Hi all Can someone please provide me with some guidance? We are working on a passive radar project and the aim is to implement a pcl radar receiver on the USRP1 using TVRx daughterboards. I have the Microtune 7901. I did some tests to see what the effects of the RF and IF amplifiers in the tuner are (linear region,etc.). I noticed that the maximum output of the TVRx is -15dBm regardless of the input signal and the gains. For this measurement, I've got the test circuit that they have in the Microtune datasheet with the Minicircuits RF transformer to match the 200 ohm output of the TV tuner to 50 ohms. Also, I captured a signal from the TVRx and looked at it in Matlab. The maximum I value is 17 and the maximum Q value is 16, which means that we are not close to using the whole dynamic range. I also did a test with the BasicRx db. This time I injected a 13dBm signal into RX-A. The maximum value in Matlab is 7919 which looks much better. Now I have 2 questions: -Does anyone know anything about the TVRx and why I only get an output of -15dBm? The Microtune datasheet is a bit vague. It only says that the signal level for 1dB RF gain compression is 72dBuV (-113dBm) with AGC deactivated. Maybe the TVRx wasn't designed to utilise the entire dynamic range of the ADC?? -What is the maximum signal level after the DDC+filtering,etc? My guess is around 2047*8 because of the bit-shifting at the ADC interface. Sebastiaan -- Sebastiaan H. Radar and Remote Sensing Group, University of Cape Town Tel: +27 83 305 5667

on 2009-02-25 16:57

on 2009-02-26 09:23

Sebastiaan H. wrote: > in the Microtune datasheet with the Minicircuits RF transformer to > match the 200 ohm output of the TV tuner to 50 ohms. Also, I captured > a signal from the TVRx and looked at it in Matlab. The maximum I > value is 17 and the maximum Q value is 16, which means that we are not > close to using the whole dynamic range. There are a couple of things to note here. First, the 200 ohm output of the microtune is plenty to drive the ADCs which have an input impedance close to 1K. So using a transformer to step the impedance down to 50 ohms gives you a loss of 6dB (2x voltage). Second, At 0dB of PGA gain, the maximum input to the ADCs is 2Vpp differential, which is 10dBm in a 50 ohm system. The programmable gain amplifier gives you 20dB of gain, so with the gain at max, the input needed to clip would be -10dBm, which is about right for the max output from the TVRX. > I also did a test with the BasicRx db. This time I injected a 13dBm > signal into RX-A. The maximum value in Matlab is 7919 which looks > much better. 13dBm will clip the ADC. 10dBm is the real max full scale value. > Now I have 2 questions: > -Does anyone know anything about the TVRx and why I only get an output > of -15dBm? The Microtune datasheet is a bit vague. It only says that > the signal level for 1dB RF gain compression is 72dBuV (-113dBm) with > AGC deactivated. Maybe the TVRx wasn't designed to utilise the entire > dynamic range of the ADC?? With the PGA, it uses most of the dynamic range. Matt

on 2009-03-06 14:45

Matt Sorry to bother. Please tell me why I lose 6dB when measuring the signal at pins 17 and 18? I have the RF transformer from Minicircuits that is supposed to match the 200ohm output of the TVRX to a 50ohm so that I can measure it on a spectrum analyser. Because it is matched, I should not have a 6dB loss? I believe the input impedance of the ADC is around 220ohms? How does the BasicRx work? The input is 50ohms, so is there an RF transformer on the BasicRx that matches this to the ADC? Thanks in advance. Sebastiaan

on 2009-03-08 09:22

Sebastiaan H. wrote: > Matt > > Sorry to bother. Please tell me why I lose 6dB when measuring the > signal at pins 17 and 18? I have the RF transformer from Minicircuits > that is supposed to match the 200ohm output of the TVRX to a 50ohm so > that I can measure it on a spectrum analyser. Because it is matched, > I should not have a 6dB loss? While the transformer does not lose power, you have to understand that the ADC is a voltage input device. A transformer that converts 200 ohms to 50 ohms divides the voltage in half. When we say that the full scale level is 10 dBm, it actually means 2V peak to peak. Once you put it into the transformer, you'll only have 1V peak to peak. > I believe the input impedance of the ADC is around 220ohms? How does > the BasicRx work? The input is 50ohms, so is there an RF transformer > on the BasicRx that matches this to the ADC? It is a voltage input. We match the input with a 50 ohm terminating resistor. Matt

on 2009-03-08 20:44

Which can easily be seen if you realize that in ideal Passive transformer no power is added or lost. Remember : P = (U*U)/R No let's use it for the transformer: Pin = (Uin*Uin)/200 Pout = (Uout*Uout)/50 Passive no loss transformer: Pin = Pout So: (Uin*Uin)/200 = (Uout*Uout)/50 <=> (Uout*Uout) = (Uin*Uin) * (50/200) = (Uin*Uin)/4 So: Uout = Uin/2 There you have the factor 0.5! Kees (W1/PE1LDW)