Section IV Models 78100A, 78101A
Principles of Operation
multivibrator (and the multivibrator frequency deviations) inductive path to ground to cancel stray board capacitance
are thus controlled by the value of the currents passing which would otherwise shunt the feedback signal. CR8 is a
through Q 12 and Q 13. hot carrier diode. Its turn-on voltage is lower than the base-
collector saturation voltage of Q21, thereby assuring that
4-16. Electrode Inoperative Circuit. Q21 will not saturate. Saturation of Q21 could result in
?squegging,? a low frequency amplitude modulation of the
4-17. If an electrode should become detached from the oscillator output which produces many undesirable side-
patient, at least one side of input transistor Q7 would be bands on the transmitted signal, which could interfere with
cut off. That emitter voltage would drop, saturating one or other telemetry channels, or other broadcasting.
both sides of Q8 and increasing the voltage across R46. This
would turn on Q11 which is normally held off by the turn
on voltage of CR6. R62 stabilizes the voltage on the 4-22. Transistor Frequency Doubler.
emitter of Q1 1. This current from Q11 pulls up and clamps
the voltage on the output of U1C. This clamped voltage is 4-23. Q22 is a times-two frequency multiplier, with its
level shifted by R61 and R59 so that the bases of Q12 collector tuned (by L5 and C28) to twice the crystal
and Q13 are pulled up, so they conduct about 4/10 of their oscillator frequency. The amount of second harmonic is
normal average current. Therefore, the nominal frequency increased by operating Q22 so it is biased off by L4 and
of the multi-vibrator is reduced to about 400 Hz. turned on only at the peak of each oscillator cycle. C26
418. Subcarrier Filter. couples and, with C34, matches the two stages in im-
pedance. The current pulses through Q22 are averaged by
4-19. The output of the multivibrator is attenuated and C27, causing a dc voltage across R55 that tends to turn off
shifted in level by R31 and R32. These resistors form one Q22 further. This negative dc feedback establishes a bias
of the elements in a two-pole active filter employing C12, point for Q22 for a particular crystal oscillator output
C13, R33, emitter follower Ql8, and load resistor R34. power. R55 adjusts the dc current drain.
Q19 and Q20 are used in a second two-pole fdter. They are
connected in a unity gain configuration with R39 futing the 424. Step-Recovery Diode Frequency Doubler. t
current in Q19. R37 in series with R38 fixes the total bias
current in Q19 and Q20. R35, R36, C14 and C15 fix the 4-25. Power from Q22 reaches the antenna via the non-
location of the poles. In cascade, these two active fdters linearity of step recovery diode CR7. This generates the
form a four-pole Butterworth low pass fdter with a -3 db second harmonic of the times two output and therefore
point at 1.7 kHz. Potentiometer R38 controls the ampli- drives the antenna at four times the frequency of the
tude of the output. oscillator. L6, C29, and C30 couple and impedance match
Q22 and CR7. R56 biases the cathode of CR7. The output
420. Carrier Oscillator. is double tuned by tank circuit C31 and L7, and by tank
circuit L8 and C33 in parallel with output feedthrough
4-21. Q21 is a common-base, voltagecontrolled crystal capacitor A2A2C3. The two tank circuits are coupled by
oscillator which operates at one fourth of the carrier C32.
frequency. It is biased by R5, Q5, and R54. C22 grounds
the base to ac voltage. The collector is loaded by R53 and 4-26. Power Supply.
tuned in parallel by L3, C23, C24, and C25. In addition,
C23 and C24 step down the output impedance to drive the 4-27. The battery negative supply is regulated with a
feedback network consisting of CR1, L1, C2 1, and Y 1. The simple comparator circuit. The two PNP transistors of UlD
feedback is series resonant at the frequency of oscillation are closely matched, so the same collector current flows in
(for one particular frequency, the emitter of Q21 is each of them. Thus, if the output voltage of the regulator
connected to C23 and C24 through a low impedance). The exceeds the breakdown voltage of VR1 plus one diode
frequency of series resonance is primarily dependent on Y 1, drop, the current in both sides of UlD increases. This
but can be varied slightly by CRI and C21. L2 cancels the increases the drop across R40 and decreases the base-
shunt capacitance of Y1. Varactor diode CRl acts as a emitter voltage of Q1, reducing its output current, and
voltage-variable capacitor which modulates the oscillator therefore the output current of Q2. The output voltage is
frequency. The dc voltage across CRl is determined by reduced. By this negative feedback, the output voltage is
R52, R57, and the power supply voltage through L11 and maintained at the breakdown voltage of VRl plus one
R50. The ac subcarrier signal is superimposed on the dc diode drop.
through C16. C21 is a trimmer capacitor for setting the
oscillator on the exact frequency. L11 and C20 form an 4-28. C40 assures frequency stability. L9 and C17 fdter
any RF which may be picked up on the battery line. R2
4-2 078 100-2 |
ACEC
