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[Linrad] Re: Linrad/sdr-IQ level adkustement?


There are 3 points at which you can control the gain of the SDR-IQ.
See the pdf manual.

1. A 10dB In/Out attenuator at the input.
For EME you do NOT want this attenuator in, unless you have
extraordinary gain before the SDR-IQ.

2. A variable gain in the AD8370 preamp, 6 to 34 dB.

3. The RCF shift of the 24 output bits to select which 16 bits you
will send to Linrad.

Spectravue labels these things differently, but the above is how it is
in the SDR-IQ :-)

In Linrad, Leif has configured the "Centre freq/Gain Box" to operate
over a gain range from 0db down to -30dB (I think).
This is a smooth control in 1dB steps (unlike Spectravue which makes
only 6dB steps available).
At some selectable point in the Linrad control, going down 1dB more
switches in the 10dB attenuator and
also increases the preamp gain by 9dB. You can select where that
My advice to you is to set that point to -25dB, so that effectively
the 10dB attenuator is never used.

The SDR-IQ should be run hard with lots of input and gain, to minimize
quantization noise.
Leif has described above how you might set that up.
You want to run the SDR-IQ just a few dB below the point where the
output overloads (red light).
You don't need a lot of headroom - EME signals are barely above the
When local power line noise starts up here, my red light used to come
on due to the noise
in the 2MHz or so BW of my preamp and transverter.
I put back a 200kHz cavity filter after my preamp to limit the BW, and
that fixed the PLN overload.

When driven close to output saturation, the SDR-IQ delivers too much
signal to Linrad.
The RCF shift allows you to select which 16 bits you want, probably
the top 16 or nearly so.
I was previously using 5 for this, Joe has suggested 6, which reduces
the output a bit more - 6 is good.
For EME, you want the Linrad wide graph to sit between 20 and 25dB.
Use the Linrad gain box to put the level there, I use about -15dB of
Linrad gain.

Good luck,

Guy  VK2KU

On Mar 9, 5:57 am, "f6dro" <f6...@xxxxxxxxxx> wrote:
> SDR-IQ IF gain? SpectraVue or Windows? Or do you mean any of the the gain
> settings in Linrad?
> I did my measurements on Spectravue as I wanted to vary the gain , it was an
> easy place to do this , while being able to use the " continuum" information
> which is very usefull.
> The gain I varied was in SDR-IQ setup:
> 1) RF gain : this one , I must not change it as I've found it's kills my
> noise floor
> 2) 6620 IF gain , originaly 24db , going to 0 in 6db steps , I noticed it
> changed almost nothing on the noise floor ( still preamp ON/preamp OFF
> measurement).
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