CIRCUIT DESCRIPTION

AUDIO/LOGIC DESCRIPTION (ALL MODELS)

frequencies below 300 Hz that could cause interference with LTR data and Call Guard signals.

Gate U308B blocks the microphone signal when microphone audio is not transmitted such as during the data handshake to set up the call. A high signal on pin 5 closes the gate and passes the signal. This gate is controlled by the Q6 output (pin 13) of latch U110. Transistor Q307 functions as an inverter and level translator.

3.5.2SUMMING AMPLIFIER (U303D), PRE- EMPHASIS/LIMITER (U303A)

U303D amplifies the transmit audio signal and also the transmit data signal from an optional modem if one is used. Jumper R398 is installed to route the modem signal to U303D and the filtering circuitry, or jumper R399 is installed to route a wideband signal directly to the synthesizer so that it bypasses the filtering circuitry. U308A closes when wideband (25 kHz) channels are selected. This bypasses R403 which provides a higher deviation level with those channels.

NOTE: If the wideband data input is used, the external device must provide FCC-approved modulation limiting and splatter filter circuitry and a stable DC level.

The output signal from U303D is then routed via the option wireouts to U303A which provides limiting and 6 dB per octave pre-emphasis. This stage is an amplifier which limits by saturating. Limiting prevents over-modulation caused by high-level input signals. R370 and R378 set the input level to the next stage, and C334 provides DC blocking.

3.5.3 SPLATTER FILTER (U302B, U302C)

U302B and U302C form a five-pole, low-pass splatter filter which attenuates frequencies above

3 kHz. This prevents adjacent channel interference. Frequencies over 3 kHz may be produced if limiting occurs in the limiter stage just described. The signal is then fed to digital potentiometer U802 on the RF board which sets the deviation level. Refer to Section 3.7.4 for more information.

3.5.4TRANSMIT DATA CIRCUIT (U302D, U302A)

The transmit LTR data and Call Guard tone/data signals are generated by the microcontroller on pins 37 and 38. The four logic combinations possible with these two outputs are applied to a resistor network consisting of R389, R392, R386, and R395. This network creates a four-step pseudo sine wave from the digital outputs. This signal is applied to a low-pass filter formed by U302D and U302A. This filter attenuates harmonics present in the signal which provides smoothing of the stepped sine wave.

The passband of this filter is controlled by Q306 which switches additional capacitance into the circuit. When LTR or digital Call Guard data or low- frequency tone Call Guard signaling is being trans- mitted, Q306 is turned on and the cut-off frequency decreases to approximately 150 Hz. Then when a high-frequency tone Call Guard signal is being trans- mitted, Q306 is turned off and the cut-off frequency increases to approximately 220 Hz. Q306 is controlled by the same signal used to control Q300 in the receive data circuit (see Section 3.4.3).

U308C provides gating of the transmit data signal. When the control input (pin 6) is high, the gate is closed and the signal is passed. Test gate U307A is used in the test mode to bypass the data filter to provide the wideband data signal required for setting modulation balance. Q303 and Q308 provide level translation and inversion. The transmit data signal is then fed to digital potentiometer U802 on the RF board which sets the data deviation level. Refer to Section 3.7.4 for more information.

3.6 DISPLAY BOARD

Control of most display board functions is provided by microcontroller U2. This device contains a 2K byte ROM and 128 byte RAM and has 20 I/O lines. It communicates with microcontroller U101 on the audio/logic board via the SPI serial bus consisting of SCK, MOSI, and MISO lines (see Section 3.3.1). When there is data to send to the audio/logic board, such as if an option switch is pressed, U2 issues a service request on the Service Request Out line (J1, pin 6).

 

February 2001

3-10

Part No. 001-9800-001