General
Theory of Operation 5C.3-9
2.15 Static Random Access Memory (SRAM)
The SRAM (U0103) contains temporary radio calculations or parameters that can change very
frequently, and which are generated and stored by the software during its normal operation. The
information is lost when the radio is turned off. The device allows an unlimited number of write
cycles. SRAM accesses are indicated by the CS signal U103-20 (which comes from U101-CSGP2)
going low. U0103 is commonly referred to as the external RAM as opposed to the internal RAM
which is the 768 bytes of RAM which is part of the 68HC11K1. Both RAM spaces serve the purpose.
However, the internal RAM is used for the calculated values which are accessed most often.
Capacitor C0133 serves to filter out any ac noise which may ride on +5V at U0103.
CONTROLLER BOARD AUDIO AND SIGNALLING CIRCUITS
3.0 General
3.1 Audio Signalling Filter IC (ASFIC)
The ASFIC (U0201) used in the controller has 4 functions;
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RX/TX audio shaping, i.e. filtering, amplification, attenuation
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RX/TX signalling, PL/DPL/HST/MDC/MPT
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Squelch detection
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Microprocessor clock signal generation (see Microprocessor Clock Synthesizer Description
Block).
The ASFIC is programmable through the SPI BUS (U0201-E3/F1/F2), normally receiving 21 bytes.
This programming sets up various paths within the ASFIC to route audio and/or signalling signals
through the appropriate filtering, gain and attenuator blocks. The ASFIC also has 6 General Control
Bits GCB0-5 which are CMOS level outputs and used for AUDIO PA ENABLE (GCB0) to switch the
audio PA on and off, EXTERNAL ALARM (GCB1) and B+ CONTROL (GCB2) to switch the voltage
regulators (and the radio) on and off. GCB3 controls output GPI/O (accessory connector J0400-12),
HIGH LOW BAND (GCB4) can be used to switch between band splits and GCB5 is available on the
option board connector J0102-3. From version 0102726B09_Cntl on, output GCB4 controls gate
U0171 via U0172-1 which enables the µP to receive data from the control head. The supply voltage
for the ASFIC has additional filtering provided by Q0200, D0200, R0200, L0200 and C0200. Diode
D0200 increases the voltage at the base of Q0200 about 0.6 volts above the 5 volt supply voltage to
compensate the base - emiter voltage drop of Q0200.
3.2 Audio Ground
VAG is the dc bias used as an audio ground for the op-amps that are external to the Audio Signalling
Filter IC (ASFIC). U0251-1 form this bias by dividing 9.3V with resistors R0251, R0252 and buffering
the 4.65V result with a voltage follower. VAG emerges at pin 1 of U0251-1. C0253 is a bypass
capacitor for VAG. The ASFIC generates its own 2.5V bias for its internal circuitry. C0221 is the
bypass for the ASFIC’s audio ground dc bias. Note that while there are ASFIC VAG, and BOARD
VAG (U0201-1) each of these are separate. They do not connect together.
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