[minima] Re: KISS Mixer Musings
- From: "Joe Rocci" <joe@xxxxxxxxxx>
- To: <minima@xxxxxxxxxxxxx>
- Date: Wed, 5 Mar 2014 15:39:28 -0500
JoshMy Spice modeling indicates that you need about 5Vpp of drive on the J310
gates in order to get bestinsertion loss and IMD. The DC bias should be
adjusted so that the positive peaks of the LO on the gatesare just below the
point of gate conduction, or about +0.5Vpk referenced to the source (around
2.0VDC bias).You should be able to get the excess insertion loss down to better
than -1.0dB relative to the 6dB theoreticalcase, meaning better than 7dB
overall. Please try this if you re-run your tests; you can possibly lower the
turns on your gate drive xfmr if your 8640 generator doesn't have enough
"soup". Also, please see my previous postabout modifying the circuit so that it
automatically biases itself for the amount of LO
available.JoeW3JDR-----------------------------------------------Mark and group,
So I re-ran some tests, and I can answer (I think!) a number of questions
that I had about my first set of data. First of all, my bias issues are
entirely due to my drive levels being low, since I used a 50-ohm signal
generator for the LO port and my input transformer is lossy. The SI570
output of the Minima should generate much higher voltage swings (with much
lower LO power altogether!), which is an ideal drive for these parts. At
some point I'll switch to a square-wave drive and possibly a better input
transformer, and I think I'll need bias. So ... we can leave that question
aside for now; in the Minima, it should behave reasonably as long as your
FETs don't have a large pinchoff voltage. My best-case conversion loss was
7.45dB, when I was driving into the mixer with +22.49dBm of LO power
(again, a 50ohm generator sine wave - not at all ideal for this circuit!
At this level, I probably should have used some gate bias, but I didn't -
too many moving parts and too little test time...); this is the maximum
power I can get out of my generator setup, and the 8640 angrily flashes its
"reduce output power" light :-) With +10dBm LO input, my conversion loss
is 9.32dB, which is not horrible.
My 1dB compression point for this mixer, when driven at +10dBm LO power, is
around +1.5dBm (no compression is seen until the input power reaches
-6dBm). Again, this seems like a nice mixer for its parts count and
simplicity.
As far as the harmonics and unwanted products go, I ran a test with 14MHz
RF (-10dBm) and 34MHz LO (+10dBm); the conversion loss in this case was
11.6dB, as my transformers aren't super-hot down "that low" (this is an LO
input limitation, as both the 20MHz and 48MHz outputs are the same power).
Given the conversion loss, my IF output was -21.6dBm; the most powerful
unwanted output was at 28MHz, where there was a -49dBm spur (-26.4dB from
the wanted IF output). After that, there was a 40MHz spur at -55.7dBm
(-34.1dB from the wanted IF output), LO leakage at -57dBm (-35.4dB from the
wanted IF output), an RF->IF leakage at -58dBm (-36.4dB from the wanted IF
output), and that's pretty much it under 50MHz.
Overall, the unwanted signals aren't too ugly. I at some point will re-run
the chain in the reverse direction; if it's symmetric, and 2*IF->RF is the
only real spur of note "nearby", then there might not be a strong need for
extra bandpass output filtering on TX - because 2*IF is already above
30MHz, so the normal LPF will knock it down. The LO leakage might be the
biggest concern, when the LO is near a filter edge...
Anyway, more data. Sorry if this is beyond the scope of the list - I might
actually build a Minima at some point, but mostly I was very intrigued by
the building blocks! I've been wanting to build a JFET mixer for years,
and this was the first design that I ran across that motivated me enough to
build it.
Josh, KB8NYP
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