PolyPhase or N-Path Mixer - Page 1
Forum section: electronics / rf-microwave

author: mawyatt
author_rank: Super Contributor
timestamp: December 22, 2020, 08:02:58 pm
content: There has been lots of discussions on this subject over on another thread which is about filters not mixers, so please take a quick view there first. In respect for the OP and the filter thread, I decided to start a new thread on these new type passive mixers.https://www.eevblog.com/forum/rf-microwave/an-alternative-to-quartz-filters/I've attached a couple of the earliest public domain publications on the subject from Drs Molnar and Andrews from Cornell in the IEEE SSJ and CAS publications. Our first public discussions were in 2009 at the IEEE ISSCC where Molnar and Andrews gave a presentation. Today there are 100's of papers on the subject and it's derivatives and I believe a few of the PPM (PolyPhase Mixers) have made it to the commercial world in some SDRs I've been told.Anyway, happy viewing.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: December 23, 2020, 04:17:29 pm
content: Here's a couple low resolution images of an early wide-band tuning receiver based on the PolyPhase Mixer (PPM). Created by Cornell in 65nm CMOS, and photographed with a developing technique utilizing custom lenses and precision camera/lens control.Here's a brief list of the proprieties of the PPM I can think of, some of which are quite unique and make this device a fascinating subject.1} Better than theoretical passive Bi-Phase Mixer noise figure! Less than 2dBmeasuredNF without LNA, you can't do that2} Very high input referred TOI without preselector filter, even for passive mixer. ~30dBm!!3} Wide tuning range overdecadesin frequency!4} Narrowband passive impedance match at antenna port without inductors or transmission lines!5} Input match follows effective mixer LO over multiple decades without varactors or other tunable impedance!6} Complex input antenna port +-j impedances achieved from baseband without inductors or transmission lines!7} Direct I and Q downconversion! Basis for wide tuning range Direct Down Conversion Mixer First Receiver (see images)!8} Compatible with small feature CMOS, performance scales with 1/feature size!9} Bi-linear transformation from baseband to antenna (tracks LO)! Directly allows higher order bandpass filtering at antenna port with baseband low pass transfer functions without inductors!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: December 24, 2020, 02:47:03 am
content: This trick (the PolyPhase Mixer) is new to me, thank you for opening a thread for it.Didn't parsed yet the 2 attached PDF from here, will look at them, but first I want to ask why the switched approach and many paths of the signal was preferred, instead of a single path with one analog multiplier -> analog filter -> analog multiplier and sinusoidal LO?Is this some technological choice, like for example preferring a switch over a Gilbert cell, or is it something else about the topology of PolyPhase mixer that I missed entirely?

author: mawyatt
author_rank: Super Contributor
timestamp: December 24, 2020, 03:36:52 am
content: Quote from: RoGeorge on December 24, 2020, 02:47:03 amThe many paths are fundamentally why the PPM can achieve a better than theoretical NF (2/pi) for a passive Bi-Phase Mixer. The PPM can approach a 0dB NF as the n-paths approach infinity. The many paths also push the harmonic response upwards generally well out of the region of interest.By up converting the baseband LPF due to the bi-laterial transformation PPM properties this allows a very large shunt capacitor as "seen" from the antenna port with Out-of-Band signals. This large capacitor attenuates these OoB signals before they have a chance to enter the mixing process because of the low mixer input impedance (S11~0dB) for OoB signals. In effect the PPM has pushed a baseband integrating capacitor LPF directly to the antenna port symmetrical around the LO. From a system design perspective this is exactly what you want for high dynamic range, force the signal filtering process directly upon the input signal at the very input before any active signal processing (gain). OoB signals get attenuated just like a preselector filter before any amplification, except this PPM filter is the baseband signal filter, or a major part of such, and tracks with the LO unlike the usual fixed frequency preselctor filter. We came up with a way that allowed doubling of the bandwidth of the receiver without effecting the bandwidth of the PPM input and TOI, and effecting the NF by <0.1dBThe Gilbert Mixer suffers from shot noise due to the active switching of bias currents, this imposes a NF limit that is higher than conventional passive mixers, and much higher than the PPM. The MicroMixer is another brilliant mixer from Barrie Gilbert that has a theoretical infinite DR, due to the unlimited current handling capability, of course real transistors have current limits. Another unique attribute of this mixer is that the gain increases with the input signal level! We've utilized these in many applications with SiGe bipolar devices and they work very well, but still suffer the same noise issues as the classic Gilbert Mixer.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: December 24, 2020, 03:40:48 am
content: Quote from: RoGeorge on December 24, 2020, 02:47:03 amYou are welcome. BTW the PPM is more that just a trick circuit, it employs DTCA signal processing to it's fullest as your soon to discover with the reference papers attached above which just scratch the surface....thus my reference to a "deep dive" in the other filter threadThere's a reason why Apple acquired a small startup Passif and made a bright young PhD student Dr. Caroline Andrews a multi-millionaire a year after graduatingBest,

author: MikeP
author_rank: Regular Contributor
timestamp: December 24, 2020, 05:08:21 pm
content: Mike, thank you so much for posting these documents!

author: mawyatt
author_rank: Super Contributor
timestamp: December 24, 2020, 05:44:12 pm
content: You are quite welcome.Have fun with this PPM, it's beyond an intellectual curiosity that challenges conventional thinking, but a highly useful and practical circuit that lends itself to modern CMOS scalingBest,

author: ejeffrey
author_rank: Super Contributor
timestamp: December 24, 2020, 06:01:03 pm
content: Quote from: RoGeorge on December 24, 2020, 02:47:03 amAn analog multiplier has a NF limit because when the LO is zero you are throwing away signal. A switched mixer appears at first glance to avoid this as one switch is always on but in fact has exactly the same limit only the "lost" signal goes into LO harmonic products. The multipath mixer always has one phase on like the biphase switched mixer but phase shifts each to combine to the output . This it both avoids (or reduces for finite phases) the harmonic products and puts that power in the signal band.In principle a quadrature pair of analog multipliers could get the same effect, but it's hard to make lossless noiseless continously variable couplings.Interestingly you can also build active circulators with a similar technique where the symmetry is broken not by a magnetic material but by the rotation of the switch pulse.

author: mawyatt
author_rank: Super Contributor
timestamp: December 24, 2020, 07:16:48 pm
content: Quote from: ejeffrey on December 24, 2020, 06:01:03 pm

author: RoGeorge
author_rank: Super Contributor
timestamp: December 27, 2020, 03:56:32 am
content: Meanwhile browsed the two paper and the thesis attached in the other thread. It all starting to make sense, very clever indeed, especially as an antenna filter.However, I still don't get how to avoid overlapping all the multiple of LO bands down to zero.My reasoning is like this (thinking first only about one phase/path and one low pass filter):- a switch is a multiplier of Vin with either 0 or 1, therefore we can think about it as a normal mixer- a normal mixer fed with f1 at one input and f2 at the other will produce two distinct tones f1+f2 and f1-f2 at output- we plan to tune our LO such as f1-f2 will be zero- the other tone, f1+f2, will be cut out anyway by the low pass filter following after multiplier- since a square wave is not a pure tone, but a bunch of many harmonics, we can take the spectrum of the 0/1 waveform tone by tone and multiply each frequency with Vin- since any 2 distinct frequencies are orthogonal to each other (doesn't "influence" or mix to each other in a linear circuit) we can just apply them all at once- the spectrum of a pulsed square waveform is an infinite series of multiples of the base frequency, the amplitudes for each harmonic varies, but the overall envelope is a sin(x)/x with the same period as the width of one pulsesThe spectrum for a square wave signal with duty cycle 50%, 25%, 12.5% and 6.125% (that will correspond to 2, 4, 8 and 16 phases/paths) will look like in the attached animated gif (made with real signals from the soundcard).The first plot is the signal waveform at 4 different duty factors (from 50% to 6.125%), and the following two plots are the corresponding spectrum for these duty factors. Both spectrum plots are the same, just that one is with the spectral lines in Volts, and the other with the spectral lines in dB.We see that the case for n=2 (50%) is the only one that has the most spaced apart harmonics, because it has only odd harmonics. All other duty factors will have both odd and even harmonics, and the bigger the n, the more equal in amplitude these harmonics will be. We note that all the harmonics are equally space at an fLOdistance, and comparable in power (except for the missing one at each nthof LO multiple, where n is the number of paths/phases).This is a concern because our multiplier will shift to zero not only the frequency of interest, LO, but any other multiple of LO frequency will be shifted to zero. Once we do that, there is no way to distinguish what signal came from which multiple of the LO frequency. So, this is a rather a comb pass any-LO-multiple-shifted-to-zero type of filter.In other words, once we see a signal at the output, we have no clue from which frequency that signal came. It can be from any multiple of the LO. I am not sure yet how to get rid of all the other teeth of the comb and keep only the first one.Is this type of filter used only where it doesn't matter from what multiple of the fLOour signal of interest is coming?

author: mawyatt
author_rank: Super Contributor
timestamp: December 27, 2020, 05:56:01 am
content: Remember the PPM is a completely different type of mixer which has sampled memory and the bi-linear transformation property allows simultaneous down and up conversion, this property is responsible for the nulling of the harmonic responses at the antenna port. Fourier analysis shows that N +-1 do not get nulled and thus in the 8 phase PPM the 7th and 9th harmonics are not suppressed. Later work has shown the nulled harmonics attenuated over 60dB.If you study both the IEEE papers attached above by Molnar and Andrews, this harmonic response is discussed and in the 8 phase version of the PPM the "Specifically, the 8-phase passive mixer eliminates content at the 3rd, 5th, 11th, 13th harmonics and so on" from SSJ paper C.Benefits of 8-Phase Mixingpg 2699 and fromC. Harmonic Suppression, pg 2705,"For the 8-phase case, which actively rejects these harmonics, the output due to the 3rd harmonic was 35.4 dB less than the fundamental and that due to the 5th harmonic was 42.6 dB less...... However, recent works have presented harmonic rejectionschemes which provide 60–80 dB of rejection (depending on implementation) , . These techniques could easily be applied to our design as well, providing a similar degree of rejection".pg 2705.In CAS paper"One such structure is the 8-phase passive mixer shown in Fig. 16, driven by eight nonoverlapping LO signals similar in concept to “harmonic suppression mixers” recently discussed in the literature , , , ..... Thus , which takes the form shown in Fig. 17, has a Fourier series which only contains half of all odd harmonics, driving the terms corresponding to n=3,5,11,13,... to zero."pg 3101Be sure and follow up with the references mentioned in these 2 IEEE papers, as well as later works using A. Molnar, C. Andrews, N-Path, Mixer 1st and Polyphase Mixer searches on the IEEE site, as there was a flurry of activity after these 2 IEEE papers were published. I recall a paper reporting a measured NF of ~1.8dB over 8 years ago, but can't recall the paper name or authors.As mentioned the PPM is a very very deep dive, and takes considerable time and effort to truly get an understanding of what's going on. I would forgo the simulations until you get a firm grip on what's really going on, simulations can easily mislead, obscure the details, and send you down the wrong path.After some time studying the PPM I think you'll agree that this is one of the most amazing circuits I've ever come across, deceptively simple, frustratingly complex in behavior, many unique useful properties, and outstanding performance.Best & Happy Holidays,

author: mawyatt
author_rank: Super Contributor
timestamp: December 27, 2020, 01:45:16 pm
content: Quote from: RoGeorge on December 27, 2020, 03:56:32 amPlease remember thisPPM(PolyPhase Mixer {PPM} or N-Path Mixer or Mixer First, which I refer all as PPM) is aMixerthat imposes a filter like function at the antenna port rather than just a filter. The PPM does frequency translation to baseband I and Q, thus is aMixer, whereas a basic filter has no frequency translation.This mixer can be used in most places because the first harmonic response is the 7th, and second harmonic response is the 9th harmonic for the 8 phase PPM version. The other harmonics of the LO clock are suppressed and effectively nulled by the unique behavior of this mixer.BTW nice graphs, how do you create those with a sound card?Best & Happy Holidays,

author: RoGeorge
author_rank: Super Contributor
timestamp: December 27, 2020, 04:20:24 pm
content: Indeed, mixer, I was incorrectly using the term filter (thinking mostly about what I wanted to use it for, instead of what it is).That was just a sloppy usage of words, sorry for that, but I am still very bamboozled by the whole circuit, because apparently it shouldn't work. That is why I was detailing so much each step of my reasoning. I like to think based on intuition and visual elements, rather than based on math and formulas. I know this habit can easily lead to wrong conclusions, but has its advantages, too.Since the circuit works in practice, yet it looks to me like it shouldn't, I'm certainly loosing some very important aspect here. Will read again, and pay more attention to the referred bibliography papers, thank you for pointing them.About the animated picture, I did that because I was not very sure about the theory. I only vaguely recalled from the uni days that the envelope for the spectrum of a square wave is sinc(x). But I like visual representations rather than formulas, so I started to tinker some theoretical spectrum plots with Python and Matplotlib. Since I'm not a software developer, formatting the plots and eventually adding some sliders to change the duty factor, soon turned into a programming chore.I would have very much preferred to touch some real knobs and buttons instead of Python simulations, but I have no spectrum analyzer. Then, I remember about an old oscillocope/generator free tool for controlling the sound card, a tool that was already having all the required features I was looking for (https://www.zeitnitz.eu/scope_enmade by Christian Zeitnitz )- can plot the spectrum or the waveform- can generate square waves with a given duty factor- can save the plots on diskThe "Scope" program was made for Windows (its look tells it was made with NI's LabVIEW). Luckily "Scope" still works under Linux, too, with Wine. So I made a few plots at different duty factors and saved them. Since they were way too many to attach them all, I stitched them together (the oscilloscope view and the 2 spectrum views) with Gimp in 4 images, one image for each duty factor (I was wanting to see the spectrum up to n=16, because it happens that I have some old 16:1 analog switches mux/demux ICs - MMC4067 - from the CMOS 4000 series).To illustrate even better how the spectrum evolves with doubling n each step, the 4 stitched pics from Gimp were grouped together in an animated gif, with these few lines of Python:Code:#!/usr/bin/env python3# -*- coding: utf-8 -*-from PIL import Imageimport glob# Create the framesframes = []imgs = sorted(glob.glob('*.png'), reverse=True) # sorted by namefor i in imgs:new_frame = Image.open(i)frames.append(new_frame)# Save into a GIF file that loops foreverframes.save('png_to_gif.gif', format='GIF',append_images=frames[:], #frames is twicesave_all=True,duration=500, loop=0)

author: mawyatt
author_rank: Super Contributor
timestamp: December 27, 2020, 05:58:47 pm
content: I agree about the math, it's the intuitive understanding that is what comes first, then the math follows to have a proof that the intuition is right. Just about every break-thru was by intuition, or by accident-then intuition, then the math.You are approaching the PPM in the right way IMO, get a seat-of-the-pants understanding and then the math will make more sense. Your initial assessment is likely from experience and what to expect, this simple circuit with a few switches , resistors and capacitors can't work as described, yet it does work and works incredibly well indeedThe key to understanding the PPM is to realize this is not a simple switch with a shunt capacitor and "bleed" resistor, or what one might expect of a sample and hold with a leak path resistor. Using superposition think of this as a means for the signal at the antenna to experience effects of what's on the other side of the switch, so as the input signal is experiencing down-conversion it is simultaneously experiencing up-conversion from the baseband. Once you get a feel for this then consider that using multiple switches time sequenced can create a somewhat continuous effect as "seen" from the antenna port due to one switch is always on.Because of the up conversion the baseband can influence what's "seen" at the antenna port and this is how the impedance matching takes place, and since this follows the switching process it follows the effective LO or frequency of the clock divided by the number of switches. The matching network tunes with the center frequency, with no inductors, transmission lines, varactor diodes, or active circuitry required, just passive switches, Rs and Cs!!! Complex R +-jX impedance matches can be created at the antenna port by manipulating the baseband impedance magnitude and phase.The noise properties are another fascinating effect, defying conventional thinking just like the impedance matching. The 2/pi theoretical limit is bypassed because the mixer has memory and allows the RF signal to correlate with the clock/N period. As the RF signal approaches the center frequency the correlation gets stronger reaching a peak when the RF signal period equals the N cycle period of the mixer, at this point the correction effectively becomes the auto-correlation function. So the input RF waveform is cycling over and over again thru the switches and synchronously stored on the shunt capacitors and experiences a noise reduction effect as predicted by correlation.For signals that are far out of band the PPM antenna port "looks" like a huge shunt capacitor with the switchonresistance is series. With a switch resistance of an ohm or less, this produces a significant mismatch for the out of band signals and they don't couple to the antenna port. It's only when the input signal gets close to the center frequency does it begin to see a decent impedance match and begin to couple to the port and experience the downconversion to baseband. This is a massive benefit for the PPM based system as the out of band signals that cause problems with desensitization and blocking experience a large attenuation right at the antenna input before any active circuitry, just like a preselector filter, but without a dedicated preselect filter and all the issues that brings! Another not so obvious benefit of the PPM that is likely not observed with initial review and study.There is much much more but I'll leave this for startersThat's interesting about the use of the sound card, thanks.Best & Happy Holidays,

author: KE5FX
author_rank: Super Contributor
timestamp: December 28, 2020, 12:04:48 am
content: That's a nice way to put it -- to the extent the signal from the antenna 'agrees' with the charge already on the capacitor that is connected to it at the moment, less current will flow into or out of that capacitor. As the RF and LO frequencies start to diverge, the capacitors draw more current as they try to follow the resulting voltage fluctuations from one LO cycle to the next, and the shunt impedance drops.I think someone mentioned Dan Tayloe's work with a similar approach on the other thread. Googling for info on the "Tayloe mixer" may give newcomers a better introduction than wading through a bunch of IEEE or BSTJ papers full of math, because Tayloe's work introduced the technique to amateur radio circles. It's interesting stuff.I'd point out, however, that the main reasons for the post-2010 renaissance of interest in commutating RF circuits are VLSI-friendliness and power consumption. This topology doesn't give you any magical new capabilities, or open up corners of the performance envelope that were previously forbidden. If you aren't building cell phones or otherwise obsessing over getting the most IP3 per mAh, then multipath mixers and filters are just new rabbit holes that eventually lead to the same places where the existing ones do, but with less literature and prior art to guide the way.

author: mawyatt
author_rank: Super Contributor
timestamp: December 28, 2020, 04:21:17 am
content: Quote from: KE5FX on December 28, 2020, 12:04:48 amI'm not aware of any prior art before 2009 in the public domain that addresses these features, if you have references/papers please provide those. Dan Tayloe's work is the closest I'm aware of before 2009 but his patents don't address these features either. I never seen any reference before 2009 that discusses a passive mixer with a sub 3.92dB noise figure, nor a baseband passive inductor-less matching network that is used to track an LO and match an antenna port with a complex impedance. There was prior work on the harmonic rejection mixers tho, with references in the IEEE papers attached earlier.I've never seen DARPA create a workshop specifically for a circuit concept, and they did so with the Mixer First workshop which was entirely dedicated to this topic. DARPA wouldn't be wasting time with just another RF circuit if this wasn't some new and undiscovered capability lurking behind the commutating concepts that future radios could benefit from. Soon after the workshop others got involved adding more credibility to this new type mixer.Having the small CMOS available today has certainly helped fuel these types of commutating RF circuits recently and bring them into the public view. 20 years ago ~200GHz devices were available tho and we were doing commutating RF circuits back in ~1980 (Chirp Z based real time SA), so this technology has been available lurking in the background for many decades. I think the real reason for the post-2010 renaissance are more folks are involved, more $, and wireless technology is now an everyday discussion.Best & Happy Holidays,

author: KE5FX
author_rank: Super Contributor
timestamp: December 28, 2020, 07:00:08 am
content: Quote from: mawyatt on December 28, 2020, 04:21:17 amPoint being, those goals are all achievable with traditional filter/LNA/mixer and ADC+DSP signal paths. Commutation is mainly useful if you're trying to shrink size and power consumption, which is always of keen interest to DARPA. Otherwise, it's technically very interesting, but not a game-changer.Nobody here is designing their own chips... and you can bet that the ones that are being designed are patented out the yin-yang. This sort of thing getsreallyinteresting when I can buy it off the shelf from Mouser.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 28, 2020, 11:27:54 am
content: I found my mistake!The very first assumption I made, the one that a switch is the same as an analog multiplier is wrong. They might be considered the same, but only for a pure rezistive load, and not when a capacitor or an inductor is present in the load.For an analog multiplier, when an input is zero volts the output is also forced to zero volts, and that will forcefully start to discharge any energy stored in the load. For a switch, when the switch is open the output is disconnected from the load, and any energy already present in the load stays there. So, the voltage is undefined (in regards to the inputs) and depends of the load only when the switch is disconnected. Here, in the case of a capacitor as a load, the output remains at the same voltage until the switch connects the Vin again.This changes everything, the essential thing I was missing is the behavior of aswitchedcapacitor, which is not the same as a permanently connected capacitor to a pulsed source.Some very interesting things happen in a circuit containing switched capacitors!Doh, I never thought before about switches that way, as connecting/disconnecting energy storage components to a circuit. Usually all the parts are permanently connected. Switching is somehow similar with changing a circuit's topology on the fly.Need to experiment a little with that in order to build an intuition, and think more about the consequences of switching.

author: gf
author_rank: Super Contributor
timestamp: December 28, 2020, 12:02:38 pm
content: QuoteOff-topic here, but that's exactly what the switches in a SMPS do as well - connecting/disconnecting energy storage components, or re-connecting them in a different wayAnother point is IMO, it is not sufficient to consider a single path alone. The interaction of the paths matters. If this were not the case, there were no reason to use N paths instead of a single one. Cancellation of particular harmonics eventually happens at the summing node.

author: mawyatt
author_rank: Super Contributor
timestamp: December 28, 2020, 02:07:16 pm
content: Quote from: KE5FX on December 28, 2020, 07:00:08 amBut everything electronic is pursuing shrinking the size, power, and cost, so this is a potential game-changer!! I have a "hint" that the PPM is in the Apple AirPods, Watch, and a few commercial SDRs, no verification (yet), just a "hint"Edit: I would like to see what a"traditional filter/LNA/mixer and ADC+DSP signal paths"that can cover over ~2 decades in frequency, have a ~2dB Noise Figure, ~30dBm TOI, and direct downconversion to I & Q would look like? Likely large, complex, expensive, and consume a few wattsQuoteNot advocating folks design their own chips, but you can buy the components at Mouser for a moderate frequency PPM. A few resistors, capacitors, a CMOS bi-lateral (or discrete NMOS) switch (CD4016 or 66) and a shift register (74-594, 595) is all that's required for < $2. Certainly within the capability of most folks that can play around to get a "feel" for what's going on with the PPMBest,

author: mawyatt
author_rank: Super Contributor
timestamp: December 28, 2020, 02:23:44 pm
content: Quote from: RoGeorge on December 28, 2020, 11:27:54 amThe shunt resistor called Rb in the PPM is to "bleed" off charge from the capacitor between the switch phases. This causes a bi-directional charge flow from the antenna input that follows the signal which is responsible for creating the impedance effect as seen from the input. For example without Rb, if the RF signal were at the LO frequency then the charge distribution stored on each capacitor would be fixed and no significant change in charge would occur at the input during or between clock phases, thus the impedance "seen" would appear very high. Rb causes a charge change and the impedance "seen" as Vinput/dQ/dT, where dQ/dT is the charge change and Input is the RF input voltage.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: December 28, 2020, 02:30:25 pm
content: Quote from: gf on December 28, 2020, 12:02:38 pmAnother reason for using N paths is the impedance "seen" at the input would be highly discontinuous with just one path since the baseband would be disconnected half the time. With N paths greater than 1, then the baseband is always connected by one switch.Of course the summing node just happens to be the antenna portBest,

author: gf
author_rank: Super Contributor
timestamp: December 29, 2020, 12:28:37 pm
content: Quote from: mawyatt on December 28, 2020, 02:30:25 pmOne summing node is the antenna port of course, and the other one is obviously the "Harmonic Recombination" stage (from the 2nd attached PDF), where appropriate weighting of each path seems to be important for the harmonic suppression.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: January 08, 2021, 10:15:48 am
content: Interesting discussion.See this link this should help some better understand what's happening:https://youtu.be/MP7m5OjXWUgThanks for the interesting post

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2021, 02:34:24 pm
content: You are welcome.BTW great find, excellent discussion by Dr Nauta!!He and a few others in Europe became involved after the initial discussions and papers at the ISSCC in ~2009.Best,

author: R_G_B_
author_rank: Frequent Contributor
timestamp: January 08, 2021, 04:27:54 pm
content: Interesting how you applied this theory for your application. Analogue is indeed not dead. You can do some clever tricks with analogue signal processing.Understanding correlation is a very useful tool. Understanding the time and frequency domain and how they relate goes a long way. You don't always need to relyOn a dedicated digital signal processor or even a digital signal processor to do digital signal processing. Creative thinking and willingness to experiment with the unconvential is your friend. Surprised this discussion a short one.

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2021, 05:45:29 pm
content: When we developed a hand held real time Spectrum Analyzer in ~1980 based upon the Chirp Z algorithm we utilized Discrete Time Continuous Amplitude (DTCA) signal processing because DSP couldn't approach the power consumption levels required. This had the benefit of working in both domains, analog and digital, but without the need for ADCs and digital processors. So when the requirement to find narrow tones in real time scattered throughout the spectrum came up, and without the benefit of an ADC nor digital processor, we looked for unconventional means and discovered the earlier work on the commutating filter mentioned in another thread and referenced in Dr Nauta's video.What we didn't realize for some time was this very DTCA filter could morph into a mixer with almost unbelievable characteristics.....and yield a single CMOS chip high performance fast frequency hopping receiver with few external componentsAs the old saying goes, "necessity is the mother of invention", and folks needed a different way to construct radios for some applications and the PPM provides one such means.Agree about the lack of involvement, but this is a complex and deep dive topic, so many may not want to expend the effort, but glad a few like yourself have.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: January 08, 2021, 06:26:47 pm
content: Quote from: R_G_B_ on January 08, 2021, 10:15:48 amNice finding, that's a very clean explanation, indeed!The same video from the 'ISSCC Videos' channel was also posted on the 'icdutwentenl' channelhttps://youtu.be/tYgc5mbdXMwIt seems like it is/was a workshop for the N-Path filter (video from Sept 2020), but I couldn't find the workshop:https://youtu.be/OTxGakNCeeAThe N-Path filter is a very clever circuit. While it feels like most of the aspects are understood by now, this looks like the kind of circuit full of surprises, and this is before even starting to experiment with it.What intrigues me the most at the moment is the fact that I can change the cutting frequency of an RC filter without varying the R or the C value!Not sure if this is a well known thing, my guess is I am probably rediscovering the hot water here, but this alone can have a lot of applications. Maybe it's just me who missed this obvious aspect, but as close as two weeks ago I would have bet my head on the validity of the fc = 1/(2*pi*R*C). Would have never thought this can be changed with a switch.My first prediction was a switched signal should have affect only the amplitude, and not the fc.That is why I was saying last time that a switch is very different from a PWM source.In fact, when I read in the pdf thesis linked in former posts that "The short exposure to the input signal divides the time constant by the duty cycle" and therefore it will change the fc, it was so obvious yet so unexpected that I thought it's a mistake, or maybe I am reading it out of context. I had to make a time domain LTspice simulation to convince myself:The 4 colors are the frequency response for 50%, 25%, 12.5% and 6,125% on time for the switch:What exactly I was missing all this time? What part of EE do I need to study more so to not be surprised about this change in fc? Asking because the variable fc, or the change of tau in an RC cell, is stated in all the papers, yet no author seems to make a big deal out of it.Another thing I don't understand is why the fc is much lower in that simulation? I see about 400kHz (first blue plot), for the switch with 50% Dwell time, but my calculation for half of 1/(2*pi*R*C) with 1k and 100pF is 796kHz, about 2 times bigger than measured.Letting aside for a moment why does the formula and the simulation show different fc, the main question is why I don't see this switching principle exploited more often? Or is it exploited, but I didn't noticed it?What other related applications are out there that tune by fast switching instead of changing analog values?

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2021, 06:49:44 pm
content: Not sure about your simulations (haven't looked closely tho) but if you think of the switch in series with R, then the average current that flows is dependent on the switch duty cycle. For example if the switch duty cycle is 50% then the R resistor is only conducting current 1/2 the time, so the average current will be 1/2 the switch "ON" current and the resistor will look like a resistor of R/D, where D is the duty cycle. Of course this assumes a switch period much smaller than the RC time constant to get the averaging effect.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2021, 07:05:39 pm
content: Quote from: RoGeorge on January 08, 2021, 06:26:47 pmAnother good find!!This video shows how easy a commutating filter can be constructed with cheap readily available components, available to anyone interested.Hopefully more folks will build a few of these commutating or N-Path filters to play around withThen maybe venture onto the PPMBest,

author: RoGeorge
author_rank: Super Contributor
timestamp: January 08, 2021, 07:16:26 pm
content: About the fc calculation, indeed 1/2, but I found it to be 1/4 when simulated, instead of 1/2. I thought maybe this is because when the switch disconnects the source, nobody discharges the capacitor, but if I apply this idea then the other duty factors does not match with the simulation results.I need to experiment with real circuits in order to understand more, right now I don't know if I'm seeing some simulation artifacts or there is an additional 1/2 in there. Also I need to review the simulation (the one posted is from 31 of Dec, made in a hurry and didn't have time for it since then).The fun part is this idea of switching (or pumping chunks of energy, or time bounder-ing a quantity) looks possible to apply not only for RC, and not only in EE, but in many, many other fields and applications.If my intuition is correct, it should be possible to make for example a tunable Quartz crystal.

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2021, 09:31:45 pm
content: Try this simple example:Place a 1 ohm R in series with a switch, and a 1F C shunt to ground. Now connect a voltage source of 1V to the switch other side. Make this a 1 volt step that happens at 1 second. Force the switch to be ON all the time. Now run a Transient Analysis (TD) sim, the voltage across C will display as classic step response of simple 1st order RC LPF with a time constant of 1 second. Now clock the switch ON/OFF with a duty cycle of 50% at 1KHz and rerun the sim, the result will show a voltage across C that has a response similar to a 2 second RC time constant. Now change the duty cycle to 25% and rerun, you should see a result that resembles a 4 second time constant.Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2021, 10:07:08 pm
content: Quote from: RoGeorge on January 08, 2021, 06:26:47 pmSimple -- with no load resistance, the gain is always asymptotic 1.0 at DC. With a load, gain will be PWM'd down.Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: January 09, 2021, 12:04:12 am
content: Quote from: mawyatt on January 08, 2021, 09:31:45 pmMade a similar test, and something wrong happens with the formula from the behavioral source.V=5V*sin(2*pi*(1kHz +10MegHz*(time/maxt))*time)My idea was that the variable 'time' will be in the range 0..10ms for a 10ms transient simulation ('maxt'=10ms). That would make time/10ms to browse the range 0..1, and from here [0..1] multiplied with 10MHz and shifted with 1kHz would have made the frequency sweep from 1kHz to 10.001MHz.However, V=Asin(2*pi*f*t) is about instantaneous values, and if I make f as a function of the same t, then the rate of change for f also matters. Some sort of aliasing happens between the rate of change for f and the rate of change for t.In the end, the behavioral source with that formula was not the same as a slow frequency sweep. Some phase inversions happens, and if I look at the very last period of the B1, it is of 50ns (20 MHz) instead of the 100ns (10 MHz) I was trying to get.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: January 09, 2021, 12:22:19 pm
content: Here's another example using a simulation on n path filtershttps://youtu.be/fHbkLNlac_w

author: mawyatt
author_rank: Super Contributor
timestamp: January 09, 2021, 02:38:28 pm
content: I expect these filters have similar characteristics to the PPM, and in the video the phase switches are shown as symmetrical transmission gates utilizing PMOS and NMOS devices. We found, and confirmed by other researchers, for a given chip area the single NMOS device produces a lower NF (transmission loss), better TOI and don't require a complementary clock drive. We also found that triple well NMOS devices produce the best overall performance for the PolyPhase Mixer application, however many CMOS semiconductor processes don't support triple well NMOS.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: January 27, 2021, 07:25:53 pm
content: Some more docs, the Wireless Engineer May 1947 paper "Narrow Band Pass Filter Using Modulation" by N.F. Barber (see page 36 of 86)https://worldradiohistory.com/UK/Experimental-Wireless/40s/Wireless-Engineer-1947-05.pdf(thanks to this nice archive of Experimenta Wireless / Wireless Engineer journalshttps://worldradiohistory.com/Experimental_Wireless.htm).Also, a 40 minutes video from two weeks ago, about the N-Path Filter seminar by Bram Nautahttps://youtu.be/L3wJ1XedpSoHad a lot of fun so far studying the theory, quite vast I would say for such a simple circuit. Still learning.Did some experiments (in the kHz range) with a CMOS analog mux/demux on a breadboard, and saved some oscilloscope printscreens.However, while reading and experimenting, I think I found something new that could significantly improve the circuit. Looks like it should work (on paper), and can hardly wait to experiment with this new idea on a breadboard (in the MHz range).

author: R_G_B_
author_rank: Frequent Contributor
timestamp: February 11, 2021, 11:56:23 am
content: Be interesting to see what you have found

author: OwO
author_rank: Super Contributor
timestamp: February 12, 2021, 04:42:03 am
content: Is there any chance this could be implemented using LNB mixer FETs like the NE3503M04? It has gain to 12GHz so I think it may work in switching mode up to 1GHz. There's about 0.3pF capacitance gate to channel. I will have to do some experiments...

author: SuperFungus
author_rank: Contributor
timestamp: February 12, 2021, 05:40:36 am
content: Isn't the problem of generating N accurately phased and duty cycle aligned clocks each at N times the desired fundamental a much bigger problem than finding a fast enough fet?

author: OwO
author_rank: Super Contributor
timestamp: February 12, 2021, 01:28:09 pm
content: I was thinking of using a LVDS serializer IC to generate the polyphase clocks, e.g. MS90C385B but it's a 7:1 serializer so that means using 7 phases rather than 8. It's a 1000Mb/s serializer so it would be able to do up to 140MHz.Some more ideas: if the capacitor on each phase is replaced with a LC tank, the mixer input impedance would peak at two frequencies f_lo + f and f_lo - f. But if delay lines and buffer amplifiers are added between each phase, it can force one "direction of rotation" of the "standing wave" on the baseband phases, and possibly act like an image reject mixer rather than a direct conversion mixer. The problem with the direct conversion approach is that it converts signals near the center frequency to DC or near DC, and any gain stage after the mixer will have 1/f noise. So for narrowband receivers you won't be able to get a low noise figure, sure the mixer has low NF but whatever follows must amplify a near DC signal and will have a much higher NF. That's why I still look for high IF approaches rather than direct conversion.

author: mawyatt
author_rank: Super Contributor
timestamp: February 12, 2021, 02:11:50 pm
content: Quote from: OwO on February 12, 2021, 04:42:03 amAny FET should work but simple large W/L NMOS produces the best results. CMOS NMOS devices in a triple well process where you can access the channel, well and substrate individually produces the best results. 3rd (odd) order non-linearity is the limiting factor and these type FETs produce the best results. We did some detailed analysis and experimentation over 10 years ago that showed NMOS outperformed any other semiconductor devices including all the much praised 3/5 compound devices.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: February 12, 2021, 03:12:25 pm
content: Quote from: OwO on February 12, 2021, 01:28:09 pmThere were a number of papers beginning around ~2011 on using more complex functions than simple shunt caps to produce different results as "seen" from the antenna, I reviewed a USC PhD student paper than won the best student paper of the IEEE ISSCC way back that addressed various quadratic functions at baseband. These were RC and trans-impedance amplifier implemented quadratic functions on each phase allowing full integration on the chip. Using inductors would require N inductors operating at baseband, thus high values, and not integratabtle (everything we did was chip related), but might yield some interesting possibilities for non-chip implementation. To achieve the high input TOI the baseband function must "look" like a low impedance at the LO frequency, thus the shunt C approach and a "Pi" type capacitive input LC filter considered.To move the information away from zero IF (LO center), you might consider a low IF approach where the information is at an offset frequency. If the information is FSK type, then a bandpass function at baseband would produce a double +- two peak response away from the LO center (this was done long ago for a "ring tone receiver" where the signal transitioned between leading and lagging I and Q at baseband). Another use of the PPM was to implement a quickly tunable notch filter to suppress an interferer, and many other adaptations which I can't discuss.This is a fascinating circuit can lead to many different directions and adaptations, good to see more interest from folks.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: February 12, 2021, 03:38:13 pm
content: Quote from: SuperFungus on February 12, 2021, 05:40:36 amActually this isn't much of a problem since fast logic is available even in discrete form, and the PPM is not very sensitive to phase inaccuracy. Since the PPM input clock is N times the LO center frequency, the logic must operate with a max clock rate of N*LO max frequency. In the original works one concern was what was called "shoot thru current" where if the clock phases had a phase overlap this would momentarily shunt two adjacent phases together and cause additional disturbance at the antenna port (we were concerned about antenna LO leakage in some applications).Dr. Andrews (author of original IEEE papers referenced from Cornell) proposed an elegant solution where each even and odd clock phase were split to create an even antenna port and an odd antenna port. These ports were then linked to the input antenna port by 2 small on-chip inductors. Since the "shoot thru current" is very narrow (~picoseconds) and always occurs from odd > even or even > odd clock phasing, the inductors isolate the even and odd switches and suppress the shoot thru current which would flow between the even and odd switch sets if phase overlap was happening.Best,

author: dmendesf
author_rank: Frequent Contributor
timestamp: May 28, 2021, 02:54:41 am
content: This made me remember the attached paper from 1981 (in Portuguese, unfortunatelly). It describes a lockin amplifier made with a very similar architecture (capacitors and switches).

author: mawyatt
author_rank: Super Contributor
timestamp: November 23, 2023, 05:19:01 pm
content: While posting on another thread recalled a receiver concept based upon the PolyPhase Mixer (PPM) that hadn't been discussed, this dates back to ~2005 so somewhat from memory!!The receiver was called Ring Tone Receiver (RTR) after the old land line telephone Ring Tone which indicated someone is calling. The receiver was a combination of the old pager narrow band FSK architectures blended with a new twist the PolyPhase Mixer forming a new different style low power wide-band frequency hopping receiver.The concept was targeted for applications where battery power consumption and size were paramount, so everything was focused on power consumption and size minimization.The RTR utilized the Direct Down conversion to I and Q baseband feature of the PPM in a Mixer First (no LNA) receiver architecture but still achieved a high sensitivity. This was due to the very low PPM inherent Noise Figure, and good out of band rejection also due the unique properties of the PPM. Baseband I and Q were first RC passive filtered (for antenna port narrowband impedance matching) then active filtered with gain and low bandwidth around the +- FSK deviations. After active filtering the I and Q signals were limited and used as clock and data in a simple self clocked logic setup which would indicate if "I" preceded "Q" or "Q" preceded "I" as digital data bits. The self clocking serial data stream was simply compared to a specific code to identify the required action, which included activating a brief return transmitter acknowledge burst and/or activating a high data receiver to process much more data and such. The RTR was also quite frequency agile (due to the unique PPM properties) and could bounce multiple decades to avoid jamming (intentional or not).Anyway, thought some folks might be interesting in this somewhat unique PPM based receiver concept!!Best,

author: tggzzz
author_rank: Super Contributor
timestamp: December 18, 2024, 08:30:49 pm
content: Cross-reference, solely to avoid losing other useful references in the noise of that thread:https://www.eevblog.com/forum/rf-microwave/mixer-noise-figure/msg5752197/#msg5752197

author: mawyatt
author_rank: Super Contributor
timestamp: December 19, 2024, 03:48:36 pm
content: Yeah, should add this reference here also as the most consolidated source for PPM (N-Path) we've found, not surprised it's Molnar's group at Cornell and still quite a bit of activity over the past few decadesUnfortunately most is hidden behind the IEEE requirements (fee). Would be curious to "see" what's transpired over the years but even being a life IEEE member, still must pay for paper accesshttps://molnargroup.ece.cornell.edu/publications/Best

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2024, 07:39:25 pm
content: My bet these series of "switched capacitors" tunable filters are made that way (I was using them as 1.8-2.5kHz low-pass filters for my SSB radios).https://www.analog.com/media/en/technical-documentation/data-sheets/MAX7400-MAX7407.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/MAX7401-MAX7405.pdf