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Locked 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

,----------------------. High Performance Mixers and
/ What's all this \ Amplifiers for RF Communications
/ extinct stuff, anyhow? /
\ _______,--------------' Chris Trask / N7ZWY
_ |/ Principal Engineer
oo\ Sonoran Radio Research
(__)\ _ P.O. Box 25240
\ \ .' `. Tempe, Arizona 85285-5240
\ \ / \
\ '" \ IEEE Senior Member #40274515
. ( ) \
'-| )__| :. \ Email: christrask@...
| | | | \ '.
c__; c__; '-..'>.__

Graphics by Loek Frederiks

----- Original Message -----
From: "Steve Ratzlaff" <steveratz@...>
To: <loopantennas@...>
Sent: Sunday, June 10, 2007 9:17 AM
Subject: [loopantennas] Re: Trask Active antenna design #2 tested


Hi Chris,
Your revised circuit is just built and tested.
Same testing configuration as before (10 pF series input cap with 51 ohm
termination at input, to generator; 1 uF output cap; jfet gate bias
adjusted
for best 2IMD. -10 dBm each tone at the output to the spectrum analyzer.)
+21.5 dBm OIP3, +45 dBm OIP2.
Vcc 12.82 volts, 26 mA.
Drain 4.58 volts, 12 mA approx jfet current
Emitter 5.21 volts, 13.6 mA bipolar current
Source 1.80 volts

Gain/Frequency response
20 kHz -8.5 dB, 50 kHz -4.1 dB, 100 kHz -2.9 dB, 1 MHz -2.5 dB, 10
MHz -3.0
dB, 20 MHz -3.2 dB, 30 MHz -3.7 dB, 40 MHz -4.9 dB

73,
Steve


----- Original Message -----
From: "Chris Trask" <christrask@...>
To: <loopantennas@...>
Sent: Saturday, June 09, 2007 12:24 PM
Subject: Re: [loopantennas] Re: Active antenna design


Okay, I see what I did here that made a mess of things. I had
hurredly
adapted a two bipolar transistor circuit used for low power applications
by
substituting a JFET for the NPN device and then changing the bias
resistors
without properly thinking about it. With the JFET, it turns out that
the
emitter of the PNP needs to be degenerated so as to accomodate the wide
range of Vgsoff of the JFET. Doing so allows me to increase the JFET
drain
resistor so as to improve the open loop gain. The gain is now
around -0.5dB. And you can now vary the bias current from 5mA each leg
to
12mA by applying a voltage to the JFET gate instead of changing out the
resistors as before:



Chris

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