Reconfiguring with a 1 uF monolithic input cap, resetting jfet gate bias
for
minimum 2IMD:
+25 dBm OIP3, +48 dBm OIP2. Vcc 12.87 volts, 26 mA. Drain 4.51 volts,
source
1.80 volts, emitter 5.16 volts (almost the same as before). -10 dBm output
tone levels, at SA, same as before.
Gain/frequency response: 20 kHz -1.6 dB, 50 kHz -0.2 dB, 100 kHz 0 dB, 1
MHz
0 dB, 10 MHz -0.4 dB, 20 MHz -0.3 dB, 30 MHz -0.2 dB, 40 MHz -0.9 dB.
(Of course the circuit is no longer equivalent to an active whipamp
circuit.
I can build a jfet-input active whipamp circuit with greater than +45 dBm
OIP3, +85 dBm OIP2---tested with the same testing configuration using 10
pF
input capacitor simulating a one-meter whip.)
Yes, I'm beginning to see why there is such a substantial difference
between the two. I've spent the entire efternoon with my PSpice models
trying to locate the source of the IMD problem and correct it.
I took the PSPice models for both circuits and did a reverse
transmission test, discovering that the reverse transmission for my circuit
is twice that for the Lankford circuit regardless of whether a 0.1uF or 10pF
coupling capacitor is used. Thinking that this would be due to no small
part by the signal voltage on the J309 drain being coupled to the gate, I
devised a third circuit which adds a 2N2222 common base stage between the
J309 and the 2N2907 (making it a cascode section), which substantially
reduced the J309 drain signal voltage when the coupling capacitor was 10pF.
This combination now makes the reverse transmission of the first and third
circuits virtually identical.
My reasoning here was that in the second circuit (my first), there will
be substantial IMD voltages at the J309 drain that are a result of
correcting for the differences between the J309 gate and source signal
voltages, which will then be coupled to the gate and subsequently amplified.
The 10pF coupling capacitor makes this signal path more substantial since it
unloads the gate and lets more of the IMD signal at the drain be coupled to
the gate.
I then took all three circuits and replaced the J309 with a 2N2222. For
the Lankford circuit, this resulted in little performance change for either
forward or reverse when using the 10pF coupling capacitor but improved the
performance when using the 0.1uF coupling capacitor. For my two circuits,
when using the 0.1uF coupling capacitor the forward gain was almost 0dB and
the reverse isolation for my second circuit was improved.
I can see two things from all this. First, a JFET is the lesser choice
for the first stage in an amplifier such as this as it results in lower
forward gain. Secondly, the use of a cascode for the first stage reduces
the reverse transmission and may subsequently reduce the IMD problem by
reducing the IMD voltages at the input transistor drain (or collector) which
are fed back to the unloaded gate (or base).
This is something that I had not considered in low power amplifier
design as I'm generally dealing with 50-ohm terminations. Now, I can see
that there are at least two things that need to be considered in the design
of active short dipoles (or monopoles), which was what I was focused on when
this began. The first of these is that high antenna impedances, such as
from an untuned short dipole, can cause additional IMD problems in the
amplifier by way of improperly loading reverse IMD products that are
transmitted to the amplifier input. The second is the design of the
amplifier itself, requiring that the source of the IMD products that can be
conducted to the input need to be reduced.
The low power amplifier design itself is not that much of a problem as
for now it appears that the IMD source can be controlled by using a cascode
first stage and the overall reverse isolation can be improved by using
bipolar devices. This will be especially true if it is intended that the
amplifier have signal gain.
The matter of the high antenna impedance which results in unloaded
reverse IMD products makes the interface between the antenna and the
amplifier difficult. Given the ramifications of high input impedances,
parallel tuning (or no tuning) is out of the question. And the remote
adjustment of variable inductors can be a bit of a task, especially for wide
bandwidths since transductors (aka saturable reactors) have a limited
practical range of variation.
All of this being the case, I'm going to have to take all the notes I've
made for remotely tuned active short dipoles and just start all over again.
Looking at all of this, designing active loops was simple as you are dealing
with low impedances and inductive reactances.
I'm glad, though, that this all took place because I would have spent a
lot of time in designing these and would have come up with something that
was far less than ideal. Less than ideal is a practical goal.
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 4:25 PM
Subject: Re: [loopantennas] Re: Trask Active antenna design #2 tested
Hi Chris,
Reconfiguring with a 1 uF monolithic input cap, resetting jfet gate bias
for
minimum 2IMD:
+25 dBm OIP3, +48 dBm OIP2. Vcc 12.87 volts, 26 mA. Drain 4.51 volts,
source
1.80 volts, emitter 5.16 volts (almost the same as before). -10 dBm output
tone levels, at SA, same as before.
Gain/frequency response: 20 kHz -1.6 dB, 50 kHz -0.2 dB, 100 kHz 0 dB, 1
MHz
0 dB, 10 MHz -0.4 dB, 20 MHz -0.3 dB, 30 MHz -0.2 dB, 40 MHz -0.9 dB.
(Of course the circuit is no longer equivalent to an active whipamp
circuit.
I can build a jfet-input active whipamp circuit with greater than +45 dBm
OIP3, +85 dBm OIP2---tested with the same testing configuration using 10
pF
input capacitor simulating a one-meter whip.)
73,
Steve
----- 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
,----------------------. 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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