Frequency doubler 10 - 20 MHz - Page 1
Forum section: electronics / projects

author: bob91343
author_rank: Super Contributor
timestamp: June 26, 2021, 05:31:44 am
content: I picked up, very cheaply, a couple of OCXO units that generate 10 MHz. I want to adapt one as a master frequency for my Kenwood TS-940S amateur radio transceiver, which requires 20 MHz.Does anyone have a simple frequency doubler circuit? I have seen lots of complex ones but this application needs to be cheap and simple.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2021, 05:39:53 am
content: Well, you can't get simpler than a harmonic doubler.Starting from a square wave, you'll need to filter it to something sinusoidal, then chop it with some offset so as to produce a different duty cycle (ideally 25%). Typically a class C amplifier will do the job, giving a lopsided sinus thing -- rich in even harmonics. A tuned load selects the 2nd harmonic, then is followed up with another amp (if possible, a limiter, to remove amplitude modulation -- the intermediate signal will be double-humped as it's pumped by the first amp's pulses), then whatever output conditioning the receiver needs (buffering, filtering, level shifting, etc.).Now, that's less than a half dozen transistors all in, but it's the passives around it that are key. How much complexity are you looking for in terms of tuning versus It Just Works(TM)?If you need something fancier, you'll need a PLL. You may prefer a PLL anyway, just because you can get a multiplier or synthesizer chip for a few bucks, that's many hours less time building and adjusting than the classical equivalent.Tim

author: bdunham7
author_rank: Super Contributor
timestamp: June 26, 2021, 05:40:45 am
content: https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3327.htmlNot sure if the jitter would be acceptable with that design...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2021, 05:42:46 am
content: Quote from: bdunham7 on June 26, 2021, 05:40:45 amIf you don't mind that every other edge is RC timed, it's pretty good. Nice thing about this application is, the input frequency has no range, so your only barrier is RC stability and the varying input threshold of the gate following it (preferably, use a line receiver or comparator, for a better defined threshold than a plain old logic gate).Tim

author: HB9EVI
author_rank: Frequent Contributor
timestamp: June 26, 2021, 01:45:50 pm
content: other way without active electronics is a DBM; just feed the 10MHz to LO and RF and you get 20MHz at the IF port

author: jonpaul
author_rank: Super Contributor
timestamp: June 26, 2021, 02:05:50 pm
content: 10MHz>>RF transformer>>full wave rect (2-4 diodes) >>HP filter

author: bdunham7
author_rank: Super Contributor
timestamp: June 26, 2021, 02:35:10 pm
content: Quote from: T3sl4co1l on June 26, 2021, 05:42:46 amThe falling edges of the output are generated by both the rising and falling edges of the input, the negative pulse width and thus the rising edges of the output are RC timed. The jitter I was referring to would result from any asymmetry of the input, which would produce output pulses in pairs that are closer together, resulting in a strong subharmonic at the input frequency. I'm not sure this would work for the OP's purpose without some additional filtering on both ends.

author: bdunham7
author_rank: Super Contributor
timestamp: June 26, 2021, 02:42:15 pm
content: Quote from: jonpaul on June 26, 2021, 02:05:50 pmExactly my first thought! However, there would have to be a lot of filtering and gain stages to actually make it work.

author: TimFox
author_rank: Super Contributor
timestamp: June 26, 2021, 03:33:39 pm
content: MiniCircuits sells a range of passive frequency multipliers that are essentially DBMs optimized and specified for doubler, tripler, etc. service.https://www.minicircuits.com/WebStore/Multipliers.htmlThe doublers typically have a loss between 10 and 15 dB, with the fundamental suppressed between -25 and -45 dBc.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 26, 2021, 05:11:32 pm
content: You could refer to this thread:https://www.eevblog.com/forum/projects/1-5-to-3-mhz-frequency-doubler-circuit/

author: bob91343
author_rank: Super Contributor
timestamp: June 26, 2021, 05:55:32 pm
content: Just as I feared, there appears to be no circuit that meets my requirements. I just want it simple and not needing tweaking. I want something approximating a sine wave at 20 MHz without having to amplify, filter, or any such stuff.I am contemplating making a mixer from a few diodes but a bit unsure as to what circuit might be suitable.I looked at some minicircuits parts but can't find the prices. ebay yields nothing. I don't know how much amplitude output I need. I can probably use anything in the range of a Volt or less.

author: bd139
author_rank: Super Contributor
timestamp: June 26, 2021, 06:00:53 pm
content: This will do the trick. Couple of toroidal cores and diodes:https://www.qsl.net/pa3fxo/frequencydoubler.html1n5711 diodes are fine for this. Don’t need anything special.The radio’s input signal conditioning stuff is probably going to be good enough to avoid using filters or anything fancy.

author: fourfathom
author_rank: Super Contributor
timestamp: June 26, 2021, 06:04:01 pm
content: Quote from: evb149 on June 26, 2021, 02:59:00 pmYes. I'm using the ON Semi NB3N502 to take a 10 MHz clock and multiply it to 25 MHz. This same part will also do 10 -> 20. This looks quite similar to the Renasas devices. Squarewave output though.The typical way to do this in the RF domain is with a "push-push doubler". Filtering is required. There are many implementations, active / passive / vacuum tube / IC / etc. Here's a simple one:

author: bob91343
author_rank: Super Contributor
timestamp: June 26, 2021, 06:10:29 pm
content: Now we are getting somewhere! I can wind a couple of transformers with cores I have laying around. I can use 1N4148 diodes (I have about 5000 of them!). I might try that soon.Thanks to those who jumped in to help. If and when I get something to report, I will do so.

author: bdunham7
author_rank: Super Contributor
timestamp: June 26, 2021, 06:27:36 pm
content: Are your 10MHz OCXOs a sinewave output around 0-2dBm into 50 ohms? If so, the best simple, passive and cheap solution is the one thatTimFoxproposed. $6 direct from MiniCircuits.https://www.minicircuits.com/WebStore/dashboard.html?model=AMK-2-13%2BIf your OCXO output is 0dBm, you should get a fairly clean sine at -12dBm from that. If that is too low, you can add an inexpensive LNA that you get on eBay for $10.The other solutions are going to result in a greatly attenuated signal that will require filtering if you actually need a sine.If your 10MHz OCXO does not have a full power sine output, you may have other issues.

author: TimFox
author_rank: Super Contributor
timestamp: June 26, 2021, 07:03:15 pm
content: Quote from: bob91343 on June 26, 2021, 06:10:29 pmThe 1N4148 PN switching diodes may have too high a voltage drop. The MCL units I referred to, and the circuits posted by others, normally use Schottky diodes (e.g., 1N5711).

author: fourfathom
author_rank: Super Contributor
timestamp: June 26, 2021, 07:13:58 pm
content: Quote from: TimFox on June 26, 2021, 07:03:15 pmTrue, but if you're winding transformers and aren't trying for maximum power transfer just put more turns on the center-tapped secondary. This will step up the voltage for the diodes, compensating for the forward drop. I've made push-push doublers using '4148's -- accepting or otherwise dealing with the Vfd.

author: Bud
author_rank: Super Contributor
timestamp: June 26, 2021, 07:34:06 pm
content: Quote from: bob91343 on June 26, 2021, 05:55:32 pmNot possible. Period.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 26, 2021, 08:16:27 pm
content: Quote from: Bud on June 26, 2021, 07:34:06 pmIndeed, but reading the other thread can give you an idea or two.It's possible to use an XOR gate and a delay line. Of course if you're using an integrated delay line, this is still some kind of "filtering", but doesn't require external passive components to trim.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 26, 2021, 08:45:32 pm
content: Note that, if it's a square wave output,the square of a square wave is DC.Mixer methods are useless. This was hinted at earlier, but perhaps bears emphasizing.Tim

author: bob91343
author_rank: Super Contributor
timestamp: June 26, 2021, 09:21:20 pm
content: For clarification, the OCXO has a square wave output, with a little overshoot. I don't know what the rig requires but it may be sine wave. The square is pretty symmetrical so there aren't any strong even harmonics.The oscillator output is around a Volt into a 50 Ohm load, so I should have no problem with the 1N4148s. If worse comes to worst I probably have a few germanium diodes in my stash but I'd rather not use those.I will next rummage through my toroid core collection and see what might be good for 10 MHz or so and still be nice and small. And look through my wire for a few turns to put on. I must compute the inductance I need, based on 50 Ohms and 10 MHz for the first transformer and 20 MHz for the output one. I may even resonate the output winding with a capacitor to get a cleaner output.So I don't think two homemade transformers, two diodes, and a capacitor are too complex.If this thing turns out simply, I may build a few and try to sell them to hams with this transceiver. Not that I expect it to make money, just cover my cost and make a few people happy.

author: TimFox
author_rank: Super Contributor
timestamp: June 26, 2021, 09:39:18 pm
content: You probably need to resonate the secondary of the input transformer, since your source is a square wave. As pointed out above, passive diode doublers are essentially full-wave center-tap rectifiers, and a square wave will give only the DC component after the diodes. A square wave with slow rise and fall times will give spiky peaks at twice the input frequency, but the output spectrum will be low at the second harmonic, with lots of power at higher even harmonics. Anyway, any doubler (passive or active) will need filtering to reduce the unwanted harmonics. A really good ideal (active) multiplier, fed by a good sine wave, will theoretically give only DC plus second harmonic sine wave (which follows from simple trigonometric identities). Note that the basic MCL specifications I cited only give a maximum level on the unwanted fundamental in the output (governed by balancing of the diodes and transformers), but you may still have higher even harmonics. Hence, the need for a filter, which need not be complicated.

author: fourfathom
author_rank: Super Contributor
timestamp: June 26, 2021, 09:42:40 pm
content: Quote from: T3sl4co1l on June 26, 2021, 08:45:32 pmTrue, a pure squarewave won't work. I was thinking that the squarewave might have a slow enough rise and fall time that the full-wave-rectified output would still have enough 2nd harmonic to be usable. If not, a little low-pass or bandpass filtering at the input would help. Perhaps the transformer primary or secondary could be resonated at 10 MHz with a capacitor, but this would depend on the output Z of the 10M oscillator and the leakage reactances of the transformer windings.The easiest and cheapest way is probably one of those 8-pin SOIC parts already mentioned.

author: bob91343
author_rank: Super Contributor
timestamp: June 27, 2021, 03:15:10 am
content: Some good points here. I dug through my drawer of toroid transformers, sometimes common mode chokes, and found a few that might be worth a try. I can certainly resonate the two transformers. As usual, the workbench is crowded with several simultaneous projects.

author: fourfathom
author_rank: Super Contributor
timestamp: June 27, 2021, 04:20:07 am
content: You don't necessarily need the transformer at the output (junction of the two diodes). A resistor to ground there might do the job, depending on what you are feeding with the doubled output.

author: bob91343
author_rank: Super Contributor
timestamp: June 27, 2021, 05:11:08 am
content: I haven't checked the transceiver schematic diagram. I suspect resonating the output transformer might afford some filtering and thus minimize spurious synthesizer products.I am unsure whether I will even do this. It just came to me in a moment of joy when I saw how good these cheap oscillators are.My thoughts are to unsolder one side of the master oscillator crystal and inject the signal at that point. I don't know the configuration of the oscillator. Its frequency is standardized by a tiny variable capacitor that must have a poor solder joint and thus intermittently goes a tad off frequency, so I should open it up anyway.The particular oscillator that is under test right now requires about 2.38 Volts to control its frequency to spot on, with an almost insignificant change with voltage. So I may just put in a divider from the 5 V power source. The OCXO draws a little over 600 mA on startup and eventually drops to around 180 mA. While I can probably safely draw that from the internal supply, I think it wiser to pipe in some energy from outside. It takes only a minute or two for it to settle. Amazing piece of work.

author: bdunham7
author_rank: Super Contributor
timestamp: June 27, 2021, 05:32:22 am
content: Quote from: bob91343 on June 27, 2021, 05:11:08 amWhat model are they?Two random comments--if it is a square wave HCMOS output, that really needs a buffer as it isn't intended to drive 50 ohms. And many OCXOs have a Vrefoutput for the adjustment divider, you should use that if it has it.

author: bob91343
author_rank: Super Contributor
timestamp: June 28, 2021, 02:00:31 am
content: It's this one.https://www.aliexpress.com/item/4000751047931.html?spm=a2g0s.9042311.0.0.187a4c4d7wdmbh

author: fourfathom
author_rank: Super Contributor
timestamp: June 28, 2021, 04:28:06 am
content: Quote from: bob91343 on June 28, 2021, 02:00:31 amI can't find a spec for that particular model, but that class of OXCO often has a sinewave output capable of driving 50 Ohms. If so this would make the simple push-push diode doubler a reasonable solution. You may still need to boost and filter the output.

author: bdunham7
author_rank: Super Contributor
timestamp: June 28, 2021, 04:57:45 am
content: I don't have a pinout, but if it is the standard I'm thinking of, there should be 4 volts output on the pin kitty-corner from the RF out, that is where the control voltage should come from, not the +5V supply.It's HCMOS. It looks like enough voltage, but it can't drive 50 ohms very well.The specs look pretty good for five bucks...

author: bob91343
author_rank: Super Contributor
timestamp: June 28, 2021, 06:25:44 am
content: Mine puts out a square wave and seems able to drive 50 Ohms.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 28, 2021, 07:10:08 am
content: For on-chip doublers we typically use a differential pair. Drive deferentially (which with a 10 MHz square should be doable with an inverter, though you'd want to filter out the higher order harmonics so you really just get the fundamental). Just tie together the differential outputs and you get double the frequency at the output. Must be doable with discretes too, I imagine.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 02:09:16 pm
content: The differential pair doubler (often called "push-push") requires a roughly sinusoidal drive. If driven by a good square wave, the same problems occur with only DC at the output.

author: bdunham7
author_rank: Super Contributor
timestamp: June 28, 2021, 02:11:40 pm
content: Quote from: bob91343 on June 28, 2021, 06:25:44 amAt what level? It isn't intended for 50 ohms, so there could be other issues as well, or it may simply tolerate the load indefinitely.

author: bob91343
author_rank: Super Contributor
timestamp: June 28, 2021, 06:05:04 pm
content: It was just a casual test. I have no data.As my motivation comes and goes, I will eventually make some progress here. First I will get some transformers that can resonate at 10 and 20 MHz respectively and roughly. Then interconnect them with diodes. Should work. The level remains to be seen.I am thinking about rewiring the TS-940S to utilize the transverter socket for input, output, and power of this oscillator. I will never use that jack and don't even know if a transverter is available. The hole in the cover for master oscillator calibration could be used to access a pot, also. Maybe there is room for a little USB power supply to run the oscillator, thus reducing the required external connections to just output for calibration. But then, I could set it up once and probably never need to do it again, totally eliminating the need for an external connection.Just thinking out loud.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2021, 06:08:04 pm
content: Quote from: bob91343 on June 28, 2021, 06:05:04 pmAs in a mains power adapter with USB output? Beware those are rarely well filtered, you're introducing a huge source of noise inside your radio. Plan on putting it in a shielded enclosure with filters on both ends. It'll be more effort, I think, than simply investigating the existing radio to find a supply rail of adequate rating to run the oscillator.Tim

author: bob91343
author_rank: Super Contributor
timestamp: June 28, 2021, 08:06:53 pm
content: Good point, Tim! Maybe I will run it from an external supply. I don't want to load any of the internal ones with an extra few hundred milliamperes. Of course the factory vaporware TCXO may draw that much but I don't know how to tell. I know there is 8V available for the microphone preamplifier.Nothing is ever simple.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 08:10:36 pm
content: Because of the heater, TCXOs draw a surprising amount of current at low voltage, especially at turn-on. Microphones do not.

author: bob91343
author_rank: Super Contributor
timestamp: June 28, 2021, 09:14:49 pm
content: This one draws 660 mA at turn on. So my plan is to bring in 5V from outside the radio. As it is, the radio will have to tolerate the one Watt or so of heat generation from the oscillator.I tested one of the common mode toroid chokes scavenged from something or other and it self resonates at 9.6 MHz. That will do for my input transformer. Since it has no secondary center tap, I will just use four diodes in a bridge. Now I need to find an output transformer to tune in the 20 MHz.

author: TimFox
author_rank: Super Contributor
timestamp: June 28, 2021, 10:17:55 pm
content: If the toroid self-resonates at 9.6 MHz, you cannot tune it higher (e.g., to 10 MHz or 20 MHz) with tuning capacitors.

author: bob91343
author_rank: Super Contributor
timestamp: June 29, 2021, 05:34:31 am
content: I realize that. But it's a broad resonance so it will be fine for the input transformer, which receives the 10 MHz square wave. Then I built a bridge of 1N4148 diodes and now have to cobble up an output transformer, which has to resonate at 20 MHz or so.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2021, 09:09:45 am
content: Mind that its resonant impedance is probably very high (>600 ohms) -- don't expect much power output from that doubler.Tim

author: bob91343
author_rank: Super Contributor
timestamp: June 29, 2021, 06:14:44 pm
content: I will check that before I go much farther. I am lucky to have the equipment to make all these measurements. Or maybe smart enough to accumulate good stuff when I can. So far I don't have an output transformer. I suspect that using toroid cores at these frequencies may not be the best idea. But I have a few to try. I have used them as high as 30 MHz so it should be okay. And I have seen gear at much higher frequencies. Distributed capacitance of the winding is the problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2021, 08:02:13 pm
content: Powdered iron of RF types (mix #2, etc.) are preferred at this frequency. Closed ferrite is right out. Type #61 and 67 ferrite, in wide gap structures (rod or threaded-slug most often), are alright.CMCs are made from ungapped ferrite, to minimize energy storage (maximize series impedance as a CMC, minimize magnetizing current as a transformer). They're also typically wound with split windings to maximize leakage, raising the characteristic impedance (in terms of a transformer) and lowering the cutoff frequency. So depending on which characteristic you're measuring (Cp + LL cutoff, or Cp + Lm impedance peak), you'll have an impedance somewhere between high (100s) and very high (kohms).Air core inductors are also quite reasonable, and easy to construct. You'll probably want to have a design ready to go before committing the time to build them, of course. Or buy an inductor kit; wound chips on ferrite or ceramic (air core) are compact, perform alright, and aren't too expensive. Give or take how much more of this you might be planning on doing in the future.Tim

author: bd139
author_rank: Super Contributor
timestamp: June 29, 2021, 08:21:24 pm
content: For personal stuff I have a selection of 43 and 61 cores and a NanoVNA. That’s close enough to bodge stuff rather than production engineering at under 50MHz or so.

author: bdunham7
author_rank: Super Contributor
timestamp: June 29, 2021, 08:40:39 pm
content: Quote from: bob91343 on June 29, 2021, 06:14:44 pmI wonder at what point does a DC bias create problems on a toroid that small?Perhaps you could make them like an adjustable IF transformer--an inside threaded tube, windings on the outside and a ferrite slug that can be adjusted. You'd probably get a lot sharper peak. And perhaps you could add a 12AT7 for some gain before and after....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2021, 09:17:12 pm
content: Ferrite toroids with mu ~ 1000 typically take a few ampere-turns to saturate. So, not a whole lot for a fair number of turns, but also still unlikely to be a problem for signal level stuff.Multilayer ferrite beads can saturate in just a few 100mA, particularly in higher Z@100MHz values (more turns), or even 10s mA for small chips (< 0603).The saturation flux is easier to calculate for a toroid or shape core: measure the cross-sectional area and multiply by 0.3T or thereabouts (some MnZn go up to 0.45T, some NiZn peter out at 0.2T). Note this is the flux per turn: multiply by turns to get flux at the winding terminals.Inductance is the conversion factor between flux and current: once you know flux, divide by inductance (H == Vs/A) to get current. High mu cores are nonlinear so the exact current varies (and depends on history, i.e. hysteresis), but this will give a reasonable value when used with the average inductance.Air gap acts to reduce inductance, replacing some core loss with lossless air, so that the Q factor generally rises as well. Note that cross section does not increase when gapping a shape core, so the saturation flux remains constant -- only the saturation current increases, because of the drop in inductance.Tim

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: June 29, 2021, 09:35:21 pm
content: This is the frequency doubler for my OCXO support board, in this case 5->10 MHz,10->20 works, too if the traps are adjusted. There is no low pass filter in ordernot to spoil the phase stability, only traps that work only locally but never atthe nominal frequency.The doubler can also operate as a push-pull amplifier, delivering up to 20 dBm.As a doubler I only got 17 dBm, but that is still a lot. The traps can either becheap crystals or Amidon ferrite rings, red or yellow for higher frequencies.On the pic of the bottom of the board, the double/amplifier is the small stripetowards you.The board can lock the crystal oven to an incoming reference, currently 10 MHz,when I find the time also to an incoming 1pps. The phase comparator is in theCPLD that can also create a local 1pps.The ferrite transformers are COTS from Digikey or Mouser, €1.50 or so.The data sheet has specs on DC bias.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: June 29, 2021, 10:06:03 pm
content: There is also a frequency doubler in my modded GPS receiver, using JFETs.<http://www.hoffmann-hochfrequenz.de/downloads/DoubDist.pdf>cheers, Gerhard

author: vk6zgo
author_rank: Super Contributor
timestamp: June 29, 2021, 11:13:22 pm
content: An "old school" way, which still works, is to differentiate your 10MHz square wave, giving two spikes percycle, of different polarities.Clip one of them, & use the other spike to "sync" a free running 20MHz oscillator on every second half cycle.This works remarkably well.

author: Bud
author_rank: Super Contributor
timestamp: June 29, 2021, 11:18:33 pm
content: OP said he does not want "stuff", didn't he. The expectation was to feed 10MHz into a magic component and get a clean sine of same amplitude on the output.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: June 30, 2021, 07:08:52 am
content: Not metrology grade, but this could fit the bill:<https://www.digikey.de/products/de?keywords=NB3N511><https://www.digikey.de/product-detail/de/renesas-electronics-america-inc/511MILF/800-1039-5-ND/1915340><https://www.digikey.de/product-detail/de/renesas-electronics-america-inc/511MILF/800-1039-5-ND/1915340>

author: eb4fbz
author_rank: Regular Contributor
timestamp: June 30, 2021, 09:27:46 am
content: RF Splitter and balanced mixer = Frequency doubler with good rejection to other unwanted products. Loss will be about 9-10dB.

author: bob91343
author_rank: Super Contributor
timestamp: June 30, 2021, 05:35:32 pm
content: I am still leaning toward the two-transformer-diode-bridge circuit. The Renesas 511MILF might work but its output is a clock, not sinusoidal.

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2021, 06:30:52 pm
content: By coincidence, I received a promotional e-mail from Pasternack about their frequency multipliers. Interestingly, they have doublers that are all passive and therefore have insertion loss, but their triplers are powered (+12 V) and have insertion gains. The range of doublers goes down to 10 MHz input, but the triplers start at 250 MHz. I didn't check, but I assume they are more expensive than the MCL units I cited in my first response, and are built in aluminum boxes.

author: bob91343
author_rank: Super Contributor
timestamp: July 01, 2021, 10:23:25 pm
content: Everything from Pasternack is outrageously expensive. I am still working on my own doubler using a bridge rectifier but so far have nothing that works well.

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2021, 10:31:37 pm
content: Pasternack stuff is high priced, but very high quality. The MCL units are reasonably priced and can be trusted.I found it interesting that all of the triplets were powered, but all the doublers were passive.

author: bob91343
author_rank: Super Contributor
timestamp: July 01, 2021, 11:14:30 pm
content: The reason for such strange situations is usually that someone had a requirement, they built a unit, and then put it in the catalog. Apparently nobody wanted a powered doubler.

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2021, 11:35:36 pm
content: More likely, there is a good reason for powering the triplers, and they advertise a series of units covering a range of frequencies. Pasternack makes custom-length cables, but most of their other stuff is catalog items. Also, the doublers go down to 10 MHz, but the triplets are higher frequency with relatively low bandwidth.(Stupid spellcheck changed that to “triplets”.)

author: bob91343
author_rank: Super Contributor
timestamp: July 02, 2021, 03:59:49 am
content: Well I am not looking for a 'reasonably priced' item from Pasternack. This is a hobby and I have very little discretionary money for trinkets. If I were a working engineer again, it would be different.

author: TimFox
author_rank: Super Contributor
timestamp: July 02, 2021, 01:24:41 pm
content: Then buy one from MCL, like I posted much earlier. "MCL" is MiniCircuits Laboratires, not a part number from Pasternack. I only mentioned Pasternack as an example of what the professional vendors are supplying and said it would be more expensive than MCL. My exact wording was "The MCL units are reasonably priced and can be trusted.", as a comparison to the expensive premium parts from Pasternack.In general, when dealing with non-linear circuits like frequency doublers, you will need both gain (from an active circuit) and a resonant circuit or other filter to remove unwanted harmonics.

author: bob91343
author_rank: Super Contributor
timestamp: July 02, 2021, 06:29:51 pm
content: Thanks for the clarification, Tim. Where can I buy that unit? How much are we talking about, for cost?Meanwhile I am struggling to make this home made circuit work. I might succeed, when I get to spend more time on it.

author: TimFox
author_rank: Super Contributor
timestamp: July 02, 2021, 06:57:51 pm
content: The MCL website sells the lower priced units (roughly $6.00) with minimum quantity of 10. Several of the usual vendors (Mouser, Newark, etc.) come up in a Google search for MiniCircuits vendors, but I haven’t chased further. MCL has a huge catalog, based on their core technology of toroidal RF transformers.

author: Bud
author_rank: Super Contributor
timestamp: July 02, 2021, 08:49:43 pm
content: You can buy Minicircuits stuff on eBay but do not buy from China. It will be counterfeit and not to specs.

author: bob91343
author_rank: Super Contributor
timestamp: July 02, 2021, 10:57:42 pm
content: Well it looks kind of grim. MiniCircuits is cheap but I have to spend $60. No way. Chinese vendors are counterfeit so I can't buy from them.Looks like a no-win situation.

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2021, 12:32:31 am
content: All very true. I was hoping to avoid an active solution. The only reason I suspect a sine is best is because this is going to substitute for a crystal. I could of course leave the crystal in and drive it with 20 MHz which would certainly provide the accuracy I want. (The driving signal will be very accurate.)Since it's a crystal oscillator load, it should end up just being synchronized by the source, which is fine for my purpose. But I can't synchronize it with 10 MHz. At least I don't think so.

author: TimFox
author_rank: Super Contributor
timestamp: July 03, 2021, 03:34:22 am
content: OK. Take a single transistor, bias it appropriately, and run it into a tuned circuit (tuned to the second harmonic).Have you tried Google to locate a vendor who will sell you a cheap MCL doubler in singles? I leave it as an exercise for the reader.Several of my classic test instruments that use an internal 10 MHz clock can take an external 5 MHz drive and run it through a 10 MHz crystal to get a high-quality clock.

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2021, 05:00:49 am
content: Tim, tell me more about the synchroniation you mention in your second paragraph. If they can synch a 10 MHz xtal with a 5 MHz signal, I should be able to do the same at double those frequencies.

author: TimFox
author_rank: Super Contributor
timestamp: July 03, 2021, 01:45:45 pm
content: It's not really synchronization, it's using the 10 MHz crystal in a crystal filter circuit following an input amplifier that distorts the 5 MHz sine wave so that it contains second harmonic content.

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2021, 05:17:38 pm
content: I was thinking along those lines. Do you have more information?

author: TimFox
author_rank: Super Contributor
timestamp: July 03, 2021, 05:28:17 pm
content: This will take some time, but I’ll try to refer you to an instrument manual.

author: TimFox
author_rank: Super Contributor
timestamp: July 03, 2021, 07:24:02 pm
content: See the manual for the PTS-160 frequency synthesizer,http://manuals.repeater-builder.com/te-files/MISCELLANEOUS/PROGM%20PTS%20160%20Instruction%20Manual.pdf, from Programmed Test Sources (ca. 1980).The manual comprises separate sections for different modules, with the pagination restarting at each. The maximum output frequency is 160 MHz.The relevant section is pp. 65 to 80 of the .pdf file, with the crystal filter shown in the schematic on p. 77.As described in the manual, either 5 or 10 MHz can be applied from an external standard, and the filter produces a 10 MHz reference for use by the modules.PTS made several variations on this design, with different frequency ranges, and at my former employer I believe we had a 40 MHz version.

author: TimFox
author_rank: Super Contributor
timestamp: July 03, 2021, 08:02:55 pm
content: The PTS-160 is all BJTs and digital ICs.Gerhard_dk4xp, in reply #50 to this thread, mentioned a multiplier using a JFET.An advantage to the JFET over the BJT in this doubler application is that if you apply a decentsine waveto the gate, strong enough to get a non-linear response, but not so strong as to switch the device ON/OFF, in theory the output of the FET (quadratic response for ideal JFET) will contain only the fundamental and second harmonic spectral components, while the BJT (exponential response) will contain the fundamental and all integer harmonics.Since neither a pure sine wave nor an ideal square wave contains anything at the second harmonic, something must be done to distort the waveform before extracting the harmonic. Applying a sine wave to a device with even-harmonic distortion gives an asymmetry between the "goes up" and "goes down" parts of the waveform, shifting the zero crossing away from the center of the period, and thereby generating even harmonics. If the distortion is odd, then the "goes up" and "goes down" both get distorted from their ideal sinusoidal shape, but the zero crossing stays centered, thereby generating only odd harmonics.

author: bob91343
author_rank: Super Contributor
timestamp: July 03, 2021, 11:15:12 pm
content: It seems that there is no simple answer to this. I haven't gotten back to the circuit to try to generate 20 MHz but might do so this weekend.Even a single diode should work. Its rectified output will be unipolar, thus containing even harmonics. I might try that instead of a bridge. I have yet to study the transceiver circuit to see what might work. Up until now it's just been a thought, nothing more.

author: Bud
author_rank: Super Contributor
timestamp: July 04, 2021, 01:34:44 am
content: Quote from: bob91343 on July 03, 2021, 11:15:12 pmGlad you realized this !QuoteTo maximize 2nd harmonic content your diode should only conduct for 120 degrees out of 180 unipolar. Look up "conduction angle" in relation to harmonics content for details.

author: bob91343
author_rank: Super Contributor
timestamp: July 04, 2021, 02:02:00 am
content: Due to the forward drop of the diode, it will indeed conduct less than 180 degrees. How much less will depend on amplitude.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2021, 02:12:12 am
content: Out of a square source, you're more likely to get harmonics due to capacitive loading (asymmetrical rise/fall). Still, something.Tim

author: fourfathom
author_rank: Super Contributor
timestamp: July 04, 2021, 04:39:20 am
content: Quote from: T3sl4co1l on July 04, 2021, 02:12:12 amWith a single-ended multiplier you will get even and odd harmonics. The transistor multiplier mentioned previously is essentially a class-C stage, and squarewave drive works as well (or better) than sinewave. As you know, the beauty of a balanced multiplier is the minimization of the odd harmonics.I once redesigned a clock tripler in a piece of telecom gear: 51.84 MHz to 155.52 MHz. We had a nice square wave clock input, and the original design had a single-transistor Class-C style multiplier with fancy filtering and amplification to get rid of all the undesired harmonics (especially the 2nd). I replaced all that with a simple circuit that sent the squarewave clock into a 155 MHz SAW filter, followed by an ECL buffer to square up the 3X output. Worked great. Wouldn't have worked at all as a doubler. Fourier is your friend.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2021, 06:52:08 am
content: I was referring to the diode idea.Quote from: fourfathom on July 04, 2021, 04:39:20 amWell yeah that works great, for a sine input. For a square (not rectangular, I mean 50% square), if you prefer the Fourier picture, the harmonics mix, superimpose and cancel out. You don't get even harmonics, you just get more odd harmonics. In short, as I said at the start of this thread, you can't square (the math function) a square (the waveform), you just get DC. The class C amp is just a unipolar class D amp, reproducing the signal with actually unusually good fidelity. There is no instantaneous (stateless time domain) function, even or odd, which can break the squareness of a square wave. At best you get skewing of rise/fall times; in which case I suppose we should really be discussing a trapezoidal wave, which is the more practical case after all.Anyway, the point about a diode was, given various assumptions about the signal path, one could introduce even harmonics by unbalancing the edges. An ordinary amp with no additional assumptions, won't do that, but given similar assumptions, can do the same thing (i.e., a capacitive load driven by asymmetrical on/off source resistances, to unbalance the edge rates).QuoteIndeed, triplers work very nicely from whatever input, and you can basically use a comparator (ECL input stages are basically diff pairs) to not just generate rich 3rd harmonic (as in ~1/3 the fundamental amplitude), but by overdriving the input, the rise/fall time and amplitude becomes much less significant, i.e. it acts as a limiter too.Consider a square wave into a filter. The filter will essentially produce repeated step responses, overlapping (superimposed) as they do. The effect is that, though the 3rd harmonic has been selected as dominant, the 1st and 5th combine as sidebands, producing apparent amplitude modulation at the 2nd harmonic (being (3-1) and (5-3)). Feed this into another stage, and the amplitude modulation can be overdriven to give a flatter output, even without a follow-up filter; in Fourier terms, the sidebands have been mixed together and canceled out. Which of course works fantastic for FM radio, where insensitivity to AM (environmental fading, tuning error) is a virtue.I don't know that Fourier analysis is all that useful of a way to reason about nonlinear systems (outside of a harmonic balance analysis, and anyway, have fun doing that by hand?), i.e. as thinking about harmonics and how they mix. The toy, the cartoon picture really, of multiplying sines, is only valid for what it is -- a translinear multiplier applied to pure tones. It's more complicated when multiple tones are present, and much, much more complicated when highly nonlinear mixers are added on top of that. Higher order mixing products will completely destroy your image rejection, in the context of radio design. (For multipliers, who cares, at least all the products are synchronous!)Tim

author: fourfathom
author_rank: Super Contributor
timestamp: July 04, 2021, 02:50:00 pm
content: Quote from: T3sl4co1l on July 04, 2021, 06:52:08 amTim, we have been in general agreement since your observation that a pure square wave into an ideal push-push diode doubler gives a DC output, not 2x. But fed a pure square wave a simple transistor (or diode) stage will generate strong 2nd harmonics (and 3rd, 4th, etc) if you adjust the conduction angle appropriately. Some input filtering and bias control is probably needed to get the right waveshape.

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2021, 03:45:03 pm
content: With a true square-wave drive, one cannot adjust the conduction angle of an active device, only the output amplitude.With an approximately sine-wave drive, adjusting the bias works well for class-C amplifiers tuned to the fundamental, or with different bias settings to optimize the output into a circuit tuned for the second or third harmonic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2021, 03:46:01 pm
content: Ah, okay. Then you're following the second part -- adding some capacitance or whatever to draw out the waveform.I would rather not use terms such as "conduction angle" as this implies it's a linear (other than cutoff) amplifier, with no extenuating (stateful) conditions. You can have an ideal class C amp (an instantaneous, zero-capacitance, nonlinear dependent source, say Iout = f(Vin) for Vin > Vthreshold, 0 otherwise), and it will never, ever modify a square wave. But you can have a nonideal one, say with a BJT which overly saturates and thus incurs storage time at turn-off, with no corresponding delay at turn-on, which therefore acts to increase the conduction angle. And it really is the conduction angle in the BJT case, as it conducts until t_stg has passed; it's not just the output hanging around because current dropped and there's capacitance on the node or whatever.But the same is likely not true of a MOSFET amplifier. Mind, given a highly nonlinear Coss typical of power MOSFETs today, it might look suspiciously like it, anyway (i.e. drain voltage seeming to sit around for a while, before shooting off).Eh, perhaps I'm being too technical / picky / pedantic with the levels of abstraction that come to mind when I see amplifiers and classes.Tim

author: fourfathom
author_rank: Super Contributor
timestamp: July 04, 2021, 03:46:20 pm
content: Example: A very simple doubler. 3V 10 MHz square wave input, 20 MHz sine output. I made absolutely no effort to optimize this, I just threw some "probably close enough" values at LtSpice. Note the R/C coupling into the diode -- this turns the square wave into pos and neg pulses. The output tank circuit is tuned to 20 MHz.

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2021, 03:53:46 pm
content: Yes, the R/C coupling changes the square wave into something not square.Conduction angle was a common parameter in class-C vacuum tube amplifiers, where sine waves got converted into sine tips that produced the desired output when filtered by a tank circuit.For non-linear amplification with solid-state devices and tuned loads, I recommend the discussion (trigger warning: Bessel functions) inhttps://idoc.pub/documents/communication-circuits-analysis-and-design-clarke-hess-6nq8yjekrqnw

author: fourfathom
author_rank: Super Contributor
timestamp: July 04, 2021, 04:58:50 pm
content: QuoteWhich is what I meant when I previously mentioned input filtering and adjusting the conduction angle. Thanks for the link -- looks interesting!

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2021, 05:48:18 pm
content: I love that book for the topics it covers. However, I recommended it to someone here earlier, and I think he freaked out when he saw the Bessel functions in the calculation of the harmonic content of the collector current in a BJT driven by a sinusoidal voltage.

author: bob91343
author_rank: Super Contributor
timestamp: July 04, 2021, 07:35:19 pm
content: Where do you get a 633 pF capacitor?Well that looks like the best idea so far. I will try it; I have all the parts.

author: fourfathom
author_rank: Super Contributor
timestamp: July 04, 2021, 08:05:57 pm
content: Quote from: bob91343 on July 04, 2021, 07:35:19 pmYou don't, except in a simulation. Instead, you will probably use a trimmer capacitor or tunable inductor. BTW, that 633 pF cap should probably be smaller by about 1pF, to compensate for the diode capacitance. And you would also need to compensate for the load capacitance, and impedance. My simple unloaded tank needs a very high-Z load to work as shown.But that circuit was just a demonstration of the concept. You might want to use a transistor with a collector tank-circuit rather than the diode. If I were doing this I would probably use a double-tuned tank circuit to improve the filtering and make tuning easier. I will try to put together a simulation of that and post it here.

author: bob91343
author_rank: Super Contributor
timestamp: July 04, 2021, 09:08:00 pm
content: It would be an understatement for me to say that I don't trust simulations. Too many assumptions, and too little attention to details.I am more a seat of the pants designer, setting things up and testing, then seeing where I went wrong and correcting it.

author: fourfathom
author_rank: Super Contributor
timestamp: July 04, 2021, 10:25:27 pm
content: Quote from: bob91343 on July 04, 2021, 09:08:00 pmMe too, but simulations can be extremely useful. The trick is to know what factors may differ between simulation and the real world. Model behavior, component tolerances, poorly-known parameters, etc.For example, I don't know what the output characteristics are for your 10 MHz oscillator. One of the spec sheets posted up-thread may have the data, or at least some of it. And I have no idea about the input impedance of the circuit you plan to drive with this 20 MHz signal, or what the desired drive level is.Even then you should only trust the simulation so far, but for passive LCR stuff it's quite good. Experts (not me) can do much better in all of this.Just for fun, I simulated a trifilar-wound transformer driving two diodes in the push-push doubler we previously discussed. By putting a small capacitor in series with the input we get a differentiated pos and neg spike from the 10 MHz square wave. Adjusting the capacitor value I can get a pretty decent duty-cycle on the 20 MHz output. Adding a 20 MHz tuned circuit at the output cleans up the waveform nicely. The available output voltage will depend on the load Z. I haven't tried resonating the trifilar secondary, but that might be helpful in purifying the output.

author: TimFox
author_rank: Super Contributor
timestamp: July 04, 2021, 10:40:25 pm
content: With Spice analysis, the .AC mode is essentially the algebra you would do to solve the usual equations, after any active devices (transistors, etc.) have been "linearized". The results from accurate values of R, L, and C should be exact. If you apply 1 V directly to the base of a transistor, the calculation should give you an accurate gain for small signals, (i.e., 100 V at the output, even though you have a 5 V supply, means that 1 mV in should give you 100 mV out) but you need a .TRAN analysis to get a reasonable result when actually applying 1 V to such a device.

author: fourfathom
author_rank: Super Contributor
timestamp: July 04, 2021, 11:53:29 pm
content: Here's another approach, this time with a transistor. I have a somewhat more reasonable loaded "Q" on the tank circuit. Again, the input is differentiated, this time to avoid 10 MHz half-cycle conduction. Instead, we want about 25% (90 degrees) for the transistor on-time. I am assuming a 500 Ohm load. The output shows some ring-down on the waveform, another filtering stage would help this, or a higher Q on the single filter.

author: bob91343
author_rank: Super Contributor
timestamp: July 05, 2021, 02:04:30 am
content: I can't read values on the schematic, and the waveform in the middle has no legend so I don't know what it is showing. Some spectrum I guess.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2021, 03:05:07 am
content: Try loading the image in a new tab? (from thumbnail state, middle-click; from enlarged state, right-click view image)Tim

author: bob91343
author_rank: Super Contributor
timestamp: July 05, 2021, 05:57:03 am
content: Okay that worked but what does pp mean on that capacitor?

author: fourfathom
author_rank: Super Contributor
timestamp: July 05, 2021, 06:37:35 am
content: Quote from: bob91343 on July 05, 2021, 05:57:03 amIt's a typo that LTspice ignored. It should read 150p (picofarads).If you want to try that circuit, here are some things you can play with:I have a fairly low "loaded Q" on that tank circuit. This lets it work in the case of slight component tolerances and load parameter difference, but some tuning might still be required. If you have a hand-wound inductor, try squeezing or spreading the turns to get peak output.You can improve the filtering by increasing the loaded Q. You can do this by changing the C7/C8 ratio, and probably adjusting the inductor. Or reduce the inductance and increase the capacitance.If you change the Q, you may want to adjust the transistor current. Adjust R2 to obtain the maximum undistorted output waveform.The middle plot shows the spectrum at the output port. You can see that the 10 and 30 MHz signals are at least 25dB below the desired 20 MHz signal. Increasing the tank circuit Q will improve this further, but at the expense of more finicky tuning (including any temperature coefficient-related mistuning)

author: TimFox
author_rank: Super Contributor
timestamp: July 05, 2021, 01:36:55 pm
content: In order to maintain backwards compatability with Spice files dating back to the Fortran-based program and Hollerith cards, Spice parses the component values using only the prefix, case-insensitively, (p, m, k, etc.), obtaining the unit (V, F, ohms, etc.) from the initial letter of the component name (VIN, C12, R3, etc.), requiring MEG for mega, since m or M always means milli. "pF" is optional, since the F is ignored.

author: bob91343
author_rank: Super Contributor
timestamp: July 05, 2021, 09:58:11 pm
content: I have no problem tweaking component values once the circuit appears to work. Even if it works it still may be some time before I implement it in the radio. I don't know how many hams read these things but I am sure they know how messy it gets when you want to pull a radio and put it on the bench. Reinstalling is an excercise in 'what is this wire for anyway?' - at least that's how it works for me.

author: TimFox
author_rank: Super Contributor
timestamp: July 06, 2021, 07:24:52 pm
content: This information is not directly related to your original problem, but it may illustrate the importance of filtration with frequency doublers.For an unrelated project, I needed a OCXO at a frequency near 78 MHz. There are many OCXOs on eBay that have been removed from equipment; most have "normal" values like 10 MHz, but there are some oddball frequencies available. Since these odd frequencies were originally special order, the exact part numbers cannot be found online, and the eBay vendor may have incorrect parameters (except for the frequency, which is marked on the unit), including what type of output (sine or square). Therefore, I need to check them before proceeding.The unit I tested (Isotemp OCXO 131-132 at 77.76 MHz) worked fine at 5 V supply, pulling about 0.55 A at turn-on and 0.21 A after an hour. It is hard to judge that waveform with only a 200 MHz scope bandwidth, but it looked like a distorted sine wave at approximately 3 V pk-pk, connecting through 510 pF to either a properly-terminated 50 ohm line, or after 3 ft of RG-58 into a high-Z load (roughly 120 pF including scope input). The voltage changed only slightly (1.3 dB) with the termination.The frequency was correct, to within the calibration of my frequency counter (only 2.5 ppm low), with no attempt to use the electronic tuning (if installed). So far. so good.Measuring the spectrum with a 400 MHz spectrum analyzer, from 2 to 202 MHz (50 ohm input impedance), I noticed a strongsubharmonicline in the spectrum, at half the nominal frequency. Since 78 MHz is a bit high for a crystal, the construction must have a 39 MHz (rounded) oscillator with a tuned doubler for 78 MHz. The spectrum contained all of the harmonics of 39 MHz (up to my scan limit), with 78 MHz the strongest. Specifically,39 MHz: -36 dBc (1st)78 MHz: 0 dBc (2nd), +13 dBm = 1 V rms at this frequency.117 MHz: -42 dBc (3rd)156 MHz: -29 dBc (4th)195 MHz: -44 dBc (5th)Note that the suppression of the fundamental and all but the 4th harmonic is quite good, about -40 dBc, but the 4th harmonic (2nd harmonic of the desired output) is stronger than the other undesired frequencies. I shall therefore apply some basic filtering centered at 78 MHz, since I need to amplify it anyway.

author: fourfathom
author_rank: Super Contributor
timestamp: July 06, 2021, 08:46:51 pm
content: Yes, on the undesired output spectra. To the OP, what exactly is this new 20 MHz signal supposed to feed? Some circuits are fairly tolerant of impure clock inputs, but with others these spurs (harmonic and subharmonic in this case) can cause problems. For example, if the clock is driving a mixer, this can result in spurious responses at the output.And while you're at it, what is the desired level and impedance of the 20 MHz clock?

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2021, 10:54:45 pm
content: fourfathom, I have not determined the required parameters. This is meant to replace the master oscillator in a Kenwood TS-940S. That is a 20 MHz crystal oscillator. There was a TCXO designed for the radio that got mounted in place of the original oscillator. I want to modify the radio as little as possible so am thinking along the lines of disconnecting one side of the original crystal and driving with the OCXO at 10 MHz with a frequency doubler.I haven't looked closely into the original circuit and so cannot answer your reasonable questions, which are in my mind also. I have the manual and will take a look before long. First I want to find or wind a 100 nH coil to use in a 20 MHz tuned circuit.

author: TimFox
author_rank: Super Contributor
timestamp: July 06, 2021, 11:11:40 pm
content: The inductance calculator found inhttps://daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtmluses a simple formula that is far more accurate than it looks, for an air-core solenoidal coil. It looks better in inches than in mm.L= (d2xn2)/(18d+ 40l)where:Lis inductance in micro Henrys,dis coil diameter in inches,lis coil length in inches, andnis number of turns.Note that diameter and length are measured to the center of the wire, not the inside diameter of the coil.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 07, 2021, 02:19:44 am
content: And if you want enterprise grade accuracy, there's always:http://hamwaves.com/antennas/inductance.htmlTim

author: fourfathom
author_rank: Super Contributor
timestamp: July 07, 2021, 03:44:11 am
content: And if you want to use toroids for the inductors, here's a calculator:https://www.changpuak.ch/electronics/amidon_toroid_calculator.phpThere are several others, but they all give the same results. You could do worse than having a small assortment of T50-2, T12-2, T50-6, and T12-6 cores for HF use. You can squeeze or spread the turns to adjust the inductance.You will generally get the highest Q with an air-core solenoid inductor, but an iron-powder toroid (like the ones above) can be quite good, and there is less coupling of the magnetic field to surrounding metals. For lower inductance values you can also use the #0 core material, which is just a low-loss insulator. Or use a plastic washer (some plastics are less lossy than others at the higher frequencies).

author: TimFox
author_rank: Super Contributor
timestamp: July 07, 2021, 03:47:44 am
content: Both air-core and toroidal coils work well at 20 MHz. Each has its advantages. For tuning, you can tweak the windings of an air-core coil, but toroids are stuck with integer numbers of turns. Usually the best ways to tune a resonance are either slug-tuned coil forms or variable capacitors with a fixed coil.

author: fourfathom
author_rank: Super Contributor
timestamp: July 07, 2021, 04:21:58 am
content: Quote from: TimFox on July 07, 2021, 03:47:44 amYes, but I've been able to do slight adjustments to the toroid inductance by having the windings more or less-closely spaced. Of course this only works when there is enough space between turns to allow this adjustment, and enough turns where this makes a difference. A T50-2 core needs five turns to get close to 100nH, and this should work. Honestly, I don't know if this is actually changing the inductance, or just changing the inter-winding capacitance, but either way it gives some tuning capability.

author: bob91343
author_rank: Super Contributor
timestamp: July 07, 2021, 05:48:30 am
content: While air core coils have radiation to the rest of the circuit, magnetic cores produce distortion. You can't have it all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 07, 2021, 06:30:05 am
content: Turns spacing on a toroid, works when the number of turns is small (so you aren't filling it out, there's room to move) and the permeability is low (which is the case for RF mixes like #2, less so for #52, very little for ferrite as in CMCs/transformers).Key observation: the fields around the wiresdo not change when a core is introduced.The field from any given turn overlaps the field from each other, and with them being so far apart, they don't cancel out until, well, comparable distances away. So, there is considerable leakage, on the order of 1/mu_r of the total.Which means we can manipulate that leakage, by scrunching the turns together to raise self-inductance (and increase external field, approximating more closely a short solenoid).So, expect a tuning range on the order of 1/mu_r times the nominal / total.There's also a solenoidal component, axial with the toroid: consider if the thickness of the toroid were reduced to zero, so that the helix of the winding is reduced to zero. Now you have a single turn, going around the toroid's center line: a loop with field parallel to the toroid['s axis of rotational symmetry]. This is reduced if the winding doubles back on itself (which is less preferable for RF purposes, due to interlayer capacitance and proximity effect), or returning the winding end along the core (without winding more turns along the way).Tim

author: Old Goat
author_rank: Contributor
timestamp: July 07, 2021, 02:17:04 pm
content: I would consider a MiniCircuits doubler first:https://www.minicircuits.com/pdfs/MK-3.pdf

author: fourfathom
author_rank: Super Contributor
timestamp: July 07, 2021, 03:08:00 pm
content: Quote from: Old Goat on July 07, 2021, 02:17:04 pmAs has been mentioned, the OP is using a 10 MHz source having a squarewave output, so a low-pass filter would be required to provide a 10 MHz sinewave input to that doubler. That filter could be pretty simple, perhaps a simple Pi network. Also, that doubler is designed for 50 Ohm input/output, which may require matching. It also has a pretty strong 4x output (down 10 dB from the 2x), so output filtering is probably needed. This could be incorporated into any output matching network. The 2x output is down 10dB from the input, so some gain is probably needed unless the desired output impedance is high enough that you can get enough voltage gain in the output matching network/filter.None of this is particularly difficult, and may be easier than the simple transistor doubler I showed up-thread.

author: bob91343
author_rank: Super Contributor
timestamp: July 07, 2021, 09:56:24 pm
content: I have had nothing but trouble finding that mini-circuits unit. And it's too expensive when you factor in shipping. I would much prefer building my own, which is on my project list.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2021, 11:59:06 pm
content: I was looking for something else, and found two MCL doublers, RK-2 and RK-3, at Surplus Sales of Nebraska:https://www.surplussales.com/Mini-Circuits/Misc.htmlfor $9 and $11 in singles. The listings are slightly inconsistent, so you should look up the items in the real MiniCircuits Laboratories catalog.

author: bob91343
author_rank: Super Contributor
timestamp: July 12, 2021, 04:14:48 am
content: Thanks Tim. For ten bucks, maybe plus shipping, I'd rather know for sure these will do the job. Otherwise I am still planning to build the doubler. I will perhaps filter the square wave then rectify and filter it to get the doubled frequency. Depending on amplitude, I may have to add a 2N3904 amplifier or something.First I need to dig out the diagram of the TS-940S oscillator and decide how to drive it with the presumably more precise frequency. It's in the realm of not fixing what ain't broke, since the dial readings on the rig are damned close as it is.Tonight I worked Sweden and the South Cook Islands, diametrically opposite directions, in immediate sequence. Band conditions are the best they have been in years.

author: bob91343
author_rank: Super Contributor
timestamp: August 20, 2021, 04:19:05 am
content: I have rediscovered the MC1496 IC. It appears that it can operate as a frequency doubler. I have used this device in the past but don't remember how. I wonder if they are still available and, just as important, whether they will do the job.