[USRP-users] TwinRX Gain

Marcus D. Leech mleech at ripnet.com
Thu Jul 13 20:31:46 EDT 2017

On 07/13/2017 07:57 PM, Eugene Grayver via USRP-users wrote:
> Hello,
> I am trying to understand the ‘actual’ gain in TwinRX.  
> Experimentally, I see that a 0 dBm input requires gain of +46 dB to 
> give a full-scale (-1/1) output. Interestingly, the two channels have 
> about 3 dB of difference in gain (I would have expected a lot less).  
> Since the A/D expects about 0 dBm (based on the datasheet), it appears 
> we only have 50 dB of actual gain available.  Is that correct? What 
> does the 95 dB of gain mean?
> Thanks.
> _______________________
> Eugene Grayver, Ph.D.
> Aerospace Corp., Sr. Eng. Spec.
> Tel: 310.336.1274
> ________________________
That's the gain-control range, distributed among (AFAIR) a couple of 
different variable-gain stages.

The common-mode reference voltage on the X310 ADC (I assume you're 
talking about an X310 motherboard?) is, AFAIR, set to 1.5V, which amounts to
   about +7dBm.  The ADC is 14 bits, so the smallest signal it can 
resolve is about -73dBm.  This is a very rough estimate.

In front of this, there are fixed and variable gain stages, including a 
variable gain stage in the ADC itself.   The mixer stage(s) may also 
have a small
   amount of conversion gain, I don't know off the top of my head.

But the 95dB figure is the total gain-control range, which is the sum of 
the gain controls as distributed through the conversion and gain chain 
on the card.

Since RF components will typically have actual delivered gain that may 
vary from "nominal" by anywhere from 0.5 to 2dB in the same part, you cannot
   reliably predict how much actual gain any given gain *setting* will 
produce, even at the same frequency  (RF parts also vary in gain with 
   so for a widely-tunable card like TwinRX, you cannot assume that the 
*actual* gain will be the same across all frequencies for the same gain 

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