[time-nuts] Sub Pico Second Phase logger

[Bruce Griffiths]

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.
However if the sound card input noise dominates reducing the mixer
effective output noise wont help.
> 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.
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.
>>> MiniCircuits AN-41-001 "FAQ about Phase Detectors" has on page 2 a 500 
>>>       
> ohm 
>   
>>> resistor to ground and a 5000 ohm resistor to the first filter 
>>>       
> capacitor, 
>   
>>> so the capacitor is isolated from the IF port by the resistors.
>>>
>>>
>>>       
>> I wouldn't take too much notice of that recommendation as I have little
>> confidence in the author's experience/knowledge.
>>     
>
> Well, OK, but:
>
> Stephen Kurtz says the same thing on the third column of the third page, a 
> bit above Figure 6.
>   

Off 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.
> Nelson and Walls (paper 971), Figure 4, also shows the low pass filter 
> arranged to absorb the sum signal, not allowing it to be reflected back 
> into the mixer.
>
>  
>   
>>>> Supposedly an SRA-1, but some caution is in order as some 
>>>>         
>> statements as
>>     
>>>> to the effect of the input offset of an opamp based IF preamp in the
>>>> same application note were of dubious veracity unless one 
>>>>         
>> were to use an
>>     
>>>> inverting opamp input stage.
>>>>
>>>>         
>>> This issue was mentioned in another app note, but their main issue 
>>> appeared to be that the opamp bias currents could cause an offset.
>>>
>>>
>>>
>>>       
>> But the circuit they suggest has no effect on bias current induced
>> offset, the same current flows into the mixer and termination impedance
>> independent of the series resistance.
>>     
>
> You're right that the proposed remedy didn't make sense.  I don't know 
> that this is a big problem with modern opamps, especially FET input ones 
> (if needed).
>
>   
The only configuration for which it makes any sense is an inverting
input amplifier with a finite input voltage offset.
>> Yes, I should have said that when the 2 input signals are in quadrature,
>> any capacitive crosstalk will have little effect on the phase shift.
>>     
>
> Ah.  Because the capacitor coupling adds a second 90 degree shift, 
> bringing the total to 180 degrees.
>
> But crosstalk by ground coupling will be unaffected.  As will crosstalk by 
> transformer action.  Those boards are pretty crowded.
>
>
>   
Yes its better to measure it rather than relying too much on conjecture.
>> The AP192 has a somewhat higher interchannel isolation than that, the
>> interchannel crosstalk spec is about -120dB.
>> With a sufficiently large number of samples the its easy to see
>> artifacts as low as -140dBFS.
>>     
>
> Yep.  Seems like a very good card.
>
>
>  
>
>   
>> 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.
>   
>> 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.
>   
>>>> Other cards using AKM 24 bit ADCs should also be suitable.
>>>>
>>>>         
>>> Who is AKM?
>>>
>>>
>>>       
>> Asahi Kasei EKM
>>
>> http://www.asahi-kasei.co.jp/akm/en/
>>
>> http://www.asahi-kasei.co.jp/akm/en/product/proaudio.html
>>
>>     
>
> Thanks.  I'll look into their data.
>
>
>   
>>> 20 Log[ 2^24 ] = 144 dB, so something else will be the limit.
>>>
>>>
>>>
>>>       
>> Actual ENOB ~ 19 to 20 bits.
>>     
>
> Makes sense.  20 Log [ 2^19 ] = 114 dB.  Still plenty good enough.
>
>
>   
>>>> Ideally an external sound card with balanced  XLR inputs would be 
>>>>         
> best.
>
> Yes.
>
>
>
>   
>>>> HP produced a number of different phase comparators each with a
>>>> different type of phase detector.
>>>>         
>
> OK.  And the PLL folk must have a million designs.
>
>
>   
>>
>> Can alleviate it 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.

Bruce