Circuit : Andy Collinson
Email :
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.
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.
| Circuit Exchange International | Return to Testgear | https://www.cxi1.co.uk |