User's Manual

RPT 633
TM1184 Issue 1 Page 3 - 5
Transmitter Keying
11. Normal transmitter keying is achieved by fitting a shorting link across LK1 pins 2-3.
In the quiescent state the Tx Key line is at 5V, the output of IC6f is at 0V and the
bias voltages of TR5 and TR8 are shorted to 0V. When the VOX keying module
sets the Tx Key command to logic low the output of IC6f goes high impedance.
Forward bias is applied to TR5 and TR8 providing the PLL is in lock.
12. Provision has been made for an alternative method of keying the transmitter which
keys the VCO off when the transmitter is not in use, thus removing any carrier
components in the standby mode. This is achieved by fitting LK1 pins 1-2. A 0V
signal on the Tx Key line then turns on TR3, via inverter IC6f, which in turn turns on
TR1 and supplies 8.2V to the VCO transistor TR2.
Frequency Multiplier
13. Transformer T1, enabled by the in-lock signal turning on TR5, provides a balanced
feed to the multiplier diodes D3 and D4 which pass the desired transmit output
frequency and reject the fundamental frequency and odd harmonics of it. The
desired frequency is passed to TR6 for amplification
Band Pass Filter
14. This is made up of two cascaded two-section helical filters FL1 and FL2. The filters
reject any VCO leakage and any unwanted frequencies generated by the
multiplication process.
Driver Stages
15. Transistors TR7, TR8, TR9 amplify the filtered signal up to a level of approximately
250mW, inter stage matching being optimised by VC1 and VC2. The output power
from TR9 and ultimately the output power of the transmitter, is controlled by RV1,
while the bias for TR8 is controlled by the keying line ensuring a high attenuation
of the output signal when the transmitter is keyed off.
Output Amplifier
16. Matching between TR9 and TR10, operating as a class C amplifier, is optimised
by VC3. The output of TR10 is matched to the output filter by C50, C51 and a
length of micro-strip line, the filter being formed by C53 to C60 and the micro-strip
lines. Any harmonics of the output frequency are attenuated by the filter and any
residual VCO signal is attenuated by VC4 and L13. The output power at SK4 is
2.5W.