Just for completeness, I should mention that I use 3 and 4 MHz tones for all my IMD testing, and for these circuits, set for -10 dBm each tone at the output, as seen on the spectrum analyzer.
Steve
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----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 3:14 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested
Prior to breadboarding this, I went back and forth with the PSpice
models to determine where the discrepancies. I went so far as to remove the
2N2907 emitter bypass capacitor, but that resulted in a loss of -6dB. So, I
rearranged the input to place the 50-ohm termination ahead of the blocking
capacitor, changed the blocking capacitor to 10pF, then added a 100K
resistor from the JFET gate to ground, and then I saw the -3dB of midband
loss.
So now I see why my model gain and your measured gain are so far apart
this last time around. I was measuring the gain grom the JFET gate to the
output, and you're measuring it from the 50-ohm load AHEAD of the 10pF
capacitor to the output.
That being the case, I now have to wonder what effect this 10pF coupling
capacitor has on the IMD performance. So, could you take both of these
circuits, replace the 10pF capacitor with 0.1uF, and then measure again? My
suspicion here is that the JFET drain voltage in my circuit is feeding back
to the gate and combined with your 10pF coupling capacitor is causing the
IMD performance to degrade.
Chris
