TX Preemphasis, Filter and Limiting Amplifier
Audiofromthe microphone(MICHI)isappliedtotheaudio
board at J703-8. The 560-ohm resistor provides bias forthe mic
element. Input to the active filter is AC-coupled through C301.
Operational amplifier U301B and associated circuitry oper-
ates as an active 6 dB/octave preemphasis fil-ter (preemphasis
from 300 to 3000 Hz), and limiting amplifier. The output of
U301B is applied to a post limiter filter circuit.
Clamping diodes D301A and D301B with the voltage di-
vider R302G through R302J, bias the U301B inverting Op Amp
at 4 volts, and limit the output to a nominal 3.6 volts peak-to-
peak. Controlled limiting is provided by use of regulated power
supplies for this circuit.
Post Limiter Filter
The Post Limiter Filter (PLF) following U301B consists of
two sets of High/Low Pass filters. Between the RC filters is a
third order, multiple feedback low pass filter (U301A). A Che-
byshev low pass filter was chosen to minimize passband ripple
(0.5 dB) and provide sharp roll off at the high end of the voice
band.Rollofffor thePost LimiterandSummingamplifieris28.3
dB/octave.
SummingAmplifier
ThePostLimiter voiceoutputissummedinto U302A,along
withsubaudible tonesand MTXdata.Subaudible tones areaudio
signals generated on the Logic Board, applied to the SignalTone
input, and passed through the 200Hz Low Pass Filter to remove
the unwanted harmonic energy in the voice band. U302A sums
all the TX signals.
CompensationAmplifier
When the DPTTline islow(inthetransmit state), thefiltered
tones are summed into the compensation amp then coupled
through the Modem Filter to the "TX MOD" output. Aconstant
set level for these tones is assured with the use of a regulated
power supply for the microcontroller and latches on the Logic
Board.
MTX Data from the Logic Board’s Modeni IC is normally
in a high impedance state except when enabled to send data at
standard logic levels. Tomaintain asymmetrical swing about the
quiescent bias level, the unloaded input is biased at approxi-
mately 2.3 Vdc. Potentiometer R324 adjusts the Data Deviation
level. After the MTX Data is biased, it is summed into the
Summing Amplifier.
FET Gates
Field Effect Transistor (FET) gates operate as Pchannel
switches. FET Q30l operates as a gate for switching the TX
Audio signal on or off, and is controlled by the
Mic Mute signal from the microcontroller. When turned off,
FETtransistorQ302(controlledbytheDPTT line) allowsSignal
Tones to be summed. MTX Data is summed when Q303 is
activated by the TX DATA MUTE input.
Software prevents high speed data (MTX) from being trans-
mitted simultaneously with voice or signal tones.
Modem Filter
Modem Filter U303A and associated circuitry con-sists of
two second order, low pass (Butterworth) filters with unity gain.
The circuit provides flat passband response, and provides addi-
tional transmit path high end roll off (12 dB/octave). From the
post limiter filter to the ModemFilter output at J703-11, roll off
at 3000 Hz is 36 dB/octave. The 20 kHz attenuation for the radio
is 84.3 dB minimum, with a 1kz reference. Nominal passband
gain is 0.222 V/V.
RECEIVE AUDIO CIRCUITS
Receive audio consists of audio, signal tones, 4800 Baud
data (MRX), and squelch detector (CAS). The receive audio is
coupled through a 300 Hz, high-pass tone reject filter, and then
applied to an audio amplifier with the required deemphasis.
Subaudible data is coupled through a 200 Hz low pass, voice
reject filter and limiter.
MRX data is AC-coupled to a3400 Hzlow pass filter (data
limiter), and thento aninverting comparator with hysteresis. The
receiver squelch path consists of a 4.2 kHz high pass filter, a
noise rectifier/amplifier and a comparator.
A Block Diagram of the receiver audio circuits is shown in
Figure 3.
300 Hz High Pass Filter and Audio Amp
The Discriminator input signal at J703-7 contains both
receive audio (voice) and data. The voice path consists of two,
third order,300Hzhighpass(Subaudible Reject)filters.Thefirst
is a Butterworth filter (U602A) to provide low ripple in the
passband. The second, a Chebyshev (U602B) is for greater
attenuation of the low frequencies. The two filters provide 18
dB/Octave of low frequency attenuation and unity gain.
S Data/Voice Mute controls FETtransistor Q602 to pro-
vide audio mute. RX Mute allows both voice audio and
controller-generated Signal tones to be summed at Speech
Audio Amplifier U603A. Capacitor C617 provides 6 dB/Oc-
tave speech deemphasis for the circuit. The RX Audio output
at J703-6 is biased at 5 volts.
200 Hz Low Pass Filter and Tone Limiter
The discriminator audio from the RF Boardor the signal-
ling tones from the Logic Board are switched to U605A input.
During receive, the discriminator audio at J703-7 passes
through the 200 Hz Low Pass Filter and Tone Limiter for
Channel Guard or low speed data decoding. During transmit,
the signalling tones atJ703-9 pass through the 200 Hz Low
Pass Filter for addition of Channel Guard or low speed data
onto the mic audio.
The signal from U605A is passed to the 200 Hz Low Pass
(Voice Reject) Filter. U604A and U604B make up Frequency
Dependent Negative Resistance 1 (FDNR), of the filter. The
FDNRmimicsanLCladdercircuit. FDNR1isusedtoprovide
for a sharp roll off at high (voice) frequencies. U604C and
U604D of FDNR 2, allowfor smoothing of the passband, and
continued attenuation of frequencies above 200 Hz.
The FDNR filters are used for transmitter subaudible
wave shaping (Channel Guard or low speed data) or receiver
subaudible filtering. The receiver subaudible filtering is sent
to the Data Limiter (U601D and associated circuitry) for
Channel Guard or low speed data decoding.
The output of the filter is passed through buffer amplifier
U605B to Data Limiter U60ID. The Liniiter provides wave
shaping (5 volt p-p) for the RX Tone Data output at J703-l2.
High Speed Data Limiter
4800 baud MRX data in the Discriminator Audio is AC
coupled to an RC type, 3400 Hz low pass filter consisting of
R636 and C625. Inverting Comparator U601C provides hys-
teresis at a nominal 20 dB level below the data signal magni-
tude.
4200 Hz High Pass Filter
Thediscriminator output isalsoappliedto a4200Hz,fifth
order, multiple feedback high passfilter consisting of U605C,
U605D and associated circuitry. The filter rejects all RX Tone,
Voice, and Data signals.
Squelch Rectifier/Amplifier and Comparator
The noise output of the 4200 Hz highpass Chebyshev
filter is applied to Rectifier/Amplifier U603B. The gain of the
amplifieriscontrolled bySquelch AdjustpotentiometerR628.
The rectified noise is filtered by R630 and C623 to
provide an average DC level proportional to the noise level.
This DC level is applied to the inverting (-) input of compara-
tor U601A. The non-inverting (+) input of U601A is refer-
enced to 5.7 Vdc.
The gain of U603B is adjusted by R628 such that when a
signal is applied to the receiver and 8 dB SINAD is measured
the DC noise level will just fall below the reference voltage
and the CAS line goes to + 5 Vdc. When the CAS line is high,
the microprocessoron the logic boardknows that the channel
is busy with a carrier.
As the noise level increases (weaker signal), it will take a
level of noise larger than the reference to squelch the radio.
The difference between the CAS turn-on and turn-off levels
provides sufficient hysteresis to prevent the squelch circuit
from chattering.
Thermistors R645 and R646 vary the reference level of
U601A over temperature variations to compensate for tem-
perature variations in the receive circuitry, allowing the
squelch threshold to be stabilized over a wide temperature
range.
QUICK CHECKS
1. Refer to the Block Diagrams (Figures 2 and 3) for
proper signal levels and gains for the vanous audio
paths.
2. Note the state of the FET switchesfor muting.These
switches are controlled by the Logic Board. If amute
line is high (+ 5 Volts), ground that pin and monitor
the results. However if a mute line is low, the line
maynotbepulledhighunless firstdisconnectedfrom
the Logic Board.
3. All bias points (+ 5Volts) shown on the Audio Board
Schematic Diagram are generated by Op Amp
U303B. The other Op-Amp circuits will not operate
properly without this voltage.
4. When measuring signal levels, remember that invert-
ing(-)inputsof operational amplifiers with feedback,
are "virtual grounds". No AC voltages should be
present at these ports.
Copyright© March 1991, Ericsson GE Mobile Communications, Inc.
LBI-38546
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