[hpsdr] ADC Question

[Larry Gadallah]

(This is a long overdue response to Andrew's question)

My casual and somewhat disorganized research and readings regarding this
lead me to a few conclusions:

  1. People want the maximum dynamic range possible, to allow operation in
crowded and/or high-signal environments
  2. People want low noise and maximum sensitivity to allow detection and
demodulation of very weak signals
  3. The want all of the conveniences and "bells and whistles" that SDR
allows

There is little doubt that current amateur radio SDR systems are already
close to meeting items 1 and 3 above. I have my doubts about item 2, for
the reason that Phil mentions: The ADCs that our DDC receivers use have a
relatively high noise figure. Sure, you can add preamplification ahead of
the ADC to help mitigate this, but that would degrade item 1 above.

What I have not found, which might put this question to bed, is an
explanation of the limits of how good the noise figure of a real 100 Msps
ADC implementation can be. If we are close to the limit already, then
perhaps this is a moot point. I suspect that there are some fab and
materials issues that cause these kinds of limitations, so perhaps if some
enterprising person designed a high speed ADC in GaAs or something like
that, we could get better performance, but I'd imagine that it would come
with a government/military price tag, not the cellular phone price tag that
we are used to now :-)

As to the difference in performance between SDRs and analog receivers, my
estimation is that the best HF analog receivers have a noise figure in the
range of ~10 dB, and a dynamic range of about 90 dB. Our SDRs have much
higher noise figures, in the 25-30 dB range, and a dynamic range of ~100
dB. I think the SDRs probably would outperform many analog receivers in
high-signal situations, but I think in many cases the analog receivers
would be better in low signal/low noise environments. It is a gross
oversimplification, but the SDRs are better in the high end of the
operating range, and the analog radios are better in the low end of the
operating range.

I have also read a lot of comments from people who seem to look at the ITU
atmospheric noise curves for suburban or rural areas and conclude that you
don't need a lot of sensitivity or a low noise figure, especially at lower
frequencies. However, I'd argue that top-band operators, MW DXers, and
anyone who has experienced a beverage antenna might disagree and there are
situations where an exceptionally low noise figure is a big benefit.

Hello Larry,
> I'm intrigued by what you mean by 'bit closer to analog performance'
> sounds like a backward step to me!?? As it is, you are very unlikely to
> suffer intermodulation issues or phase noise issues as Mercury / Hermes is
> way ahead of practically any analog receivers unless you run a really
> larger antenna system or like in an exceptionally high signal area.
>
> The IMD performance at full scale for Mercury / Hermes should be around
> 25-30dBm, which is excellent, but typical HF band noise should push any IMD
> into the noise due to the dither effect in the ADC.
> You will find it difficult to find an ADC better than the LTC2207, ADI and
> TI etc devices can't touch them.
> Andrew
> G4XZL
>
>
> ________________________________
>
>>  From: Phil Harman <phil at pharman.org>
>> To: Larry Gadallah <lgadallah at gmail.com>; Andrew <irbsurfing at
>> yahoo.co.uk>
>> Cc: hpsdr at lists.openhpsdr.org
>> Sent: Friday, 27 July 2012, 6:51
>> Subject: Re: [hpsdr] ADC Question
>>
>> Hi Larry,
>>
>> I confirm that the LTC2209 is a drop-in replacement for the 2208 and runs
>> fine at a higher clock frequency since I have one installed on a Mercury
>> board.
>>
>> The NF of the LTC2208 is about 30 dB  (as long as you don’t follow the
>> data sheet and resistively terminate the input) so is usable up to about
>> 17 m
>> without a preamp when connected to an efficient antenna.
>>
>> 73 Phil....VK6APH
>
>
Cheers,
-- 
Larry Gadallah, VE6VQ/W7                       larry AT gadallah DOT com
PGP Sig: B5F9 C4A8 8517 82AC 16B6  02B6 0645 69F0 1F29 A512
"If a man aspires towards a righteous life, his first act of
abstinence is from injury to animals." -- Albert Einstein.
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