# Transmit filtering **URL:** \ **Category:** GNU Radio\ **Created:** [September 5, 2011, 4:21pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490 "2011-09-05T16:21:06Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\ **Post date:** [September 5, 2011, 4:21pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/1 "2011-09-05T16:21:06Z")
I’m generating two NBFM signals, offsetting them plus and minus 25 kHz with FreqXlatingFilters, and combining the output in an adder which then drives a USRP sink. It seems sensible to put a bandpass filter on the result. I have a basic conceptual confusion. I would think that you’d specify a bandpass filter around zero with a low cutoff of, say, -40e3 and a high cutoff of +40e3. However, this doesn’t work – the bandpass filter seems to require a number \>0 for the lowpass frequency. So, in this situation, where I want to filter a spectrum that is centered around zero, how do I specify the filter lowpass and highpass frequencies? A second question – there is a signal component on the output directly at the USRP center frequency. It’s at most about 30dB below the desired signals. I assume this is the equivalent of LO leakage. Is there any way to filter this out or at least reduce its amplitude? As I’m writing this, I wonder if the solution to both problems is to put both my signals on one side of the USRP center frequency, say +25 and +75 kHz. That would allow a sensible bandpass filter that would remove the zero-frequency component. But the downside of that seems to be throwing away half the available bandwidth. Thanks, John ---
**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\ **Post date:** [September 5, 2011, 4:35pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/2 "2011-09-05T16:35:19Z")
On Mon, Sep 05, 2011 at 10:19:54AM -0400, John Ackermann N8UR wrote: > I would think that you’d specify a bandpass filter around zero with a > low cutoff of, say, -40e3 and a high cutoff of +40e3. However, this > doesn’t work – the bandpass filter seems to require a number \>0 for > the lowpass frequency. That would be a low-pass filter with a cutoff at 40e3. With complex-valued signals, a low-pass filter is essentially a band-pass filter centered around 0 Hz. > there is a signal component on the output directly at the USRP center > frequency. It’s at most about 30dB below the desired signals. I > assume this is the equivalent of LO leakage. Is there any way to > filter this out or at least reduce its amplitude? > > As I’m writing this, I wonder if the solution to both problems is to > put both my signals on one side of the USRP center frequency, say +25 > and +75 kHz. Yes, you could use an external analog band-pass filter if that center spur is a problem for you. ---
**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\ **Post date:** [September 5, 2011, 5:47pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/3 "2011-09-05T17:47:42Z")
On 05/09/11 10:19 AM, John Ackermann N8UR wrote: > I’m generating two NBFM signals, offsetting them plus and minus 25 kHz > with FreqXlatingFilters, and combining the output in an adder which > then drives a USRP sink. It seems sensible to put a bandpass filter > on the result. > > I have a basic conceptual confusion. I would think that you’d specify > a bandpass filter around zero with a low cutoff of, say, -40e3 and a > high cutoff of +40e3. However, this doesn’t work – the bandpass > filter seems to require a number \>0 for the lowpass frequency. > Make sure that you specify complex-taps. I assume you’re using GRC. By > default, it uses the version of > the bandpass filter with real taps. But if you specify complex taps, > you can use negative frequencies. – Principal Investigator Shirleys Bay Radio Astronomy Consortium > **[Live Draw HK Tercepat | Result HK | HK Pools | Live HK | HK Prize Hari Ini](https://www.sbrac.org/)** > > Temukan live draw hk tercepat melalui halaman hk pools yang resmi dan dapatkan result hk prize melalui live hk secara langsung di situs kami. ---
**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\ **Post date:** [September 5, 2011, 7:00pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/4 "2011-09-05T19:00:13Z")
On Mon, Sep 5, 2011 at 12:55 PM, John Ackermann N8UR [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hi Marcus – > > I am selecting complex taps from the drop down, but still get an error at > runtime if either cutoff frequency is 0 or smaller. > > Thanks, > > ## John John, That sounds like a bug in the GRC block, then. I just tried this as a complex BPF and it works fine: gr.firdes.complex\_band\_pass(1, 1, -0.1, 0.2, 0.1) However, as was already mentioned, if you are trying to create a bandpass filter from -40 to +40 kHz, that’s just a low pass filter with a bandwidth of 40 kHz: gr.firdes.low\_pass(1, 100e3, 40e3, 2e3) Will give you a filter for this purpose. Substitute your own sampling rate and transition width, of course. Tom ---
**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\ **Post date:** [September 5, 2011, 6:56pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/5 "2011-09-05T18:56:30Z")
Hi Marcus – I am selecting complex taps from the drop down, but still get an error at runtime if either cutoff frequency is 0 or smaller. Thanks, ## John ---
**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\ **Post date:** [September 5, 2011, 7:12pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/6 "2011-09-05T19:12:00Z")
On 05/09/11 12:55 PM, John Ackermann N8UR wrote: > Hi Marcus – > > I am selecting complex taps from the drop down, but still get an error at > runtime if either cutoff frequency is 0 or smaller. > > Thanks, > > ## John I just tried it, and it works just fine. I select: Complex-\>Complex (complex taps) from the menu, with a complex I/O, and I’m able to specify negative frequency components. What is the error you’re getting at runtime? – Principal Investigator Shirleys Bay Radio Astronomy Consortium > **[Live Draw HK Tercepat | Result HK | HK Pools | Live HK | HK Prize Hari Ini](https://www.sbrac.org/)** > > Temukan live draw hk tercepat melalui halaman hk pools yang resmi dan dapatkan result hk prize melalui live hk secara langsung di situs kami. ---
**Author:** ![John\_Ackermann\_N8UR](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/cdc98d/32.png) [@John\_Ackermann\_N8UR](https://www.ruby-forum.com/u/John_Ackermann_N8UR)\ **Post date:** [September 5, 2011, 7:47pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/7 "2011-09-05T19:47:51Z")
On 09/05/2011 01:10 PM, Marcus D. Leech wrote: > > > > I just tried it, and it works just fine. > > I select: Complex-\>Complex (complex taps) from the menu, with a complex > I/O, and I’m able to specify > negative frequency components. > > What is the error you’re getting at runtime? Using a band reject filter with low frequency set to -10000 and high frequency set to 10000, dropdown of complex(decimating), I get this: Traceback (most recent call last): File “/home/jra/gnuradio/fm\_rx.py”, line 166, in tb = fm\_rx() File “/home/jra/gnuradio/fm\_rx.py”, line 115, in **init** 1, 200000, -10000, 10000, 1000, firdes.WIN\_HAMMING, 6.76)) File “/usr/local/lib/python2.7/dist-packages/gnuradio/gr/gnuradio\_core\_general.py”, line 3437, in band\_reject return \_gnuradio\_core\_general.firdes\_band\_reject(\*args, \*\*kwargs) IndexError: gr\_firdes check failed: 0 \< fa \<= sampling\_freq / 2 ---
**Author:** ![AlliPlatt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/a88e4f/32.png) [@AlliPlatt](https://www.ruby-forum.com/u/AlliPlatt)\ **Post date:** [April 24, 2020, 5:03pm UTC](https://www.ruby-forum.com/t/transmit-filtering/211490/8 "2020-04-24T17:03:42Z")
Hi…I have an SDRPlay and we have this religious AM station here in DC/MD on 1120kHz and it blocks out alost every signal below 30 MHz. There is also this religious station out of SC where the guy sounds like a troll and just spews right wing propaganda, very annoying to listen to. I found two sites that sell traps but when you call they are out of business; clifton labs and some ham radio guy. I have tried everything else but the better the antenna the worse this gets. Even overloads my portable radios. I would make one but cant find the inductors/ dont have the equipment to make one.