[time-nuts] Sub Pico Second Phase logger

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Dec 17 20:43:16 UTC 2008


Joe

Joseph M Gwinn wrote:
> Bruce,
>
>
> time-nuts-bounces at febo.com wrote on 12/16/2008 10:21:55 PM:
>
>   
>> Joe
>>
>> Joseph M Gwinn wrote:
>>     
>>> Bruce,
>>>
>>>       
> [snip]
>   
> Bruce wrote:
>   
>>>> A CB stage probably isn't optimum for the mixer preamp so that lower
>>>> value caps can be used provided that they effectively short the
>>>> amplifier input resistor Johnson noise at the frequencies ofinterest.
>>>>
>>>>         
>>> But is a CB stage adequate?  Elimination of hum pickup is worth a lot.
>>>
>>>
>>>
>>>       
>> [Bruce] Text should have been:
>>
>> A CB stage probably isn't optimum for the mixer preamp so that 
>> a low noise preamp with a higher input impedance can be 
>> employed allowing lower
>> value coupling caps to be used provided that they effectively 
>> short the amplifier input resistor Johnson noise at the 
>> frequencies of interest.
>>     
>
> OK.  The form of input amplifier is one of the tradeoffs one must make.
>
>
>
>   
>>>> It may be simpler just to select a mixer for which the IF ground can 
>>>>         
> be
>   
>>>> isolated from the RF and LO grounds.
>>>> However a preamp with a transformer output may be useful if one uses 
>>>>         
> a
>   
>>>> mixer where all the grounds are connected together by the package.
>>>>
>>>>         
>>> It has to be far easier to select the right mixer than to deal with a 
>>>       
> 1 Hz 
>   
>>> transformer.  And cheaper.
>>>
>>>
>>>       
>> I've been advocating this for some time, however one can then no longer
>> just buy an off the shelf mixer complete with SMA connectors, one has to
>> design and assemble a suitable PCB.
>>     
>
> Actually, for quantity one, I don't bother with PCBs.  I use 1/16 inch 
> thick glass-epoxy Vectorboard with a 0.1" hole pitch, and thread bare wire 
> through the holes.  Given that these are effectively prototype boards, the 
> ease of wholesale change is very useful.  For transmission lines, I would 
> just run miniature coax or homebrew twisted pair from point to point on 
> the board, although it has not been neecessary yet.  If microphonics is an 
> issue, the board can be conformal coated to glue the wires in place. 
>
> The traditional coat, wax, allows easy alteration and repair.  In the 
> 1960s, a friend was building things using RTL (Resistor Transistor Logic) 
> ICs and solderable magnet wire between the leads.  What a rats' nest that 
> was.   The problem was how to package this so it wouldn't fail when 
> bumped.  The solution was to build the circuit on a sheet of vectorboard 
> in a 12" by 12" rectangular baking pan, and then fill the pan with hot 
> wax.  Whenever something had to be changed, melt the wax, pour it off, 
> make the change, pour the wax back into the pan, allow to cool.
>
> Modern ICs connected with point-to-point wires and potted in wax - it's an 
> odd mix of technology ages.  Like implementing stone-age tools with modern 
> ceramics.
>
> If I were building receivers, I suppose I would be forced to use 
> surface-mount comoponents and PCBs.
>
>
>   
>> 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.
>>     
>
> 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.


>> The noise problem with audio balanced drive chips can easily be overcome
>> with a discrete implementation.
>> That is discrete resistors and IC opamps.
>>     
>
> Yes, even on vectorboard.  I do it all the time.
>
> Eventually, the supply of through-hole components will dry up, but it 
> hasn't happened yet, and some components from the 1970s are still 
> available.  Even if the original manufacturer is long dead.
>
>
>   
>>>> I have read similar papers from that era on radar signal processing.
>>>> They either used a Hilbert transform or a pair of digital filters 
>>>>         
> whose
>   
>>>> outputs were in phase quadrature.
>>>> The quadrature accuracy for a given bandwidth depends on the number
>>>> of taps.
>>>> The beat frequencies (in a dual mixer system) won't match exactly and
>>>> some correction for the resultant phase shift errors will need 
>>>> to be made.
>>>> This may be less of a problem when the 2 beat frequency signals are
>>>> identical in frequency and just differ in phase.
>>>>
>>>>         
>>> So long as we know the exact frequency, even if it isn't the exact 
>>>       
> desired 
>   
>>> frequency, all may be well.
>>>
>>> Joe
>>>
>>>
>>>
>>>       
>> I'm reminded of some phase recovery algorithms used in phase shift
>> interferometry that largely negate the effect of small fixed 
>> phase errors.
>>     
>
> Yes.  A detailed math analysis of the test setup will be needed.
>
>  
>   
>> 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 using different 
> tools also allows us to distinguish test artifacts from processing 
> artifacts.
>
>
> Joe
>   

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

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


Test frequencies:

100Hz

1kHz

Sound card sample rate:

~24KSPS

Test duration:

1000 sec

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.

Bruce



More information about the time-nuts mailing list