660 Chapter 22
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210 °Soundwave3000 Hz 15 10 50 201000 HzFigure 22.9 : Polar response of a cardioid microphone.capacitance of a bipolar transistor connected as a Hartley or other simple oscillator tuned
to the desired carrier frequency in the FM band.
Such oscillators were prone to drift in operating frequency as the transistor characteristics
were sensitive to both temperature and supply voltage variations. This required periodic
retuning of the receiver to compensate for transmitter frequency drift. This was particularly
true of the very early units that employed germanium transistors. Signifi cant improvement
in this regard was made possible with the availability of suitable silicon transistors.
One of the authors well remembers hand crafting several body pack transmitters in 1965
for use by lecturers at Georgia Tech. The receivers employed were H. H. Scott units that
had been modifi ed to incorporate automatic frequency control circuitry to compensate
for the transmitter drift within reasonable limits. These early units had acceptable
audio bandwidths but the simple modulation technique employed did not produce large
frequency deviations, resulting in a small dynamic range of the recovered audio signal.
Those of us who have experienced the entire history of wireless microphones consider
the present-day versions to be truly remarkable. Not only have the early shortcomings
been addressed but also features not even envisioned by the early practitioners have