[minima] Re: KISS Mixer Musings
- From: Joshua Blanton <jtblanton@xxxxxxxxx>
- To: minima <minima@xxxxxxxxxxxxx>
- Date: Wed, 5 Mar 2014 19:52:40 -0500
Hi Joe,
I'm not actually using J310 FETs, as I have several hundred 2SK192A-BL
parts that I choose to try.... The ones I used, with the pinch off that I
matched, would be "best" suited with a LO peak to peak voltage of 2.4V
(pinch off to almost forward conduction). My via is actually very similar
to your self bias recommendation, but only because I need no bias 😀
I am working on an LO amp to do some impedance matching (with a much higher
output impedance, as I don't think that the mixer is actually consuming
much power -it just needs a large voltage swing. I'll also look at
transformer games, but the chores I'm using are pretty small and don't have
much room for more bifilar windings...
Thanks,
Josh
Sent from my phone - please excuse my brevity.
On Mar 5, 2014 3:39 PM, "Joe Rocci" <joe@xxxxxxxxxx> wrote:
> Josh
>
> My Spice modeling indicates that you need about 5Vpp of drive on the J310
> gates in order to get best
> insertion loss and IMD. The DC bias should be adjusted so that the positive
> peaks of the LO on the gates
> are 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 theoretical
> case, 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 post
> about modifying the circuit so that it automatically biases itself for the
> amount of LO available.
>
> Joe
> W3JDR
>
> -----------------------------------------------
>
> 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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