# Linrad Gain and Noise Floor

• Subject: Linrad Gain and Noise Floor
• From: VK2KU <clearmail.com.au; vk2ku@xxxxxxxxxxxxxxxx>
• Date: Fri, 28 Dec 2007 20:37:55 -0800 (PST)

```Hi Leif,

I am shifting this to another thread since we have moved way off the
original topic!

>If you make a graph of noise floor level as measured
>by Linrad vs gain you will find a straight line with
>a knee where the noise floor goes to zero.
>
>The knee is soft, it ranges over something like 20 dB.
>The statement that your noise floor should increase
>by 15 dB is valid only if it already is above the knee.
>Otherwise it has to increase by at least 15 dB above
>the level you would get by extrapolating the straight line
>below the knee. Use the RCF output shift to set gain in
>6dB steps and check the noise floor you get.
>Easiest if you change par_sdr14 with a text editor and

I tried this today both with the preamp power on and off,
though I think you intended it to be off.
Within the limited accuracy of my measurements, each change of 1 in
the RCF Shift gives a 6dB change in the Linrad Noise Floor.
With preamp off, noise floor runs from 39 to -2 as RCF Shift runs from
0 to 7.
With preamp on, noise floor runs from 54 to 13 as RCF Shift runs from
0 to 7.
The whole noise floor is Red when the noise floor is 54, regardless of
the setting of the 2 selective limiters, presumably because of my
value of 4000 for Selective Limiter Max Level.
No evidence there of any knee, so presumably I am above it.

However I did notice a "funny" in the main spectrum scale when I used
the arrows on the right to move the spectrum up to show below 0 on the
scale.
Instead of nice scale increments in fives, in one case I got a scale
of:
24, 19, 14, 9, 4, -0, -5, -10
and in another slightly different scale position:
35, 30, 25, 20, 14, 9, 4, -0, -5.
Sorry to keep finding these little things!