[time-nuts] Sub Pico Second Phase logger

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Dec 17 03:21:55 UTC 2008


Joe

Joseph M Gwinn wrote:
> Bruce,
>
>
> time-nuts-bounces at febo.com wrote on 12/16/2008 08:43:29 PM:
>
>   
>> Joe
>>
>> Joseph M Gwinn wrote:
>>     
>>> Bruce,
>>>
>>>
>>> time-nuts-bounces at febo.com wrote on 12/15/2008 06:42:59 PM:
>>>
>>>       
> [snip]
>   
>>>>>           
>>>> However the proposed remedy has little or no effect on the errors 
>>>>         
> caused
>   
>>>> by such bias currents (eg transistor base currents).
>>>> The series resistor could be reduced to zero without effect on the 
>>>>         
> mixer
>   
>>>> offset due to the bias current. However the preamp offset due to the
>>>> source resistance would be reduced.
>>>>
>>>>         
>>> Hmm.  It may be simpler than that.  With the TDS3012B and 34410A 
>>>       
> connected 
>   
>>> in parallel across the IF output of a mixer, the bias currents from 
>>>       
> the 
>   
>>> TDS3012B developed a voltage across the mixer load resistor, and this 
>>> voltage was sensed by the 34410A.  All the phase detector hadto do was 
>>>       
>
>   
>>> not short the bias current to ground.
>>>
>>>
>>>
>>>       
>> AC coupling?
>>     
>
> At the expense of phase shifts and temperature sensitivity, but yes.
>
> And it makes it hard to sense a DC signal, if that's the intent.
>
>
>  
>
>   
>>>>> I think that there are many top-end firewire soundcards.  Whatever 
>>>>>           
> the 
>   
>>>>> music folk like the sound of would be a good place to start - 
>>>>>           
> musicians' 
>   
>>>>> well-trained hearing can be quite good.  At least above 20 Hz.
>>>>>
>>>>> Actually, the people that make the AP192 do have firewire and usb 
>>>>> offerings:
>>>>>
>>>>> <http://www.m-audio.com/index.php?do=products.family&ID=recording>
>>>>>
>>>>>
>>>>>
>>>>>           
>>>> I've looked at all of the M-Audio offerings.
>>>> The more expensive ones have built in preamps plus 48V phantom 
>>>>         
> supplies,
>   
>>>> which can be switched off, however the presence of the switched +48V
>>>> supply is perhaps an invitation to disaster.
>>>>
>>>>         
>>> Given that capacitance to ground is more benefit than problem in this 
>>> application, I would be tempted to use a pair of back-to-back 
>>>       
> rectifier 
>   
>>> diodes as a clamp to protect the mixer IF output et al.  The 48 volt 
>>> phantom supply will be short-circuit protected, so current will 
>>> automatically limit.
>>>
>>>
>>>       
>> A pair of coupling capacitors at the preamp output combined with clamp
>> diodes to the amplifier power supply rails would work well even if the
>> +48V can't be switched off.
>> The +48V appears between the balanced pair conductors and ground.
>> Unfortunately the power available  from the phantom supply may not be
>> sufficient to power the mixer preamp.
>>     
>
> OK.  The power limit does make protection easy.  I gather that the limit 
> is a few milliamps, so even a 1N4148 would work.
>
>  
>   
>>>> I've also looked at the specs for several other high end sound cards.
>>>> Quite a few only have single ended inputs.
>>>> Maybe, I should document the various cards and highlight their
>>>> shortcomings etc for this application.
>>>>
>>>>         
>>> That would be very useful.
>>>
>>>       
>> I'll start on this shortly.
>>     
>
> Thanks.
>
>
>   
>>>>>> The 4V rms input allows the mixer preamp to use devices like the 
>>>>>>             
> THAT
>   
>>>>>> 1646 to drive the balanced sound card inputs without degrading the 
>>>>>> noise floor too much.
>>>>>>
>>>>>>
>>>>>>             
>>>>> Or build an isolation amp with some gain, and kill two birds with 
>>>>>           
> one 
>   
>>>>> stone?
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>           
>>>> A low noise isolation amplifier with a frequency response down to 1Hz 
>>>>         
> or
>   
>>>> so without using a transformer may be difficult to do.
>>>>
>>>>         
>>> People do make low noise common-base RF amplifiers, but 1 Hz would 
>>>       
> yield 
>   
>>> some pretty large bypass capacitors, even if the flicker noise can be 
>>> controlled well enough.  I would consider using ultracapacitors, which 
>>>       
>
>   
>>> didn't exist until very recently, and of course have very large 
>>> capacitance values.
>>>
>>>
>>>       
>> 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 of interest.
>>     
>
> But is a CB stage adequate?  Elimination of hum pickup is worth a lot.
>
>
>   
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.


>>>>>> With a 1V rms full scale the noise floor degradation would be very
>>>>>> obvious when using a THAT 1646 (equivalent devices are even 
>>>>>>             
> noisier).
>   
>>>>>> It may be better to use a mixer preamp with a transformer coupled 
>>>>>>             
> output
>   
>>>>>> stage using hybrid feedback to achieve a low frequency cutoff below 
>>>>>>             
>
>   
>>>>>> 1Hz together with low noise.
>>>>>>
>>>>>>
>>>>>>             
>>>>> With a transformer, even if toroidal, keeping hum out may prove 
>>>>>           
> quite 
>   
>>>>> difficult.
>>>>>
>>>>>
>>>>>
>>>>>           
>>>> High end (eg Lundahl LL1517) line output audio transformers come with 
>>>>         
> mu
>   
>>>> metal screens and metal foil interwinding shields.
>>>>
>>>>         
>>> They don't pass 1 Hz very well. I bet the rolloff is ~20 Hz. 
>>>
>>>
>>>       
>> When driven conventionally the transformer cutoff is around 20Hz,
>> however if one uses the appropriate driver with a controlled negative
>> output R to cancel the transformer primary internal winding resistance,
>> the low frequency response can be extended significantly. This also
>> reduces the low frequency distortion.
>> However individual adjustment of driver to suit transformer is required
>> and tracking the winding resistance over temperature may be an issue.
>>     
>
> That would certainly work.  See next.
>
>  
>   
>>> Certainly one can build a VLF transformer, but it will be a project 
>>>       
> for 
>   
>>> sure, and the transformer may be quite large.
>>>
>>>
>>>       
>> The transformers only weigh about 65g.
>>     
>
> This weight estimate assumes the negative resistance circuit I assume, the 
> intent being to allow use of the Lundahl LL1517 transformer.
>
>   
Yes.
> You might be interested in the following article:  "A Broadband Active 
> Antenna for ELF Magnetic Fields" by John F. Sutton and G. Craig Spaniol" 
> in Physics Essays March 1993, Vol 6, #1, 1993.  The negative-impedance 
> trick is expanded upon.  Sutton also has some US patents on this.  US 
> patent 5,296,866 covers the antenna, but is hard to understand without the 
> article.
>
>
>   
>> It may be simpler just to select a mixer for which the IF ground can be
>> isolated from the RF and IF 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.
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.

The noise problem with audio balanced drive chips can easily be overcome
with a discrete implementation.
That is discrete resistors and IC opamps.


>  
>
>   
>>>>>>>               
>>>>>> No, I meant replace his 90 degree hybrids with a digital 
>>>>>>             
> equivalent.
>   
>>>>>>             
>>>>> I believe that his 90-degree hybrids are already digital.
>>>>>
>>>>>           
>>>> I'm not convinced of that, if only because real time 10,000+ tap FIR
>>>> filters at 30+MSPS are probably still impractical.
>>>>
>>>>         
>>> I'm not convinced that one needs a 10,000-tap FIR to achieve this, and 
>>>       
> Sam 
>   
>>> Stein is one smart fellow.  I recall some NASA patents from twenty 
>>>       
> years 
>   
>>> ago on how to get I+Q data from a single ADC, and while there was FIR 
>>> processing of some kind, there were only maybe 8 or 16 taps. And 
>>>       
> Tayloe 
>   
>>> (US patent 6,230,000) gets much the same effect with one resistor, 
>>>       
> four 
>   
>>> capacitors, an analog mux, and two differential amplifiers.
>>>
>>>
>>>       
>> 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 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.

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.
This allows a wide variety of sound cards to be evaluated without one
person having to test them all.

Bruce



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