Frequency doubler 10 - 20 MHz

[bob91343]

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.

[T3sl4co1l]

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

[bdunham7]

https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3327.html

Not sure if the jitter would be acceptable with that design...

[T3sl4co1l]

https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3327.html

Not sure if the jitter would be acceptable with that design...

If 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

[HB9EVI]

other way without active electronics is a DBM; just feed the 10MHz to LO and RF and you get 20MHz at the IF port

[jonpaul]

10MHz>>RF transformer>>full wave rect (2-4 diodes) >>HP filter

[bdunham7]

If you don't mind that every other edge is RC timed, it's pretty good. 

The 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.

[bdunham7]

10MHz>>RF transformer>>full wave rect (2-4 diodes) >>HP filter

Exactly my first thought!  However, there would have to be a lot of filtering and gain stages to actually make it work. 

[TimFox]

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.html
The doublers typically have a loss between 10 and 15 dB, with the fundamental suppressed between -25 and -45 dBc.

[SiliconWizard]

You could refer to this thread: https://www.eevblog.com/forum/projects/1-5-to-3-mhz-frequency-doubler-circuit/

[bob91343]

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.

[bd139]

This will do the trick. Couple of toroidal cores and diodes: https://www.qsl.net/pa3fxo/frequencydoubler.html

1n5711 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.

[fourfathom]

Well maybe one can get simpler (or at least match it for convenience), at least in terms of BOM count and circuit construction:

https://www.renesas.com/us/en/products/clocks-timing/clock-generation/clocks-general-purpose/511-loco-pll-clock-multiplier

https://www.renesas.com/us/en/products/clocks-timing/clock-generation/clocks-general-purpose/501-loco-pll-clock-multiplier

One 8-pin IC, decoupling capacitor(s), output clock series termination resistor, et voilà!

Yes.  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:

[bob91343]

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.

[bdunham7]

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 that TimFox proposed.  $6 direct from MiniCircuits.

https://www.minicircuits.com/WebStore/dashboard.html?model=AMK-2-13%2B

If 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.

[TimFox]

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.

The 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).

[fourfathom]

The 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).

True, 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.

[Bud]

... without having to amplify, filter, or any such stuff.

Not possible. Period.

[SiliconWizard]

... without having to amplify, filter, or any such stuff.

Not possible. Period.

Indeed, 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.

[T3sl4co1l]

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

[bob91343]

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.

[TimFox]

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.

[fourfathom]

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.

True, 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.

[bob91343]

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.

[fourfathom]

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.