Alan:
TNX for kudos to the Three Gurus.
Glad you learned so much.
It is ok for the IRF510 to have different gate voltages on different units.
They are actually extremely variable and easily can run a gamut of gate voltages
from ~2v to ~5v, and still be in the "linear rf" region. If those are the voltages measured
with a proper increase to 200 ma. quiescent transmit current, then that is correct.
That being said, it is best to use MOSFETS from at least the same manufacturing
batch as they are easiest to adjust together. You have already noticed that by replacing
one IRF510 and noting different gate voltages on two devices with proper adjustment.
That is a proper quiescent TX draw. Now, if you can cause the IRF510 drain to increase
the current draw to 1.0 - 1.3 Amperes with a mic signal, that is the correct level of output.
Note that this is NOT accomplished by changing the gate voltage -- leave that alone when
it is properly adjusted to about 200 ma total draw. Change the drive from the audio/rf
chain into the PA via RV1. That should be increased until just below the level of distortion.
Don't worry if you overdrive it a little; you can always back it off a bit with no harm done.
However, note that the impedance the drain sees is very important. It MUST be close to
50 ohms, If it is low, it will probably lead to spurious oscillation. If it is high, it will probably
lead to overheating the devices...and, of course, little real RF output.
In answer to your question about "ignoring" gate voltage...the answer is no. Never ignore
the gate voltage of the MOSFETS. Because there is a narrow range of voltage on the gate
which allows them to operate as RF devices. Rarely does it exceed 5v and rarely is it less
than 3v or so. But it can be almost anywhere within the 2-7v range. The only thing you know
for sure is that there is a virtually instantaneous rise in current when the "linear" region has
been exceeded, which promptly destroys the MOSFET with too much heat generation.
So using the current draw method is really a good one as it shows when "enough" adjustment
has been made, even though this is really a voltage sensitive and functional device.
If you really want to increase the power out, increase the drain voltage (to 24+v) and increase
the heat-sink size to handle a larger heat dissipation. Leave the RV adjustments pretty much alone
once they are made the first time.
Hope this helps.
john
AD5YE
