5-9
In the audio and video generators that produce a beat-frequency, the output frequency is produced by
mixing the signals of two separate rf oscillators. One is fixed in frequency, and the other is variable. The
difference between the frequencies of the two oscillators is equal to the desired audio or video frequency.
Audio signal generators often include resistance-capacitance (rc) oscillators in which the af is
directly produced. In these signal generators, a resistance-capacitance circuit is the frequency-determining
part of the oscillator. The frequency varies when either the resistance or the capacitance is changed in
value.
In other signal generators, however, the capacitance alone is often chosen as the only variable
element. The change in frequency that can be produced by this method is limited, and it is usually
necessary to cover the entire range of the generator in frequency steps. This is usually accomplished by
providing several rc circuits, each corresponding to a specific portion of the entire range of frequency
values. The circuits in the oscillator are switched one at a time to provide the desired portion of the af
range.
The amplifier section of the block diagram (figure 5-5) usually consists of a voltage amplifier and
one or two power amplifiers, which are coupled by means of rc networks. The output of the final power
amplifier is often coupled to the output control (attenuator) by means of an output transformer.
The output control section regulates the amplitude of the signal. A commonly used af signal
generator is the model SPN audio oscillator shown in figure 5-6. The model SPN is a programmable
synthesized signal generator designed to provide a stable, low-distortion, wide-amplitude range signal
over a 1-Hz to 1.3-MHz frequency range. Additionally, the equipment provides a square wave output and
means for swept operation with an external signal and can be remotely controlled via an IEEE-488 bus.
For the proper operation of any test equipment, you should always refer to the specific technical manual
that describes its use.
Figure 5-6.—Model SPN audio oscillator.
