Frequency doubler 10 - 20 MHz

[bob91343]

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.

[bdunham7]

It just came to me in a moment of joy when I saw how good these cheap oscillators are.
 So I may just put in a divider from the 5 V power source.

What 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 V_ref output for the adjustment divider, you should use that if it has it.

[bob91343]

It's this one.

https://www.aliexpress.com/item/4000751047931.html?spm=a2g0s.9042311.0.0.187a4c4d7wdmbh

[fourfathom]

It's this one.

https://www.aliexpress.com/item/4000751047931.html?spm=a2g0s.9042311.0.0.187a4c4d7wdmbh

I 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.

[bdunham7]

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...

[bob91343]

Mine puts out a square wave and seems able to drive 50 Ohms.

[TheUnnamedNewbie]

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.

[TimFox]

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.

[bdunham7]

Mine puts out a square wave and seems able to drive 50 Ohms.

At 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.

[bob91343]

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.

[T3sl4co1l]

Maybe there is room for a little USB power supply to run the oscillator

As 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

[bob91343]

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.

[TimFox]

Because of the heater, TCXOs draw a surprising amount of current at low voltage, especially at turn-on.  Microphones do not.

[bob91343]

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.

[TimFox]

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.

[bob91343]

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.

[T3sl4co1l]

Mind that its resonant impedance is probably very high (>600 ohms) -- don't expect much power output from that doubler.

Tim

[bob91343]

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.

[T3sl4co1l]

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

[bd139]

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.

[bdunham7]

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 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....

[T3sl4co1l]

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

[Gerhard_dk4xp]

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 order
not to spoil the phase stability, only traps that work only locally but never at
the 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 be
cheap 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 stripe
towards 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 the
CPLD 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.

[Gerhard_dk4xp]

There is also a frequency doubler in my modded GPS receiver, using JFETs.

<    http://www.hoffmann-hochfrequenz.de/downloads/DoubDist.pdf      >

cheers, Gerhard

[vk6zgo]

An "old school" way, which still works, is to differentiate your 10MHz square wave, giving two spikes per
cycle, 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.