Harmonic Receivers

When there is a requirement to scan a large area of spectrum for unknown signals the harmonic receiver technique can considerably reduce counter surveillance search times compared to the conventional Superhetrodyne receiver.

Harmonic Receiver operation

A harmonic receiver operates in a similar way to a conventional superhet receiver, the main difference is in the mixing process.

The mixing process is a conventional receiver involves mixing / sampling an incoming (wanted, RF) radio signal with a spectrally pure locally generated radio signal (local oscillator, LO).

This process has the effect of translating the incoming radio signal to another intermediate frequency (IF) where subsequent processing (amplification, filtering etc.) of the signal is easier to achieve.

In a harmonic receiver the local oscillator signal is first passed though a comb generator prior to being applied to the mixer.

This effectively generates multiple harmonics of the local oscillator signal meaning that multiple sections of RF spectrum can be simultaneously captured.

Thus scan times can be greatly reduced when compared to a conventional receiver.

However capturing mutliple sections of RF spectrum effectively raises the harmonic recevier noise floor making it less sensitive than a conventinal receiver.

This trade off makes the harmonic receiver ideal for counter surveillance sweeping.

A room can be rapidly swept for 'bugging' or eavesdropping devices.

Local evesdropping devices will radiate sufficiently high power to negate the loss in sensitivity.

Further the lower sensitivity of the harmonic receiver means that the 'sweeper' will not be faced with interogating a large number of distant legitimate radio signals.

Click here to download the theory behind harmonic receivers and harmonic mixing.

Audiotel uses harmonic receivers in a number of their counter surveillance products;