[time-nuts] Spectrum Analyzer Specifications

Magnus Danielson magnus at rubidium.dyndns.org
Thu Mar 24 13:42:36 EDT 2016


Rick,

Depending on the "card" (it's more like USB dongle now) and their 
choices of ADCs and  DACs, you may or may not bypass the DC blocking caps.

I know that people playing around with laser-shows use this trick, but 
it was quite some time since I look at was is available.

Would be interesting to see what is available.

If one has ADCs and DACs, when one could implement the loop in digital, 
which have the additional benefit that adjusting it can be automated.

Cheers,
Magnus

On 03/24/2016 02:31 PM, Richard (Rick) Karlquist wrote:
> Thanks for the explanation.  I actually built
> something like that 30 years ago using an LT-1028
> op amp, only it used an HP3582 FFT box instead of
> a sound card (which didn't exist at the time).
> Does anyone have any experience with the low
> frequency cutoff of sound cards?  You would
> think 20 Hz, but maybe they go lower??  Any
> recommended sound cards (or USB things that
> work like sound cards)?
>
> Rick
>
> On 3/24/2016 4:42 AM, Bob Camp wrote:
>> Hi
>>
>> Pretty simple:
>>
>> Double balanced mixer, RPD-1 is one option, there are others.
>>
>> Fairly simple L/C lowpass between the mixer and an op-amp.
>>
>> 20 db positive (non-inverting) op-amp amplifier string after the mixer
>>
>> Output of the string goes to the sound card. Use a good (dual / quad)
>> audio op amp
>>
>> Quadrature amp picks off the output of the first op amp stage, switch
>> and resistors to set gain, pot to set op point.
>>
>> ====
>>
>> So what you have is an old style quadrature phase noise amp and “PLL”.
>> More or less a very junior version
>> of the 3048 test box. Like any setup of this sort, you check two
>> similar oscillators. They run in quadrature and
>> you do a few “measure this with switch in position A” sort of things
>> to set things up each time.
>>
>> Nothing exotic.
>>
>> Bob
>>
>>
>>
>>> On Mar 24, 2016, at 12:25 AM, Richard (Rick) Karlquist
>>> <richard at karlquist.com> wrote:
>>>
>>>
>>>
>>> On 3/23/2016 3:56 PM, Bob Camp wrote:
>>>
>>>> and standard covered. If you want to do spurs, spend $40 and build a
>>>> phase
>>>> noise test set that will drive the sound card on your PC.
>>>>
>>>> Lots of choices ….
>>>>
>>>> Bob
>>>>
>>>
>>> Any documentation on this $40 phase noise test set?
>>>
>>> Rick N6RK
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