M850-00
T856/857 Circuit Operation
C2.13
Copyright TEL 31/09/98
2.9 T857 Exciter Drive Amplifier
(Refer to Figure 2.2 and the exciter circuit diagram (sheet 3) in Section 6.3.)
A two-stage, wide band amplifier (Q350, Q355) provides an output level of approxi-
mately 1W (+30dBm) for an input of 100mW (+20dBm) from the VCO. IC300 pin 1,
Q302, and Q303 provide a 9V regulated supply for the exciter.
To reduce the spurious output level when the synthesiser is out-of-lock, the Tx-Reg. and
Lock-Detect signals are gated to inhibit the exciter control circuit and to switch off the
RF signal at the input to the drive amplifier. This is achieved by a PIN switch attenuator
(D201, D202, D203).
Cyclic keying control is provided by additional circuitry consisting of several time
delay, ramp and gate stages:
• Q301, Q302, Q303, IC300 power ramping
• Q310, Q311 Tx-Reg. and Lock-Detect
gate
• Q306, Q307, Q308, Q309 delay and PIN switch drive.
This is to allow the RF power circuits (both exciter and PA) to ramp up and down in a
controlled manner so that minimal adjacent channel interference is generated during
the transition.
R517, R518 and R519 form a 3dB attenuator to provide good VCO/drive amplifier isola-
tion as well as the correct exciter drive level
Note:
The exciter provides a DC control signal to the PA via the RF coax. This is
injected via L314.
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