Project 25 Digital Mode; Vhf Rf Board (Version C); Receiver - E.F. Johnson Company 5100 Series Service Manual

Transmit Mode
The signal from the microphone is amplified and
then routed to the CODEC chip where it is first digi-
tized 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.

5.1.3 PROJECT 25 DIGITAL MODE

Introduction
In Project 25 Digital Mode, the carrier is modu-
lated with four discrete deviation levels. These levels
are ± 600 Hz and ± 1800 Hz. Digitized voice is created
using an IMBE™ vocoder.
Receive Mode
The signal is processed in the same way as an
analog mode transmission until after the squelch func-
tion is performed. If a signal is detected to be present,
the DSP resamples the signal from 20 kHz to 24 kHz.
This is done so that the sample rate is an integer
multiple (5x) of the data rate of the digital modulation
which is 4800 symbols/sec (9600 bits/sec).
The resampled signal is then processed by a
demodulator routine to extract the digital information.
The resulting bit stream (9600 bps) is sent to a routine
that performs unframing, error-correction, and voice
decoding. The result of these operations is a recon-
structed voice signal sampled at 8 kHz. The sampled
voice signal is sent to a D/A in the CODEC to produce
an analog signal for output to the audio power ampli-
fier and speaker.
Transmit Mode
The microphone signal is processed as in the
analog mode until it reaches the DSP. At this point the
audio signal is processed by a voice encoding routine
to digitize the information. The resulting samples are
then converted to a bit stream that is placed into the
proper framing structure and error protected. The
resulting bit stream has a bit rate of 9600 Hz.

VHF RF BOARD (VERSION C)

time, into a digital level corresponding to one of the
four allowable frequency deviations. This produces
16-bit symbols with a rate of 4800 Hz. The symbols
are resampled to a rate of 48 kHz and filtered to
comply with channel bandwidth requirements. The
filtered signal is then sent to a D/A in the CODEC to
produce the analog modulation signal for the VCO.
The modulated VCO signal is then mixed up to the
final transmit frequency and then sent to the RF PA for
transmission.
5.2 VHF RF BOARD (Version C)
NOTE: The following describes the new revised
Version C RF board described in Section 1.13.1. The
RF Board is not field serviceable. It must be replaced
as a unit with a new board.

5.2.1 RECEIVER

Front End Bandpass Filter
transmit/receive switch. Following this switch a fixed
tuned bandpass filter is used at the front-end of the
receiver. This filter provides first image rejection with
minimal loss in order to provide the desired receiver
sensitivity. Following the filter a variable attenuator is
used to increase the dynamic range of the receiver
when receiving high level signals.
Front End LNA and Bypass Switching
determining the overall noise figure of the receiver
chain. An MGA-71543 amplifier (U39) provides
optimum noise figure, gain, intercept point, and power
consumption.
Post-LNA Bandpass Filters
LNA. This filter is identical to the front end filter
previously described and provides additional image
rejection.
5-2
CIRCUIT DESCRIPTION
This bit stream in then encoded, two bits at a
A harmonic filter is followed by a PIN diode
The Low Noise Amplifier (LNA) is critical in
An additional bandpass filter is located after the