Q. With suitable diagram explains the working of a sweep frequency generator.
Sol. A sweep frequency generator or sweeper is a special type of signal generator in which the output frequency is cyclically swept through a range of frequencies.
The instruments may have a display built into it to show the variation of amplitude with frequency or it may provide signals for use with an external oscilloscope. Shows the block diagram of typical sweep generator.
The time base is usually adjustable to give output sweep times in the range from 10 ms to greater than 100s. it is also often possible to control the sweep manually from the front panel of the instrument. The time base is frequently a triangular or a sawtooth waveform.
Two modes are used to set the swept frequency range:
(i) The stop- start Mode: in this mode the stop and start frequencies are set from the front panel and the instrument sweeps between these limits. This mode is used for wide sweep widths.
(ii) Delta Frequency Mode: in this mode the centre frequency and the maximum excursion about this frequency are set from the front panel. It is used for narrow sweep widths.
The frequency range of the swept frequency generator usually extends over the three bands, 0.001 Hz to 100 kHz (low frequency to audio), 100 kHz to 1500 MHz (RF RANGE) and 1200 GHz (microwave range). Three approaches may be used to cover a wide band of swept frequencies from a single instrument.
(a) Manually Switching: Between different frequency oscillators, the problem occurs when the frequency range needed overlaps two bands.
(b) Stacked Switching: The bands are automatically selected by electronic switches so that one can sweep the whole instruments range as one continuous band.
(c) Heterodyne control: Illustrated in given. Two high frequency signals are mixed to give a lower difference frequency O/P as one continuous band.
Output level control is used in swept frequency generator to keep the output amplitude to the values set on the front dial. Usually RF amplitude of the output. This is compared with a signal corresponding to the required amplitude and the error is fed back to an electronic attenuator circuit to keep the output constant.