For those who may be curious, I dug out the schematic for my version
of the quadrature-driven DBM frequency doubler and posted it to
ko4bb.com. It uses a quadrature hybrid coupler to generate +45 and
-45 degree signals at 5MHz, buffers them with emitter followers, and
applies them to the RF and LO inputs of a Level 7 DBM. The file name
is "Frequency doubler quadrature DBM" (it should be searchable once
it is moved out of the recent upload directory).
Due to the excellent balance of the quadrature hybrid coupler and the
DBM, the spurious products in the mixer output are all odd-order
harmonics of 10MHz, the strongest being 30MHz at about -35dBc. The
5MHz feedthrough is about -50dBc. Note that these depend on the
mixer balance and circuit layout, as well as the quadrature coupler
design and construction.
Also shown is an active filter/amplifier that brings the 10MHz output
back up near 1Vrms and lowers the 30MHz harmonic to ~ -60dBc and the
5MHz feedthrough to ~ -65dBc. If a simple series LC is used to feed
the load, H3 and 5MHz feedthrough can be reduced further to ~ -80dBc.
Bandpass filters at the output frequency can create phase modulation
with temperature changes. Accordingly, they are presumptively
disfavored. However, I built the whole circuit into a smallish cast
aluminum box, which integrates any external temperature changes with
a long time constant (tens of minutes), and have never had any
problem in this regard.
For many applications there is no need for any filtering (other than
a simple LP noise filter) after the mixer. Because the even-order
output products are very well suppressed, the output waveform is
nearly perfectly symmetrical. This means that it has a 50% duty
cycle and maintains its 50% duty cycle when it is AC coupled or
DC-restored, so triggering can always be stable at the center point
of the waveform.
This inherent freedom from spurious outputs and suppression of
even-order output harmonics makes the circuit attractive wherever low
spurious output is desired, compared to diode doublers or active
push-push circuits. For any required suppression of spurs, the DBM
doubler needs less aggressive filtering because of its inherent
balance. It also avoids the flicker noise that is characteristic of
diode, BJT, and FET doublers.
Of course, any design is an exercise in compromise and meeting
required performance goals, and there are always many means to reach
the end, so different designers will choose different paths. This is
one I happen to like. If you try it, you may find that you do, too.
Note: The circuit works without the emitter follower buffers, but
the spurious outputs are somewhat higher due to the nonlinear loading
on the hybrid coupler. This may still be useful if an all-passive
circuit is required.
Best regards,
Charles
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