your values seem to be different , > Hence the question. > Yes, that will be the case, as with any analog circuitry; amplifiers, > filters and such are produced with tolerances, and thus these values are > expected to change from device to device. > Is this correct or am I doing something wrong. As my values seem to be > almost 30 ~ 33 higher than ones you are getting from your tests. what could > be causing this error/ discrepancy. > 30dB _higher_? Are you sure you are inserting the same power (by the > way, -40dBm is quite some power and I would generally recommend using an > attenuator to avoid damage if the input power rises, as Lou already > warned about)? Are integrating over the same number of samples? > As a special note to your measurements: You’ll sometimes see LO leakage, > which can contribute significantly to total power. To avoid that, you > could specify a uhd.tune\_request(a,b) instead of a simple target > frequency. That would allow you to specify a digital tuning offset, > which would move the LO out of your measured bandwidth. Generally, I wonder what you use these values for, afterwards. Because they are only valid for the 250kHz bandwidth as limited by the antialiasing filter in your USRP’s FPGA; if you wanted to use this value for higher sample rates, I’d expect there to be a high level of proportionality, but I guess since your doing calibration now, it would make sense to use a filter on the PC where you can control the noise equivalent bandwidth yourself – especially since I assume you’re measuring a single tone, and that would fit in the narrowest of bandpass filters, which would avoid measuring the power of the input tone including your noise floor over your complete sampling rate bandwidth. If you were using a spectrum analyzer, that would actually shift a analog filter through the spectrum and display the energy passing that filter at every frequency, which would give you a display quite different from mag squared for the full 250kHz bandwidth of 1s of input signal samples. Greetings, Marcus ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 25, 2014, 4:24pm UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/14 "2014-08-25T16:24:22Z")
1. What you are doing is correct. What’s the power of your signal generator? I was using -40 dBm, and as Marcus says, that is strong, but you’ll need to have a good SNR since the noise bandwidth is ~250 kHz. Several posts ago you stated running -8 dBm into the WBX. You may have damaged the WBX, since that 30 dB difference is about the isolation you may get with a damaged amp. I suggest sniffing down the chain of the WBX with a spectrum analyzer, make sure the two amps are working. Also as Marcus said, you need to be using the same bandwidth and decimation, or that will change the calibration factor. I choose 250 kHz wince I was working with WBFM channels. 1. If the RX gain is changed, then the k must change to compensate. See my \*.grc I linked in the last post. I assigned RX gain to a QT slider then added it to K. The RX gain can be changed and the RX power stays about the same. 2. I’m on travel now so I don’t have access to my computer. You ought to update to GR 3.7; lot’s of nice features, and it also preserves the flow diagram even if the blocks of changed. Thanks, Lou KD4HSO Gayathri Ramasubramanian wrote > ‘channel\_power\_dBm’ value to be very close to the input signal generator > mails you had sated that you get k = -34.5 wih RX gain of 15. So in view > calibration along with the xml file like last time. Using the xml to know > Gayathri > – > View this message in context: > [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50090.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50090.html) > Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 25, 2014, 10:19am UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/15 "2014-08-25T10:19:12Z")
Hi Thank you for your note. My questions here are just based on my previous measurements and your last mail. kindly clarify the same. 1. Is it possible that different USRPN210 devices with WBX boards have the different calibration factors. I set the channel gain to 0 and still get : @ 400 MHz : k = -62.8 @ 900 MHz : k = -58.5 @ 1800 MHZ: k = -52.5 I checked the above with 3 USRPN210 +WBX devices and found the K value to be very close to the stated values. Your values seem to be different , Hence the question. My method was that I first ran the flow graph with channel gain as 0 and k as 0. Then found the power value displayed for the ‘channel\_power\_dBm’. I subtracted this value from my input signal value (from signal generator) and found out the difference. This is what I am taking to be the ‘k’ value. If I use k values (62.5 for 400 MHz) it in the block too, it give the ‘channel\_power\_dBm’ value to be very close to the input signal generator value. I repeated this for 3 devices at the 3 frequencies. Is this correct or am I doing something wrong. As my values seem to be almost 30 ~ 33 higher than ones you are getting from your tests. what could be causing this error/ discrepancy. 1. In you mails you say that we have to subtract RX gain from Calibration factor to baseline it for RX gain = 0 dB . Also in one of your previous mails you had sated that you get k = -34.5 wih RX gain of 15. So in view of these two stmts, was your ‘k’ value of -34.5 for 1800 MHz. This 1800 MHz is the only one satisfying this rule with previous data set: i.e \*-34.5 - 15 = 19.5 \*( this is close to the 1800 MHz results you have written about in your last email). I just want to check if my understanding is correct and Im doing the math right. 1. Could you kindly paste a picture of your flow graph for spectrum calibration along with the xml file like last time. Using the xml to know which bock is specified is a little tough. Having a visual flow graph along with the xml would be more effective to understand and relate between them. Please clarify above. Look forward to your response. Thanks Gayathri > On Aug 23, 2014, at 8:38 PM, madengr [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > 1. I set the gain of the WBX to 15 dB, which is why our values do not > match. > side has two fixed gain stages followed by 0 - 31.5 dB of digital > attenuation. That’s what you are setting for the gain in the UHD; a gain > of > 31.5 dB equates to 0 dB attenuation setting. Yes, you have to offset the > computed power by RX gain if you want the reading to stay constant. Note > the n\*log10()+k block does not allow k to me changed in real-time, so you > have to multiply each sample by a constant. You must do the same if you > want to calibrate the spectrum. See example here: > > [https://dl.dropboxusercontent.com/u/49570443/power\_calc\_2.grc](https://dl.dropboxusercontent.com/u/49570443/power_calc_2.grc) > > I have no idea about the differences between the WX and QT frequency > sinks. > They should give identical results with identical settings. > > 1. The channel power and spectrum are totally different computations, so > they must have different calibration factors. See my linked example. > > 2. For finding intermod levels, you use the same math for the channel > power > calculation, but narrow the channel with a frequency translating filter > and > decimation. Either tune to each intermod product or have multiple > channelizers in parallel. This is where it may be more efficient to do > the > > > lower version ( 3.6.4.1) I suppose. > > your test. > > and hence increases the strength of the signal wrt to the noise. Then > > how is it different from RX gain.? Im a little confused on > > where the gain parameters actually impact the value of the signal. > > > > 1. The QT block also seems to do an internal FFT default size of 256 and > > send out a signal. However this value does not match the value seen on WX > > FFT GUI ( the CUrsor on QT SINK actually gives a vlue very close to > > actual > > 2. Since This gives the channel power, I suppose this cant be used to > > find > > – > View this message in context: > [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50066.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50066.html) ---
**Author:** ![madengr](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ec9cab/32.png) [@madengr](https://www.ruby-forum.com/u/madengr)\ **Post date:** [August 25, 2014, 4:43pm UTC](https://www.ruby-forum.com/t/gr-usrp-and-gpib-measurements/231321/16 "2014-08-25T16:43:57Z")
-8dBm may, or may not, have damaged either of the the two LNA stages on the WBX. “Modern” WBXen have TVS and limiter diodes in front of both stages to try to ameliorate laboratory accident consequences, but they offer no firm guarantees. Nearly all of the receivers on USRP daughtercards are designed to be connected to an antenna, which means a low-noise stage “up front”. This means that using them in the lab requires some prudence and caution, such as putting a 30dB attenuator in front of the device before connecting it to a laboratory signal generator. There are several orders-of-magnitude difference between the average power levels you’d expect from an antenna, and what you’d typically be trying in the lab, so when you’re working in the lab, padding the input with 30dB of attenuation is just a really prudent practice. Modern RF LNA transistors are, for the most part, “precious little princesses”. They have gate regions that are only a few molecules thick, and “high” currents flowing in these areas will cause holes to form in the gate layer, which will quickly affect gain and noise figure, and then lead to complete device failure. You can put clamping diodes in front (as Ettus now does on its xBX daughtercard series), but such devices cannot protect in all cases, and they compromise ultimate noise figure and gain a little bit. Thus, ahem, endeth the sermon of the day. We turn now to the hymn book “Glory unto GaAs”. 🙂 🙂 On 2014-08-25 10:23, madengr wrote: > I choose 250 kHz wince I was working with WBFM channels. > Thanks, > Lou > KD4HSO > > Gayathri Ramasubramanian wrote > > > Hi Thank you for your note. My questions here are just based on my previous > > measurements and your last mail. kindly clarify the same. 1) Is it possible that > > different USRPN210 devices with WBX boards have the different calibration factors. > > I set the channel gain to 0 and still get : @ 400 MHz : k = -62.8 @ 900 MHz : k = > > -58.5 @ 1800 MHZ: k = -52.5 I checked the above with 3 USRPN210 +WBX devices and > > found the K value to be very close to the stated values. Your values seem to be > > different , Hence the question. My method was that I first ran the flow graph with > > channel gain as 0 and k as 0. Then found the power value displayed for the > > ‘channel\_power\_dBm’. I subtracted this value from my input signal value (from > > signal generator) and found out the difference. This is what I am taking to be the > > ‘k’ value. If I use k values (62.5 for 400 MHz) it in the block too, it give the > > ‘channel\_power\_dBm’ value to be very close to the input signal generator value. I > > repeated this for 3 devices at > > the 3 frequencies. Is this correct or am I doing something wrong. As my > > values seem to be almost 30 ~ 33 higher than ones you are getting from > > your tests. what could be causing this error/ discrepancy. 2) In you > > mails you say that we have to subtract RX gain from Calibration factor > > to baseline it for RX gain = 0 dB . Also in one of your previous mails > > you had sated that you get k = -34.5 wih RX gain of 15. So in view of > > these two stmts, was your ‘k’ value of -34.5 for 1800 MHz. This 1800 MHz > > is the only one satisfying this rule with previous data set: i.e \*-34.5 - 15 = 19.5 \*( this is close to the 1800 MHz results you have written about in your last email). I just want to check if my understanding is correct and Im doing the math right. 3) Could you kindly paste a picture of your flow graph for spectrum calibration along with the xml file like last time. Using the xml to know which bock is specified is a little tough. Having a visual flow graph along with the xml would be more effective to understand and relate between them. Please clarify above. Look forward to your response. Thanks Gayathri > – > View this message in context: > [http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50090.html](http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p50090.html) > [1] > Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [2] Links: