QEI 691 User manual

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QEI
MODEL
691
MODULATION
MONITOR

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SECTION 1
. GENERAL INFORMATION
1-1 DESCRIPTION
Th e QEI Model 691 Modulation Monitor/Test Set (FCC -· Type Approval
Number 3- 244 ) is an all solid state Modulation Mon1tor de~igned to
meet or exce~d the Federal Coli111un ication Commission requirements • for
measuring the modulation characteristics of FM transmitters having a
frequency range
of 88. lMHz to 107 : 9MHz both at .the -: transmitter . and off-
air. ·
The 691 is manufactured in~ 10 ½" X 19
11
rack mount. All operator
controls are located
on the front panel. AC Power, RF Inputs and Monitor
Outputs are located
on the rear panel. ·
1-2
ELECTRICAL SPECIFICATIONS
RF Frequency Range ..... . ..... . .• .... 88 .1 - 107.9MHz
(Thumbwheel selected)
RF Input (to . .g.!)arantee 66dB S/N .in a 30KHz Bandwidth)
Ant ... . ....... . .. . ... .... .. :. . .. .. 3mv to 200mv .
Sig
Sample ................... ·: .... 2mw to 50W (depending on external ·
load)
Total Modulation Display
Modulation
Meter
Accuracy .... . .........• .. . .. .... +2 % entire range
Frequency Response .... . .. ... . ... 0.2dB 30Hz - 75KHz
Range .... ......... . ..•. .•..... .. 0-133% (-20dB to +2d B)
Ballistics ......... .. ........ . .. per FCC regulations
Peak Indicators .
f
Range .. : ..... J. .................... 0-199%in 1%increments
Accuracy . .... :. . . .. .. . . .. . .. . . . . .. + 1%
Peak Counter
Set Point
...... . ........ . ...... .. . 1 00 %+ 1%
Ti me . .. . ... . . . .. . . .. . .. . .. . .; . . .. . 1 mi nute + 1 sec.
Distortion
............ . .... . ... .... • 0.05% TH D or IMO
S/N .. .•............•....• . ..... . ....
I
Test Displays (2)
better than 75dB (with 75usec de-emp)
better than 66dB in a 30KHz Bandwidth
Meter Accuracy ...• . ..... . . ...... • + 2% entire range
Peak Indicator Accuracy .... ... .•• . + 1%
Meter Range. . ..•. . . . .. . . . . •.. . . . . 1 33 %to -75dB (autorangi ng)
Pilot & SCA Injection Accµracy ... :!:_ 0.5% (6 % to 12 %)
... ........ .. ....

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Separation
L into R
or R into L.. ..•...•... . .•. . 55db
Crosstalk
Main into Sub ... .. .. ................. 65db
Sub into Main ....... ... ......... . .... 65db
SCA into Sub or Main .. ... ... . ... . .... 70db
Pilot into Sub or Main •• ........ . .. .. 70db
Monitoring Modes
1) Left
2) Main
3) Pilot Level
4) 38KHz
5) FM/S/N
6) SCA Inj. narrow
7) SCA Inj. wide
Distortion (Left or Right)
Right
Sub
Phase Cai .
Gen. Phase
AM S/N
Main
SCA Mod .
THO .•... . .......... . .. • ........ . .... .0. 05 %
I
MD . .. .• .. .. .. ..... . .. . ..... . ........ 0.1%·( SMPTE)
SCA (4KHz dev.) ........... . .. .. • ..... 1% (50 Hz . to 5KHz) (150us de-emp )
Outputs
Left.•
.. . .. ... .. .• ....•.. .• . .. .. ..... +10 dbm bal . · a~d Hi-z
Right... .
.. . ... .. . .... . .. . ........... +10 db mbal . and Hi-z
Scope ........ . ..............••. • .. .• .1 V8p
Frequency ........... . . •. . . ......... . . X. , I. F. Pilot, SCA (TTL)
Spectrum Analyzer Output .... ........... 70db; +120KHz or +400 K Hz
X (horix. ). ...........•..... • .. • ..... 4V~p
Y (vert.) ........•............. . ... •. 0.5V / 10db
1-3 MECHANICAL SPECIFICATIONS
Dimensions .. . ...... . .. . .•........... . .. lO ½" H X 19
11
WX 12
11
0
Weight ........... .. .. ............. . .... 16 lbs.
Maximum Operating Temp ... . .. • ...... . ... 122°F (5 o0 c) Ambient

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1-4 INSTRUMENT IDENTIFICATION
This unit is identif i ed by a Model and Serial Number located on the rear panel .
All correspondence to your sales representati
ve or the factory in regard to the
unit should reference the comp l ete Model and Serial Numbers.
1-5 OPTIONS
(01) SCA Monitor option (67KHz) consists of the pl ug in board assenbly A6
(02) SCA Monitor option (special frequency) (not FCC Type Approved)
(03) CCIR Specifications (not FCC Type Approved)

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2-1 INITIAL INSPECTION
SECTION 2
INSTALLATION
Check the shipping carton for external damage. If the carton exhibits evidence
of abuse in handling (holes, broken corners, etc.) ask the carrier agent to be
present when the unit is unpacked. Carefully unpack the unit and inspect all
equipment for physical damage. Immediately after unpacking any bent or broken
parts, or scratches should be noted. Keep all packing material for proof of
damage claim or for possible future use.
2-2
PREPARATION FOR USE
The unit is designed to be mounted in a standard 19
11
rack. Air space should
be provided above and below the unit so that heat generated by the circuitry
may be dissipated. Additional cooling may be required if the unit is placed
above high heat generating equisment in order to keep the ambient temperature
below the maximum specified (50 C).
Mount the unit to the rack using # 10 countersink screws and finishing washers.
The unit requires a 105-125V single phase, 50/60Hz power source or a 120-250V
single phase, 50/60Hz pow er source. The identification plate on the back
panel shows for which power source the unit is wired. See Section 5-7 for
inf ormation on changing taps.
2-3 MONITOR · coNNECTIONS
1. Connect the antenna supplied to monitor ANT connector Jll if off air
operation is desired.
2.
Connect a dummy l oad large enough to handle the RF input to DUMMY LO AD
connector J9 .• A½ watt termination is supplied. The input signal sampler
will handle
up to 50 watts with an adequate external dullllTG' load, thus
allowing
direct measurement of exciters and low power transmitters.
DO NOT OPERATE THIS EQUIPMENT WITHOUT AN ADEQUATE DUMMY LOAD INSTALLED.
3. Connect the RF sample to RF IN connector JlO.
4.
TBl-4 and TBl-5 are 600 ohm balanced left audio output.
5.
TBl-6 and TBl-7 are 600 ohm balanced right audio output.
6.
TBl-8 is 600 ohm unbalanced SCA audio (ref. to gnd.)

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2-4 REPACKING FOR SH I PMENT
NOTE: W ARRANTY MAY BE VOID IF UNIT IS NOT RETURNED IN ORIGINAL PACKING.
NOTE: Before returning a u nit for repair or ca l ibration, contact the factory
or your authorized representative for Return Authorization. Attach a tag
showing owner's name and address. Adescription of the service required
should also
be incl uded. Unit must be shipped prepaid and insured for full
va l ue. USE THE ORIGINAL SHIPPING CARTON AND PACKING MATERIAL FOR RE - SHIPMENT.

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SECTION 3
OPERATION
3-1 OPERATOR CONTROLS AND INDICATORS (FRONT PANEL)
1. POWER Switch and led AlSl and AlCRl apply line power to unit.
2. CARRIER FREQUENCY thumbwheel A1S2 sets unit on desired frequency.
3.
L.O. UNLOCK led A1CR2
Led lights if local oscillator is unlocked or reference is lost.
4. RF LEVEL leds A1CR3 (Hi), A1CR4 (LO), AlCRS (OK)
a. AlCRS (OK) will light and AlCR4 (LO) will go out when there
is sufficient RF input to allow a-curate modulation and signal to noise
measurements. This level will normally
be obtained with the antenna
supplied within the
70dBu contour.
b.
A1CR3 (Hi) will light if an excessive RF sample is applied.
Reduce the RF LEVEL pot on rear panel until A1CR3 estinguishes. AlCRS (OK)
will remain lit with Hi RF level since all required monitor functions will
be in specification. However, AM Signal to Noise will not be accurate be-
cause
of saturation of the IF amplifier. In addition, when A1CR3 (Hi) is
lit, the input mixer is being protected by an input clipper.
5. MUTE Switch A1S3
a. With MUTE switch in ON position unit wil1 mute all signals
below RF LEVEL OK threshold.
6.
COMP Jack AlJl provides a 6Vpp wideband composite signal.
7. DE-EMP AUDIO Jack A1J2 provides a 6Vpp@ 400Hz signal with 75usec
de-emphasis. NOTE: This signal is NOT filtered to remove 19KHz.
.
8. TOTAL MODULATION Displays (Yellow)
a. Meter AlMl in conjunction with MOD METER switch A1S4A provides
total modulation (main, sub and SCA) indication with FCC semi-peak ballistics
switchable to POS or NEG.
b. PEAK MOD thumbwheel AlS5 and lamp AlDSl provide a total modula-
tion peak indicator independent of polarity and settable in 1%increments
from Oto 199 %. This flasher will light from 2 to 4 seconds each time a
peak is detected.
c. 100 % PEAKS PER MINUTE Display A10S2 and RESET/OFF Switch A1S6
provide a count of 5ms 100 %peaks. The display flashes at a count of 10,
has a maximum count of 19 and resets automatically each minute. RESET over-
rides the automatic
reset function and OFF extinguishes the display. This
indication
is intended to monitor modulation peaks for Automatic Transmitter
Systems (ATS) as defined by the FCC in the ATS rules.

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d. CAL pushbutton A1S7 provides a total modulation calibrate
signal. The frequency of this signal is 32KHz . See Section 5 for further
information. NOTE: LO UNLOCK led must light when CAL is depressed.
9.
SCOPE Display (Vector and Spectrum Analyzer)
NOTE: To use these features of the 691 requires an oscilloscope
with X
and Y DC Coupled inputs. The bandwidth of the scope need only be a
few hundred kTTohertz but it must be DC Coupled on both X and Yinputs.
a. Connect VERT (A1J3) to Scope Y input. Connect HORIZ (AlJ4)
to
Scope X input. Set both inputs to DC coupled, 0.5 volts/div .
b.
To view Stereo Vector pattern, depress VECTOR button on A1S4B.
Mono modulation will give a vertical line. Any deviation from vertical in-
dicates
that the station mono balance is incorrect.
c. To use SpectrumAnalyzer feature, depress NARROW pushbutton
on A1S4B. Adjust scope horiz. (X) gain to get full width baseline. Adjust
scope
vert (Y) gain to 0.5/div. Display will be centered on frequency de-
termined
by CARRIER FREQUENCY switch and will be approximately +120KHz and
lOdB/div. Depressing WIDE pushbutton will change dispersion to-approximately
+350KHz. NOTE: WIDE position will cause TOTAL MODULATION display to pin
meter
and peak flasher if MUTE is OFF.
10. PILOT PRESENT indicator A1DS3 and 0ADJ AlRl indicate the presence
of the 19KHz pilot tone and provide monitor phase calibration in conjunction
with 0
CAL position of CHANNEL B DISPLAY SELECT. Pilot 0 ADJ AlRl is set
for a null on CHANNEL Bmeter. This adjustment is accurate even with modu-
lation applied.
11.
SCA PRESENT indicator A1DS4 indicates the presence of SCA. The
DEV switch A1S9 may be set to cause CHAN~EL B SCA MOD display to indicate
100 % at either 4KHz or 6KHz deviation.
12.
FREQ Jack AlJS and switch A1S8 provide a convenient output for an
external counter to read station frequency, pilot frequency or SCA frequency.
To read station frequency requires a counter capable of 10.7MHz. First con-
nect the counter·to the XO jack on the rear panel . If the X.O. is not exactly
8.000000MHz, adjust the piston trirrmer on ·the monitor A2 assembly until it
does. See Section 5 for details. Now connect the counter to AlJS and place
A1S8 to IF position. The counter should read 10.7MHz. If it reads lower,
the·
station frequency is higher by the same number of hertz. Conversely, if
it reads higher, the station frequency is lower. NOTE: Be sure of the
accuracy
of the counter. Also, remove modulation from the carrier when
making this measurement. To measure pilot frequency, place A1S8 in PILOT
position. Frequency read on counter will be 10 times the actual pilot.
This allows a counter with only a one second gate time to read to .lHz
accuracy.
To measure SCA frequency, place A1S8 in SCA position. NOTE:
Remove modulation from SCA when making this measurement.

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13. OUTPUT SELECT Switch AlSlO, AUDIO Jack A1J6 and SCOPE Jack A1J7
provice distortion measuring outputs and scope observation output. Switch
AlSlO in LEFT/CHAN A position provides left deemphasized audio with a cutoff
frequency of lSKHz for stereo distortion measurements at AlJ6 and whatever
is displayed on the CHANNEL Ameter at A1J7 for scope observation. When
AlSlO is in RIGHT/CHAN B posit ion, right audio is provided at A1J6 and
Channel B signal appears at AlJ7.
14.
SCA Audio Jack AlJ8 provides 150usec deemphasized SKHz bandwidth
SCA audio for distortion measurements.
1
5~ TEST DISPLAYS (CHANNEL A - green) (CHANNEL B - blue) and DISPLAY
SELECT switch AlS ll
a. DISPLAY SELECT switch controls the signal components to be
displayed.
b. Meters A1M2A and A1M2B in conjunction with+/- switches
A1S13A and A1S13B, MOD; HOLD: AUTO switches A1S14A and A1Sl4B and RANGE
(- dB) readouts A1DS5A and AlDSSB prov i de two metering displays with a
range
of Oto 133 %with FCC ballistics (MOD position) or - 70d8 to +3dB
with semi VU ballistics (AUTO or HOLD position). These displays may be
sw it ched to+ or -indications and may be deemphasized by pushbuttons
A2Sl5A or A1S15B . NOTE: FM and AM S/N are automatically deemphasized.
c.
PEAK thumbwhee1s AlS12A and A1S12B and 1 eds AlCR6A and AlCR6B
provide a peak indication of the corresponding meter display settable in 1%
increments
from Oto 199 %.
3-2
OPERATOR CONTROL AND CONNECTIONS (REAR PANEL)
1. DUMMY LOAD Jack A1J9 - A DUMM~ LOAD ADEQUATE FOR THE RF INPUT
SUPPLIED MUST BE INSTALLED AT THIS JACK . A½ watt load is supplied with
the
unit.
2. RF INPUT Jack AlJlO -Transmitter RF sample connects to this jack.
3.
CARRIER LEVEL Control A1R2 -This control is used to reduce RF
Input below RF HI threshold.
4.
ANT Jack AlJll - 75 ohm FM Antenna connects to this jack.
5.
X.O. OUT Jack A1J12 - 8MHz sample from crystal reference oscil lator
is avai1ab1e at this jack for measurement purposes.
6.
S.A. CENTER Control A1R3 -Control used to center Spectrum Analyzer
display.

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7. COMPOSITE EXT INPUT A1J l3 - Th is input all ows a composite signal
to be a pp lied direct ly to the Stereo and SCA circui ts of the monitor bypas -
sing the
RF demodu l ator.
8. COMPOSITE EXT LEVEL Control Al R4 -Sets external composite signal
l
eve l .
9.
COMPOSITE Switch AlS16 - This switch must be in INT position to
use RF d emodu l ator . In EXT position signal app lied to COMPOSITE EXT INPUT
Al J13 is f ed to stereo and SCA circuits.
3-3 MONAURAL MEASUREMENTS
1. Total Modulation
Set the
CARRIER FREQUENCY thumbwheel to the desired frequency and
MUTE switch to ON. Ver i fy . that L.O. UNLCOK l ed is out and that RF LEVEL
OK led is lit. Al l ow a few seconds for the monitor to stabil ize on the
frequency selected. Adjust the
PEAK M OO thumb wh eel to the desired level.
T
he amber PEAK MOO l amp wil l fl ash for 2 to 4 seconds each time the modu-
l at i on exceeds the preset PEAK MOD level in either a positive or negative
direction.
The TO T AL MODULATION meter responds to total modu lation with
FCC specified ba l listics and may be switched to read either posit i ve or
negative modu lation w it h the MOD METER POS or NEG pushbuttons.
NOTE
The Total Peak Mod Flasher and Total Modulat i on Meter will
only agree if the
transmitter is modulated by a low distor -
tion si ne wave. Any disto r tion wi ll cause posit i ve to nega-
tive assynmetry approximately double the amount of the dis-
tortion in percent. Remember thet the flasher reads the
highest
po l arity and that the meter reads the selected
pol
arity. The difference in readings with program material
is
due to many factors such as the amount of audio dynamic
range compression, transmitter overshoot, program assymmetry,
etc.
The 1 00 % PEAKS PER MINUTE readout counts the number of positive or negative
100 %peaks occuri ng within one minute. This counter was designed to show
compliance with FCC ATS rules. Therefore, it counts 5 millisecond peaks
as ~er FCC rules 73.342(b)3. However, the required PEAK MOD flasher must
rema in lit for 2 to 4 seconds each time it triggers per FCC rules 73.332(D)4i.
This
means that it is possible to get many ATS 5 mill i second peaks during one
flash of the PEAK MOD flasher. Si n ce the ATS rules al low 10 (ten) 100 %5 mi l li-
second peaks per minute; t he readout flashes at 10 and has a maximum count of
19. It automatical ly resets at one minute in tervals and may be manual ly reset
or turned off wi th i ts associated switch.

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2. Mono Distortion
Approximately
6V
00
@ 100 %@ 400Hz is available for connection to
a high Z distortion meter at the OE EMP AUDIO jack. This output is
deemphasized 75usec and is NOT filtered to remove 19KHz.
3. FM Signal to Noise/ AM Signal to Noise
Remove modulati on from the transmitter. Depress FM S/N - AM S/N
DISPLAY SELECT pushbutton. Place CHANNEL A and CHANNEL B METER MOD/HOLD/
AUTO switches in AUTO. FM Signal to Noise will be displayed on CHANNEL A
meter
and AM Signal to Noise will be displ ayed on CHANNEL Bmeter. The
reading will be the algebraic sum of the METER RANGE (-dB) readout and
the associated meter. Note that FM S/N is automatically measured with
75usec
deemp in a SOHz to lSKHz bandwidth . If it is desired to check
for FM noise below 50Hz, e.g. blower vibration, depress DISPLAY SELECT
MAIN-SUB and read CHANNEL Awhile ho lding CHANNEL A METER DE EMP - FLAT
pushbutton depressed. Any difference in the noise readings indicates
noise with a frequency
component below SOHz . AM S/N is automatically
measured with 75usec deemp and is accurate as long as the RF LEVEL HI
led is out and RF LEVEL OK led is lit. The residual noise may be ob-
served with a scope connected
to SCOPE jack. The OUTPUT SELECT switch
is then used to route either Channel A or Channel B to the scope.
3-4
STEREO MEASUREMENTS
1. Left and Right Channel Modulation
Depress DISPLAY SELECT LEFT-RIGHT pushbutton and place METER MOD/
HOLD/AUTO s~titches in MOD position. CHANNEL A meter will indicate
left channel modulation and CHANNEL Bmeter will indicate right channel
modulation with
FCC specified semi-peak ball istics .
A
fully modulated left or right stereo signal will indicate 90 %.
If pilot is removed, both meters will indicate the same as the
TOTAL MODULATION meter.
If MOD/HOLD/AUTO switches are placed in HOLD position, the ballistics change
to semi - VU and the RANGE (- dB) readouts will illuminate. If the METER DE EMP
pushbutton is then depressed, the meter will give an approximate indication
of the loudness perceived
at the output of a receiver.
2. Separation
Depress DISPLAY SELECT LEFT-RIGHT pushbutton and place METER MOD/HOLD/
AUTO switches in AUTO position. Apply a fully modulated left or right signal
to the transmitter. The fully modulated channel should read 90 %(-ldB). The
unmodulated channel will autorange to give the separation reading. (Algebraic
sum of the METER RANGE {-dB) and its associated meter -ldB.) NOTE: Be sure
of monitor 0 calibration before attempting separation measurements. See 3.4.7.
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3. Left and Right Signal to Noise
Depress DISPLAY SELECT L EFT-RIGHT pushbutton and p la ce METER MOD/
HOLD/AUTO sw it ches in AUTO position. Remove l eft and right modulation
from transmitter but do not remove pilot. Depress METER DE EMP push-
buttons. Meters will autorange
to give signal to noise readings.
4. Left
and Right Distortion
High impedance deemphasized aud io is available at the LEFT/RIGHT
AUDIO jack for connection to a distortion test set. The signal is
switched between l eft and right by the OUTPUT SELECT switch.
5.
Main Channel (L=+R) and Subchannel (L=-R) Modulation
Depress
DISPLAY SELECT MAIN-SUB pushbutton and place METER MOD/HOLD/
AUTO switches in MOD position. A ful ly modu lated L=+R signal will read
90 % on CHANNEL Ameter and O on CHANNEL B meter. A fully modulated L =-R
signal will read 90 % on CHANNEL Bmeter and O on CHANNEL A meter.
6. Crosstalk
(Main in to Sub and Sub into Main)
Depress DISPLAY SELECT MAIN-SUB pushbutton and place ME T ER MOD/HOLD/
AUTO switches in AUTO position. A fu ll y modulated L=+R or L=-R signal will
indicate
90 % (-ldB) on its respective meter. The other meter will autorange
to give the crosstalk reading. (A lgebraic sum of METER RANGE (-dB) and its
associated meter -ldB.)
7.
Pilot Injection and Monitor Phase Calibration
Depress
DISPLAY SELECT PILOT -0 CAL pushbutton and place METER
MOD/HOLD/AUTO switches in MOD position. Pilot injection is then read on
the PILOT scale of CHANNEL Ameter. PILOT PRESENT l amp will light at
approximately 3% pilot injection. Monitor Phase Ca libration is accomplished
by setting PILOT0 CAL control for a null on CHANNEL Bmeter. Phase Calibra-
tion may be accomplished accurately while transmitter is being modulated .
8.
38KHz Suppression
Depress
DISPLAY SELECT 38KHz - GEN 0 pushbutton and place METER MOD/
HOLD/AUTO switches in AUTO position. Modulate the transmitter with a L=-R
signal between SKHz and lSKHz. CHANNEL A meter will autorange to give
38KHz suppression reading.
9. Stereo Generator
Pilot Phase
Check monitor phase calibration according to 3.4.7. Set monitor and
modulation as in 3-4-8 above. Adjust stereo generator phase control for
minimum indication on CHANNEL B meter. This feature al l ows setting genera-
tor phase without an oscilloscope.
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3-5 SCA MEASUREMENTS
NOTE
This monitor is designed and approved only
for use with a
67KHz SCA and 10 % SCA injection
as used with stereo.
1.
SCA Injection (Unmodulated)
Depress DISPLAY SELECT SCA INJ NARROW-MAIN pushbutton and place METER
MOO/HOLD/AUTO switches in MOD position. CHANNEL Ameter w ill read SCA in-
jection on the PILOT scale through an FCC specified narrow filter. The SCA
must be unmodulated for an accurate readin g. The SCA PRESEN T lamp will light
at approximately 3% inject i on. Main channel O - 15 KHz modulati on will be dis-
played on the CHANNEL Bmeter.
2.
SCA Injection (Modulated)
Depress DISPLAY SELECT SCA INJ WIDE-SCA MOD pushbutton and place
METER MOD/HOLD/AUTO switches in MOD position. CHANNEL Ameter will read SCA
Injection on the PI L OT scale through a wide filter. This filter allows in-
jection to be monitored while modulating the SCA .
3.
SCA Modu lation
Set monitor
up as in 3-5-2 above. Place SCA DEV switch in 4KHz or
6KHz pos ition as desired. This sets the deviation f or 1 00 %. CHANNEL B
meter will indicate
SCA Modu lation with FCC semi - peak ballistics.
4. SCA Distortion
High impedance 150usec deemphasized SCA audio is available at the
SCA AUDIO jack for connection to a distortion test set.
5. SCA Signal to Noise
Depress DISPLAY SELECT SCA INJ WIDE-SCA MOD pushbutton and place
CHANNEL B METER MOD/HOLD/AUTO switch in AUTO position. Modulated the
transmitter with a
unmodulated SCA. Depress CHANNEL B METER DE EMP push-
button.
Meter will autorange to give noise reading. Reading will be made
with 150usec deemphasis.
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4-1 GENERAL
SECTION 4
THEORY OF OPERATION
The monitor is basically a speciali zed FM superhet receiver. It has
been specifically designed to operate in a high RF environment and as such
does not have an RF preamplifier. It has also been designed with a wide
IF filter in order to meet FCC requirements for FM modulation monitors .
Because of these restraints it is neither as sensitive nor as selective as
a
norma 1 FM receiver. However, it wi 11 operate in close proximity to antenna
farms and master antennas without the intermodulation and overload problems
normally encountered in these areas.
Unlike other monitors/RF amplifier systems, the si
gnal path in the
691 is the same whether the monitor is driven from the transmitter or off-
air. The only difference is a - 40d8 signal sampler when driven from the
transmitter input
(RF IN) port. This means that there is no signal degra-
dation
when used off-air above the mute level. This le vel is set for use
within the 70d8u contour with the antenna provided.
Refer
to Block Diagram 6912001. RF signal is fed to either the Antenna
or RF inputs. The signal then passes through the voltage tuned image rejec-
tion board (Al3) then to the A4 receiver assembly. The output of the phase
locked 1st Local Osci l lator is also fed to the A4 assembly producing a 10.7MHz
IF. The L.O. phase lock loop circuitry is located on the A2 assembly. The
10.7MHz IF is filtered, demodulated, and the resulting composite signal is
amplified and fed to the various filtering, stereo and SCA demodulation and
metering circuitry required. A portion of the 10.7MHz IF is also fed to a
mixer along with a swept
2nd L.O. operating at 10.255MHz + 120KHz. The re-
sulting 455KHz + 120KHz is filtered by a ceramic filter, fed to a logarithmic
amplifier, demodulated
and used to provide a spectrum analyzer presentation
on an X-Yoscilloscope.
4-2
DETAIL CIRCUIT DESCRIPTION
1. Al (Chassis) Assembly
The chassis assembly contains the power supplies (+12V, -12V, +5V),
filter, 1st L.O., meters, indicators and controls. The power supplies are
regulated
by three terminal regulators.
2.
A2 (PLL and Total Meter Driver) Assembly
The output of a high stability crystal reference oscillator (Yl, QlO,
Qll) is divided down to 5KHz. A samp le of the 1st L.O. is fed to Q9 and
then to a progranlllTiable divider. The CARRIER FREQUENCY thumbwheel programs
the divider so that when the L.O. is 10.7MHz above the thumbwheel setting,
the divider output is 5KHz. The reference signal and the programmable sig-
nal are compared in phase and the resultant error voltage is filtered by
a ctive filter Ul5 and Ul6 and then fed to the AFC port of the 1st L.O. Ul2
is used to light the L .O. UNLOCK led if l ock or reference is lost. Ull is
used to generate the 32KHz pulse for the calibrate function.

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U17 and UlS and the Total PEAK MOD thumbwheel fonn a precision positive
and negative reference for the modulation peak comparators U20. RS3 and RS6
provide reference for the 100 % PEAKS PER MINUTE comparators. Composite signal
from the A4 assembly is also fed to these comparators.
The outputs of these comparators are fed to one shots of 2 to 4 seconds
in the case of PEAK MOD and 5 milliseconds in the case of the 100 % PEAKS PER
MINUTE. The 100 % PPM one shot drives a counter made up of U22, U23 and U24.
This counter is reset every minute by a pulse derived from the A3 assembly.
Composite signal is also fed through the TOTAL METER POS and NEG
switches to the total meter driver circuit, U13, U14 and associated circuitry.
3. A3 (Dual Average Voltmeter) Assembly
This board contains two identical voltmeter circuits, one for Channel A
Display,
one for Channel B Display plus the reset generator (UlO) and decoder
driver (Ull, Ul2, U13) for the 100 % PEAKS PER MINUTE display.
Channel A designators are in units; Channel B designators are in hundreds .
Al and Ul along with their associated PEAK thumbwheel are used to drive the
TEST DISPLAY PEAK led. Signal components sslected by the DISPLAY SELECT push-
buttons
and the METER+/ -switch are amplified or inverted by U2 and fed to a
combination attenuator-amplifier. This combination provides gain in
lOdB
steps from O dB to 50 dB. The action is as follows:
RANGE
ATTEN
AMP
OdB
-30dB
+30dB
-lOdB
-20dB
+30dB
- 20dB
- lOdB
+30d8
-30dB OdB +30dB
- 40dB
-lOdB
+SOdB
- 50dB
OdB +50dB
The range changes are accomplished by the outputs of up-down counter
U6 being used to clamp the appropriate resistors to ground through U3. A
sample of the attenuator-amplifier output is fed to comparator US. The
output of US in conjunction with gates US, U6, and U7 then drives U6 up or
do~n to the required range.
The output of the attenuator-amplifier is then peak detected by Q7
and fed to meter driver U9 . Q9 is used to change ballistics from FCC to
VU as required.
4.
A4 (Receiver) Assembly
Ql and associated circuitry buffer and filter the output of the 1st
L.O. (fc + 10.7MHz). This signal and the incoming RF are mixed in balanced

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mixer Zl to produce the 10.7MHz IF. This IF si gnal is amplified by Ul,
filtered and applied to U2 which is an FM IF Amplifier and Demodulator
System. The demodulated composite output of U2 is amplified by Q4 and
Q5 and filtered by a lo w overshoot, phase compensated low pass filter.
Either the output of this filter or the composite input from the rear
panel is
supp lied to composite driver amplifier QB, Q9, QlO, Qll and
Q12. The output of this amplifier is 6Vpp@ 100 % and is used to drive
the
stereo and SCA monitoring circuits.
Q6 and filter C54, L16 and C55 convert a 32KHz pu lse from the A2
assembly to a 32KHz sine wave which is used to FM a 10.7MHz oscillator
(Q7 and associated circuitry) when the CAL pushbutton is depressed. This
si
gna l is coupled to the IF amplifier and used to provide the monitor
calibrate signal.
The RF signal level voltage at U2 pin 13 is fed to vo lt age comparator
circuit U3. The outputs of this IC are used to mute the monitor and also
drive the
RF LEVEL L.O. and OK leds .
The 10.7MHz signal from the IF filter FLI is also fed to mixer 22
through buffer Q2. There it is mixed with the output of 2nd L.O. Q13
and Q14. Th is L .O. is frequency swept by a sawtooth generator made up
of CR14, Q16 and U6. The output of the sawtooth generator is also used
to sweep the 1st L.O. in SPECTRUM ANALYZER WIDE mode through Ql5. The
455KHz output of mixer 22 is fed through buffer Q17 and Ql8 to the
spectrum analyzer
filter and logarithmic ampl i fier U4 and associated
circuitry. The output of the log amp is AM demodulated and fed through
active
filter U5 to the SCOPE VERT jack and the AM Noise measurement
circuit.
5. AS (Stereo Demodulator) Assembly
Composite signal is fed to Ql which drives the pilot filter. The
filtered 19KHz is squared by U3 after passing through the phase adjust
circuit composed of Q3, Q4 and the front panel PILOT 0 CAL control . The
19KHz square wave is used to l ock PLL U4. The output of U4 is nominally
1
90KHz. This signal is divided down to provide square waves of 38KHz@
o0, 38KHz@ 1800
1
38KHz@ 900 and 19KHz@ 900 . These signals are used to
demodulated
left and right and also to provide GEN 0 calibration and PILOT
0 calibration signals. Stereo demodulation is accomplished by choppers
QlO and Qll and the resulting left and right signals are amp lif ied by Ql2
through Ql7. The output of these amplifiers is fi lt ered and used to feed
th~
DISPLAY SELECT switch and also audio output amplifiers UlO, Ul3 and
Q19 through Q22. Left and right audio is added in Ull to produce the
vertical channel for the vector displ ay and subtracted in U12 to produce
the horizontal channel for the vector display.
Choppers Q7 and QB are fed a 19KHz square wave at 90° along with
composite
signal. The action of these choppers and U2 produces the
PILOT 0 null indication.

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Transistor Q18 is used to couple 38KHz@ 90° into the Subchannel
filter to produce the GEN 0 indication.
CRl, CR2, Ul and Q6 provide the Pilot Present Lamp threshold detect
functions.
6.
A6 (SCA) Assembly
The Composite signal is buffered by Ql and fed to the dual 67KHz
filter. One section of this filter has a bandwidth approximately lKHz
and fulfills the FCC specification for exact measurement of SCA injection.
The output of this filter section is buffered by Q2 and fed to the DISPLAY
SELECT switch. The other section of this filter has a wide bandwidth to
allow
injection measurements under modulation and also to prevent unwanted
composite signal components from reaching the SCA demodulator. U2 is a
phase locked loop demodulator.
The audio output is buffered by Q6, filtered
and amplified by Ul and U4 . Q·3 changes the 100 %deviation point from 4KHz
to 6KHz depending upon the front panel SCA DEV switch.
U3 detennines SCA presence and also drives the SCA frequency output.
Q7 mutes SCA audio if the SCA is lost. Q8 and Q9 drive the SCA PRESENT
lamp.
7. All (Display Select Switch) Assembly
This seven station pushbutton selects the signal component to be
displayed. U2 and associated circuitry fonn a high pass active filter
with a low frequency cutoff of 50Hz used to feed the FM S/N position.
Ll is a 19KHz filter that smoothes the PILOT 0 CAL null indication.
-...

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SECTION 5
ALIGNMENT AND TROUBLESHOOTING
5-1
EQUIPMENT REQUIRED BUT NOT SUPPLIED
1.
Oscilloscope, DC coupled, triggered,
min. 10 MHz BW.
dual trace,
horizontal input
2. Digital Voltmeter
3.
Audio Generator
4. Frequency Counter
5. Stereo Generator
6.
SCA Generator ..
7.
FM Signal Generator with Calibrated
RF Al tern · ator
8.
Distortion Analyzer
9. Assorted Cables
and hand tool s.
NOTE: DO NOT ATTEMPT TROUBLESHOOTING AND ALIGNMENT OF THIS UNIT WITHOUT
ADEQUATE TOOLS AND TEST EQUIPMENT.
Before starting alignment of this unit, verify that the three supply
voltages are present
and correct.
5-2 PLL AND TOTAL METER ALIGNMENT (A2 Assembly)
1.
2.
Place MOD PEAK Thumbwheel (AISS) to 100.
+
Measure voltage at A2U17-6 (TPl), this voltage should be +3.00V - .15V
3. Measure voltage at A2U1 8-6 (TP2). Adjust A2R69 until this voltage
has the s~me magnitude but opposite polarity of step 5-2 (2).
(-3.00V -,15V).
4.
Measure voltage on center arm of A2R83. _· Adjust A2R83 until its
voltage is 30mv high~r than that in step 5-2 (2) (+3.03V).
5.
Measure voltage on center arm of A2R86. Adjust A2R86 until its
voltage is 30mv lower than that in step 5-2 (3) (-3. 03V).
6. Depress NEG button on AIS4A.
7. Adjust A2Rp5 · until TOTAL MOD meter (AIM!) is zeroed.
\ \0
8. Apply a 400 Hz, 4Vp-p sine wave to COMP IN (AIJ13).

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9. Place INT-EXT switch (A1S16) in EXT position .
10. Adjust
COMP LEVEL (A1R4) until MOD PEAk lamp (AlDSl) just
lights at 100 %.
11.
Turn PPM readout switch (A1S6) to on. PPM readout should
read "00". Set
MOD PEAK thumbvtheel to 101 %. Increase audio
slightly, PPM readout should count to "19" before MOD PEAK
lamp lights at 101 %.
12. Set audio level
until MOD PEAK lamp just lights at 100 %.
13. Adjust
A2R99 until TOTAL MOD meter (AlMl) reads 100 %.
14. Depress POS button on A1S4A .
15. Adjust A2R104 until TOTAL MOD meter (AlMl) reads 100 %.
......
16. Connect a frequency counter to X.O. OUT (AlJ12).
17 . Adjust A2Cl6 until the frequency is exactly a:000000 MHz.
NOTE: This will set the L.O. exactly 10.7 MHz above the
indication on the CARRIER FREQ . thumbwheel (A1S2).
NOTE: The crystal used in the 691 Modulation Monitor is a
high a0curacy
crystal with stability of 2.5 PPM from
0 - 60 C.
5-3 DUAL AVERAGE VOLTMETER ALIGNMENT (A3 Assembly)
1. Calibrate PLL and Total Meter (Section 5-2)
2. Preliminary
set up:
a. Place thumbwheels AlS12A and A1S12B to 100 %.
.
b. Place+,- switches (A1Sl3A and AlS138) in - position.
c. Place MOD-HOLD-AUTO switches (A1S14A and A1Sl48) in MOD Position
d. Depress
MAIN, SUB button on DISPLAY SELECT switch (AlSll).
3. Remove audio input from the monitor.
4. Adjust
A3R74 until CH.A meter (AlM2A) zeroes.
5. Adjust
A3Rl74 until CH .B meter (AlM2B) zeroes.
6.
Apply 400 Hz audio input until MOD PEAK lamp (AlDSl) just
l ights at 100 %.
7. Adjust
MAIN filter control (AlR15) until CH.A PEAK l ed just lights.

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8. Adjust A3R72 until CH.A meter (A1M2A) reads 100 %.
9.
Place+, -switch (A1S13A) in+ position.
10 . Adjust A3R25 until CH.A meter (A1M2A) reads 100 %.
11. .Place
MOD-HOLD - AUTO switch (A1S14A) in AUTO position.
12. Reduce input level until CH.A meter auto-ranges. AVM display
(AlDSSA) will read 10. Increase input level until CH.A meter
auto-ranges up.
AVM display (AlDSSA) wil l read 00. Adjust
A3R60 until CH.A meter (A1M2A) auto-ranges at approximately
-
15 dB and +1.5 dB .
13. Increase input frequency
to 40 KHz. Adj u st input level
until MOD PEAK light (~lBSl) just l ights.
14 . Adjust SUB filter control (AlR13) until CH.B PEAK l ed just
li ghts.
15 . Adjust A3Rl72 until CH.B meter (AlM2B) reads 1 00 %.
16. Plade +
,- switch (A 1 S13B) i n+ positio n.
17. Adjust
A3R125 u ntil C H.B meter (A 1 M2B) reads 1 00 %.
18. Place
MOD - HOLD - AUTO switch (A1Sl4B) in AUTO p osition.
;19. Repeat step 12 of this section . Adjust A3R160 for auto r ange
set point.
20 . Set input frequency to exactly 38000 Hz. Adjust input level
until MOD PEAK light (AlDSl) just l ights.
21 . Depress 38K , GEN0 button on DISPLAY SELECT switch (AlSll ).
22. Adjust 38K filter control (A 1 R14) until CH .A PEAK led just
light~ at 100 %. CH.A meter should read 100 %.
5-4
STEREO DEMODULATOR ALIGNMENT (AS Assemb ly)
1.
Co m plete alignment of A2 assembly (Section 5-2) and A3 assembly
(Section 5-3
).
2. Apply an input signal of 19000 Hz. Adjust input level u ntil
MOD PEAK lamp just l ights at 100 %.
3.
Reduce input level by exactly 20 d B.
4. Depress PILOT LEVEL , ~ CAL button on DISPLAY SELECT sw itch (AlSll ).
5. Adjus t ASR4 u ntil CH.A me t er reads 100 %(10% on pil ot scale) .