User Manual Part 1

5.1.3 CIRCUIT PROTECTION (FUSES)

Circuit protection is provided as follows:

• An 8- or 10-ampere fuse in the power cable provides overall transceiver protection.

• A 2-ampere fuse on the RF board protects circuits on that board.

• F700 (2-ampere) on the display controller board protects the Sw B+ output of the

microphone connector.

• F1 on the logic board protects the Sw B+ output of universal interface connector J5.

• The various voltage regulators provide circuit protection by automatically limiting

current.

5.1.4 ANALOG MODE

Receive Mode

The signal is routed from the antenna connector through the PA board to the RF Board

where it is filtered, amplified, and mixed with the first local oscillator frequency generated by the

synthesizer. The resulting IF signal is also filtered and amplified and sent to the AD9864 digital

IF chip.

The signal is then mixed with the second local oscillator frequency to create a second IF

signal. The second IF signal is then sampled and downconverted to baseband. The baseband

signal is then decimated to a lower sample rate that is selectable at 20 kHz. This signal is then

routed via a serial interface from the IF chip to the DSP on the logic board.

On the logic board the DSP digitally filters the input signal and performs frequency

discrimination to obtain the message signal. The DSP first performs a carrier- detection squelch

function on the radio. If a signal is determined to be present, the audio portion of the signal is

resampled and then filtered appropriately. The filtered signal is then routed back to a D/A in the

CODEC to produce an analog signal for output to the audio power amplifier and then the speaker.

Any detected signaling information is decoded and the resulting information is sent to the

microcontroller.

Transmit Mode

The signal from the microphone is amplified and then routed to the CODEC chip where it is

first digitized and then sent to the DSP. The DSP performs the required filtering, adds the desired

signaling, converts the sample rate and then sends the resulting signal back to a D/A in the

CODEC to produce the analog modulation signals for the VCOs. The modulated VCO signal is

then sent to the RF power amplifier and transmitted.

5.1.5 PROJECT 25 DIGITAL MODE

Introduction