Where to point with power meter on RF transmitter circuit.

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1

Hi, I have a general electric circuit question.

I am building below transmitter and I think I made it. But I have no idea how to calibrate this circuit with power meter.

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This is my question. When I use this power meter, where should I put +/- clips? Does (-) should be always common ground of circuit and anode clip moves? According to original designer, with power meter I could measure and calibrate while I am moving on each trimmer, But it seems nothing comes up with my power meter unless I connect to somewhere obviously current flows, for instance collector of T4/T5. I guess it doesn't show up RF power but just dc current from main power..

Had no idea where should I ask but I guessed some people here must knows. Any tip would be appreciated. thank you!

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2

Connect red to the terminal marked "output" and black to ground. There the probe will respond to RF signals.

3

And this is not a power meter, it is a detector. Make sure to terminate your amp into a 50 ohm dummy load before testing.

4

KeithRB:
And this is not a power meter, it is a detector. Make sure to terminate your amp into a 50 ohm dummy load before testing.

It has a 50 ohm load built in. But it has no blocking capacitor so it can't be used as a test probe in a circuit with DC offsets.

5

Thank you for quick answers, then

if I want to calibrate these trimmer capacitors for getting maximum power, would it be same output point where to measure?

And what if I use thise rf probe, would be any better for my question?

6

And not for very much power...

7

And for most parts of the circuit the impedance will be higher than 50 ohms - a general purpose RF-probe
would be high impedance with buffering in the probe. You can then attach it to anywhere in the signal
path with only slight loading at that point. The buffer amp has to be very low capacitance and
high resistance on its input, which is rather specialist.

For more precise power measurement at 50 ohms the circuit used is like the one on this page: Communications

The rf pushes current round the diode loop onto capacitors whose voltage is sensed (which high impedance
amp). This can provide very good accuracy over wide range of frequencies and powers with the
right kind of diodes. The matched pair of diodes gives better accuracy at low powers where the currents
are extremely small IIRC.

The lack of loading on the output means it only measures peak power, but does so with minimum
disturbance (reflections) back to the source.

This technique works up to 10's of GHz with the right engineering of the probe (not cheap)!

8

Oh-iyoiyoi..

Sounds like high-tech.

Well. I see, I may try with simple probe one first. But thank you for detailed answer, I didn't know(might be still), but always good to learn something new.

So quick anawers, always love here, thanks

9

Put simply that detector circuit is 50 ohm and probably only suitable for looking at the output from the
transmitter circuit.

It has to be 50 ohms at the end of a piece of 50 ohm coax.

Build the same detector circuit into a probe itself and you can use a much higher load resistor (1k?)
and be able to probe more parts of the circuit without over-damping them.

Typical RF circuitry has impedances from 10k down, mostly in the 5k to 10 ohm range. You ideally want
a signal probe to have a higher impedance than the circuit its peeking at.