AlOA3
AlOA3 Phase Lock Loop 1 (PLLI) IF, Circuit Description
AlOA3 functions to mix the output of A10A4 (160.15 to 166 MHz) with the LO output from AlOAl (200 to 300 MHz). T he output of this assembly is the difference frequency suitably filtered and amplified to about -10 dBm.
LO Amplifier @
The LO amplifier consists of common-emitter amplifiers Ql and Q2. CR1 and CR2 provide limiting to prevent overdriving Q2 near 200 MHz where Ql has more gain.
Mixer @I
The double-balanced mixer Ul operates with about +7 dBm LO drive and with -30 dBm RF signal input. The IF output is about -36 dBm and covers 30 MHz to 140 MHz. The 185 MHz Low-Pass Filter attenuates the harmonics of the RF signal input. The 10 dB pad reduces the RF signal input from -20 dBm to -30 dBm.
IF Input Amplifier @
The IF input amplifier has an input filter to partially filter the RF and LO signals from the mixer. The amplifier Q3 has emitter degeneration R17 to reduce distortion.
140 MHz Low-Pass Filter @I
The 140 MHz Low-Pass Filter is a modified elliptic filter which must pass 140 MHz while rejecting 160 to 166 MHz by at least 60 dB. The three adjustable coils optimize the stopband by providing nulls at the frequencies shown on the schematic. This filter also rejects the LO frequencies (200 to 300 MHz).
IF Output Amplifier @
The IF output amplifier consists of two common-emitter stages and an output low-pass filter. The two stages are coupled by Cl4 and L17 which provide high frequency peaking.