The first time a class E rig is put into operation, you must
use an oscilloscope to ensure everything is working correctly. Do not
apply gate drive or any other voltages to your class E transmitter
without first connecting an oscilloscope to the stage in question.
After proper transmitter operation has been verified, you can tune
the transmitter using the drain current meter, output meter and/or efficiency meter.
The idea behind tuning one of these transmitters correctly is to get
maximum efficiency consistent with the desired output. When you first
put a transmitter into operation, use this step by step procedure to
ensure everything is working.
1) MOSFET Gate Voltage Check the gate waveform(s).
Connect the oscilloscope probe between the MOSFET source and gate
leads. Connect the scope probe directly to the MOSFETs if possible. If
you're using a driver stage, start with the gate of the driver MOSFET,
get the driver working correctly, and then move to the final RF
amplifier.
If the gate waveform is poor, get it right before applying any drain
voltage. If you're using a driver, and the gate voltage is OK, apply
drain voltage to the driver only, and verify that the proper
gate drive is present at the final RF amplifier and that the driver
drain current is not excessive. The driver drain voltage waveform may
not be a good class E waveform, as the driver is not necessarily
operating in class E.
If you're using sine wave drive, make sure
the waveform is at least close to a sine wave, and not too triangular.
Flat topping of the sine wave is OK. You should see something like a 10
to 12 volt positive swing, measured at the gates.
2) Verify the drain waveform Once the gate waveform has been
established at the final RF amplifier, connect the 'scope probe to the
drain, between the drain and source. If you have a dual trace 'scope,
keep one probe on the gates and monitor the gate waveform at the same
time you're monitoring the drain waveform.
Before applying any drain voltage, set the loading capacitor all the
way "in" (full capacitance). Bring up the drain voltage to 1/3 to 1/2
of the operating voltage (slowly, if possible!). If you don't start
drawing excessive current, try (when you get to about 1/3 of the DC
operating voltage) adjusting the tuning capacitor and see if you can
get something resembling the sample waveform. If you do, great!.
If not, try opening up the loading cap a bit. If you cannot get the
proper waveform, or at least something which resembles the waveform,
you may have too little or too much inductance, and/or too little or
WAY too much tuning capacitance. Usually, higher than optimum
values for the tuning and loading capacitance will still result in a
class E waveform and low relatively low drain current. If the drain
voltage is too low to get reasonable drain current, you will not be
able to get the proper waveform.
Ok, you've got something which looks OK on the scope. If you are not
already monitoring the gate waveform, check it again with the RF
amplifier operating at reduced power. Make sure the gate voltage is
still 10 to 12 volts of positive swing. If it's starting to drop
off more than a small amount as the drain voltage or current is
increased, you might have an input to output coupling problem. Refer to
the previous section, "Maintaining gate drive under modulation" for
tips on correcting the problem.
Bring the RF amp up to full voltage, watching the drain waveform on
the scope to ensure that the peaks are within safe limits. Now, adjust
the loading cap until you start to see some "back porch" or noticeable
peaks or spikes showing up on the "low" time of the drain waveform.
Watch the drain current - make sure it doesn't get too high. If it
does, re-adjust the tuning capacitor to lower the drain current. Once
you start to see the back porch on the waveform, back off the loading
cap a little, until the wave looks good again. The transmitter is
properly adjusted for the power level you're running. If you need more
power, adjust the tuning capacitor for lower capacitance - the drain
current will go up. If you start to see back porch, ringing, or other
spikes starting to show up during the drain waveform "low" increase the
loading capacitance. As described above, check the gate voltage and
waveform again - under full power.
You are properly tuned up when, for a given power level, your
loading capacitor is set such that any lower capacitance settings will
result in back porch or other anomalies during the drain waveform "low"
time. Something to note: The drain waveform will most likely become
quite distorted as you drop the drain voltage to very low values. The
current and voltage may not track in a perfectly linear fashion once
you drop below a certain drain voltage. This is not a big problem. The
drain waveform should *not* become distorted as you increase the drain
voltage. Record the settings of the tuned elements!
3) Important: Observe the gate and drain waveforms under
modulation
When applying modulation, do so first with reduced voltage applied -
just to make sure the modulator is working at all under low power.
Bring up the modulation slowly - whether under reduced or full power.
Monitor the drain and gate waveforms under modulation - at the same
time, if possible. Look for parasitics, spikes in voltage (above what
should be expected) and changes or significant reductions in the gate
drive voltage (no more than a volt or so change at full modulation).
The gate voltage should not change very much with modulation. TEST,
TEST, TEST!!!
Tuning without an oscilloscope
Using an Efficiency Meter
The Efficiency Meter (Patent Pending) (Non commercial, individual use of the circuit and method permitted) is an innovative circuit
that provides an effective and
accurate method of tuning a class E transmitter. The meter compares the
power input of the RF amplifier to the RF output and displays the result
on a panel meter. The RF output voltage is generated by rectifying and filtering
a small portion of the RF output.
Tuning is simply a matter of obtaining the
highest indication on the efficiency meter at the desired drain current. Since
the RF output from the class E amplifier will change considerably as the
optimum tuning point is reached, an RF level control is provided to keep the
levels within the indication range of the meter. At optimum tuning, the
meter will be sensitive to efficiency changes of under 1%.
A PC board implementation of the Efficiency Meter is available. In addition
to the Efficiency Meter, the board also contains an overload shutdown system
that can be tied into most transmitters, providing very effective protection
in the event of an overload.
Tuning with the Drain Current Meter
Once you have verified your transmitter operation with an
oscilloscope, you may be able to tune the transmitter using the
drain current meter, and an RF output indicator (SWR bridge, etc.).
With most class E transmitters, the output or "loading" capacitor is
generally set for the lowest capacitance consistent with a good
waveform. In many, but not all transmitters, there is a peak in the RF output when the optimum
value is reached. Further decreases in the output capacitor will result
in a lower RF output. The optimum value will be just before the point
where the RF output begins to fall off.
When tuning up, Operate the RF amplifier at about 1/2 the normal
voltage - no modulation. While monitoring the drain current and the
RF output, adjust the capacitance of the tuning capacitor so the drain
current is about 1/2 the normal, full voltage operating value. You
should show some RF output. Now try reducing the capacitance of the
loading capacitor. The output should increase (as should the drain
current), and at some point while reducing the capacitance, the output
should begin to fall off. Increase the loading capacitance slightly, so
the capacitor is set to the point just before the RF output drop off.
Re-adjust the tuning capacitor so the desired drain current is 1/2 the
normal, full voltage operating drain current. Re-adjust the loading
capacitor as necessary. At this point the transmitter should be tuned
up. You should be able to apply full drain voltage, and modulation.
Re-check the tuning when operating at full power.
