[time-nuts] Sub Pico Second Phase logger

Joseph M Gwinn gwinn at raytheon.com
Thu Dec 18 22:20:07 UTC 2008


Bruce,

time-nuts-bounces at febo.com wrote on 12/17/2008 03:43:16 PM:

> Joe
> 
> Joseph M Gwinn wrote:
> > Bruce,
> >
> >
> > time-nuts-bounces at febo.com wrote on 12/16/2008 10:21:55 PM:
> >
[snip]
> > 
> >> [BG] Obtaining suitable mixers for 5MHz and 10MHz input frequencies 
or even 
> >> 100MHz is easy.
> > 
> >> However for the higher microwave frequencies most mixers come 
complete
> >> with connectors attached and share a common ground.
> >> 
> >
> > [JG] True.  However, I don't think we will be going from 1 GHz to 1 Hz 
in a 
> > single step, and the last mixer can have separate grounds.
> >
> >
> > 
> An upper limit of at least 100MHz should be feasible for the final 
mixer.
> A dual conversion scheme will be essential if one uses a triple balanced
> or similar first mixer that has an IF response that doesn't extend down
> to the low frequencies that a sound card can use.

Yes.  We will see if it's needed.


> > 
> >> [BG] If we can devise a suitable test setup then one could just log 
the
> >> samples to a file for whatever sound card one has and make the data
> >> available to others for analysis.
> >> 
> >
> > Yes.
> >
> >
> > 
> >> This allows a wide variety of sound cards to be evaluated without one
> >> person having to test them all.
> >> 
> >
> > And evaluation of the same test data by multiple people usingdifferent 

> > tools also allows us to distinguish test artifacts from processing 
> > artifacts.
> >
> 
> [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?

 
> 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.

 
> Test frequencies:
> 
> 100Hz
> 
> 1kHz

Why no 10 Hz?  (Well, 20 Hz.)

 
> Sound card sample rate:
> 
> ~24KSPS

I assume that this is the lowest rate supported, and certainly is overkill 
for 1 KHz.

 
> Test duration:
> 
> 1000 sec

At least initially, but we will need longer datasets to see thermal 
effects clearly.

 
> File format:
> 
> Wave file??
> Resolution 24 bits for 24 bit sound cards, 16 bits for 16bit and lower
> resolution sound cards, etc.
> 
> Some refinement of sample rates and test duration is required to keep
> the data file sizes manageable.
> With a 24 bit sound card sampling at 96KSPS or 192KSPS for 1000sec can
> produce file sizes of 1GB or more.
> Some preprocessing (low pass filter and decimation) may also be 
required.

I agree that a simple preprocessor will be needed.  This would be the 
place to convert from the raw adc 16 or 24 bit format into something 
universal, perhaps 24 bit or 32 bit (with zero padding as needed).  It 
probably should be written in C, for speed and portability.  I expect that 
there are open-source libraries available to read and write wav files, and 
many analysis programs will accept wav.  However, it would be easy to make 
the preprocesor able to emit other formats as needed. 



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