Ramsey Electronics FZ-146 Specifications download pdf (Page 23)

FX-146 • 23

thereby increasing available modulation voltage.

The VCO requires a very pure source of well-filtered DC, free of AC hum,

alternator whine or other disturbance. R19 and the 47 uf C40 form a basic

low pass filter. Transistor Q4 serves as an electronic capacitance multiplier.

The actual effect of the filter is that the beta of Q4 multiplies the 47 uf for a

virtual capacitance effect of a much larger device.

Stage G: The FX- Transceiver Synthesizer PLL

The MC145152 IC incorporates the equivalent of 8000 individual transistors

and contains the following circuits:

• A crystal reference oscillator governed by Y2, 10.24 MHz.

• A counter or "frequency divider" circuit set externally to divide the

crystal oscillator output by 2048, for a Reference Frequency output

of 5 KHz.

• A second counter or frequency divider that divides the frequency

from the Prescaler (U3) by the externally programmed number that

we call "N".

• A third frequency divider ("A") also used for programming

• Control logic circuitry which permit the "N" and "A" counters to

work together for channel programming.

• The Phase Detector (or "phase corrector") which compares the 5

KHz Reference Frequency with the "intended" 5 KHz output of the

N-divider and sends correcting pulses to the VCO to keep the

output of the N-divider right at 5 KHz.

• A "lock detect signal" circuit. The reference oscillator is internal to

U6, governed by Y2.

The precision of the 10.240 MHz reference oscillator can be adjusted by

trimmer C81. The R divider feeds 5 KHz to the phase detector section of U6

(10240 KHz divided by 2048).

The output of the TD6128 Ã64/65 prescaler U3 is AC coupled via C57 to pin

1. U3 is a dual modulus prescaler, controlled by pin 9 of U6. The prescaled

output of the VCO is fed to the A and N counters. The "N" number

programmed on the diode matrix is predetermined to divide this frequency

down to 5 KHz for phase comparison with the 5 KHz output of the crystal

controlled reference divider. Maximum "N" is 65,535, achieved by switching

on all 16 parallel inputs.

Unlike simpler PLL IC's, U6's phase detector has TWO outputs at pins 7

and 8. These outputs go through very simple low pass filters (R44-C68, R53-

C91) to cut back the 5 KHz whine sound of U6 at work. Op amp U5:A sums

together the phase detector outputs and the output of U5:A is passed

through a network of 2.2 uf electrolytic capacitors (C67,70,90,92) to smooth

out the phase detector pulses to clean DC for controlling the VCO.

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Ramsey Electronics FZ-146 Specifications Page 23