AEA PAKRATT PK-232, Technical Reference Manual

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PK-232 TECHNICAL MANUAL CHAPTER 3 – THEORY OF OPERATION
PK232TM Rev. A 5/87 3-2 Page 12
3.2 Analog Subsystem
Each of the PK-232's operating modes requires different bandwidths, as well as varying
AFSK tone and speed characteristics. The analog circuitry is adapted to these differences by
using program-controlled switches to place precision resistors in parallel with the fixed com-
ponents of the circuits. In this manner, under software command, the analog circuits are op-
timized for the mode selected.
3.2.1 Receive Function
Audio signals from the selected RADIO 1 or RADIO 2 input pass through a rolloff filter to
compensate for VHF receiver characteristics and are then applied to a buffer amplifier to
limit audio signal amplitude.
The signal then pass through a bandpass filter whose center frequency tuning and band-
width are controlled by the OPMODE selection for optimum characteristics.
The bandwidth-limited signals pass through an amplitude limiter to remove variations in
audio levels. Mark and Space tones are separated in the Mark and Space resonators. The
separated Mark and Space output signals are rectified in a diode discriminator circuit.
The detected data is filtered in an active lowpass filter and compared with a reference
voltage in a slicer circuit to digitize the data. The resulting data is then shaped and in-
verted for passing to the internal modem circuitry.
When a full word of data (eight bits) has been assembled in the modem circuitry, an inter-
rupt is generated to the microprocessor which then processes the data.
3.2.1.1 Receive Circuits
The received audio signals are routed through one of two radio connection receptacles,
J4 (RADIO 1) or J6 (RADIO 2), or through the respective paralleled radio input cable
jacks, J3 or J5. The radio is selected by SW2, the RADIO 1/RADIO 2 push-button switch
on the front panel. Radio input and output connections are bypassed for RF by C64 and
C63.
The audio input consists of resistor-capacitor combination R34 and C54 to compensate
for the audio characteristics of the average VHF FM radios by emphasizing higher-fre-
quency tones and attenuating lower-frequency tones.
Input buffer amplifier U28-D limits received audio signal levels to prevent overloading
multiple-resonator feedback-type 0.5-dB-ripple Chebyshev active bandpass filter formed
by operational amplifiers U23 and U26.
Analog gates (FET switches) U22, U24, U25, U26 and U27 select filter center frequency
and bandwidth. Gate selection is a function of the operating mode.
Bandpass-filtered output signals are amplified and limited by U28-A to remove ampli-
tude variations and passed to MARK and SPACE resonators U30 and U32. Gate U29 de-
termines the tone-pair resonator tuning.
For CW signals, U31 transfers the bandpass filter output directly to the MARK channel
discriminator. The SPACE channel is decoupled from the circuit and ignored.