TSP

In this episode Shahriar takes a deep dive into near-THz sub-harmonic mixers. To demonstrate the concepts, measurements and teardowns a set of Tektronix Sub-Harmonic mixers have been acquired (WM 490-F & WM 490-W) which operate from 90-140GHz & 75-110GHz respectively. This video is organized as follows:
00:00 - Introductions
00:50 - Acquired near-THz sub-harmonic mixers & diplexers
01:26 - Theoretical background, non-linear devices, harmonic generation
06:05 - Spectrum analyzers utilizing sub-harmonic mixers, signal ID
08:27 - Tektronix sub-harmonic mixer design, schematic & principle of operation
15:11 - Mixer diode characterization using Keithley SMUs & I/V tracer application
19:59 - Mixer teardown, analysis and microscope view of THz diode
28:07 - Diplexer architecture & characterization using a 4-port Siglent network analyzer
33:24 - Complete near-THz down-conversion setup using a x6 mm-wave multiplier source and Tektronix MSO68-B oscilloscope, conversion loss, harmonic number verification, signal ID process
45:35 - Using a ~100GHz carrier to determine the RPM of an optical chopper
48:15 - Concluding remarks
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[@lbgstzockt8493]

Honey wake up, new Signal Path video just dropped

[@campbellmorrison8540]

The most impressive pile of test equipment on KZbin! And one of the best guys to explain their operation

[@superciliousdude]

I wish this kind of material existed when I was studying EE in university. Thanks for making yet another awesome video.

[@Xsiondu]

WOW I am staggered by how interesting this whole video is.

[@preste3]

Great vid as always. I have made external mixers for my old Tek 492 using these diplexers. Just a tiny diode at the end of rigid coax and a flashlight reflector for a horn. Works to 61GHz using a radar module in test cw mode. There is a conversion loss list somewhere in Tek docs but 15th harmonic etc wont ever be strong.

[@OneBiOzZ]

Im just going to give a shot out for the 18:10 - 18-11 changing of mixers and having them be so precisely aligned on camera that it fried my brain for a moment

[@jessicav2031]

Thanks for continuing to put this kind of "real" content on an otherwise mostly silly platform. I'm going to have to watch this two or three more times (while trying to hold back the typical lab equipment jealousy). Suffice to say I don't think I will be using a 10ghz scope to measure a 200hz encoder wheel anytime soon, let alone generate a 100ghz signal 😆

[@frankfox4366]

Lots of "real" content on this platform. KZbin will recomend you stuff, if it has got idea you like. You give it idea by searching for things.

[@glasslinger]

Aw come on! Cat videos are really fun! :)

[@JeffCowan]

Thanks for the great explanation regarding how a nonlinear devices actually result in mixing behavior.

[@florianhofmann7553]

I've finally found a place to learn some RF magic in a practical and engaging manner. Many thanks!

[@trevorvanbremen4718]

Very impressive (or at least I THINK it is???)... It motivated me into building my own ultra high frequency oscillator... (around 454THz I believe) . . . . I grabbed a red LED and hooked it up to a battery thru a resistor.

[@fjs1111]

Actually - you're spot on... LEDs are in fact oscillators. They obviously generate visible light, but in the process create ultra high frequency oscillations in the electrical side too at the same frequency as the electrons jump in the junction. Not many people realize this, so good for you!

[@BrightBlueJim]

I worked for Tek at one of their field offices back in the 80s, and worked on 492 series spectrum analyzers for several years, but never had the opportunity to play with the external mixers. Thanks!

[@vk5fe943]

Another great video, from beginning to end.

[@leocelente]

That last experiment was amazing!

[@supercompooper]

2am in Toronto watching meteors and mixers. awesome 👍😎

[@mohamedlanjri]

Great video!! Appreciate all the effort on getting that hardware and showing them up.

[@wither8]

So fucking awesome, especially the light/darkfield teardown of the diodes. Also lol @ "just a simple multimeter" re: that Keithley dmm

[@joopterwijn]

Dat is snel 🤩 morgen ochtend even kijken!

[@genericcheesewedge4870]

One issue using an endmill to cut such a tapered slot by moving it up as you move is that you would get a concave bottom, as you are now taking an angled projection of a circle for your bottom surface. Let’s say the channel was 6mm wide and you had a rise over run of 1/10, the center of your endmill would cut 3mm ahead of the sides at the same level, so as you move forward these 3mm you will have a 0.3mm hollow. That’s a reason why you normally use a ball nose endmill as the projection of a sphere stays constant, at which point your surface dishing is a function of stepover and cutter radius. So this part was probably made on a fancy 5 axis machine, or depending on age and production volume at least an angled fixture

[@adamturowski3765]

I bet it was an angled fixture. CNC machines , especially fancy ones, were not really available back then.

[@xDevscom_EE]

Thank you for explaining this so clearly. Would be nice to also see episode about Gunn diodes for generation of these high frequencies. Perhaps using EIP 578B locking counter could be great illustration of using mixer + Gunn oscillator to make programmable many-GHz clock source. Also could you use different harmonics in a mixer like this, for example 20th instead of 23th?

[@rolfdieterklein]

Excellent Video and Experiments again. I own some of those mixers up to 220 Ghz (got it from a ham radio OV working with THz). An succeded to repair the 90-140 GHz Version. The inside looked a litte bit different frm this video and the diode was in series. I needed around half dozen similar diodes and H20 glue to glue it on the tip reaching the filter with the other side. I have a spectrum analyser with integrated diplexer and signal id which helps (also biasing), -- Great to use a SMU for i-V curve, I will try to get this app for my 2650. Makes it safer to test the diodes. But at the time of my repair I didnt had one - will try it for more precise characteristics... For test I used a 120Ghz Radar Chip from silicon radar.

[@chrislemus9392]

Excelente

[@RALL123456]

Fascinating stuff as always! Thank you so much for your work ❤

[@loueckert4970]

Great demonstration, thanks 73 de KT1R Lou

[@FreejackVesa]

Instrument Flex 💪🏼

[@tommihommi1]

the windows login screen on the tek scope at the end of the video :D

[@HectorLopez-jw2ke]

Genius more rare experiments

[@ctid107]

Wow, thanks for that. I was lying on my back outside here in Belfast watching the Perseid meteors last night, a fantastic natural phenomenon. Now a fantastic egineering/physics experience. I don't understand everything in your video, but there were quite a few "now i get it!" moments on first watch. I'll just have to watch it again.

[@vincei4252]

Of course over here in my corner of the world it was the worst thunder and lightning storm with insanely heavy rains all last night. SMH.

[@Richardincancale]

21:00 Great instructive video! Did you see the graffiti inside the end cover - ‘DB’ was here!

[@jamesfrancom8100]

love your channel. Especially since I can't afford Agilent test equipment.

[@hexarith]

Wonderfully detailed video. Highly appreciated. It's just a pity that your microscope camera and your recording setup didn't agree on the interlacing field order.

[@Thesignalpath]

Thanks for pointing that out. I’ll fix it for the next video.

[@paulpaulzadeh6172]

Hi Shahriar , I have 2450 and 2461 KEITHELY , but I got KickStart software which you can do all curve tracer function for transistor and diode with all 4 quad. better then App.

[@veeseir]

the amount of money in the background at 0:30 is insane

[@vincei4252]

This is super interesting! Thanks for posting this. I bought some absolutely minuscule SRD's (100G & 300G) years ago and they've been in a drawer waiting for this video! I always pondered how they'd get connected into a circuit and seeing that it is something so simple and just mechanical is awesome! I need to hone my milling skills to try to build a structure that can retain these things if only to get an IV characteristic from them. Is that IV characteristic app available for the 2450 ?

[@glasslinger]

I bought some too, from Russia! At first I thought there were just some particles of dirt in the baggie!

[@w2aew]

Really cool video. I wonder if the “damage” on the top of the diode was actually marks from a wafer prober during test?

[@Thesignalpath]

I’d imagine that would be more in the center…

[@wither8]

​@@Thesignalpath The RF absorber looks like it suffered a brittle snap, maybe from a drop. Then, like you suspected, it would have resulted in damage at the bottom of diode, as the broken gasket basically wedged the diode. If someone didn't realize that it had been damaged and then tried to use it, that could have easily blown the thin oxide layer. A standard CNC probably wouldn't get you a surface finish R_a good enough for RF purposes. A surface grinder, well-designed jigs that you can precision pin to align against so you the right angle tapers, and a few thousand dollars in metrology equipment would probably get you on-par (or better) than what they were producing. But the diode is where all the magic really happens. What you should try to do is surgically remove/repair where the IC has localized damage and repeat the experiment Either way, as usual great stuff. Every episode that features failure analysis with the microscope earns you an A+ in my book

[@djohnsto2]

You could consider designing some unique pieces of lower-cost equipment using your knowledge and creativity - Similar to the NanoVNA, using existing tech for innovative purposes to bring gear to groups who otherwise wouldn't have access - That would be amazing.

[@nebicicek7404]

Is there a reason why the bias-t was connected to the diplexer IF port, rather than directly inline with the mixer?

[@fjs1111]

Those Keithley SourceMeters are so awesome aren't they?

[@Factory400]

This hertz my brain 🧠 😂

[@Wtfinc]

I really need a video about how analog signals mix and how transmitters use those mixed signals, what the if is for. There’s no good videos on radio that i have found for anyone attempting to make a transmitter and receiver

[@przemyslawbrys]

Above my paygrade, but super enjoyable!

[@IW3IPD]

Thanks for the wondeful video. I was wondering: is it really such a delicate diode that cannot be tested with a multimeter? When in operation, it is biased at 10 mA, which is a lot more than was used for characterization and is also more than the test current used by multimeters, at least in my belief.

[@Thesignalpath]

The main issue is the initial discharge into the diode. A lot of multimeters produce an open voltage of several volts. The moment you connect the diode the inrush current can be high and damage the diode.

[@jimgeorge8956]

Is the high loss seen when introducing a gap in the waveguide due to free-space propagation loss or is it also due to the mismatch? If you have a pair of WR10 horns, I wonder if that would reduce the amount of loss. My lab had an old Tek 492 with a couple of mixer modules and the diplexer. We surplussed it a while ago, our mixers only went up to ~40 GHz as I recall.

[@kurtiunlisted8589]

I was wondering, on the diagram at 1:30, wouldn’t there also be an „RF+LO“ tone between 2LO and 2RF?

[@luc7luc7]

Apparently the diode (a beam lead diode) was bent in the wrong direction. There is usually a minuscule spacer that sits on top of the rod and the diode lead is bent to sit on top of it. Then it would be possible to apply a small pressure to promote the contact of the LPF hat to the diode without smashing the diode, by the way beam lead diodes are extremely fragile. Maybe the diode is still good... even if not a new diode is very easy to apply and restore the mixer to the former glory.

[@glasslinger]

Do you hire fleas to do the work on the diode assembly? :)

[@garci66]

Great video as usual! Just one question, when you show the transfer function of the step diode, shouldnt it be a square wave rather than a super narrow pulse? I understand the srd actually has a time dependancy which maybe helps create yhe pulse, but at first sight the vertical.respinse curve would create a square, wouldnt it?

[@Thesignalpath]

The response goes to infinity vertically, there is no horizontal increase once it starts going up. As a result you can’t have a secure wave. If order to get a square wave, the I/V curve would have to look like a step function.

[@BrightBlueJim]

en.wikipedia.org/wiki/Step_recovery_diode shows very sharp, narrow pulses as well. I think the pulse is due to inductance in the circuit. The step-recovery diode turns off extremely quickly, so any inductance at all in the circuit produces a pulse much higher in voltage than the forward voltage drop of the junction, like on the order of several volts. But this dissipates quickly, depending on that same inductance. This is the main application of step recovery diodes, since those very fast rise times transfer to very rich harmonic content.

[@garci66]

@@Thesignalpath Thanks! I guess I need to think it a bit more.

[@AlexRian_]

1. Would there be any benefit in having a diode with a P/N junction width equal to a multiple or integer factor of the wavelength to be measured? 2. "How" tiny is the diode in the parts you have? How much current can it carry before it gets destroyed? Is it on the order of single digit mA or will it only carry a few uA before being damaged?

[@LutzSchafer]

Shariar I wonder about the construction of this diode. I would expect a whisker tip type like the old ceramic coaxial germanium mixer diodes. This would ensure a very small junction capacity. But I could be wrong. Do you know more about this?

[@luc7luc7]

This type of diode is called "beam-lead" (just google images and you will see a lot of them). These are planar diodes where a metal bridge connects to a very small epitaxial junction with capacitance about 50 femto farads or so. I believe the diodes used in these mixers were either made by Alpha(formerly TRG) (later SkyWorks) or Metelics [but I am not 100% sure]. The whisker type of diodes you mention were used by tektronix in the previous generation of the harmonic mixers employing the 1N26B and the 1N53B.

[@der.Schtefan]

I started a GoFundMe for the poor diode....

[@AdityaMehendale]

The lowermost (yellow) trace at 47:28 appears asymmetric, although the chopper-wheel's apertures look symmetric. What could be the source of this asymmetry? Would it reverse if the chopper-wheel's direction were to reverse?

[@BrightBlueJim]

Direction shouldn't affect this. It looks to me like the chopper pulses are saturating the detector, and it's taking some time for it to recover.

[@karlbesser1696]

According to the dimensions, the mixer should have a cut-off frequency of 40 Ghz.

[@WolfmanDude]

What are the use cases for those super-high frequencies? I can imagine radar, but what else?

[@BrightBlueJim]

These days, the so-called mm-wave bands that 5G wireless uses. In the era when these were made, the basic research that has made these frequencies usable today.

[@ThatEngineerGuy_]

At the very least let the ads roll boys! Free for you and $$ for him.

[@peterlustig457]

25:34 bet could be made on manual mill

[@leandrocartes534]

the signal phat plis láser module ando plis

[@leandrocartes534]

plis module lasr

[@stepannovotny4291]

How may of the 8 billion people on this planet have an RF lab equal to or better than this one? I would like to know the answer to that question.

[@phoneaccount6907]

What did they do in 100 GHz 50 years ago?

[@8BitNaptime]

Carcinotron and particle accelerators.

[@xDevscom_EE]

Many quantum voltage standards used (and still use) 70-90 GHz still , since Josephson Junction device is in a nutshell discrete quantum-accurate AC current to DC voltage converter.

[@luc7luc7]

Lots of applications, I add 2 more: - Plasma physics measurements (interferometry, and slightly later reflectometry) - Supply pump for parametric amplifiers (in the tens of GHz)

[@dabdoube92]

Long time no vids. Everything alright ?

[@Thesignalpath]

Yes! Thanks for checking. Just lots of travel and family stuff.

[@c4t4l4n4]

The "Harmonic Mixing Method" slide @6:30 had me a little bit confused. You used commas on the left side (7,328 GHz + 1,875 GHz) and on the right side you used a decimal for the IF (1.875 GHz). You also used a decimal point for your LO frequency. That is how rocket systems abort prior to launch or experience a "Rapid Unscheduled Disassembly" during ascent! 😁🧡

[@tommihommi1]

as long as you never use either comma nor dots to separate thousands, it's not confusing

[@rcpmac]

That’s the most microwaves I’ve ever seen! Does he repair them?

[@Thesignalpath]

I repair everything! ;)

[@lukahierl9857]

​@@Thesignalpathwe need an 1. April video in wich you change the high voltage fuse in a microwave.

[@TheBarretNL]

30:36 ... 30:42 hehe :)

[@Thesignalpath]

Missed in the edits. :)

[@GeorgeGeorgalis]

Another very interesting video, thanks! Reminds me of an experiment to measure the speed of light... kzbin.info/www/bejne/mJSulZttmK52Zrc it would be interesting to see how you would do that with your bench? Shorter distance, more accurate, etc?

[@Kevin_geekgineering]

where the hell you get all these instruments man

[@Ferraday]

You’ve made a mistake at 5:30, referring to individual pulses as different harmonics (on an X-t graph), when in reality every harmonic should be localized at each pulse, and will show up in a similar looking graph when plotted as X-f.

[@Thesignalpath]

I am referring to the equivalent series of pulses in the frequency domain, not the time domain one of course.

[@TheHuesSciTech]

Very cool!! Just a couple of notes: you're pronouncing "ridged" incorrectly, it's not the same as "rigid". And the low-mag microscope footage seems unusually poor quality (massive interlacing artefacts maybe??), but I assume you already noticed that during the edit. But those are just some minor constructive feedback notes, absolutely amazing content as always!

[@Thesignalpath]

Yes, it is more like Rijd. I don’t know what the issue with the smaller microscope is, I’ll look into it. Thanks!

[@stevengpreston]

Talking faster than I can listen.

[@Thesignalpath]

Have you tried 0.75x speed? Some people have mentioned they prefer it. Although I have had people who told me they listen at 1.5x...

[@SDX2000]

@@Thesignalpath Yes, listening at 1.5x. I loved this video BTW. Also why do we care at what voltage the diode turns on as long as the "knee" is sharp enough 18:30? Is a higher DC bias voltage bad somehow?

[@adamturowski3765]

​@@SDX2000To get as high power in the high harmonics as possible, LO signal should swing as close to the maximum nonlinearity of the diode as possible. The maximum nonlinearity happens to be at the knee. The diode rectifies LO and that generates positive DC component moving operation point away from the knee. Negative external DC bias counters that bringing operation point close to maximum nonlinearity: the knee. And that is why adjusting external DC voltage affects mixer conversion loss and thus IF level.

[@SDX2000]

@@adamturowski3765 thanks for the explanation. I get that but it still does not explain why the actual value of the turn on voltage is significant. Can't you apply a negative DC offset of lower magnitude in the case of the diode with the higher turn on voltage?

[@adamturowski3765]

@@SDX2000 Turn on voltage is significant because it determines how much LO voltage swing gets rectified and turned into DC that needs to be compensated for with external bias voltage. Is that more clear now?