[time-nuts] Spectrum Analyzer Specifications

[Bob Camp]

Hi

Most of mine used OP-27 and OP-37 op amps. The typical destination was a 3561
in most cases. It was nice because it had built in “normalize to 1 Hz” capability. 

Many sound cards will go below 20 Hz. The problem is often that the noise rises 
fairly quickly as you go below 10 Hz. You also get into some interesting settings issues
with the normal operating system based drivers. “Twitter needs to change the audio level
to send you an alert” is a gotcha.

Bob

> On Mar 24, 2016, at 9:31 AM, Richard (Rick) Karlquist <richard at karlquist.com> 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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