[time-nuts] Double balanced mixer question

[Magnus Danielson]

Hi Attila,

On 2020-07-24 02:00, Attila Kinali wrote:
> On Fri, 24 Jul 2020 01:37:04 +0200
> Attila Kinali <attila at kinali.ch> wrote:
>
>
>> Due to a diode mixer not being a good multiplier (it only
>> multiplies the signs and adds up the amplitudes... and has
>> offset voltages when the switching happens) you will
>> get quite strong second order components like 2*LO-RF.
>> The only way to dampen them is to make the mixer as symmetric
>> as possible or to use a push-pull kind of architecture to
>> cancel out second order components (e.g., a double-double-balanced
>> mixer)
> Blub.. I should not write mails when I'm tired.... Let me correct this:
> It would not be 2nd order components, but 3rd order components 3*LO-RF
> and 2nd order harmonic (2*LO, 2*RF) leakage.
>
> While the latter can be reduced by better symmetry, the former is
> intrinsic in the working of the double balanced diode mixer. I don't
> know of any way how to reduce that, but I am not a specialist in
> mixer design, so that doesn't mean anything.

The double-balanced mixer is able to suppress the leakage of the two
input signals and also even order mixing product. Higher order products
you either use or avoid, and to avoid them you simply reduce drive level
of one port. By lowering drive level 3 dB, the useful product lowers 3
dB but the 3rd distorsion products lowers 9 dB so the distorsion
improves by 6 dB. This is common to all non-linear distorsion. This
lowers both overtones and intermodulation mixing of 2*LO-RF and 2*RF-LO.
Higher overtones can often be suppressed just by filtering, but
intermodulation products often lie within the band. This is all the same
as IP3 and you have pretty known properties as you back down from your
IP3 point. The properties reduce even quicker for higher
non-linearities, and well, the harder you push the signal the deeper
into non-linearity one gets and more indermodulation products become
apparent. So, pushing hard on the mixer for signal to noise becomes an
issue of balancing with the intermodulation products, and the IP3 point
becomes relevant. There is mixers able to tolerate mode, simply by
dropping in more diodes, so they have higher drive level. You pay some
to get some.

I went into ham radio to learn more stuff about practical RF, and I
ended learning more about this kind of stuff as I asked myself what
makes up a radio with good large signal properties, asked about which
books to read, got them and learned. Hanging out at a contest station
and all the issues we learn there, going back home to think forces me to
learn new stuff. At the same time, the mixer and a PA is both mixers,
and intermodulation will destort readability of signals. Many does not
think about the PA as a mixer, but they do act like it, which gets very
apparent as one combines transmitters using a combiner, and this
combiner leaks too much energy between the transmitters and then the mix
products creates various intermodulation products and presto you send
sideband signals none of the transmitters was designed to do. Improved
isolation is done by additional filters. Hanging out with FM transmitter
operators and hearing their challenges helps too.

So then as you go back to tutorials an books, you get more respect for
what they actually say. It is very easy to ignore the details, but as
you face the actual problems and try to address them you learn very
useful experience. There is still more to learn for sure.

So, I ended up learning much more of RF issues in the process, so that
goal was achieved.

Cheers & 73,
SA0MAD Magnus