Service manual

Theory of Operation

DCB, DCFCB and DCFCTB Models

If the "INT" side is jumpered then the on-board receiver has priority. When it asserts

carrier detect the low passes through D2 and R9 turning on Q2. This allows the

received audio to pass through Q2, C8 and R39 to the decoder circuits. The low also

passes through R11 to Q3 which forces Q4 off preventing audio from the external

transceiver from coming through.

If both sides of JP4 are jumpered then the two receive audio signals are mixed

together. This is done if the SINAD option is used as it allows receive audio from the

external transceiver jack to pass through Q4 and Q2 back to the on-board receiver

connector to feed the SINAD option board. JP1 must also be jumpered when the

SINAD option is installed.

VOX

Receive audio is routed from the external transceiver connector to buffer amplifier

U2A, which drives RMS to DC converter U5B. The DC voltage at the cathode of D6

represents the Level of the incoming receives audio. U5A acts as a comparator who’s

output pulls low when the receive audio exceeds about 45 mV

p-p

or about 750 Hz of

deviation (350 Hz for narrowband systems). This passes to JP3 which can be

jumpered to provide VOX carrier detect for radios that do not provide carrier detect.

DTMF

Receive audio enters the DTMF decoder IC (U7) from the high pass filter thru C40

and R74. When a proper DTMF digit is being decoded, TP10 goes low and a binary

output appears on pins 18, 19, 20, and 20 of U7.

Tone Decode

Receive audio from the high pass filter U3B, is passed on to low pass filter U3C.

These form a band pass filter from 300 to 3000 Hz. When the processor is decoding a

tone greater than 2100 Hz, it turns Q6 on. This makes the band pass filter from 1900

to 3000 Hz, which aids in the decoding of high frequency tones. When the processor

is decoding a tone less than 500 Hz, it turns Q7 on. This makes the band pass filter

from 300 to 500 Hz, which aids in the decoding of low frequency tones. After the

band pass filter the tone is amplified by U3D then passed to U6B, a comparator that

converts the tone to a square wave, and feeds it to the microprocessor to be measured.

The square waves can be seen on TP4.

EAS Decode

Receive audio from the high pass filter U3B, is passed on to low pass filter U3C.

When EAS functions are programmed, the processor turns Q2 on. These form a band

pass filter from 300 to 3000 Hz. After the band pass filter the tones is amplified by

U3D then passed to U6B, a comparator that converts the tone to a square wave, and

feeds it to the microprocessor to be decoded. The square waves can be seen on TP4.