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Re: [Discuss-gnuradio] USRP / DBSRX calibration

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] USRP / DBSRX calibration
Date: Wed, 26 Dec 2007 15:01:37 -0800
User-agent: Thunderbird (X11/20071115)

Jens Elsner wrote:

I noticed that (with gain 52) the USRP goes into saturation above -50
dBm and does not quite show linear behaviour below -100 dBm.
Not sure what you mean above. With gain of 52, the system gain is pretty high. For a strong signal like -50dBm you would want to have a lower gain.

I was trying to measure signals starting at roughly -100 dBm (400kHz).
That's why I chose a high gain. The "saturation effect" I'm seeing
around -50 dBm might well be due to numerical limitations of the
algorithm (integral over PSD estimate) used to calculate the power in
the band. I have to check on that again.
Should I be able to measure linearly over more that 50 dB dynamic range
with the USRP/DBSRX combination?

I would think so. Also, one thing to keep in mind is that the DBSRX has a very complex collection of settings for gain. You have 4 interacting parts --

- RF Gain (GC1, set by a DAC)
- Baseband gain (GC2, set by serial bus)
- DL, a 1-bit scale control set by serial bus
- Programmable gain amp, in the AD9862 ADC, set digitally

The current formula for setting for all of these is most likely NOT optimum. In fact, it may be quite far from optimal. If you care about dynamic range, you should take a look at the code in gr-usrp/src/db_dbs_rx.py and optimize it. You might even want to call the individual gain controls from your app instead of the main gain control function.

Furthermore, -165 dBm/Hz is a better sensitivity than my spectrum analyzer offers, which I find rather odd. Do these figures make sense?
Spectrum analyzers rarely have good noise figures. -165dBm/Hz translates to a roughly 9dB noise figure, which is a little high for the DBSRX, but not unreasonable. It is better than my spectrum analyzer.

Thank you very much for that insight. I was rather suprised, given the
20dB$ price difference.

I like the concept of dB$ !!!

In any case, typically the spectrum analyzer has a step attenuator at the front, and on most it is intentionally difficult to set it to 0dB. Mine has a minimum of 10dB, which just adds 10dB to the noise figure right there.


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