[HBR] Mixers

[Walt Hutchens]

Donald asked:

> What is your favorite mixer, and why?

Anything that is non-linear is a mixer if you put in two inputs.   There are
a VAST number of such circuits and each has advantages.   Picking the best
one depends on your application.

Low noise mixers have a big payoff at 10M and up, perhaps sometimes as low
as 20M.  The Pullen circuit would be an example.  Below 20M, atmospheric
noise will overwhelm tube noise.  The basic paper describing the circuit can
be found with a Google for 'pullen triode mixer.'

High gain mixers can be very useful in simple designs.   Pentodes are a
common example and the W6TC designs all use them, as does the HR-10 and many
other (mostly lower end) ham receivers.  They are, however noisier than
triodes and easier to overload.

Balanced (and 'somewhat balanced') mixers have the virtue that some inputs
and certain mixing products are cancelled at the output.   These shine in
receivers for strong signal or very crowded band conditions, also for some
transmitter designs where spurious outputs are an issue.

Favorites?   In simple/low cost receivers, any of the common pentode
circuits.   Pentodes usually have both signals applied on the same grid so
oscillator isolation is an issue -- it's a good idea to use a buffer stage
or an ECO. 

Even the lowly pentagrid coverters -- 6SA7, 6BE6, 6BA7 -- have a place for
extreme simplicity when desired signals are reasonably strong and some
performance compromise is tolerable.

Triode mixers can be configured for low noise -- the Pullen configuration --
or good signal handling ability.   They also work at higher frequencies than
multigrid tubes.  

In the usual HF ham receiver I prefer a push-push dual triode mixer -- say a
12AT7, 6J6. or 6DJ8 with the plates and cathodes in parallel, the received
signal on one grid, and the LO on the other.  This is 'somewhat doubly
balanced' -- that is, both input signals tend to cancel in the output -- and
it handles large signals well.  This is a very simple circuit -- always an
advantage.  Mixing gain is relatively low and for sensitivity/low noise
operation it needs an appropriate RF stage.

The beam deflection mixers -- 6JH8, 6ME8, 6AR8, and 7360 -- combine
sensitivity with singly balanced operation with respect to the signal on the
control grid AND good large signal handling capability.   Output is
push-pull -- a slight complication -- and a relatively high signal level on
the deflection plates (usually the LO signal) is needed to get good
sensitivity.   Some of these tubes connect a signal electrode to one end of
the filament so they can only be used in a configuration where that pin can
be grounded.  

For 'the most ham receiver with the fewest tubes' I'd start with a beam
mixer and no RF stage.   A 12AT7 as a push-pull LO.  You might even get by
with a single IF stage by using one of the frame grid pentodes such as the
6KT6.  I believe you can use AGC on the beam tube control grid so that would
give you two controlled stages.

Solid state diode mixers have a place: They've been used in microwave work
since WW II because vacuum tubes fade away rapidly above 1000 Mcs or so.
The modern solid state mixer circuits are certainly excellent and easy to
use.

The choice comes down to what best fits what you're trying to do -- and of
course, what interests you.

Walt 
KJ4KV