[DSP-10] Gain in receive chain question for W7PUA

[Bob Larkin]

Hi Courtney,

Great to hear from you.

First a couple of gain checks.  This is for the EZ-Kit, as the DSPx 
version is slightly different.  Put a 50 Ohm load on the antenna input, 
and bring up the bottom-line A/D and D/A bar-graphs.  With the RF gain 
full up at 100, I see all 5 white bars and 1 green bar of the "A/D 
Level" showing, and the noise number in the lower right corner is 7.0 
+/- (it varies some with tuning because of the crystal filter shape).

If I lower the RF gain to 64 the  A/D Level bar graph still shows 1 
green bar, but the level in the corner is down to about 3 dB.

As a cross check, with the RF gain at 100, the dB Offset at zero (ALT-2 
and -3) and 5 dB per division (ALT-1), the noise is in the bottom 
division, but somewhat above the center of the bottom division.

If you are far from this point, there is something wrong in the hardware 
area, most likely.

Yes the gain for the A/D is set near it's low level (I think 3 dB), 
which is the same as the EZ-Kit Lite uses for the monitor and related 
programs.  Itis not simple to control the gain dynamically, and higher 
values use up dynamic range.   Thus the single setting.

I hope this is all correct.  Its been a while ;-)

73, Bob  W7PUA

On 10/01/2011 06:55 PM, Courtney Duncan wrote:
> Bob, (and all)
>
> I've been studying the DSP-10 receive chain to understand how it works with respect to gain and noise figure.
>
> It doesn't look like there's enough gain on the boards I built, just 32 dB at RF, some filter and mixer losses, then 50 dB at 15 KHz.
>
> Then I looked at the AD1847 data sheet http://alsa.cybermirror.org/manuals/ad/ad1847.pdf and noticed that there is an ADC Programmable Gain Amplifier that runs 21-24 dB gain.  With that additional gain, I can see that it should work.  And it does work because I've been using it for six years....
>
> Do you use the gain in the AD1847 ADC and if so what do you have it set to?
>
> Courtney, n5bf/6
>
>
> "It's science. They don't know anything; they just make a lot of educated guesses."  -- Viannah Duncan
>
> Courtney Duncan, n5bf/6
> cbduncan at earthlink dot net
> 1296.100, 1.810.
> n5bf at amsat dot org
>
>
>
>
>
>
>
>
>
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