Re: [Discuss-gnuradio] AGC on the RFX900 board

[Marcus D. Leech]

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

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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.

-- 
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Shirleys Bay Radio Astronomy Consortium
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