Hi > On Jan 16, 2020, at 10:51 AM, Attila Kinali <att...@kinali.ch> wrote: > > On Thu, 12 Dec 2019 06:53:41 +0100 > Gerhard Hoffmann <g...@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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.