Ramsey Electronics FZ-146 Specifications download pdf (Page 24)

FX 146 • 24

R48 and C85 form yet another low pass filter to ensure that any 5 KHz

"whine" will not get into the VCO. Because the DC charge developed in C85

(.1 uf) would slow down the PLL during major frequency swings, such as just

going from transmit to receive, D8 and D10 are set up back-to-back across

voltage dropping R48. Whenever there is a major frequency shift (which

means a significant VCO control voltage change), one way

or the other, one diode or the other is switched on to

short out R48 and discharge C85. This lets the PLL

relock instantly; C85 recharges and the diodes become

no factor in the circuit.

The lock detect output (pin 28) gives a strong series of pulses when the PLL

is unlocked. When the PLL is locked, only a tiny sawtooth wave appears at

pin 28. The "lock detect" voltage is watched by U5:B. If "unlock" pulses

appear, they are integrated through R90 and C96 as a fairly clean DC

voltage charge built up in C96. If this charge causes U5B to swing low, bias

is removed from Transmit Buffer Q10, preventing transmitter damage and

unwanted emissions.

Stage H: The Diode Matrix and PLL Synthesizer Programming

There are two diode-matrix programming areas on the PC board. The

obviously larger area is for frequency channel programming. The second

space is for offset programming added in by U7-U10.

The 19 100K resistors at the frequency programming matrix and the 14

100K resistors at the offset matrix are "pulldown resistors," to ensure

positive logic switching action of U6.

Q15 and its associated switching diodes ensure that the desired offset is

switched in during transmit, that offset programming does not interfere when

simplex is desired and that the offsets do not interfere with receiver

operation and that receiver programming (21.4 MHz lower) does not

interfere with transmit operation.

A variety of techniques are possible for binary programming of U6's 16

paralel inputs. We focus on the diode programming approach with some

brief suggestions on externally-controlled switching. It is very intentional on

our part to leave innovative programming schemes up to FX transceiver

users, because there's no single best way to do it for everybody.

FREQ. N = BINARY PROGRAMMING VALUES

144.000 28,800

148.000 29,600

0111 0000 1000 0000

0111 0011 1010 0000

146520 (KHz)

5 (KHz)

1 2 ... 19 20 21 22 23 24 25 26 27 28 29 ... 144 145

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Ramsey Electronics FZ-146 Specifications Page 24