
Signal sources 229
frequencies below 100 Hz) to the finite thermal
time constant of the thermistor.
The Wien bridge oscillator shown in Figure
9.11b uses a two-gang variable resistor to vary both
resistors of the frequency determining network
simultaneously. This keeps constant the attenu-
ation through the network at the zero phase shift
frequency. It also provides a 10 to 1 frequency
tuning range for a 10 to 1 resistance variation, as
can be deduced from the formula for the frequency
quoted above. There are numerous sine wave
oscillator circuits which provide frequency vari-
ation using only a single variable resistor, but in
these the frequenc ...