Technical Manual

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11-75-0000 SYNTHESIZED PAGING TRANSMITTER TECHNICAL MANUAL Issue 0600

Sea Air and Land Communications Ltd Page 3

1.1 11-75 Circuit Description:

Q7 VCO oscillates at the output frequency determined by L2, C68 and D5 capacity. L2 is trimmed to adjust get the oscillator to a

frequency close to the output frequency and output from the synthesizer keeps the frequency phase locked to the reference frequency of 6.25kHz or 12.5kHz.

The synthesizer derives a voltage from it's phase comparator and adjusts the capacity of D5 via the VCO loop to keep the oscillator phase locked to the

reference frequency. Reference frequency is derived by the synthesizer dividing down the 9.6MHz crystal oscillator.

Output from the VCO is transferred via C71 to a buffer/amplifier stage which splits the signal, part is fed to the synthesizer to be divided down for the phase

comparator and part goes to Q9 RF amplifier. The signal is further amplified by Q10, Q12, Q13 to provide a 4 Watt maximum output. A 3 section filter L14,

L15, L16 and associated capacitors filters the output harmonics to less than -30dBm.

RV3 adjusts Q13 quiescent current to 150mA cold.

Q15, Q16, Q17 provides Output Power Adjustment via the Setup Program which controls Q15 via R85 and IC8 pin 9. Q15-Q17 is also controlled by an 'out of

lock' signal from IC7F & IC7E and an over temperature signal from IC7D. Q14 provides instant control over Q13 bias current from PTT or Out-of-lock.

POCSAG modulation is applied as dual-point modulation to D3 (Low Speed), and to D6 (High Speed). Deviation + and - is set via the Setup Program and IC8,

Q3, Q4, control that level. RV2 adjusts the proportion of HS and LS modulation to keep the demodulated waveform as close to a square wave as possible.

Over temperature cutout:

When the unit transmits at full power in a hot environment, a protective cutout may operate. This will reduce the output power to

a safe level, and will reset when the unit temperature has fallen to below 60 deg. This cutout is designed to avoid permanent damage to the transmitter.