then rectified. This voltage is compared with a voltage supplied
under software control, D/A. on the ASP’s. The output of this
comparator is passed to the microprocessor on the ASP CAS
(ACH.6) A/D input line. It is then used in conjunction with the
tracking data to determine if a signal is present. The response
time of the circuit is controlled by external capacitor C640 in
fast squelch mode with C641 being paralleled by ASP switch
SW2 (Pin 16) for the slow squelchfunction. An external resistor
network including thermistor R642 is used to trackout tempera-
ture variations. The resistor network interfaces with pins 53-55
of the ASP’s “A” op amp. Squelch hysteresis is assumed to be
a software function.
Demodulated audio also enters the ASP chip RAFI (Pin 44).
It passes through a gain stage and then through a 300 to 3000
Hz bandpass filter before exiting the ASP as HP10 on Pin 45.
Software switches provide for bypassing the bandpass filter.
This is desirable when looking for high speed (9600 baud) data.
In this case, the output HP10, is applied to limiter comparator
NRI (Pin 31) against a 1 uF averaging capacitor at pin 32.
Something wrong in here. ASP switch SW5 (Pin 13) is used to
block the output of the busy tone filter when looking for high
speed data. The output of this comparator is called LOSC (Pin
21). It is buffered by transistor buffer V606 and applied to the
data input of the modem ASIC and the ASP-LDSC (P4.2) input
of the microprocessor.
The notched audio returns to the ASP on pin 28, VGAF. It
then goes into a switch array. This array decides which device
to connect to the speaker. The choices are the unnotched output
of the bandpass filter, the audio from the notch filter, an ASP
generated alert tone, or the output of the DTMF generator as a
side tone. Two external resistors allow adjustment of the alert
tone level. The DTMF input is on pin 29, DTMF.
The de-emphasis network is after the switch. It can also be
switchedinand out.Thevolume controlis afterthede-emphasis
network. It allows 32 levels of volume under software control.
The digital volume control allows for separate levels for normal
operation and operation in a vehicular charger. The audio passes
through one more mute switch before leaving the ASP on pin
27, RXAF. From here, the audio goes to the expander portion
of a NE578D compander IC, D605. This IC attempts track out
large swings in the volume level to provide a constant level into
op amp N604, which is used as a driver for the earpiece audio.
The compandor function can be defeated by ASP switch SW0
(pin 18). Earpiece audio also goes out the UDC connector as
EXT-SPKR.
The audio input at RAFI (pin 44) can also be connected to
a 210 Hz low pass filter. This strips voice information off to
leave Channel Guard or low speed data information. This infor-
mation exits the ASP on pin 37, CGO, and passed through
limiter comparator C3 with a 10 uF averaging capacitor, C660.
The data input is on pin 35. The averaging input is on pin 34.
The output of the comparator, LCGO (pin 22) is applied to
microprocessor pin ASP-LCG (P4.3). The low speed data de-
code or Channel Guard detection is accomplished in software.
The logic board contains a buzzer, H803, which is used to
provide an audible alerting signal. This is accomplished under
software control by toggling microprocessor port BUZZER
(P1.7) on and off at the desired frequency. The port pin, D701-
29, drives transistor V802 which, in turn, switches the 5v supply
on and off through a 10 ohm resistor.
Transmit Audio
Audio from the internal mic is received by the ASP ASIC on
pin 74, IMIC. Audio from the UDC connector is received by the
ASP ASIC on pin 75, TTON. Both mic inputs provide a bias
voltage from the ASP generated TX BIAS supply. The mic
inputs are then fed to a switch. A third input into this switch is
the DTMF encoder chip on the EMIC (73) pin. (There is a 20
dB pad between the DTMF IC and the ASP input.) After the
switch, the audio passes through a software selectable gain
stage. It then passes through the pre-emphasis filter and then a
300 Hz high pass circuit to keep voice from conflicting with
Channel Guard or low speed data. From there, the audio leaves
the ASP on pin 70, TONE and applied to the compressor side of
the NE578D compandor, D605. The compressor provides an
AGC function to hold the mic audio more constant. From the
compressor, the audio is returned to the ASP on pin 57, TIN. It
passes through a limiter circuit and summed with the data and
tone components of the Tx modulation at a common summation
junction.
High speed data from the modem ASIC is received by the
ASP on D601-80 as IDAT. It passes through a data filter before
being combined with the Tx audio at the common summing
junction. It is possible to bypass the data filter.
Channel Guard and low speed data are generated at digital
levels by the microprocessor on walsh bit outputs WB1 (P5.0)
and WB2 (P5.1). The walsh bits are combined in a resistor
network to act as a two bit D/A. Fortone generation, a sine wave
is approximated in six steps to create a more easily filtered
signal. The combined walsh bits enter the ASP at pin 38, TOIN,
for Channel Guard and low speed data.
The DTIN input to the ASP, D601, connects directly to the
common summing junction through a switch. Its audio fre-
quency tones are sufficientlyfiltered by the post limiter filter in
the ASIC. The low frequency TIN is passed through the 210 Hz
low pass filter to filter out the waveform. This is required since
higher level harmonics tend to significantly degrade hum and
noise. Note, this is the same filter that is used on the receive
side. The filtered signal exits the ASP on pin 37, CGO. It then
goes to ASP D601-58, CGIN, and into C3, limiter comparator.
CGIN is connected directly to the common summing junction
by a switch.
A3000Hzlow passpostlimiterfilterfollowsthecommon
summing junction. (This filter can be bypassed when sending
high speed data.) Following the filter is a 32 position devia-
tion control. The output of this attenuator is passed through a
switchbefore exitingthe ASPon pin60, TXAU.Itthenpasses
through a resistor network before going to the modulation
circuits on the RF board.
RF BOARD
Refer to schematic diagram 188D6251. Each sheet of the
drawingcontainsa majorboardfunction. Thesefunctionsand
locations are listed below:
•Sheet 1 RF board control and block diagram.
•Sheet 2 Transmitter
•Sheet 3 Transmit synthesizer
•Sheet 4 Receiver synthesizer
•Sheet 5 Receiver
Power Distribution
DC power is supplied to the radio by a NiCd battery At
full charge the battery delivers 7.2 volts to the radio. If the
output voltage falls to is less than 5.3 volts, the Audio/logic
board turns the radio off.
DC voltage from the GEL Cell battery pack is applied to
the RF board through J103.1, 2 and fuse F101. Test points
TP168 and TP108 allow you to monitor the battery voltage
on either side of the fuse. Unregulated, unswitched voltage
only is applied to the transmitter power amplifiers U200,
Q200, and to the Audio/logic board through J101.8. Battery
voltage is connected to accessory connector X201 and to 5
volt regulator and switching circuitry consisting of V804,
MBT3904 and MBT3906. This switching circuitry generates
SWDC (switched 5 Vdc) which is returned to the RF board
on J101.1 as SWDC. SWDC is applied to Power Regulator
U161, Bertram RF Controller (U101-3) as VBAT and pin 1
as STARTUP, and to DC control Transistor Q131. Control
transistorQ131supplies SWDCto5 voltregulatorD100. The
output of the regulator is distributed as VREG to the receiver,
TCXO, Bertram U101-5 (TP131) and pins 16 and 37 through
respective voltage dividers, and to MOSFETS Q511 and
Q531.
VRX is originated by U161 and applied to receiver syn-
thesizer U400, RXVCO U410 and driver U430 and to pass
transistors Q161 and Q162. The pass transistors are control-
led by the Bertram RF Controller and supply voltages VTX1
(TP162) and VTX2 (TP164) to the transmitter circuitry:
U200, U300, TXVCO U310, and MOSFET driver Q330.
C116 provides low frequency filtering while capacitors
C101 through C115 decouple any noise transients that may
be present on the control lines to the Bertram Radio Interface
Controller. TP109 allows the technician to monitor the POW-
DET line.
Transmitter:
The transmitter contains preamp Q200 and power amp
U200, with a broadband 815 MHz filter feeding the antenna
connection. The transmitter PAdelivers 0.75 watts (0.4 watts
on low power)to the antenna.
Bertram Radio Interface Controller U101 receives in-
structions form the Audio/Logic board via serial data link,
J101.11, identified as “DFO”. Information is returned to the
Audio/Logic board through J101-13 and is identified as
“DTO”.BothDFO andDTOareclockedby J101.12(C1008).
Transmit on-off switching is controlled by Bertram Radio
Interface Controller using two switching circuits. Bertram
output, Dout4 (U101-20), controls voltage switching transis-
tor Q161 (TP162). Q161 switches power on/off to transmitter
PA U200, transmit synthesizer U300, RF power sense D240,
RF amplifier Q200, and buffer Q330. TXON control from the
Bertram (DOUT1, U101-41) controls transmit on-off switch
Q201. Q201 switches amplifier Q200 on and off as required
by the current status of the radio.
Transmitter output power is regulated to its high or low
tracking data setting (see Audio/logic board) by sensing
(transformer W202/W203) and detecting (D240, C241) final
power(Bertop amp #1pin 38, 39,and40) relativetotheDAC
setting (Bert output pin 35) obtained by Bertram from the
Audio/Logic board.
Tx-Synthesizer:
The channel frequency is set by loading transmit synthe-
sizer chip U320 from the Audio/Logic board via J101.5 and
J101.6, “Syntcl” and “Syntdata”. The synthesizer chip sets
the channel center frequency through its fN pin U320-10 and
monitors/locks the frequency by “listening” to the VCO out-
put via C312. Audio signals from the audio/logic (J101.7
“Mod”) varies the VCO (U310) frequency according to the
audio amplitude set on the Audio/Logic board. Transmit
synthesizer chip voltage is switched on-off in the same fash-
ion as the transmitter power out, by Bertram ‘Dout3’, via
voltage switching transistor Q162. The switched voltage is
designated as VTX2. Q330 is used to buffer the VCO out, and
feeds the transmitter pre-amp. Power to buffer Q330 is
brought in from VTX1 under control of Bertram ‘Dout4’.
AE/LZB 119 1645 R1B
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