[time-nuts] Sub Pico Second Phase logger

[Bruce Griffiths]

Joe
Joe Gwinn wrote:
> This is from home.  I'll not be at work until next year.
>
> At 11:48 PM +0000 12/18/08, time-nuts-request at febo.com wrote:
>   
>> Message: 4
>> Date: Fri, 19 Dec 2008 12:17:33 +1300
>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>> Subject: Re: [time-nuts] Sub Pico Second Phase logger
>> To: Discussion of precise time and frequency measurement
>> 	<time-nuts at febo.com>
>> Message-ID: <494ADA0D.3060004 at xtra.co.nz>
>> Content-Type: text/plain; charset=ISO-8859-1
>>
>> Joe
>>
>>
>>  >>>      
>>     
>>>>  [BG] Proposed test setup:
>>>>  (preliminary to be refined)
>>>>
>>>>  Drive 2 sound card inputs in parallel with the same source.
>>>>
>>>>  Source amplitude:
>>>>  Max sound card input -3dB
>>>>    
>>>>         
>>>  What kind of dB?
>>>
>>>  
>>>       
>> Peak input signal voltage = 70% of sound card maximum peak input voltage.
>> Just to leave some margin for gain tolerances.
>>     
>
> OK.
>
>
>   
>>  >> Sources:
>>     
>>>>  1) Wien bridge or equivalent (eg state variable oscillator with soft
>>>>  clamping) relatively low distortion oscillator.
>>>>
>>>>  2) Buffered low pass filtered output of binary divider driven by a
>>>>  crystal oscillator
>>>>    
>>>>         
>>  > RC oscillator sounds far simpler and more flexible.
>>
>> A Wien bridge using a lamp is perhaps the simplest.
>> I'll create a circuit schematics for this using an OPA2134 (dual lowish
>> noise JFET opamp).
>> One opamp for the oscillator one to drive the sound card (attenuation of
>> the oscillator output will be required for some sound cards and it is
>> desirable to have a low output impedance driver).
>>     
>
> Jim Williams of Linear Technology had a very good low-distortion AGC 
> controlled Wein Bridge oscillator.  If I recall, he used a photo-FET 
> or the like as the servoed resistor in the bridge.  There may be an 
> application note on the LT website, but I saw it in a chapter of a 
> book on analog circuitry, the chapter author being Jim W.
>
>
>   
I have the application note.
>>  >> Test frequencies:
>>     
>>>>  100Hz
>>>>
>>>>  1kHz
>>>>    
>>>>         
>>>  Why no 10 Hz?  (Well, 20 Hz.)
>>>
>>>  
>>>       
>> No particular reason other than some complications if a lamp stabilised
>> oscillator is used.
>> A diode soft (series R) clamped RC oscillator is more flexible in this
>> regard.
>> I'll also produce a circuit schematic for one of these oscillators.
>>     
>
> Jim Williams' circuit would handle 10 Hz if I recall.
>
>
>   
>>  >> Sound card sample rate:
>>     
>>>>  ~24KSPS
>>>>    
>>>>         
>>>  I assume that this is the lowest rate supported, and certainly is overkill
>>>  for 1 KHz.
>>>
>>>  
>>>       
>> It varies with the sound card.
>> I just suggested that for a starting point in the discussion.
>>
>> For an AP192 the directly (without sample rate interpolation) available
>> output sample rates are:
>>
>> 192, 96, 64, 48, 32, 8 KSPS.
>>     
>
> OK.  I would start with 8 ksps.  We will end up decimating below that 
> anyway, except for 1 KHz test signals.
>
>
> Joe
>
>   
I'll breadboard an oscillator over the holidays and take some samples.

Bruce