I agree with Manfred's assessment of temperature sensors around RF amps. The BE
FM broadcast transmitters always had an LM35 mounted on a small PCB above the
cavity amplifier exhaust RF shielded port - since 1980 design time. It ran to
the central controller as a diagnostic (is it tuned up wrong and dissipating a
lot?). No problems with RFI, with 100 MHz filtering on the board.
However, my recent amplifier is 30-40 dB higher output power, pulsed, and with
these levels, it doesn't take much leakage at all to have some finite but safe
signal power. Thermocouples are worst in RF, as they tend to bias from the RF
at the junction. The semiconductor sensors that came with DMM's like Flukes are
notorious about detecting RF and biasing the reading. Nowadays I prefer the
platinum RTD (PT100) as they have a nice low Z output, and are fairly easy to
linearize and use with meters. They make some that mount with a screw. Like
Manfred said, you can do brute force filtering around your sensor to prevent
conducted RF from getting in and out via the leads. Mounting right at a
transistor flange might not be possible due to the nearby high RF current in
the circuit.
73
John
K5PRO
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