Range Indication; Supply - Philips PM5324 Instruction Manual

30
Due to the curved characteristic of m ix i ng d iode 409 l ow-frequent difference signals appear as so'on as the
frequency of the h.f. osc i l l ator approximates to one of the frequenc ies of the ca l ib ration oscil lator o r its
harmonics.
The difference frequencies are suppl ied to the I.f. amplifier and l im iter 324 v ia low-pass filter 770/552 (cut-off
frequency about 25 kHz).
3_ 1 _ 6. 3. L. F . ampl ifier, l i m iter and ind icator
The band-pass curve
The gain and thus the adaptation to the input signal is determ i ned by resistor 655.
The output stage of th is i ntegrated circuit can be overloaded easily . The cut-off frequency is decreased with
capacitor 557. At the col lector of tra nsistor 3 1 3 a square-wave signal is avail able which contro l s transistor 3 1 3,
fu nctio n i n g as a switch, in such a way that capacitor 561 is d ischarged periodica l l y .
The charge is comp l e mented v i a ind i cator 8 1 5 and resistor 660. The indicator deflects i f there is a d i fference
frequency. If the d ifference frequency is zero or very sma l l - if the h .f. signal equ a l s one frequency of the
ca l ibrator or o n l y sl ightly deviates
deflect. The same applies if the difference frequency of the m ix ing product i s o utside the band-pass cu rve of
the I.f. ampl ifier. Thus the equ a l ity of h .f. and one of the c a l i bratio n frequencies can be recognized by a c lear
minimum between two maxima.
The places where a "zero-i ndication" may be expected du ring ca l ibration are indicated by sma l l triangles on
the corresponding scal es.
I f button CA L is not depressed, the i nput of the I.f. amplifier is bl ocked via d i ode 4 1 5 and resistor 679 .
Fig. 28_ Band-pass curve indica tor
3. 1 . 7. RA N G E I N DI C A T I O N
The six l ight-emitting diodes 49 1 . . . 496 on u n i t 3 serve to indicate the frequency range used .
Synchronously with the range switching of the h . f . osc i l lator, one of the L E D 's is switched on via resistor 60 1
(on unit 3) and a combination of switches of u n i t1 .

3. 1 . 8. SUPPLY

The plus potential of the - 1 2 V and - 1 8 V supply a re connected to c i rcuit zero. Both sources are sta b i l ized.
The u nstabilized voltages are produced by rectifier 405 on unit 2. The output voltage, d iv ided by resistors
637/638/639, is compared with the reference voltage across zener diode 404 via operational ampl ifier 322.
The output voltage of operational a m p l i fier 322 contro ls seri es transistor 3 1 1 v ia current amp l ifier 3 1 2 .
The offset-voltage of the operational ampl ifier is reduced b y resistor 64 1 . The short-circu i t cu rrent of the - 1 2 V
supply is l imited by resistor 645.
The sta b i l izing circuit for the - 1 8 V supply is the same as given for the - 1 2 V supp l y . The reference voltage is
the - 1 2 V supply. The output voltage is compared with the reference voltage via vol tage d ivider 635/636.
A separate secundary wi nding of the m a i ns transformer suppl ies the vol tage for the sca le il l u m ination ( lamps
8 1 1 ... 8 1 3) a nd for the il l u m i n ation of the indicator via switch 803/1 A ( button CAL).
of I .f. amplifier 324 as a function of the h.f.
- the frequency
signal is represented in F ig . 28.
i s suppressed by the I .f. ampl ifier a nd the i nd icator w i l l not
f I-1F > I CA '-
8204
MA