Modulation Indicator for Broadcast Transmitters

The need of a means of measuring the ratio of modulation and carrier voltages in present-day broadcast transmitters is obvious, but a few of the more important reasons why such an indicator is necessary will be reviewed here, With the growing tendency which now exists toward higher percentages of modulation the importance of such an instrument is increasing and makes a means of measuring this ratio indispensable in transmitters of any importance, High modulation is undoubtedly desirable. Recent tests made on a 380 meter 50 K W transmitter show that a 20 K W carrier modulated 52 % or a 10 K W carrier modulated 78 % is the equivalent of a 50 X W carrier modulated 31 % for producing a given output from the average receiving set, In adjusting the transmitter it is imperative that a means of measuring percentage modulation be available, so that the capability of the transmitters to handle high modulation and the capability of the modulators to produce the audio voltage be checked. In operation of the transmitter a percentage modulation indication is necessary to adjust the input to the speech amplifier.

At present oscilloscopes are used in these measurements. A coil is coupled to the tank of the transmitter, the picked-up current rectified by means of a two-element vacuum tube and fed into the vibrator coil of the oscilloscope. The beam is adjusted so that with no pickup it strikes on 100 %, and the amount of pickup is varied until with normal carrier and no modulation the beam strikes on 0 modulation. When the carrier is modulated the beam oscillated back and forth across the screen, producing an image which appears to be a solid straight line. The length of this line is proportional to carrier voltage and therefore indicates percentage: of modulation.

The object of this paper is to present a new method of measuring modulation percentage, give a general explanation of how it works, list its advantages over the oscilloscope and give full practical data on the test indicator built.