Hi, I wonder if you could achieve this easier with a "brute force" approach and digitize directly at the mixing frequency? Then do the rest with math in an FPGA. --mike
On Wed, Feb 10, 2021 at 1:20 AM Magnus Danielson <[email protected]> wrote: > Attila, > > On 2021-02-09 21:15, Attila Kinali wrote: > > Ciao Mattia! > > > > > > On Tue, 9 Feb 2021 10:58:20 +0100 > > Mattia Rizzi <[email protected]> wrote: > > > >> I had a look at the literature and I found a paper [1] that put an > >> upperbound between AM/PM and OIP3. Therefore I am looking for triple > >> balanced mixers or DBM with high IP3. > >> My question is: in your experience is that all or there's something > else? > > Uh.. this is a difficult question. > > First of, let me start with a few questions: what is the general > > circuit you are working with? What are you trying to synchronize? > > Is it synchronization or syntonization? Will you steer the phase > > difference to zero? > > > > Next: Forget everything you think you learned about diode mixers. > > All texts I've read on them are strong simplifications of what > > is going on, in order to make the problem of describing them > > tractable. > > (I have not looked at Gilbert cell mixers yet, so I cant's say > > anything about their analysis) > > There is more methods available than Gilbert cell mixers. For many > purposes you do not need to go full Gilbert cell, which is a 4-quadrant > mixer, but can satisfy with a simpler 2-quadrant mixer. Both kinds is > really a transistor pair and they do indeed to proper multiply. > Additionally linearizing diodes can be used to reduce the distorsion > from the arctanh distorsion (by letting the diodes perform logarithm), > and thus one can push them to higher ampiltude without too much > distorsion, but higher still remains relatively low, which remains the > fundamental SNR issue. The main claim to fame of these is really that > you can do them on silicon as integrated chip without any transformers > involved. That has it's uses, but really not what brings you best > performance. > > A better approach is the Drawmer VCA, which has way better SNR than > Gilbert cell type, but they typically do only lend themselves to > 2-quadrant as the control is exponential. For mixers with very good > dynamics and big signal support, the H-bridge mixers seems to be the > king of the hill these days. > > > > > > First thing to note, a diode mixer does not multiply. It only > > multiplies the signs of the signals, but adds the amplitudes. > > Actually, it's a bit more complex than that, if you do not have high > drive-level. It actually is able to do a multiplication, but it is not a > very good one. It ends up doing the logarithm of the two input sources, > add them and then do the exponential, all through the same NP junction > exponential response, as it finds it's balance-point in the setup. It > might sound mind-boggling, but if one comes from the right direction on > it, it would make kind of sense. To improve things, you can increase the > drive-level and well, there is a reason we do that. Then you can > consider the simplified model you advocate. > > Any NP junction will to the mixing, and this is a major issue in mobile > towers, causing passive intermodulation (PIM). > > > This fact alone, while not invalidating the general principle, > > makes the used description hard to use for precision applications. > > Add to that, that diodes are not ideal switches. Even a "slow, but > > symmetric switching" description does not capture them properly. > > Switching is asymmetric, due to the space charge zone and it > > bounces like a mechanical switch due to parasitic inductances > > and diffusion time constants. The non-linearity in the switching > > behaviour will cause headaches once you try to get to a good model > > of phase linearity in mixers. > > > > With that in mind, it becomes obvious that a diode DBM is > > pretty aweful (in the nutty time-nut sense) when it comes > > to phase linearity. But, a lot of the non-idealities cancel > > out or become insignificant, once you steer the phase such, > > that you are at certain sweet spots (e.g. 90°). > > > > But, from what you wrote, you are not concerned about using > > a mixer as a phase detector, but as a frequency translation > > device in two parallel branches. There things change a bit. > > Foremost: DC offsets are of no consequence. This simplifies > > a lot in the analysis. But it also causes problems: now > > you have shifting phases, hence you can't stay at a sweet spot. > > But with this, all you care about is noise, of any form, ending > > up in the IF signal band. To analyze this you can look at the > > frequency domain behaviour of the mixer, like I did in [1] for > > the sine-to-square wave converter. > For DMTD a problem is that the two different beat-cycles integrate only > partly the same noise from the common source, and hence there is a > decorrelation loss occurring which raises the leakage of the transfer > oscillator noise into the noise-floor. When the beat is near each other, > that problematic effect ends being minimized, but so would the time > difference, which may or may not be what you want. > > The lessons are also pretty similar: Avoid even order > > harmonics. All even order harmonics will lead to (correlated) > > noise and signal being brought into the signal band, which will > > lead to phase offsets (including AM to PM conversion). > > The above is also the reason why high IP3 seems to help: All > > devices that have high IP3 (relative to the operation point, > > or to the 1dB compression point) have also a high IP2, This > > means, that high IP3 devices are low in even harmonics. > > And it also explains why a double balanced mixer has less noise > > than a single balanced mixer: The two paths, that are driven > > 180° out of phase, lead to the cancelation of even order harmonics, > > thus get less (sub-harmonic) down conversion of noise into the > > IF band. > The cancellation of even harmonics comes because the main wish to cancel > the input terms, as the unbalanced mixer lets both through, the balanced > mixer blocks one and the double-balanced mixer blocks both inputs. > > > > I wanted actually to sit down and repeat the analysis of [1] > > for the diode DBM, but never got around it. If you are interested > > in doing that, we could work together. > > > > An additional note, as you care about sub-0.1° phase differences: > > To achieve this, you will need to either control or compensate > > the temperature dependent phase shift of mixers. 1-5ps/°C is > > what I have seen in various papers. For two similar paths, > > you can expect a 1:10 to 1:20 matching. If not controlled, > > this will lead to a phase shift in the order of 0.01-0.03°/°C > > of differntial phase shift between the paths at 3GHz. > > > > Attila Kinali > > > > [1] "A Physical Sine-to-Square Converter Noise Model", IFCS 2018 > > http://people.mpi-inf.mpg.de/~adogan/pubs/IFCS2018_comparator_noise.pdf > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
