[Moon] [Moon-Net] SDR IQ stability

Peter Blair pkb100 at btinternet.com
Thu Apr 9 22:49:42 CEST 2020


Thanks for the suggestions Joe, yes we are all OK here thank you, all locked 
down and with wonderful neighbourly support. I hope its that it's the same 
at your end. Lets hope that we can all get through it safely.
Vy 73 Peter G3LTF

-----Original Message----- 
From: Joe Taylor
Sent: Thursday, April 09, 2020 2:55 PM
To: Peter Blair ; freetalk moonnet ; Moon-Net
Subject: Re: [Moon-Net] SDR IQ stability

Hi Peter,

On 4/9/2020 07:49, Peter Blair G3LTF via Moon-net wrote:
> To save me repeating what others may have already done....has anyone 
> actually looked at the gain stability (at the 0.1 / 0.05 dB level)  of an 
> SDR IQ with a Spectravue continuum display with time? Im thinking about 
> supply voltage effects, for instance is it better supplied directly rather 
> than via USB from PC, or temperature effects.

> I’m looking at radio stars for calibration, like Taurus at 13cm, so 
> typically 0.4dB or so with 190kHz BW. I dont think this is what SDR IQ was 
> designed for! I know from other work that things like the 78 series 
> voltage regulators have a dV/ dT change which can affect stage gain.
> I know everything else in the system is prone to gain drift but I want to 
> know if the final “measuring instrument” is stable and over what period.
> 73 Peter G3LTF

I hope you are well and staying safe.

Probably you are already doing these things, but just to be sure:

For basic radio astronomical measurements it's best not to assume or
depend on gain stability in the Rx chain.  Instead, make differential
measurements, for example:

1. Observe the power difference between "on source" and off source by
+/- several beamwidths in Azimuth.

2. Point several beamwidths "ahead" of your target source in hour angle,
and let Earth rotation move the source through your beam.

In either case, you measure power difference between an off-source
baseline and "on source".

-- 73, Joe, K1JT 




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