[time-nuts] Sub Pico Second Phase logger

Joseph M Gwinn gwinn at raytheon.com
Wed Dec 17 02:19:12 UTC 2008


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.


> >>>> 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.

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.

 

> >>>>>
> >>>>> 
> >>>> 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




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