[time-nuts] Spectrum Analyzer Specifications

Tom Miller tmiller11147 at verizon.net
Thu Mar 24 13:44:36 EDT 2016


Most of the USB sound cards have a capacitor coupled input. If you jumper 
the cap they will go down to DC.

T
----- Original Message ----- 
From: "Richard (Rick) Karlquist" <richard at karlquist.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Thursday, March 24, 2016 9:31 AM
Subject: Re: [time-nuts] Spectrum Analyzer Specifications


> 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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