[time-nuts] Sub Pico Second Phase logger

Joseph M Gwinn gwinn at raytheon.com
Thu Dec 11 22:19:34 UTC 2008


Bruce,


time-nuts-bounces at febo.com wrote on 12/10/2008 08:38:13 PM:

> Joe
> Joseph M Gwinn wrote:
> > Bruce,
> >
> > 
> >> Reflecting the sum frequency back into the mixer is actually 
necessary
> >> to reduce the noise at the IF port.
> >> I believe that one of Agilent's simulation application notes mentions
> >> this effect but I don't recall the actual application note number.
> >> This will affect the mixer RF and IF port impedance so adding a 
series
> >> resistor may be required to improve the SWR.
> >> 
> >
> > How big an effect is this?  Is the absolute noise decreased, or does 
it 
> > remain the same while the signal increase?
> >
> > 
> With the same difference frequency IF port termination impedance,  noise
> is actually decreased along with the mixer conversion loss.

OK.  Complicated beasts, those mixers.  Do you know of a paper (or book) 
on the subject?


> However if the sound card input noise dominates, reducing the mixer
> effective output noise won't help.

Yes.  In the plots you posted in a different email, there was a big rise 
below 1 KHz (scan stopped at 1 KHz, so don't know the shape).  Why is 
this?


> > If I'm understanding Walls and Stein (paper 112) correctly, the 
advantage 
> > is because with the capacitor load the beatnote waveform approaches 
> > square, thus increasing the zero-crossing speed and therefor the phase 

> > sensitivity.  This is no doubt true, but the question was if this also 

> > caused a small everything-dependent phase shift, something that would 
not 
> > have mattered in the measurement of phase noise.  The object of paper 
112 
> > was to remedy a 10 to 20 dB error in phase noise measurements.  The 
> > critical words are in the lower left column of page 337, in the 
paragraph 
> > beginning "If the mixer is terminated ...".
> >
> >
> > 
> Saturating the RF port has a similar effect.

Yes.  But there are tradeoffs pushing the other way.


> If one is time stamping the zero crossings an increased zero-crossing 
slope is an advantage.
> For relative phase measurements a trapezoidal beat frequency waveform 
may be less useful.

Fitting to the approximate waveshape, sine or trapezoidal, should yield a 
very robust estimate, due to the large data support, and zero-crossing 
slope won't much matter.  Hmm.  Actually, if the slopes of the trapezoid 
are too steep, we may not have all that many slope samples.


[snip]
> 
> Of course with a capacitive IF port termination, matching the RF and LO
> ports becomes more critical as does the reverse isolation of the various
> amplifiers driving the RF and LO ports.
> It may be simpler in fact to use a level 17 mixer with high LO to RF and
> LO to IF isolation with the RF port unsaturated as it relaxes the
> reverse isolation specs for the isolation amplifiers.

Another tradeoff.  I'll have to think about it.

I'm thinking of 6 db and 10 db attenuators on the LO and RF ports 
respectively, but no isolation amplifier.


[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.



> > 
> >> It's hard to find such Firewire systems without such unnecessary 
frills
> >> (for this application) as high gain preamps.
> >> 
> >
> > The AP192 has high-level inputs, but I don't know if this bypasses the 

> > preamps, or attenuates.  Given their target market, I'd bet it 
bypasses.
> >
> > 
> There are no preamps other than an external differential input amplifier
> that translates the 4 Vrms FS inputs at the input connector to a level
> that the ADC can handle.
> The ADC chip itself has no preamps built in.
> There have been numerous complaint about this by some audio nuts,
> however for this application not having such amplifiers is ideal.

Bingo!  Good to know.

 
> >> The gain tempco and linearity of some variable gain audio preamps is
> >> somewhat suspect.
> >> 
> >
> > I would think that none of these cards has a good tempco of anything, 
> > given the lack of necessity in their market.
> >
> > I would think that linearity would be quite good, given the horsepower 

> > competitions on linearity.
> >
> > 
> Since the 2 ADCs share the same reference their gain tracking tempco
> should be quite good given that they use capacitors rather than
> resistors within the ADCs.

A happy accident, but we'll take it.


We are converging on a soundcard wishlist:

1.  True balanced inputs on XLR connectors.  And good ground design, so we 
aren't bedeviled by ground loops.

2.  24-bit ADCs, and similar DACs.

3.  Very good isolation all around. 

4.  Digital access via firewire (or USB3 I suppose), with the soundcard in 
its own box.

5.  High-level input direct to the ADCs.


While use of AKM ICs may be a very good idea, it is not a requirement per 
se.

[snip]
> >> Can alleviate [oddities at end of phase range} to some extent by 
driving 
> >> a pair of such phase detectors so that their outputs are in 
quadrature.
> >> One just selects the phase detector output that is in the linear 
range.
> >> The quadrature outputs also allow unambiguous assignment of the sign 
of
> >> any phase change.
> >> 
> >
> > The Symmetricom 5120A does something very clever to alleviate this 
> > problem.  Explained in US patent 7,227,346 and "Direct-Digital 
Phase-Noise 
> > Measurement"; J. Grove, J. Hein, J. Retta, P. Schweiger, W. Solbrig, 
and 
> > S.R. Stein; 2004 IEEE International Ultrasonics, Ferroelectrics, and 
> > Frequency Control Joint 50th Anniversary Conference, pages 287-291.
> >
> > Joe
> >
> > 
> I've read the patent.

The 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.


Joe




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