Radar System Engineering

508 THE RECEIVING SYSTEM—INDICATOR [SEC.13.9
In certain applications where the clamp is used as a series element,
it is essential that the current drawn from the reference point be as small
as possible. Even such small currents as those drawn by the grid circuits
of Fig. 13”25a may be objectionable. In the circuit of Fig. 13”26,
the transformer-coupled grid circuits form closed loops which draw no
current whatever from the reference point.
This circuit is widely used as a demodulator in cases where the phase
of the carrier with respect to the reference signal is of importance. The
waveform to be demodulated is applied to the input terminal of the circuit
and the transformer is excited by an unmodulated wave which is
exactly in or exactly out of phase with the carrier. Because of grid
current, the two tubes will bias themselves to such a point that they are
turned on only at the peak of the keying waveform;
hence the device is a peak detector.
The polarity of the rectified signal at the out-
~ put terminal will depend upon which of the two
phase relationships exists. This property js
useful in such applications as the demodulation
of a carrier wave modulated by a synchro.
As pointed out in Sec. 13.4, the output signal
J.E& at a given phase of the input signal has oppo-
Flo. 13.26.—Two-wa Y site polarity on either side of a null position of
double-triode clamp with
the ~ynchro rotor; in other words, the carrier
tran8f0rmer+0uplcd switching.
undergoes a 180° phase change as the synchro
passes through a null. Thus, when the circuit of Fig. 13.26 is used as a
demodulator, the output voltage is positive or negative depending upon
whether the synchro rotor is oriented positively or negatively with respect
to the null position.
The switching signal always “leaks” through the clamp tube to some
extent by capacity coupling, particularly at the “off-point .“ For this
reason the switching signal should be no steeper or larger than necessary.
Because of this, pentodes are sometimes used in such circuits as those of
Fig. 13.24a and 13.25a. This increases the impedance of the positive
clamp, but is only necessary when the load impedance is also high.
Gated Amplifiers.-An important form of electronic switch of the
nonclamping vanet y is a ‘‘ gated” amplifier tube, in which the switching
signal is applied to an electrode in such a way that the electron current to
the plate is interrupted and no signal can pass. Figure 13”27 illustrates
various methods which are more or less self-explanatory. 1 All of these
are widely used as pulse coincidence circuitsz or to select the pulse or
1It ia possible to combine two or more of these methods so that three or more
electrodes have signals.
z In the case of. pulse coincidence there is, of course, no distinction as to which
electrodeiEbeing switched.