Modulation Meter


The FM detector uses a pulse counting discriminator which must be driven by a rectangular waveform with fast rise and fall times. A Schmidt trigger is normally used here, driven in turn by the limiting IF amplifier. A simple tuning indicator is required so filtered DC outputs from the AM and FM detectors can be used with "Tune" positions for AM and FM on the mode switch.

The AF amplifier & linear detector can use circuit 2 here for measuring the averaged modulation. However, if it is required to measure the AM peak and trough or the FM + and - deviation separately then a half wave detector must be used with a switchable inverter in the audio preamplifier to allow the peak or trough to be selected for measurement.

The tunable local oscillator must cover a fairly wide range of frequencies and spurious amplitude and frequency modulation must be kept to a minimum by using a screened and rugged construction and a stable and low noise regulated DC supply. The oscillator may have a number of switched ranges, a single range which is multiplied and divided to cover the required ranges or use a single range fairly low frequency tunable oscillator with a harmonic generator, a sampling mixer and a frequency locking system. However, the latter method would not be suitable for monitoring SSB or some data transmissions.

Sample IF and AF outputs may be provided for connection to external test equipment.

More information to follow.

Block Diagram

The following diagram shows the functions required to make a simple, manually tuned analogue modulation meter. The arrangement is that of a single superhet with an IF of 150KHz, both amplitude and frequency modulation detectors and an AF amplifier driving a detector and meter.

The local oscillator may be 150KHz above or below the incoming frequency. The IF is very low in frequency so the signal will appear at two closely spaced points in the tuning range. The IF is also fairly broad so tuning is not too critical - selectivity is determined by simple filtering immediately after the mixer.

The IF feed to the AM detector must be taken before the limiter to avoid any distortion.