Vco; Vco Supply - Tait T880 II Series Adjustment Manual

M880-00
2.4

VCO

(Refer to the VCO circuit diagram (sheet 3) in Section 6.2.)
The VCO comprises the oscillator and three stages of buffer and gain to achieve the
required power level and reverse isolation at the output of the VCO.
The oscillator transistor (Q309) operates in a common-base configuration, utilising a
quarter-wave square ceramic resonator coupled between its collector and emitter to pro-
vide the feedback necessary for oscillation. The VCO control voltage from the loop filter
(IC750) is applied to the varicaps (D300-D302) to facilitate tuning within an 8MHz band
of frequencies. A high-Q sapphire trimcap (CV300) is used for coarse tuning of the
VCO. The typical output power at the oscillator stage is 0dBm.
The output from the oscillator drives a cascode amplifier stage (Q302, Q303) which is
designed to provide good reverse isolation from variable impedances of the following
stages. The isolation of the circuit is typically 40dB, including 0-3dB of associated gain.
An attenuated sample of the cascode output (Q302) is fed back to the synthesiser
(IC640) through a divider buffer (Q795) for phase-locking.
Following the buffer is a broadband MMIC amplifier (Q308) which functions as a gain
block to provide the drive to the final power stage (Q319). This stage typically provides
approximately 11dB of gain.
The final stage of the VCO is a power amplification stage. The power transistor (Q319)
and its associated circuitry increases the output power of the VCO to approximately
+20dBm. The output power is then attenuated to +6dBm (approximately), which is the
input level required to drive the RF power module.
Note: This power stage (Q319) is retained in the T881 transmitter to keep the VCO
architecture the same as the T885 receiver, which does require a +20dBm
output.
The VCO is an on-channel design, i.e. there are no multiplier stages to obtain the on-
channel frequency. It is modulated by superimposing the audio signal onto the control
voltage and by phase-modulating the reference signal.
2.4.1

VCO Supply

The VCO is supplied from two switched +9V supplies under the control of the Tx-Reg.
supply.
The VCO and cascode amplifier are supplied from one +9V switched supply by Q321
via the C multiplier (Q316, C365).
The MMIC and final amplifier are supplied from the other +9V supply by Q314.
A delay circuit holds the VCO on for a short time after the Tx-Reg. supply has been
switched off. This is to allow the RF power circuits to ramp down in the correct manner
before the VCO is switched off.
Copyright TEL
T881 Circuit Operation C2.7
10/07/00