Ramsey Electronics FZ-146 Specifications download pdf (Page 22)

FX 146 • 22

connection as well as +DC, D6 is grounded through RF choke L17, which

prevents the antenna RF from being shorted to ground.

During Transmit, D7 passes RF from the transmitter to the antenna, and L17

again prevents loss of RF to ground. During transmit, D6 is blocking

transmitter RF from the receiver circuit. For maximum protection of the more

delicate receiver circuit, D2 is turned on during transmit to ground any stray

RF.

Stage E-F The FX Transceiver VCO

The VCO (Voltage Controlled Oscillator) provides basic frequency control for

both transmit and receive modes. It is essential to understand its function in

the transceiver circuit. Q7 is the oscillator transistor. L7, D3 and D23 are key

VCO components.

After the VCO is assembled on the PC board, the interested builder is given

the option of experimenting with it in receive mode before working on the

PLL synthesizer. This is done by applying a variable DC control voltage

through a pot to TP1. Otherwise, TP1 is available for checking VCO control

voltage during initial alignment. TP2 permits checking VCO frequency output

with a frequency counter. TP3 permits checking the output of the TD6128

Ã64/65 dual modulus prescaler (U3)

The control voltage for the D3 and D23 varactor diodes is supplied through

R47 and R25 by the output of U5:A in the PLL synthesizer circuit.

There must be a 21.4 MHz difference between the receive and transmit

frequencies of the VCO. This swing cannot be accomplished by PLL

programming alone. The VCO must be able to stay "in range" with the

synthesizer. D3 and D23 work in series during transmit, which reduces their

capacitance per the standard formula. For example, if a given control voltage

runs both diodes at 5 pf, the actual capacitance is 2.5 pf. In receive, the +8R

through D1 causes D3 to be shunted by C39, which causes D23 alone to

control the VCO L-C circuit, introducing twice as much capacitance and

thereby lowering the frequency.

Q5 is a common base buffer which affords good isolation, low input

impedance and broadband characteristics. The buffered output from Q5 is

fed into U3, TD6128, a dual modulus Ã64/65 prescaler, the output of which

is fed to the A and N counters in U6. The output is further buffered and

amplified by Q16, the VCO buffer which couples through C35 for receive,

and Q10 through C56 for transmit.

The VCO is is frequency modulated by microphone amplifier U4. D5 and

R31 perform an interesting function. Remember that the VCO control voltage

has a range of about 1.0 volts DC (low frequency) to 7.0 volts (high

frequency). Therefore, more modulation voltage is needed at the higher

frequencies. As the VCO control voltage increases, D5 turns on and places

R31 in parallel with R33, reducing the resistance in the line to half and

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Ramsey Electronics FZ-146 Specifications Page 22