The Master Drive Unit (MDU) is a 4U-high 19 inch rack mount unit. Its function is to generate all of the frequencies required in a HiFix chain. It generates the pattern frequency and the signalling frequency, both of which are amplified to a suitable level (1.5-2 volts RF into 100 ohms) for driving the transmitter unit. It also generates the master timing sequence, and finally it can be used as a signal generator for setting up other components of the system. To perform these functions it contains three crystal oscillators and a ring-of-four timing circuit. It also has a built-in power supply unit that draws 1.5 amps from a 24-volt, positive earth supply.

The output frequencies of the MDU are derived by mixing. Two crystal oscillators, the pattern oscillator produces 132.840 kHz and the signalling oscillator produces 132.900 kHz. Either of these can be mixed with the output of the heterodyne oscillator, which generates a frequency in the range 1833-2133kHz. The difference frequency forms the output of the MDU. The heterodyne oscillator uses one of four switched crystals, thus enabling operation on one of four RF channels. The unit can be aligned on a 25 kHz segment of the band 1700-2000kHz. The four channels must be within this 25kHz sub-band. The various crystals within the MDU are contained in a crystal oven. This si maintained a temperature of 70°C by an electronic feedback control circuit using transistors.

A ring-of-four oscillator provides outputs that key the signalling and pattern frequencies in the correct sequence. Time constants in the oscillator give a 100mS on period for the signalling (trigger) pulse and the following 300mS main pattern pulse. During the signalling period, the signalling oscillator is selected to mix with the heterodyne oscillator, producing output 60Hz below the pattern frequency.

The MDU uses 10 valves (Mullard EF730, EF731 and EF734) and 16 transistors

Last Modified 16/11/07