Frequency doubler 10 - 20 MHz

[bob91343]

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.

[TimFox]

The inductance calculator found in  https://daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml  uses 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 = (d² x n²)/(18d + 40l)

where:

L is inductance in micro Henrys,
d is coil diameter in inches,
l is coil length in inches, and
n is number of turns.

Note that diameter and length are measured to the center of the wire, not the inside diameter of the coil.

[T3sl4co1l]

And if you want enterprise grade accuracy, there's always: http://hamwaves.com/antennas/inductance.html

Tim

[fourfathom]

And if you want to use toroids for the inductors, here's a calculator: https://www.changpuak.ch/electronics/amidon_toroid_calculator.php  There 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).

[TimFox]

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.

[fourfathom]

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.

Yes, 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.

[bob91343]

While air core coils have radiation to the rest of the circuit, magnetic cores produce distortion.  You can't have it all.

[T3sl4co1l]

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 wires do 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

[Old Goat]

I would consider a MiniCircuits doubler first: https://www.minicircuits.com/pdfs/MK-3.pdf

[fourfathom]

I would consider a MiniCircuits doubler first: https://www.minicircuits.com/pdfs/MK-3.pdf

As 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.

[bob91343]

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.

[TimFox]

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.html
for $9 and $11 in singles.  The listings are slightly inconsistent, so you should look up the items in the real MiniCircuits Laboratories catalog.

[bob91343]

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.

[bob91343]

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.