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

[Courtney Duncan]

On Oct 6, 2011, at 2:59 PM, Bob Larkin wrote:

> 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).
> 

OK, I did this.  A/D is 5 bars.  Level (for SpecAve 8 6, SpecAnl Non 4800) in lower left corner is about 9.5

> 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.
> 

With RF Gain 64, everything else the same, I get 5 bars and lower left corner 4.0 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.
> 

My noise is right above the bottom 5 dB division.  I would call it 6 dB with some filter ripple rolloff on the edges.

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

So it is working as designed.

The problem was not a suspected malfunction with the box, which has always worked great, the problem was understanding how we're getting from the power output of that terminator, something like -137 dBm in the whole sampled bandwidth (counting first stage noise figure), up to something that is enough to sample with at least five bits showing on the A/D level, that would have to be around a millivolt at 10K ohms, or about -70 dBm (am I calculating that right?) with only the 82 dB gain mentioned in the article (32+50).

Now that I've said it that way, it looks plausible, so maybe I've answered my own question, especially with the 3 dB in the A/D.

I'll look at it some more.

I've just closed out construction on the 1296 unit today, with help from the designs of W1GHZ, W6PQL, VK5EME, and DEMi among others.  It copies a weak beacon 200 miles away at -130 to -150 dBm, depending on conditions, and talks to people on the Friday CW/SSB local net putting out about 20 watts to a medium gain antenna.  I'll have the story up on my website (recently moved) in a few days, then after Sweepstakes plan to work on some DSP-10 software, in some form.

Thanks for the help,

Courtney, n5bf/6

> 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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"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