Frequency multipliers and mixers

Enthalpy

Greetings to all HF enthusiasts!

This is a first frequency multiplier, but beware I haven't tested it (and won't in a foreseeable future), so whether it improves over existing diagrams remains to be seen.

This diagram is to harvest an odd harmonic; connect the collectors together at one end of the coil to harvest an even harmonic.

A balanced mixer could be built similar to the multiplier, with the LO input between the input secondary's middle point and the ground; adding two PNP would make a doubly-balanced mixer. I doubt such a mixer has advantages over a diode bridge.

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Unusually, the multiplier has bipolars in common base without a power supply. All power comes from the input signal.

• The bipolars conduct at some 0.7V input voltage but drop only about 0.4V; this behaviour is more brutal than a diode, and shall transfer a bigger proportion of the input power more concentrated in the harmonics, so I hope losses improve over a diode frequency multiplier;
• The emitter current determines essentially the collector current, so imbalances between the transistors are less important than with the traditional two-transitors multiplier; operation nearer to Ft improve as well;
• The output power is well defined and should depend less on the input power than with diodes or traditional transistor diagrams.

Feel free to match the transistors by Vbe, Ft, capacitance... or use a matched pair in a case. I estimate that the fully balanced connection of the transformers, with a cold point at the middle, brings more to an odd-only or even-only output spectrum.

Stability isn't guaranteed, as the input signal brings power to the circuit. Beware many recent bipolars accept only a common-emitter configuration. I'd take only the necessary Ft, and wouldn't put the output tuning capacitor near to the collectors.

Marc Schaefer, aka Enthalpy

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Enthalpy

My second frequency multiplier relies on the ability of either gate of a tetrode MOS to pinch the drain current, which here passes by zero twice in an input period. Beware again, I haven't tested this circuit. It is meant for even multiplication only; I expect a clean second harmonic and little else.

Because the two gates aren't identical, I've put two transistors in a symmetrical circuit to minimize the odd harmonics.

Injecting the signal in the upper gate isn't perfect for gain nor stability. A cascode would improve that, possibly shared by both tetrodes, using a bipolar if stable in common base, a single gate HF MOS if you find one, or a dual gate MOS. Bias as normally the second gate would be. Especially then, the multiplier would accept a small input power.

Feel free to add in each source a capacitor and a resistor, or much better, a precisely matched integrated current source. This is interesting with other diagrams as well.

Replace the MOS by MESFET, HEMT and other variants as you like.

Marc Schaefer, aka Enthalpy

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Enthalpy

The second frequency multiplier would ideally use a matched double pentode component, still to be made, like on the diagram below.

The third gate would make the cascode without cabling penalty.

Two transistor on one chip would be matched by fabrication. Or even, have four transistors cabled internally in pairs in a cross pattern, as in op amps, if this improves further.

MESFET, HEMT and the like can improve over MOS.

Maybe these component and circuit will be used first as part of a more complex integrated circuit and be made available later just as a matched pair.

I have found no use whatsoever for a filament, alas.

Marc Schaefer, aka Enthalpy

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Enthalpy

The same matched double pentode would make a nice mixer. Tetrode MOS are already excellent mixers, whose intermodulation is second to diode rings only, provide gain instead of loss hence noise, and need little LO power. A symmetric circuit can only improve intermodulation.

Whether the local oscillator best drives the gates 1 or 2 against intermodulation is unclear; against injection, gates 2 are better. Bias should also be optimised against intermodulation. Again, the sources may have a capacitor and a resistor or a current source.

Here also, MESFET, HEMT and the like can replace MOS. Low-capacitance outputs should be preserved, as the output signal is fast at some uses.

Marc Schaefer, aka Enthalpy

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Enthalpy

An even more symmetric mixer circuit would be even better. Similar symmetry to the SO42 which calls itself "doubly balanced". It uses four matched transistors, preferably on a single chip which shall also hide the cabling mess from the user, see the diagram below...

Here also, MESFET, HEMT and the like can replace MOS. Low-capacitance outputs should be preserved, as the output signal is fast at some uses. The third gate saves some power from the local oscillator if the intermediate frequency doesn't differ much; its bias acts on intermodulation. Maybe the four transistors in cross pattern suffice to compensate all linear geometric gradients in the production process; cable 16 transistors on the chip if not. In any case, I wish that such a mixer component remain usable as a frequency multiplier as well.

Marc Schaefer, aka Enthalpy

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Enthalpy

With valves as with Jfet, symmetric preamplifiers were built to improve linearity at receiver input. I don't understand the benefit, since this reduces only the even nonlinearity which isn't a big drawback at a preamplifier, where transmodulation results from odd distortion. In case I got it wrongly, the matched pair of multigate transistors would be usable as a preamplifier as well.

A high impedance shared by the sources of transistors working symmetrically reduces further the even nonlinearity. Multigate transistors are more stable here as well. The impedance can be a coil, a resistor, a current source if not capacitive... this latter being more likely if integrated in the same chip as the HF transistors.

This can be interesting for an odd frequency multiplier, and also for the doubly balanced mixer with four transistors sketched above (not for the two-transistors mixer), where all four sources can be tied together (possibly through capacitors) to share a common constant current, say from current source(s). It would linearize the circuit further, improving the intermodulation behaviour at the signal input.

Marc Schaefer, aka Enthalpy