Absorption Wavemeter

Circuit : Andy Collinson

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Description
The absorption wavemeter circuit is a useful piece of test equipment for anyone working with transmitters, It is similar to a field strength meter but with one difference, the circuit is tuned and therefore will detect both signal strength and indicate frequency. The circuit is below:

Notes
This circuit is very similar to the Field strength meter circuit, link here. The primary components are L1 and C1. These form a parallel tank circuit whose frequency is tuned by adjusting C1. The coil, L1 is mounted externally and is also used to pickup the signal. In use, the circuit is built into a case with the coil mounted externally. It is held close to the transmitter and C1 is adjusted. When at resonance, the waveform is "absorbed" by the tuned circuit and indicated by a reading on the signal meter, M1. The frequency can be read from a calibrated dial, the relative signal strength is indicated by the deflection on the signal meter. If used for example in tuning a transmitter, a stronger transmitter signal gives a higher deflection on the meter.

The tuned circuit will respond to unmodulated or modulated carrier signals. The signal is rectified by a germanium detector diode D1 and presented as a load across R1. The FET operates as a common source amplifier and the weak gete voltage causes a change in drain current displayed on the signal meter. The signal meter should have a sensitivity of 250 uA. Designed to be powered from a 9 Volt battery, the potentiometer VR1 allows the meter to be set to zero and also acts as a crude gain control.

Calibration
The hardest part of this circuit is the calibration. Ideally you need a RF signal generator set at a known frequency and then tune C1 to this frequency. A paper or cardboard dial needs to be fixed under the variable capacitor control knob and calibrations marked accordingly.

Covering Multiple Ranges
To cover multiple ranges it will be necessary to change the coil L1. Contacts A and B on the schematic can be wired to a suitable socket like a two pin crystal holder or similar.

To calculate frequency ranges there are some javascript calculators on my tuned circuit page.
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