TX Techniques TX FM1, Technical Manual

Page: 17 / 33

14
TX-FM1/S/R/A Technical Manual www.txtechniques.co.uk
A 5V logic signal from “FM1-EXCI” arrives at PL2 when the PLL is locked . The RF
on/off control signal is sent back to the modulator board via the same connector.
Voltages proportional to the forward and reflected RF levels present on the “FM1-RF”
board arrive at PL4. These voltages are sampled by an on-chip D to A converter in
IC1.
R2 and ZD1 create a reference voltage for IC1 from the 5V supply rail which is
smoothed by C10.
IC1 directly drives sounder S1, and performs the decoding for the front panel
encoder ENC1 and the push button switches SW1 and SW2.
6.7 RF Amplifier
The RF amplifier design uses a Motorola RF MOSFET device. C1, L1, C2, and L2
form an input matching circuit. ZD1 rectifies the RF forming a bias for TR1 and R1, in
conjunction with the board circuit pad presents the correct input impedance to TR1.
L5,C4,L5,C5 form an output matching circuit for TR1 and C7, L7, C8, L8, C9, L9,
C10, L10 and C11 form a 7-pole Chebyshev RF output filter.
6.8 RF Measurement
C3, C4, R4 and R5 create a decoupled -40dB sample of the coaxial output line. C1,
C2, T1 together with R1, R2, D1 and D2 provide voltages proportional to the forward
and reverse RF power levels. The ‘reflected’ voltage is nulled by VR1 which is
connected to the RF output via R3. IC1 and IC2 buffer the forward and reflected
voltages, which are kept below 5V, under all conditions, by ZD1 and ZD2.