amplitude of
±
9V at the base of VT51. VT51 is an emitter follower, the output of
which is applied to a smoothing circuit R134 C28 having a time constant of about
0.4 second. The output of this passes to a super alpha pair VT52, VT53 and thence
to the centre zero meter. This is a 0.5, 0, 0.5mA meter the resistance of which is
built out by R137. Its zero reading is set in the absence of signal by selection of
AOT5. In the presence of signal at input B but none at input A, the resulting
symmetrical waveform at the emitter of VT51 produces no deflection of the meter. If
this input signal is sufficient to produce a negative swing greater than about 0.5V at
input 5, then VT47 is turned on to provide an incremented of current to the inhibitor,
this being the fourth such transistor operated by signal in Forward Path B. (With
increasing signal level it is actually the first one to operate.)
If there is signal in the path and none in the B path, the base of VT46 is
driven but no deflection results on the meter.
If there is signal in both paths simultaneously the squared signal in the A path
operates VT46 as a switch to turn the squared signal in the B path on and off.
If the difference of level in the two channels exceeds 20dB, the inhibitor
switches VT48 on to prevent any signal going further.
If the two signals are random then the signal at VT49 and VT50 will on the
average be equally distributed on the positive and negative sides of its zero signal
datum, and the meter will therefore remain on zero.
If the signals in the two channels are identical then the signal in the B path
will be switched on when it is negative and off when it is positive. This turns VT50
on, thereby pulling the base of VT51 positive and giving a positive deflection of the
meter.
If the signals are identical but in opposite phase then the signal in the B path
will be switched on when it is positive and negative deflection of the meter will
result.
Since the device is to intended to be a linear phase meter but is required to
be a indicator of in-phase or out-of-phase signals in the two channels, the scale
shape is deliberately distorted by clamping diodes D15 and D16. AOT7 and AOT6
are selected for positive and negative full scale deflection respectively.
12.18.3.7
Power Supply
The correlator derives its supply from the stabilised supply on the Control
Room Monitor Cassette. As this unit contains squared waveforms, which contain
considerable amounts of high order harmonics of the normal signal waveform in the
Mixer, it is liable to introduce spurious signals into other parts of the cassette, due to
the common impedance of power unit and/or wiring, or to coupling in the wiring. To
prevent this the correlator includes filtering stabilisers. Those for the Forward Path
A and the Inhibitor are on the board carrying the latter. Those for the rest of the
circuitry are on the gates board and they are identical with the previous ones except
for the values of two resistors.
Each stabiliser has a series transistor, acting as a constant current source,
and a shunt transistor, acting as an emitter follower. The upper positive stabiliser
on the circuit diagram has a series transistor VT26, of which the emitter is fed
through R66 and the base potential is determined by D8, R70, R71 and AOT1, D8
being included for temperature compensation. The base of the shunt transistor
VT27 is held at +17.4V by R74 and R75, C15 being included to remove any supply
borne interference. R67 is included so that the current in VT27 can be measured by
a voltmeter and set to the desired value of 3mA by selection of AOT1. This ensures
