[time-nuts] Phase Detectors/Mixers for DMTD and PN measurements

[Bob kb8tq]

Hi

> On Jan 16, 2020, at 10:51 AM, Attila Kinali <attila at kinali.ch> wrote:
> 
> On Thu, 12 Dec 2019 06:53:41 +0100
> Gerhard Hoffmann <ghf at hoffmann-hochfrequenz.de> wrote:
> 
>> It seems the mixer noise cannot be ignored.
>> 
>> I wonder then  why nobody  takes the mixer to cross correlation land, 
>> and maybe even the driver amplifier.
> 
> Mostly because it doesn't do as much as you think it does.
> 
> If you want to do cross-correlation you have to duplicate the
> whole chain from the mixer onward, which adds a lot of cost.
> Otherwise you gain at most 3dB by splitting the mixer and
> combining the signal immediately. At the same time you get
> into trouble because now the IF outputs of the mixers are
> now coupled which leads to periodically chaning impedance
> as the mixers are not perfectly identical, which will
> detoriate LO isolation. 
> 
> It is by far easier and more fruitful to work on the mixer itself
> instead. E.g. the output capacitor, that Tobias asked about, smoothes
> out the waveform of the IF signal, thus leading to a more linear
> behavior of the mixer and lower harmonics (less noise down-folding).
> Much simplified, the goal is to have a high impedance at the IF
> frequency (maximizes output voltage, or rather minimizes loss)
> and have a low impedance at LO, RF and 2LO/2RF/LO+RF frequencies
> (to minimize harmonics and improve linearity). Of course there are
> lots of other factors that come into play, like LO to RF isolation
> (back feeding a signal close to the oscillator frequency is never
> a good idea) and these are contradictory.
> 
> Another trick you can employ is to pass the LO through a limiting
> amplifier. The idea is that the main contributor to noise in a
> diode ring mixer is the noise (or uncertainty) of switching delay
> of the diodes (which is not symmetric, btw). By having an LO with
> a high slope, this uncertainty is reduced. But high slope means
> high power, which might damage the mixer. Using a clamped signal
> instead limits the power into the mixer and thus prevents it from
> frying while giving the advantage of having fast rise/fall times
> for the diodes to switch with (note that the swtiching time is
> limited by how fast the space charge zone can be built up and removed).
> Big disadvantage of this is, that now the LO has lots of harmonics
> which will lead to noise down-folding. As 1/f noise is the limiting
> factor for DMTD, especially even harmonics have to be kept as low as
> possible, ie the duty cycle (or DC offset) of the LO signal should
> as close to 50% (or 0, respectively) as possible. A nice side effect
> of this is that the switch-on-switch-off asymmetry of the diodes is
> reduced by having a higher slope, thus reducing even harmonics.

If you look at the response of a normal diode ring mixer, it already 
*has* responses at the odd harmonics of the LO signal. The reason is
pretty simple - the diodes have turned the drive into a square wave, regardless
of it being a sine wave to start out with …..

Bob


> 
> For DMTD applications, another limiting factor is phase shift
> variation through the mixer. I haven't looked yet at which factors
> contribute to this, but from the data I have seen, my guess it's
> mostly temperature induced. A way to combat this is to inject a
> pilot signal into both paths of the DMTD and use that as a reference
> to cancel differences in the phase shift. With this most of the
> temperature dependence can be removed. But this will introduce
> a leakage path from one channel to the other, which will cause
> problems if not properly handled. For details see [1].
> 
> 			Attila Kinali
> 
> [1] "2π Low Drift Phase Detector for High-Precision Measurements"
> by Jablonski, Czuba, Ludwik and Schlarb, 2015
> https://doi.org/10.1109/TNS.2015.2425733
> -- 
> It is upon moral qualities that a society is ultimately founded. All 
> the prosperity and technological sophistication in the world is of no 
> use without that foundation.
>                 -- Miss Matheson, The Diamond Age, Neal Stephenson
> 
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