Thanks, Marcus, for your prompt and detailed response. If I understand you correctly:
1. The best case noise figure of the RFX900 daughterboard is about 3-3.8 dB. This occurs when the mixer amplifiers are set to their highest gain setting. 2. These gain settings can be fixed via software (via UHD). 3. UHD distributes the single programmed gain value over the mixer and the PGA to optimize for the noise figure. I looked up the UHD app. notes. For RFX boards (as you probably already know :)) there is a single gain setting: PGA0. I found this at: http://www.ettus.com/uhd_docs/manual/html/dboards.html#rfx-series So if I want the best case noise figure, would it be sufficient to set PGA0 to 70 dB? As a side note, we found that for a given USRP motherboard (and hardware settings), the received power as seen in software changes from one RFX900 board to another by as much as 10 dB. Is this expected? Thanks again, Regards Sundar On Sun, Jul 3, 2011 at 8:42 PM, Marcus D. Leech <mle...@ripnet.com> wrote: > On 07/03/2011 08:05 PM, Sundaram Vanka wrote: > > Hi, > > I have a few questions about the (analog) AGC on the AD8347 mixer chip on > the Flex900 daughterboard. > > 1. Given a signal received with some known power, is it possible to read > the programmed AGC gains (and thereby find the mixer's noise figure)? > 2. Is it possible to lock these gain settings? If not, can the AGC > functionality be disabled? > 3. How fast does the AGC track changes in signal power? > 4. If (1), (2) are not possible, is it still possible to know the RX noise > figure? > > I'd appreciate some inputs :) If I understand correctly, to calibrate the > receive SNR, we need to know the RX noise floor (seen in software). For this > we need to know the RX noise figure... > > To provide some context, the following is our immediate application: we > have a coded 16QAM system implemented on GNURadio. We want to measure the > Frame Error Rate vs. Es/No ("per-symbol SNR") for this system. The TX and RX > have separate USRP boards, each with a RFX900 daughterboard. A coaxial cable > connects the TX and RX antenna ports. With this setup, we vary the transmit > power and measure the frame error rate. To compare these results with a > Matlab model, we need to know the received Es/No: for that we need the RX > noise figure...and hence my question above. > > I'd appreciate your help. > > Thanks in advance, > Regards > Sundar > > _______________________________________________ > Discuss-gnuradio mailing > listDiscuss-gnuradio@gnu.orghttps://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > The gain control on the RFX900 is all purely manual, unless you modify the > hardware for AGC. > > Figure 19 on the AD8347 datasheet shows noise figure vs gain settings for > the mixer, but on the > RFX900, there's an LNA in front of it, which at least for higher mixer > gain settings very much > offsets the conversion noise. The very best noise figure on the mixer is > at highest gain, giving roughly > 5dB for the mixer. Dividing this by the 13.2dB LNA gain and adding the > noise of the MGA82563 LNA > (2.2dB) gives a best-case noise figure of something on the order of 3dB. > In reality, you're likely > looking at closer to 3.8dB noise figure, due to connector and trace > losses in front of the LNA, etc. > > The gain setting in UHD/Gnu Radio should correspond fairly-precisely to the > gain setting on the > RFX900 mixer, which provides something like 69dB of total gain range. In > most cases in RF systems > like this, gain-control is implemented using variable attenuators in > front of the gain element, so > effective noise figure becomes worse as you lower the gain. I believe > that UHD attempts to optimize > its gain distribution for noise figure, but you can also manually set the > individual gains that > are available. On the USRP, there's a PGA in front of/part-of the RX > ADC, in addition to the gain control > on the AD8347 mixer chip, and UHD will distribute gain settings over > these elements. > > -- > Principal Investigator > Shirleys Bay Radio Astronomy Consortiumhttp://www.sbrac.org > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
_______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
