GE LBI31072 Phoenix User manual

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PHOENIX*
MAIIIITENANGE MANUAT LBI3 TTI72
450470 MHz 20-WATT MOBILE COMBINATION
GEr{ERAL@ETECTRTC

SYSTEM SPECIFICATIONS .
COMBINATION NOMENCLATURE .
DESCRIPTION ...
LBr31072 TABLE OF CONTENTS
1ii
iv \/
1
OPERATION 2 ,'
INITIAL ADJUSTMENT
Transmi tter Adjustment
Receiver Adjustment
Re-Installation
CIRCUIT ANALYSIS
Transmitter.
Recei ver
Channel Gua rd
Power Distribution
MAINTENANCE ...
Disassembly ....
Driver & PA Transistor Replacement ...
Removing ICrs .
TEST AND TROUBLESHOOTING PROCEDURES
Service Techniques
Test Points .. ..
Transmitter Alignment Procedures
Receiver Alignment Procedures
Receiver Test Procedures
Receiver Audio and Squelch lltaveform Checks
OUTLINE DIAGRA.MS
2 ,\-
2--
2
,
z
6_
7'
o
10
10
11
12
72
12
12
15-16
L7
l8
l9
Transmitter/Recei
I n t ercon nec t /Osc iver Board ......
llator/Channel Guard Board 24
24
23
25-26
27 -29
30
30
3L-34
32
SERVICE SHEET
Hookswi tch . .
Externa I Speaker
SYSTEM INTERCONNECTION DIAGRAM
SCHEMATIC DIAGRAMS
Interconnect/Osci 11ator/Channel Guard Board
Transmitter/Receiver Board (Sheets 1, 2 &,3)
PARTS LIST
PRODUCTION CHANGES
ILLUSTRATIONS
1 - Transmitter/Receiver Block Diagran
2 - Power Distribution ..
3 - Q251 Lead Identification
4 - Mechanical Layout ....
5 - Test Equipnent Set Up
6 - Transmitter Troubleshootlng
7 - Receiver Troubleshooting
Figure
F i gure
Fi gure
Fi gure
Fi gure
Fi gure
Fi gure
J
o
11
L4
l6
2L
Although the highest DC voltage in this mobile equipment is supplied by the vehicle
battery, high currents may be dravrn under short circuit conditions. These currents can
possibly heat .metal objects such as tools, rings, watchbands, etc., enough to cause
burns. Be careful when working near energized circuits!
High-1eve1 RF energy in the transmitter Povrer Amplifier assembly can cause RF burns
upon contact. Keep away frorn these circuits when the transmitter is energizedl
11 Copyrignt @ tgBl cenerat Etectric Company

L8r31072
SYSTEM SPECIFICATIONSI
FCC IDENTIFICATION NUMBER
GF, IDENTIFICA?ION NUMBER
FFEQUENCY RANGE
Recei ver
Sque I ched
Unsque lched
Transmi tter
FREQUENCY STABILITY
TEi{PERATURE RANGE
DUTY CYCLE
DTMENSTONS, LESS ACCESSORTES (g X W X D)
WEIGHT, LESS ACCESSORIES
TRANSMI TTER
AXAgMZTRlO 1A
TR1O1A
450-470 I'IHz
250 milliamperes
650 milliamperes
5.5 Amperes @ 13.8 Volts
0.0005%
-30"C (*22"F) to +60oC (140"F)
2O% Transmit, 100% Receive
65 MM X 190 MM X 240 MM
(2.55 X 7.5 X 9.4 inches)
2.O7 kg (4.5 pounds)
POl{EB OUTPUT
CONDUCTED SPUFIOUS AND
HAR['ONIC EMISSIO}I
IiODLJLA T I ON
AUDIO SF.NSITIVITY
AUDIO FREQUENCY
CHARACTER]STICS
DI STORT I ON
DFVIATION SYi\4METRY
MAXIMTIII FREQUENCY
S PREAD
RF OUTPUT IMPEDANCE
20
-70
r4,5 kllz (+3.5 kHz volce
nodulation and 0.75 kHz
CG modulation )
65 to 72O Millivolts
at J91 1-4
3.5 to 6 M'i I I ivolts at
Within +1 dB to -3 dB of
a 6 dB per octave pre-
emphasis fron 300 to 3OO0 Hz
per EIA standards. Post
limiter filter per FCC and
Less thA! 3% (1000 Hz)
Less than 5% (300 to 3000 Hz)
0.5 kHz naximum
5.5 MHz
50 ohms
AUDIO OUTPUT (to 4.0
ohms speaker)
SI]NSITlVITY
12 dB SINAI)
20 dB Quieting
Method
SELECTI V I TY
EIA Two-Signal
Me t hod
SPURIOUS RESPONSE
INTERMODULAT I ON
MODULATION ACCEPTANCE
SQUELCH SENSITIVITY
FREQUENCY RESPONSE
RF ]NPUT IMPEDANCE
-85 dB @kHz
-85 dB
-75 dB
+7,O kEz
<8 dB SINAD
Within +2 and -8 dB of a
staodard 6 dB per octave
de-emphasis curve from
300 to 3000 Hz (1000 Hz
reference )
50 ohms
0.4
o .45
* These specifications are intended
appropriate Specifications Sheet pri.marily for use of the serviceman. Refer to the
for the complete specifications.

DESCRIPTION
Genera I Ill ect ri c Phoeni x mobi Ie combi-
na.tions are ful1y transistored -- utilizi-ng
L.,oth di screte components anci integrated
circujts (IC's) for high reliab'i1ity. The
rarJio i s a sel f-contained, FM transmitter/
recejver with built-in controls and speak-
er. Its sma I 1 size makes it idea-I for
fr:ont mounting in conventional vehicles.
The stanclarrl combinations are equipped with
the following:
o One receive frequencY, and two
transmi t frcquencies
o P1 ug- j n crYsta I s for +0 ' 0005%
oscil lator stabi litY
o Channel Gr-tard (tone squelch)
ll'he raclio consists of an effective
hea t-di ssipating, aluminum die cast rrHrr
frame on which two ci rcuit boards are
mountecl. The transmitter/receiver board is
mounted on the bottom of the "H" frame and
includes FF and receiver audio circuitry'
The Interconnect/Oscillator/Channel Guard
( IOC) board contai ns a 1 I interconnections,
oscillator circuits, transmitter audio pro-
cessor , mi crophone preampl ifier , and Chan-
ne1 Guard ci rcuitry. Al 1 externa.l connec-
tors, controls and indicators are mounted
c] irectl)'on the two boards for rellability
anC ease of di sassemblY.
\* The hoarcls plug into each other, elim-
i na ting the need for i nterconnecting wires.
The only wires used in the ra.dio are the
plug-jn leacls for the internal speaker.
The top anr! bottom covcrs enclose the rrHrr
frame ancl provide optimum protection for
the radio. The internal speaker mounts on
the jnside of the top cover"
The front control panel is made of
highly durable plastic wlth rounded corners
and recessed controls for passenger safety
requirements. The panel provides access to
four standard operator controls: a POWER
crn/Off pushbutton, a momentary MONITOR
pushbutton (fixecl squelch and Channel Guard
monitor), a rotary, an edge-mounted Volume
contr()1 , and a two-posi tion frequency se-
I ector (RpTR-DIR) . A red Transmit indi-
cator t.FD (Light Fmj tting Diode) and a
green power on indicator are provided'
No pou'er supply is required since the
highest supply voltage used in the radio js
providr.d by the vehicle battery. The radio
i s de.si gned for operation only in 12 Vo1t,
negative ground vehjcle systems.
The'radio is of modular construction.
Both major modules and tuning adjustments
are easi 1y accessiblt'. Loosening the two
screws i n thc' rea r of the top cover pro-
vides access to the inte'rconnect or multi-
frequency/interconnect board.
LI_) 1:i 10 7 2
cover provicles access to tLe transnri ltcr/
receiver board.
An optional set of test probes can be
plugged onto the test pins on the board for
alignment and troubleshootlng. Illea.sure-
ments can be made using GE Test Set 4EX3A11
or a mul-timeter,
TRANSil{ I TTER
The transmltter consists of an FM ex-
citer with an audio processor and a broad-
band, fixed-tuned power amplifier. The RF
power output level is pre-set lnternally to
rated power. Once the 1eve1 1s set, a
sensing control circuit holds it constant
as temperature and/or voltage vary within
specified limits.
Frequency stability for both the
transmitter and receiver is maintained by
an electronic compensati-on network.
RECE IVER
The dual conversion receiver consists
of a front end section and two mixer/IF
sections operating at 21.4 NlHz and 455 kHz.
The receiver also contains a squelch and
audio section. The audio section provides
a 3-watt audio output into a 4-ohrn load.
AC POV/ER SUPPLY OPTION
To use the radio as a base station, an
optional 121 Volt AC, 60 llerLz power supply
is available. An eight foot cable connects
the power supp)-y to the radio. The cable
length permits the power supply to be 1o-
cated away from the radio. A green Power
On LED is located on the front panel of the
power supply.
M ICROPFIONE
The standard mobile combinations use
an electret microphone, The mi-crophone is
housed in a sturdy case, and the extendable
coiled cord plugs into a jack at the back
of the radio. The microphone is secured to
the radio by means of a strain relief hook
on the microphone cabIe. A microphone
hanger is supplied with the rnicrophone.
HOOKSWITCII OPTION
1n Channel Guard appllcations, a
microphone hookswitch may be suppli.ed wi-th
the radio. The hookswitch is equipped with
a Channel Guard disable switch,
Placing the switch in the "up"
position (towards the smaIl speaker syrrbol)
dlsables the receive Channel Guard. Iti-th
the switch in the "down" position, the
Loosen i ng
the two screws in the rear of the bottom

LnT31072 DESCRIPT ION
Channe.l CrrarC is disabled when the
microphone js removed from the hookswitch.
EXTERNAL SPEAKER (OpTTONAL)
A five-inch speaker, contained in a
LFXANo housing, provides an audio output of
3 watts. The nomina I speaker impedance is
3.2 ohms. The speaker leads are connected
to pin-s 3 and 7 of Systems plug P910. IVhen
the Externa I Speaker is used, the jumper
from H13 to H14 on the (IOC) board can be
removed or P904 unplugged to disconnect the
hui I t-in speaker. A LEXANo bracket is sup-
p) j ed for moun t i ng.
OPERATION
Complete operating instructions for
the Two-lVay Radio are provided in a sepa-
rate Operatorts Manual. The basic proce-
dures for receiving and transmitting
messa ges fo 1 1 ows :
TC RECEIVE A }.4ESSAGE
1. Turn the radio on by pushing in
the POWER pushbutton.
2. Push in the I,{ONITOR button to dis-
able the squelch circuit and Chan-
nel Gua rd decoder. Adjust the
volume control for a comfortable
I istening level and then release
the I\{ONITOR but ton f or norma 1
operation.
The radjo is now ready to receive
messages from other radios in the system.
TO TFANSI4IT A I\,IESSAGE
'| Turn the radio on as clirected in
the "To Receive a lr,lessage" sec-
tion.
2. Press the PTT switch on the micro-
phone and speak across the face of
the nnicrophone jn a normal voice
level. Release the PTT switch as
soon a s t he messa ge ha s been
given. The red inriicator I ight on
the control panel will glovr eacb
tirne the microphone PTT switch is
pressed, inr!icating that the
transmitter is on the air. The
receiver is rnuted when the trans-
mitter is keyed.
INITIAL ADJUSTMENT
After the radio has been installed (as
described in the Installation I\4anual), the
following adjustments should be made by an
2
electronics technician who holds a Firsl or.
Second Class FCC Radiotelephone license
(where requi red ) .
TRANSTI I TTER ADJUSTIVIENT
Adjustments for the Lransmi tter in-
clude measuring the forward and reflected
power and adjusting the antenna length for
optimum VSIJR, then setting the transmitter
to rated power output. Next, measuring the
frequency and modulation and entering these
mea suremen t s on the FCC requi red sta t ion
records. For the complete transmitter ad-
justment, refer to the Alignment Procedure
(see Table of Contents).
RECEIVER ADJUSTMENT
The initial adjustment for the recei-
ver includes tuning the input circuit to
match the antenna. For the Receiver Ad-
justment Procedure, refer to the Alignment
Procedure (see Table of Contents).
CHANNEL GUARD DISABLE
All radios are equipped with Channel
Guard. 1n applications where Channel Guard
is not desired, disable the Channel Guard
circuit by connecting a jumper from J910-B
to J910-9. Also, remove the versatone if
present.
RE- INSTALLATION
If the mobile combinati.on is ever
moved to a different vehicle, always check
the battery potarity of the new system.
CIRCUIT ANALYSIS
TRANS}{I TTER
The transmitters utilize a crystal
control led, frequency modulated exciter for
two-frequency operation in the 4bO-4TO trtiHz
frequency band. The solid state transmit-
ter uses integrated circuits and discrete
components for increased reliability. The
transmitter consists of audio processor
U1O1; osci I 1a tor Q151, and Q15t, buffer
Q153, exciter stages e201 through e2O4; pA
ampl i fier Q251 through Q253, and power
control circuit q2S4 through e287. The
exciter provides approximately 150 milli-
wa t ts of modula ted RF to the pA which
provides rated output power. Figure 1 is a
block diagram of the radio showing both the
transmitter and receiver.
I\{ICROPHONE PREATIPLIFIER
A preamplifier stage (e901 and
associated circuitry) is provided for the

FF- 45o-47o MHz 2t.4 uxz
[c,
R ECE
--lI
k- so.o-*z.zzz MHz
AUDIO
-J
rx xrAL FREa. = F3
Rx xrAL FRE. = E#!I:
RCONNECT/OSC /CG
TR AIiST
MUTE GATE
- l5O,O - 1 56.566 MHz

LB13 1072
455 KHr .--l
R 521
AUDIO
LEVEL
VOL /SOUELCH HI
;EIVER
cERAt'llc
FILTER
(455KHd
2502
IF LIM /FM DET/
AUDIO PRE AMPL
u30?
e!!4 osctttaloRz
MIXER/IF AMPL
u 50r
FROM CHANNEL GUARD
A UDIO
MUTE
GATE
o 605
AUDIO
AMPL
u6 0l
SMI TTER
POWER CONTROT
0254- A257
Z HELICAT
RESO -
NATORS
t2t4,L2t5
45O - 47O MHz
RC4240
Figurel-BlockDiagram
sxclrEn e-L+ PwR. AMPL.

L8I31072 CIRCUIT ANALYSIS
standard electret microphone wi.thout a
built-in preamplifier. The preamplifier
circuit is located on the IOC board.
With this microphone, MIC HI is
coupled througb J911-5 to the preamplifier
stage. The ampl i fied output is coupled
through R908 and C905 to the audio pro-
cessor.
For optiona 1 microphones with a
bui I t-in preampl i fier, audio is coupled
through J977-4, bypassing MIC PRE AMp e901.
AUDIO PROCESSOR U1O1
The audio n"o".=.o. provides audio
pre-emphasis with amplitude limiting and
post limiter filtering. A total gain of
approximateTy 24 dB is realized through the
audio processor. 20 dB is provided by
U101B and, 4 dB by U101A.
The 8.5 Vol t regula tor powers the
audio processor and applies regulated +8.5V
through p9O3-2 to a voltage divider con-
sisting of R101, R111, R110 and R109. The
+4.25V output from the voltage divider
establishes the operating reference point
for both operational amplifiers. C1O6
provides an AC ground at the summing input
of both operational amplifiers.
Resistors R109, R110, R111 and diodes
D101 and D102 provide limiting for U101B.
Diodes D101 and D102 are reverse biased at
+7.7 VDC. Voltage divider network R109,
R110 and R111 provides +b.9 VDC at the
cathode of D101 and +2.6 VDC at the anode
of D102. The voltage at the junction of
D101 and D102 is 4.25 VDC. C104 and C108
permit a DC level change between U101B-z
and the voltage divider network for diode
biasing.
When the input signal to U101B-6 is of
a magnitude such that the amplifier output
at U1018-7 does not exceed 4 volts p-p, the
amplifier provides a nominal 20 dB gain.
Itfhen the audio signa 1 level at U101B-7
exceeds 4 volts PP, diodes D101 and D102
conduct on the positive and negative half
cycles providing 100% negative feedback to
reduce the amplifier gain to 1. This
limits the audio amplitude at U1018-Z to b
volts PP.
Resistors R105, R106 and R10? and ClOz
comprise the audio pre-emphasis network
that enhances the signal to noise ratio.
R107 and C1O7 control the pre-emphasis
curve below limiting. R106 and CLOT con-
trol the cut-off point for high frequency
pre-emphasis. As high frequencies are
attenuated, the gain of U101 is increased.
Audio from the preamplifier or
microphone is coupled to the input of
operational amplifier U101B-6. The
amplified output of U1018 is coupled
through R114, R7I2 , R104 and R11Z to a
second operational amplifier U101A.
The Channel Guard (CG) tone input is
applied to U101A-2. The CG tone is then
combined with the microphone audio.
A post limiter filter consisting of
U101A, R112-R114, C10B and C110 provides 12
dB per octave roll-off. R104 and C102 pro-
vide an additional 6 dB per octave roll-off
for a total of 18 dB.
The output of the post limiter filter
is coupled through C110 to the temperature
compensated transmitter oscillators Q151
and Q152.
TRANSMIT OSCILLATOR
A tempera ture compensa ting network
consi,sting of R151, R152, R1b3, R154, D1b1
and C151 maintains oscillator frequency
over a temperature range of -30o to +60.C.
The temperature compensating DC voltage and
audio is applied to FM modulator DIS2
through MOD ADJ control R155 and R1b9.
Modulator varactor D151 varies the transmit
frequency at the audio rate applied from
the audio processor.
Q151, Y1b1 and associated circuitry
comprise a Colpitts oscillator which g.ne:
rates the ninth subharmonic (50 MHz) oi the
RF carrier frequency. The transmit
osci I lator is adjusted to the assigned
operating frequency by L151. The oscil-
lator output is applied to buffer e153, and
is then coupled through p1b1 to the exciter
circuitry transmitter/receiver (Tx/Rx)
boa rd .
The second oscillator stage is
identical to the F1 oscillator. The
desired frequency is selected through Tx
frequency swi tch S1b1 and channel Common
switch Q154.
Q154 is normally turned on so that the
emitter of the selected oscillator is
grounded through S151 and Q1b4, tqrning the
selected oscillator on.
SERVlCE
R112-R114 are 7% resistors. This
tolerance must be maintained to
a ssure proper opera tion of the
post limiter filter. Use exact
replacements.

SERV I CE
Tbe channel common circuit is for
factory test PurPoses onlY. The
circui t permits both frequencies
to be checked regardless of the
posi tion of S151. The circuit
consists of Q154, Rt72 and C168.
Groundirg channel common lead
J911-6 turns off Q154 and Permits
each transmit oscillator to be
turnecl on by grounding P911-7(F1)
and then P911-8(F2 ).
CIRCUIT ANALYSIS L8r31072
EXCITER
The exciter consists of a 1.50 llHz
amplifier a (150 \llIz to 45O MHz) tripler
and two 450 I{Hz amplifiers. The exciter
takes the output from the osci I 1a tor and
buffer circuits (2 milliwatts minimum) and
multiplies and amplifies 1t to provide 140
milliwatts minimum (45O-47O I{Hz) to the
power amplifier circuitry. In addition to
these functions, the exciter contains aIl
of the filters whicb determine the band-
width and spurious characteristics (exclu-
sive of harmonics) of the transmitter'
The output of the osci I 1a tor and
buffer stages is coupled to the input of
the exciter via J151, which is connected to
a tap on L2O1. This tap also supplies
voltage to the buffer transistor Q153.
L201 along with L2O2, selects the third
harmonic (150 \!lHz) rvhich is present aL
J151.
C2O6 and C2O7 natch the output of thls
two pole filter to the base of Q201' which
provides approximately 10 dB of gain. C273
and C2l4 match the collector of this tran-
sistor to the input of another two pole
filter, consisting of L2O4 and L205. The
emitter voltage on LzOl (AMPL-1) can be
monitored at Tp201, and is typically +0.3
vcl ts.
C216 and C2I7 match the output of L205
to the base of the tripler transistor'
Q202. The base voltage on Q2O2 ean be read
at'lP2O2, and is t1'pically -1.2 vo1ts. The
tripler has approximately 3 dB of gain.
A matching network consisting of L206
and C222 matches the 450 MHz output of the
tripler to the first two-po1e helical fi1-
ter, L2O7 and L2O8. The output of this
filter is tben matched by C223 and L209 to
a two stage, broadbanded 450 [4Hz amplifier
consisting of Q203 and Q204 . C227 , C228 '
L210 and L211 provide proper matching from
the collector of Q203 to the base of Q2O4,
providing the necessary bandwidth as we11.
Although this amplifier is capable of pro-
viding 12-I5 dB of gain, both stages are
hiased to provide adequate saturation to
ensure no AM components are present at the
exciter output. Typical actual gain is 7-9
dB, and typical power output is 150 mW.
The collector of Q204 is matched to
the input of the second two pole helical
filter (L214 and L215) by C233-C235, L272
and L213. An RF "sniffing" circuiL con-
sisting of C236, C237, D2OI, and R221 allow
relative power available at the input of
this filter to be monitored at TP203. +0'6
volts is typically present at thj"s point
after tuning the exciter.
There is no test point that indicates
rela tive power coming out of this fil ter
TP251 in the PA sect.ion indicates relative
power coming out of Q251 (Amp1 4) , and
final tuning for L214 and L215 should be
done while monitoring this test point. No
test points require the use of special RF
detector probes.
POWER AMPLIFIER
The three stage power amplifier con-
sists of Amplifier Q251, driver q252 and
power ampl ifier Q253 and associated cir-
cuitry. Collector voltage for driver Q252
is applied from A+ through power control
transistor q257 and L256, L257 and R256.
The colleetor voltage for Q252 is a result
of the output power setLing and voltage
variations at any given time. The output
of driver q252 is coupled to the base of
Power Amplifier Q253 through an impedance
matching network consisting of W254, C292,
C267 , C268 and C269.
Collector voltage for Q253 is provided
frorn A+ through L267, L262 and R258.
The 20 watt output of the PA is con-
nected to the 1ow-pass filter by W256 and
then to antenna relay K601.
RF POWER ADJUST CIRCUIT
The output power adjust circuit al1ows
the transmitter to be set to rated output
power. The power adjustment is attained by
control ling the DC col lector voltage to
driver Q252 through pass transistor q257.
The pass tra nsistor i s control I ed by a
feedback loop consisting of Q254 through
Q256. The power is set by potenLiometer
R260.
SERVICE
There are 4 points in the exciter
which are approximatelY 50 ohrns.
W201, W2O4, W205 and W251 (which
interface the 2 helica I fi1 ters
to the other exciter circuitrY).
These points can conveniently be
used to monitor or inject signals
for troubleshooting and testing
using 50 ohm sources and termi-
nations.

L8r31072
A change in output power is sensed by
D252 causing the base voltage of q254 to
change accordingly. For example, if the
output power increases, the base of Q254
goes more positlve, causing it to increase
conduction which lowers its col lector volt-
age. Q254 controls Q255, therefore as Q254
increases conduction, Q255 decreases con-
duction. This raises the voltage applied
to the ba se of Q256. Tbe conduction of
Q256 Cecreases proportionally, lowering the
base voltage of pass transistor Q257. The
resulting decrease in conduction of Q257
lowers the col lector voltage of driver
q252, tberetry lowering the output power in
proport ion to the excessive power ori gin-
a1ly sensed by the base circuit of Q254.
RECE I VER
The receivers are dual conversion,
superheterodyne FI\'I receivers designed for
one-frequency operation in Ihe 42O-572 \lHz
frequency range. A regulated 8.5 volts is
used for all receiver stages except for the
audio PA IC, whjch operates from the A+
supply.
'The receiver has intermediate fre-
quencies of 21.4 liHz and 455 kHz. Adjacent
channel selectivity is obtained by using
two band-pass filters: a 21.4 MHz crystal
fi I ter and a 455 kHz ceramic fi 1 ter.
Al1 of the receiver circuitry except
the osci 1 1a tor is mounted on the
transmit ter/receiver (Tx/Rx) board. The
receiver consists of:
Receiver Front End
21 .4 MiHz 1st IF circuitry
1st and 2nd Oscillators
455 kHz 2nd IF circuitry with FM
Detector
Audio PA Circuitry
Squel ch Ci rcui t ry
RECE]VER FFONT ENI)
An RF signa 1 from the antenna is
conpled through antenna relay K601 and two
helical resonators (L401 and L4A2) to the
t'ase of RF amplifier Q401. The output of
Q401 is coupled through three more helical
resonators consisting of L4O5-L4O7 to the
gate of 1st Mixer Q402. The front end se-
lectivity is provided by the five helical
resona tors.
OSCILLATOR & IVIULTIPLIER
Q301 , Y301 a nd a ssocia ted ci rcui try
make up a Colpitts-oscillator. The
o
frequency is control led by a third mode
crystal operateci at one ninth of the
required injeetion frequency. Voltage-
variable capacitor D301, L301 and Y301 are
connected in series to provide compensati-on
capability. The compensation voltage used
to control the transmitter oscillators is
applied to D301 to maintain stability.
L301 is adjustable to set the oscillator
frequency. R301 is in parallel with Y301
to insure operation on the third overtone
of the crystal.
The output of Q301 is coupled through
C304 to the emitter of buffer Q302. The
output of Q302 is coupled through P301 to
two tuned circuits (L351 and L352) on the
Tx/Rx board. L351 and L352 are tuned to
the third barmonic of the oscillator fre-
quency which is applied to the base of
Q351. The output of tripler A351 is
coupled to the source input of mixer Q402
through helical resonators L354 and L355.
L354 and L355 are tuned to the operating
frequency minus 27.4 MHz which is the ninth
nultiple of the crystal frequency.
The DC leve1 of the osci 1 Iator /
multiplier chain can be monitored at TP351.
The meter reading at this point is typical-
ly 0.8 VDC. The RF frequency from the
oscillator/multiplier chain and input level
to the mixer can be measured at TP401. The
meter reading at TP401 is typically 0.5 to
1 volts as measured using the Rx RF Detec-
tor Probe.
1ST I{IXER
The 1st mixer uses a FET (Q402) as the
active device. The FET mixer provides a
high input impedance, high power gain and
an output relatively free of intermodula-
tion products).
In the mixer stage, RF from the front
end helical filter is applied to the gate
of the mixer. Injection voltage from the
multiplier stages is applied to the source
of the mixer. The 21.4 MHz mixer 1st lF
output signal is coupled frorn the drain of
Q402 through an impedance matching network
(L409 and C410-C413 ) to crysta 1 filter
2501.The highly-selective crystal filter
provides the first portion of the receiver
IF selectivity. The output of tLre filter
is coupled through impedance-matching net-
work L501 to the 1st IF amplifier.
IST & 2ND IF & DETECTOR STAGES
1st IF Amplifier e501 is dual-gate
I{OSFET. The crystal filter output is ap-
plied to Gate 1 of the amplifier, and the
ampli-fied signal is taken from the drain.
The biasing on Gate 2 and the drain load
determines the gain of the stage. The
CIRCUIT ANALYSIS