[time-nuts] 5>10 doubler
csteinmetz at yandex.com
Tue Jan 27 14:25:54 EST 2015
>Now I have some 5MHz DOCXO. I have started to experiment with them
>and I would like to build a frequency doubler.
> * * *
>By the way, I see that really many of the 10MHz reference out there, are in
>effect doubled 5MHz ones so build a doubler seems reasonable for me.
One thing to watch for is the 5MHz leakage component. If you are
going to use the 10MHz standard for time-nuts experiments, the 5MHz
component needs to be WAY down (< -80dBc) or you will get funny
periodic ripples in stability plots. Despite having two 5MHz traps,
one recently published design suppresses the 5MHz component only
about 52dB below the 10MHz output, and the 20MHz and 30MHz components
are also only -50 to -55dB.
For this reason (and some others, see discussions over the last
several months in the archives) I prefer a doubler built with a
quadrature hybrid coupler and a balanced mixer. There is a write-up here:
I recently revived an old, stalled project to develop a JFET
push-push doubler for use at 5MHz (see schematic below).
FETs with very high transconductance and very small pinchoff voltage
(what a tube designer would call a "sharp cutoff" characteristic)
(e.g., 2SK369, BF862, etc.) are attractive on first look because they
can operate with lower conversion loss or even some conversion
gain. However, they are not well suited for doubler duty for two
reasons: (i) their characteristics have a very short range of
2nd-order curvature, so in order to keep noise down they must be
driven into regions of higher-order distortion and therefore generate
lots of spurious energy; and (ii) they are devilishly hard to match
well enough to suppress the input frequency feedthrough. Note that
you also need to put enough voltage on the FET drains to get them
well into the saturation region -- a Vcc of 5v is not enough. Again,
the penalty is lots of spurious energy. So, the lower conversion
loss of sharp-cutoff FETs is not the benefit it might at first appear
to be -- it is much easier to add gain after the doubler than to
remove unwanted spurious mixing products.
The design below uses medium-cutoff FETs and a Vcc of 15v (I found
that J111 and J310 work best and can be matched sufficiently with a
one-point match; 2N4416 and others also work, but are fussier and
would benefit from a 2- or 3-point match). At an input of 500mVrms,
their long 2nd-order characteristic is used efficiently to generate
10MHz with relatively little spurious energy.
I had no problem finding one or more FET pairs matched to within 1mV,
given 20 devices from the same lot (YMMV). With properly adjusted
traps at 5, 20, and 30MHz, all spurious responses were below
-80dBc. The inductors can be commercial RF parts with Q of 200 or so
(I used some high-quality through-hole RF inductors I had on hand --
I doubt any SMD inductors will work). The trap capacitors should be
C0G/NP0 ceramics for the bulk of the capacitance, plus very small
trimmers (I used 27pF, 27pF, and 100pF plus 0.2--6pF glass piston
trimmers). I wound the two transformers on Mix-61 toroid cores (each
winding is 20 turns on a FT37-61 core -- the inductance is a little
lower than called out). Mini-Circuits parts (or equivalents) may also work.
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