Figure 3-18K.Gated-beam detector waveforms.
Advantages of the Gated-Beam Detector
The primary advantage of the gated-beam detector lies in its extreme simplicity. It employs only one
tube, yet provides a very effective limiter with linear detection. It requires relatively few components and
is very easily adjusted.
There are more than the three types of fm demodulators presented in this chapter. However, these are
representative of the types with which you will be working. The principles involved in their operation are
similar to the other types. You will now briefly study PHASE DEMODULATION which uses the same
basic circuitry as fm demodulators.
What circuit functions does the tube in a gated-beam detector serve?
What condition must exist on both the limiter and quadrature grids for current to flow in a
Name two advantages of the gated-beam detector.
In phase modulation (pm) the intelligence is contained in the amount and rate of phase shift in a
carrier wave. You should recall from your study of pm that there is an incidental shift in frequency as the
phase of the carrier is shifted. Because of this incidental frequency shift, fm demodulators, such as the
Foster-Seeley discriminator and the ratio detector, can also be used to demodulate phase-shift signals.
Another circuit that may be used is the gated-beam (quadrature) detector. Remember that the fm
phase detector output was determined by the phase of the signals present at the grids. A QUADRATURE
DETECTOR FOR PHASE DEMODULATION works in the same manner.
A basic schematic is shown in figure 3-19. The quadrature-grid signal is excited by a reference from
the transmitter. This may be a sample of the unmodulated master oscillator providing a phase reference
for the detector.