[time-nuts] Sub Pico Second Phase logger

[Bruce Griffiths]

Joe

Isolation from mixer RF to LO port may be too low when the mixer input
frequencies are different.
Injection locking can then occur all too easily (just ask Ulrich about
this) when the mixer RF ports are driven by 2 separate OCXOs.

Detailed in line post follows:

Bruce

Joseph M Gwinn wrote:
> 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?
>
>   


Not offhand, but this crops up in lots of places usually when one least
expects it..


>> 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?
>
>
>   


I'll expand the frequency scale and take another snapshot for the region
below 1kHz.
This rise may be due to ADC and/or input differential amplifier flicker
noise.


>>> 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.
>
>
>   

If one believes the NIST papers the trapezoid zero crossing slope only
increases by a factor of 3.
If one uses a cascaded filter limiter the slope gain can be adjusted for
optimum results.



> [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.
>
>   

You may get away with that if you use mixers with very high RF to LO
port isolation.
Minicircuits have at least 3 level 17 mixer models that typically have
80dB LO to RF isolation at 10MHz.

Using a passive splitter for the LO drives will gain at least another
30dB in isolation between the 2 RF inputs if you use an appropriate
splitter.


> [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.
The only risk with a noninverting amplifier, is that under fault
conditions (missing supply) a very large current can flow back (with
some low noise opamps as Enrico has experienced) into the mixers and
destroy them.

For this particular application the mixer preamp gain need only be
sufficient to boost the mixer phase detector output (1V pk?, 350mV pk??
depends on mixer and its operating conditions) to the sound card input
(FSR ~ 5.6V pk for an AP192). The resultant preamp gain is relatively
low ( 5 - 15X depending on the mixer etc) and the sound card noise will
dominate (~ 100nV/rtHz midband for an AP192) thus using an ultra low
noise mixer preamp isnt necessary.


>>>> 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]
>   


Optical isolation of the ADC from the noisy digital interface to the PC
would also be nice.

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.



>>>> 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.
>
>
>   


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.
Of course 64 bit or higher precision arithmetic is then mandatory to
avoid excessive calculation roundoff noise.


> Joe
>
>