[time-nuts] Sub Pico Second Phase logger

Joseph M Gwinn gwinn at raytheon.com
Tue Dec 16 23:54:29 UTC 2008


Bruce,


time-nuts-bounces at febo.com wrote on 12/15/2008 06:42:59 PM:

> Joe
> Joseph M Gwinn wrote:
> > Bruce,
> >
> >
> > time-nuts-bounces at febo.com [Bruce] wrote on 12/15/2008 04:56:26 PM:
> >
 
> >>>>> [snip]
> >>>>>
> >>>>>
> >>>>> 
> >>>>>> The only configuration for which it makes any sense is an 
inverting
> >>>>>> input amplifier with a finite input voltage offset.
> >>>>>>
> >>>>>>
> >>>>>> 
> >>>>> Why would non-inverting not work?  Both inputs source or sink bias 

> >>>>> currents, and non-inverting presents a very high impedance.
> >>>>>
> >>>>> 
> >>>> Non inverting amplifiers usually have lower noise and generally 
work 
> >>>> very well.
> >>>> 
> >>>> I was only trying to come up with a preamp circuit for which the
> >>>> comments in the Minicircuits application note on the effect of 
> >>>> amplifier input offset voltage made any sense.
> >>>>
> >>>> 
> >>> Ah.  It may be hopeless.
> >>>
> >>> My reading was that they were worried about bias currents from the 
amp 
> >>> flowing into the mixer and causing offsets, not amplifier offset 
voltages 
> >>> per se.  The amplifier offset voltage does not cause a mixer offset, 

> >>> and may be reduced by use of a chopper amp or very good balance.
> >>> 
> >
> > By the way, I've noticed that Tek TDS3012B oscilloscope inputs can 
cause 
> > offsets as well, again I assume from the bias currents.  The circuit 
has 
> > the scope input in parallel with the Agilent 34410A 6.5-digit 
voltmeter. 
> > With scope input set to DC, big effect.  Set to AC, small effect.  Set 
to 
> > Gnd, no effect.  (Input is not grounded, so voltmeter is still happy.) 

> > Didn't try changing the input volts/cm scale.  Anyway, I think that 
this 
> > effect is what the mystery app note was trying to say.  A bias current 

> > from the scope would cause a voltage offset that depended on the DC 
> > resistance through which the bias current flowed, the DC load of the 
mixer 
> > in this case.
> >
> > 
> 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 had to do was 
not short the bias current to ground.



> >>>> If we design our own PCB then the AD7760 series ADCs are another
> >>>> possible option.
> >>>> These have a built in differential input differential output 
amplifier.
> > 
> >>> Yes.  But aren't we trying to use commonly available soundcards?
> >>>
> >>>
> >>> 
> >> Ideally yes, but they all seem to have built in performance 
limitations.
> >> AFAIK the AP192 with its 4Vrms full scale balanced inputs with no
> >> variable gain preamps or +48V phantom supplies seems to be one of the
> >> best for this application.
> >> Its major drawback is that its a PCI card located within a noisy PC.
> >> 
> >
> > 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.


> 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.

 
> >> 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.


> >> 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. 

Certainly one can build a VLF transformer, but it will be a project for 
sure, and the transformer may be quite large.



> >>>>>> 
> >>>>> The [5120A] paper is also worthwhile, and available on the web 
somewhere 
> >>>>> (don't recall where, but google found the pdf).  I had to read the 
patent 
> >>>>> multiple times to figure out what's going on.  The correlation 
> >>>>> approach is old as the hills, and only the digital phase detector 
was patentable.
> >>>>> 
> >>>> It may be feasible to achieve the same effect by purely digital 
means 
> >>>> at least for low sample rates where FIR filters with tens of 
thousands 
> >>>> of taps are feasible.
> >>>>
> >>>> 
> >>> It *is* feasible, and Sam Stein is doing it.  I've perhaps lost the 
thread 
> >>> here.
> >>>
> >>>
> >>> 
> >> 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.

How accurately must the quadrature delay be achieved?  If I recall, the 
patent or paper implies that it need not be exact.

I also recall thinking that he could implement the quadrature delay 
digitally.  I don't recall the details, but it depended on cutting time up 
into one-second batches and processing each batch independently of the 
others.  I suppose one can use a FFT-Multiply-IFFT to do it directly.


Joe




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