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Re: System Gain
- Subject: Re: System Gain
- From: Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>
- Date: Fri, 21 Dec 2007 21:52:22 +0100
Hi Guy and Dave,.
> I'm not sure exactly how the SDR-14 differs in architecture from my
> or how Leif has implemented the control of gain/attenuation through
Linrad receives 16 bit integers as they are sent by the AD6620
and converts to floating point.
It is possible to introduce quantization noise within Linrad
by setting a small gain and then force conversion from float
to 16 bit integers (for mmx routines) but that would be mis-use
of Linrad. In normal operation, the gain issues are exclusively
a SDR-xx thing.
> or indeed how the 144MHz input works - presumably through a converter
> down to 28MHz.
No. One uses aliases. The analog bandwidth is 200 MHz (or more)
The sampling clock at 66.6666 MHz has an overtone at 133.333 MHz.
If one sets the NCO for 10.767 MHz one will receive signals from
many alias frequencies one of which would be 144.1 MHz.
One needs a selective preamplifier to remove undesired aliases.
> But ... the first thing I would be checking is that you are not
> inadvertently using some
> front-end attenuation in the SDR-IQ, as I found I was with the SDR-IQ.
> When I unplug the 28MHz input to my SDR-IQ, the Linrad noise floor
> drops by about 17dB.
There is no such thing on the direct input. It goes straight into
the A/D converter chip.
Leif / SM5BSZ
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