Motorola 146-174 MHz User Manual
4-2
6880904Z07-O
August, 1996
Theory of Operation
GP350 Portable Radios Service Manual
Transmitter
The insertion loss of this filter is approximately 1.9 db for
VHF and 3.5 db for UHF. The filter is designed to be termi-
nated with the amplifier output impedance on one side, and
50 ohm on the other.
VHF and 3.5 db for UHF. The filter is designed to be termi-
nated with the amplifier output impedance on one side, and
50 ohm on the other.
The net gain from the receiver module is about (12.2 db
VHF) (10.8 db UHF) in the center of the band and about
(10.7 db VHF) (9.5 db UHF) at the band edges. The net cen-
ter of the band noise figure is approximately (5.5 db VHF)
(5.2 db UHF). This is sufficient to achieve a typical center of
the band sensitivity of 12 dbs.
VHF) (10.8 db UHF) in the center of the band and about
(10.7 db VHF) (9.5 db UHF) at the band edges. The net cen-
ter of the band noise figure is approximately (5.5 db VHF)
(5.2 db UHF). This is sufficient to achieve a typical center of
the band sensitivity of 12 dbs.
The double balanced mixer is composed of the two baluns,
T1 and T2, and the ring diode IC, CR2. The mixer operates
with a local oscillator (LO) level of +6 dbm and the conver-
sion loss is approximately 7.5 db. The double balanced type
mixer (DBM) provides excellent isolation between any two
ports. And since a DBM can operate over a large bandwidth,
the same mixer can be used for UHF and VHF radios. The
DBM also provides excellent protection against receiver
spurs due to non-linearizes, such as IM and Half-IF. The
received signal mixes down to the frequency of the first IF,
45.1 MHz, and enters the IF circuitry.
T1 and T2, and the ring diode IC, CR2. The mixer operates
with a local oscillator (LO) level of +6 dbm and the conver-
sion loss is approximately 7.5 db. The double balanced type
mixer (DBM) provides excellent isolation between any two
ports. And since a DBM can operate over a large bandwidth,
the same mixer can be used for UHF and VHF radios. The
DBM also provides excellent protection against receiver
spurs due to non-linearizes, such as IM and Half-IF. The
received signal mixes down to the frequency of the first IF,
45.1 MHz, and enters the IF circuitry.
Intermediate Frequency (IF)
The Intermediate Frequency (IF) section of the portable
radio consists of several sections including, the “high” IF, the
second LO, the second IF, and the IF IC chip. The first LO
signal and the RF signal mix to the IF frequency of 45.1
Mhz, and then enters the IF portion of the radio.
radio consists of several sections including, the “high” IF, the
second LO, the second IF, and the IF IC chip. The first LO
signal and the RF signal mix to the IF frequency of 45.1
Mhz, and then enters the IF portion of the radio.
The signal first enters the “high” IF, passes through a crystal
filter, is then amplified by the IF amplifier, and then passed
through another crystal filter. The first crystal filter provides
selectivity, second image protection, and intermodulation
protection. The amplifier provides approximately 16 dB of
gain to the signal. The signal then passes through the second
crystal filter which provides further selectivity and second
image protection. The “high” IF has an approximate 3 dB
bandwidth of 7 KHz for 20/25/30 KHz models and 4 KHz
for 12.5 KHz models.
filter, is then amplified by the IF amplifier, and then passed
through another crystal filter. The first crystal filter provides
selectivity, second image protection, and intermodulation
protection. The amplifier provides approximately 16 dB of
gain to the signal. The signal then passes through the second
crystal filter which provides further selectivity and second
image protection. The “high” IF has an approximate 3 dB
bandwidth of 7 KHz for 20/25/30 KHz models and 4 KHz
for 12.5 KHz models.
The filtered and amplified IF signal then mixes with the sec-
ond local oscillator at 44.645 MHz. The second LO uses an
amplifier internal to the IF IC, an external crystal and some
external chip parts. The oscillator presents an approximate
level of -15 dBm to the second IF mixer, internal to the IF IC.
ond local oscillator at 44.645 MHz. The second LO uses an
amplifier internal to the IF IC, an external crystal and some
external chip parts. The oscillator presents an approximate
level of -15 dBm to the second IF mixer, internal to the IF IC.
The output of the mixing of the IF signal and the second LO
produces a signal at 455 KHz (second IF). This signal is then
filtered by external ceramic filters and amplified. It is then
passed back to the IF IC, sent to a phase-lock detector, and
demodulated. The resulting detected audio output is then
sent to the AFIC to recover the audio.
produces a signal at 455 KHz (second IF). This signal is then
filtered by external ceramic filters and amplified. It is then
passed back to the IF IC, sent to a phase-lock detector, and
demodulated. The resulting detected audio output is then
sent to the AFIC to recover the audio.
The IF IC also controls the squelch characteristics of the
radio. With a few external parts the squelch tail, hysteresis,
attack and delay times were optimized for the radio. The
AFIC allows the radio’s squelch opening to be electronically
adjusted.
radio. With a few external parts the squelch tail, hysteresis,
attack and delay times were optimized for the radio. The
AFIC allows the radio’s squelch opening to be electronically
adjusted.
Transmitter
The GP350 VHF and UHF transmitters contain five basic cir-
cuits: a power amplifier, an antenna switch, a harmonic filter,
an antenna matching network, and a power control Refer to
the block diagram and the schematic for more information.
cuits: a power amplifier, an antenna switch, a harmonic filter,
an antenna matching network, and a power control Refer to
the block diagram and the schematic for more information.
The power amplifier for VHF contains three stages of ampli-
fication. For UHF, the power module contains four stages.
Both modules require an input signal of 1 mW, a supply volt-
age of 7.5 volts, and are capable of supplying, at least, 7
Watts of output. The power out of both modules can be var-
ied by changing the voltage on their second stage.
fication. For UHF, the power module contains four stages.
Both modules require an input signal of 1 mW, a supply volt-
age of 7.5 volts, and are capable of supplying, at least, 7
Watts of output. The power out of both modules can be var-
ied by changing the voltage on their second stage.
The antenna switch circuit consists of two PIN diodes
(CR101 and CR102), a pi network (C119, L112, and part of
C112), and at least, one current limiting resistor (R102 for
UHF; and R102, R103, and R108 for VHF). In the transmit
mode, TX B+ is applied to the circuit to bias the diodes “on”.
The shunt diode (CR102) shorts out the receiver port, and the
pi network, which operates as a quarter wave transmission
line, transforms the low impedance of the shunt diode to a
“high” impedance at the input of the harmonic filter. In the
receive mode, the diodes are both off, and hence, there exists
a low attenuation path between the antenna and receiver
ports.
(CR101 and CR102), a pi network (C119, L112, and part of
C112), and at least, one current limiting resistor (R102 for
UHF; and R102, R103, and R108 for VHF). In the transmit
mode, TX B+ is applied to the circuit to bias the diodes “on”.
The shunt diode (CR102) shorts out the receiver port, and the
pi network, which operates as a quarter wave transmission
line, transforms the low impedance of the shunt diode to a
“high” impedance at the input of the harmonic filter. In the
receive mode, the diodes are both off, and hence, there exists
a low attenuation path between the antenna and receiver
ports.
The harmonic filter consists of part of C112, and L107,
C113, L108, C114, L109, and C115. The design of the har-
monic filter for both VHF and UHF is that of a Zolotarev
design. This particular design is similar to that of a Cheby-
shev filter except for a large amplitude first ripple (near dc).
This type of filter has the advantage that it can give greater
attenuation in the stop-band for a given ripple level.
C113, L108, C114, L109, and C115. The design of the har-
monic filter for both VHF and UHF is that of a Zolotarev
design. This particular design is similar to that of a Cheby-
shev filter except for a large amplitude first ripple (near dc).
This type of filter has the advantage that it can give greater
attenuation in the stop-band for a given ripple level.
Another feature of this type of filter is that the coils tend to
be smaller than with a Chebyshev design. The design of the
VHF filter was modified from the Zolotarev design by
slightly changing its capacitor values to yield a filter having
an input impedance which optimized the efficiency of the
power module.
be smaller than with a Chebyshev design. The design of the
VHF filter was modified from the Zolotarev design by
slightly changing its capacitor values to yield a filter having
an input impedance which optimized the efficiency of the
power module.
To optimize the performance of the transmitter and receiver
into an antenna, a network is used to match the antenna’s
impedance to the harmonic filter. For VHF the network con-
sists of C117, L111, and C122. For UHF the network is made
up of C117 and L111. Note that, in order to measure the
power out of the transmitter, one must remove the antenna
and screw in its place a special BNC-to-Phono adapter.
into an antenna, a network is used to match the antenna’s
impedance to the harmonic filter. For VHF the network con-
sists of C117, L111, and C122. For UHF the network is made
up of C117 and L111. Note that, in order to measure the
power out of the transmitter, one must remove the antenna
and screw in its place a special BNC-to-Phono adapter.
The power control circuit consists of the networks associated
with U151, Q156, Q151, Q152, Q155, and U152. The Op
Amp U151A and Q156, along with resistor R101, make up a
current-to-voltage amplifier whose gain is mainly dependent
upon the ratio of R179 to R153. The current to the final stage
of the power module is supplied through R101 (0.1 Ohms),
which provides a voltage proportional to the current drain.
This voltage is amplified and applied to the input of U151B.
The resistors at the input of U151A (R151, R152, R154, and
R155) keep the voltages at the inputs of U151A below its
maximum allowable. These resistors are 1% tolerance parts
with U151, Q156, Q151, Q152, Q155, and U152. The Op
Amp U151A and Q156, along with resistor R101, make up a
current-to-voltage amplifier whose gain is mainly dependent
upon the ratio of R179 to R153. The current to the final stage
of the power module is supplied through R101 (0.1 Ohms),
which provides a voltage proportional to the current drain.
This voltage is amplified and applied to the input of U151B.
The resistors at the input of U151A (R151, R152, R154, and
R155) keep the voltages at the inputs of U151A below its
maximum allowable. These resistors are 1% tolerance parts