[time-nuts] Double balanced mixer question

[Attila Kinali]

On Thu, 23 Jul 2020 11:01:28 -0700
<cdelect at juno.com> wrote:

> I'm feeding 5.0 MHZ and 5.000001MHz into an HP10514A mixer.
> 
> A buffer and a 12dB attenuator feed each input and a 50 Ohm buffer amp
> (10Mhz) is on the output.

[...]

> Are there mixer schemes I can use that will eliminate the amplitude
> variations?

I guess you have read Enrico's Mixer Tutorial[1]? If not, I highly
recommend it. One very important lesson it contains is that you have
to short the signal component you do not want at the mixer.
This means you have to offer the mixer a low impedance path
at the mixer output to ground. An good inductor with high
self-resonance (>20MHz) and inductance higher than ~13µH should
do the job. 

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)

As you are using a pretty old mixer, my guess it is using
discrete diodes. I would recommend replacing those either
by some matched diode array (HSMS-2827 and HSMS-2829 come to mind)
or use matched BJT arrays in diode configuration as they 
are depicted in [2] (e.g. THAT300) to make the switching more
symmetric.

What you can also try is to drive the two input ports with
square wave signals. This should minimize second order harmonics
generation within the mixer. But I have not gone through the math
for this and thus cannot say how effective it would be. 

			Attila Kinali

[1] "Tutorial on the double balanced mixer", by Enrico Rubiola, 2006
http://rubiola.org/pdf-articles/archives/2006-arxiv-0608211v1-mixer-tutorial.pdf

[2] "Residual PM Noise Evaluation of Radio Frequency Mixers",
by Barnes, Hati, Nelson, Howe, 2011
(available on the NIST document server)
-- 
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.