Spurious Signal Measurements

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Measurement of spurious signals from a receiver or transmitter may be conducted using a spectrum analyser with the appropriate frequency coverage but some special arrangements may be required depending on the signal levels to be measured. Spurious outputs from the aerial socket of a receiver will generally be very low but from those from a transmitter may be relatively high. It also depends on how far down in level compared to the transmitter fundamental output you need to be able to measure and the dynamic range of your particular analyser.

The following diagram shows a typical setup to measure transmitter spurious responses.

If a Bird 43 with a sample head is not available then it and the dummy load could be replaced with a high power attenuator with at least 50dB of attenuation.

The notch filter is featured elsewhere on this web site - see here. In use the transmitter is activated, the fundamental located on the analyser and its amplitude recorded. If its amplitude is too high for the analyser then the variable attenuator may be used to reduce the level.

The notch filter is then tuned for minimum fundamental amplitude and the individual harmonics located and their amplitudes recorded. You can then calculate the amplitude of each harmonic compared to the fundamental.

This method allows the notch filter to increase the dynamic range of the analyser.