EFJohnson 7780 service manual LOW-PASS Filter IC205A/B, Smartnet Data Processing

CIRCUIT DESCRIPTION

caused by high input levels from the internal or external microphone. The bias voltage to this stage and also to IC201A is produced by voltage divider R208/R207.

4.8.3 LOW-PASS FILTER (IC205A/B)

IC205A and IC205B form a low-pass splatter filter which attenuates frequencies above approxi- mately 3 kHz. This prevents adjacent channel interfer- ence caused by the harmonic frequencies which result from amplitude limiting.

The output signal from this filter is applied to IC209 which contains four logic-controlled potentio- meters. The transmit audio signal is applied to potenti- ometer 3 on pin 20 and fed out on pin 19 which is the wiper of that potentiometer. The potentiometers in this device are adjusted by the control logic via the serial data bus (MSCK/MSO). The transmit audio/DTMF level is set when the transceiver is aligned.

The transmit audio/DTMF signal is then fed to IC201 where it is combined with the transmit data/ SMARTNET data signal if applicable. The output signal on pin 4 of IC201 is then fed to the VCO in the RF unit and also to pins 7 and 8 of IC209. The signal fed to the RF unit (MOD) modulates the VCO, and the signal fed to IC209 is level adjusted and fed out on pin 6 and applied to the RF unit where it modulates the TCXO. The potentiometer is set by the logic during alignment, and is used to balance the VCO and TCXO modulation signals.

4.9 SMARTNET DATA PROCESSING

Both the transmit and receive SMARTNET/ SmartZone data signals are applied to a filter formed by IC112A/B and IC113B. Switching of these signals

is provided by gate IC110 which is controlled by the SN_TR signal from pin 95 of the microprocessor. When this signal is high, the transmit data signal on pin 6 is selected is routed to the filter, and when it is low the receive data signal on pin 7 is routed to the filter.

IC112B, IC112A, and IC113B form a 1800 Hz low-pass filter which attenuates unwanted frequencies above the SMARTNET/SmartZone data band. The output of this filter on U113B, pin 7 is then fed to IC114A/B which provide DC restoration when data is being received. The signal is also fed to gate IC210 in the transmit data circuit to be transmitted when applicable.

The DC restoration circuit formed by IC114A/B and IC113A is similar to the receive data circuit described in Section 4.7.1. It converts the data signal from AC floating at half supply to DC levels of 0 and 5 volts that can be detected by the microprocessor. Diodes D104 and D105 charge and discharge C158 and C157 to establish a DC reference on pin 2 of comparator IC113A that is the average of the positive and negative going alternations. Q106 turns on in the transmit mode which grounds pin 2 and disables this circuit.

In the transmit mode, gate IC210 selects either the SMARTNET data signal or the Call Guard data signal. It is controlled by the same SN_TR signal that controls IC110. The output signal on pin 1 is then applied to potentiometer 1 in IC209. Refer to Section 4.7.2 for more information on this circuit.

November 1999
Part No. 001-7780-500	4-10