Bruce wrote:
Using the square law characteristic will inevitably increase the
phase noise floor particularly in the flicker region with respect to
just using the switching characteristic of a JFET, diode or
BJT (non saturated).
Even FETs with very large transconductance and low pinchoff voltage
do not switch cleanly in a push-push mixer -- there is always plenty
of square-law behavior to add flicker noise. So the difference is
not really all that great. Furthermore, in practice whatever
difference there is will be swamped in many (if not most) cases faced
by amateur time nuts by the much larger phase noise of their sources.
There are always tradeoffs.
The only viable solution is to use better filtering of the output of
a switching multiplier.
For the reasons given above, I disagree that a switching multiplier
is the only viable solution for amateur time nuts. For NIST,
possibly. But not for amateur time nuts. Given a dirty multiplier
such as a push-push doubler with 2SK369s or BF862s, the only way to
get the fundamental and distortion products at 30MHz and below down
to suitable levels for critical work is to use a high-Q 10MHz filter
plus a series of traps. The high-Q 10MHz filter brings its own low
frequency stability problems.
There are always tradeoffs.
If you intend to use a diode ring based mixer configuration diode
connected (collector shorted to base) npns such as 2N222's are
significantly quieter (as shown by NIST) than schottky diodes for
frequencies below 40MHz or so.
Yes, that is well known. However, the transistors need to be well
matched or, once again, you end up with spurious products that are
hard to remove without causing collateral damage. (I speak from
experience -- I have built mixers that way.) This is mitigated to a
large extent with matched diode rings as used in diode DBMs. The
advantage at 10Hz with 2N2222s is only 2 or 3 dB over a good Schottky
ring, so unless one is comparing better sources than most amateur
time nuts have access to, it will be lost in the source noise (note
also that there is a crossover, above which the 2222s are about 10dB
worse). Given the relatively small potential gain and the effort
required to match four 2N2222s to sufficient accuracy not to do more
harm than good, I believe this is an exercise best left to those who
know they need the modest advantage (again, not many of us amateur
time nuts). Once upon a time, Motorola made monolithic quad BJTs --
but I'm not aware of any matched quads similar to 2N2222s at this
time. [Several manufacturers still claim to make MPQ2222s, but those
are not monolithic and not matched.]
There are always tradeoffs.
Notwithstanding theoretical objections, the described circuit works
very nicely and performs substantially better than most time nuts
need. I presume that when one is willing to afford sources that
require better performance, one will (1) know that, and (2) also be
willing to afford an expensive doubler that meets that need.
Best regards,
Charles
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