Amplitude Modulator; Frequency Modulator - Philips PM5324 Instruction Manual

3. 1 .2. AMPLITUDE MOD UL ATOR
The output signal of the h.f. osc i l l ator is appl ied to the base of transistor 308 via separating ampl ifier 303/304
( Fig. 25). Moreover, v i a stage 307 two components of the modu l ation voltage are suppl ied to m od u lation
stage 308. The working point of mod u l ation stage 308 is adjusted with potentiometer 63 1 so that d i fference
amplifier 308/309 is symmetrica l . One component of the modu lation voltage is suppl ied to th e e m itter of
modulation stage 308 via the emitter of transistor 307. The mod u lation current i L F , which is determ i ned by
resistor 626, is suppl ied by t ra nsistors 308 a nd 309; every transistor supp l ies half the current.
The current which passes through transistor 308 causes mod ification of the transconductance (sh ifting of the
characteristic) and thus modu l ation of the r.f. ampl itude. The mod u lated r.f.·signal is ava i l ab l e at resistor 634 .
The I .f. component across resistor 634 decreas i ng s i m u l taneously wou ld cause an i nterference signal superimposed
on the modul ated signal espec i a l l y at low h .f.- and at h igh
This phenomenon is compensated for by supp l ying the decreasi ng mod u lation frequency in a nti-phase - ava i l ab l e
across resistor 6 2 4 - to resistor 6 3 4 v i a resistor 6 2 5 . Resistor 6 2 5 i s d i m e nsioned i n s u c h a way t h a t the passing
current amou nts to i L F /2 thus compensat i ng for the modu lation current th rough resistor 634.
i n mode wobble (WO B . ) this stage is used for blanki ng the fly-back. In normal operation d iode 408 is blocked;
during wobbu lating the diode becomes conductive by a positive blanki ng pu lse, thus interrupting the h .f.-signal
during the fly-back time of the wobble action.
Fig. 26. A mplitude modulator

3. 1 . 3. FREQUENCY MODULATOR

F requency modu lation is effected by voltage-contro l l ed capacitance d i odes 40 1 , 403 and 402//4 1 1 //4 1 2.
The capacitance diodes influence the osc i l lating c i rcu it of ranges 75 . . . 1 1 0 M H z , 0.4 . . . 0 . 5 M H z and 1 0.3 . . .
1 1 .1 M H z v i a isolating capacitors 520, 522 a nd 52 1 //539.
The frequency sweep is adjusted with potentiometers 608, 607 and 609. Capacitor 566 raises the h igh cutoff
frequency in the VH F range.
( F ig. 26)
3 0 7
- 3 0 9
72 V
6 2 8
1. 0 8
- 78 v - 7 8 V
I.f. frequencies.
' H F
6 7 9 5 3 1 6 3 2
27
""'AS219