UBS Axcera 325A User Manual

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures

Chapter 5

Detaile d Alignment Procedures

The 325A transmitter was aligned at the
factory and should not require additional
alignments to achieve normal operation.

This transmitter operates using the
baseband audio and video inputs or, if
the (optional) 4.5-MHz composite input
kit is purchased, either a single
composite video + 4.5-MHz input or
separate baseband video and audio
inputs.

Check that the RF output at J2 of (A9-A5)
the coupler is term inated into a dummy
load of at least 500 watts. While
performing the alignment, re fer to the
Test Data She e t for the transmitter and
compare the final readings from the
factory with the readings on each of the
trays. They should be very simila r. If a
reading is off by a significant amount, the
problem is likely to be in that tray.

Switch on the main AC and the VHF
exciter circuit breakers on the AC
distribution panel behind the rear cabinet
door.

5.1 (A4) VHF Low-Band Exciter Tray
(1070820; Appendix C) with
Baseband Video and Audio Inputs

The (A4) low-band VHF exciter tray
(1070820) has adj us t me nts for v id eo
levels, audio modulation levels, and other
related parameters.

Connect an NTSC baseband video test
signal input (1 Vpk-pk) to the transmitter
video input jack J2 on the (A12) remote
interface panel. Jacks J1 and J2 on the
VHF exciter tray are loop-through
connected and the unused jack can be
used as a video source for another
transmitter by removing jumper W4 on
jack J3 on (A5) the sync tip clamp
modulator board (1265-1302). Connect a
baseband audio input (+10 dBm) to the
balanced audio input terminal block TB1-

1 (+), TB1-2 (-), and TB1-3 (ground). If
stereo /composite audio is provided,
connect it to BNC jack J6, the composite
audio input jack on the remote interface
panel. Jacks J3 and J13 on the rear of the
exciter panel are loop-through connected
and the unused jack can be used as an
audio source for another transmitter by
removing jumper W1 on jack J15 on the
aural IF synthesizer.

Look at the front panel meter on the VHF
exciter tray. In the Video position, the
meter indicates active video from 0 to 1
Vpk-pk. The normal video input leve l is 1
Vpk-pk on the meter. If t his reading is
not at the proper level, the overall video
level can be changed by adjusting video
level control R12 on the syn c tip c lamp/
modulator board.

Switch the meter to the Audio position to
show the audio deviation (modulation
level) of the signal from 0 to 100 kHz.
The aural IF synthesizer board was
factory set for a ±25 kHz deviation with a
balanced audio input of +10 dBm. If the
reading is at not the correct level, adjust
balanced audio gain pot R13 on the aural
IF synthesizer board, as needed, to attain
the ±25 kHz deviation. The aural IF
synthesizer board was factory set for a
±75 kHz deviation with a composite
audio input of 1 Vpk-pk. If this reading is
not correct, adjust composite audio gain
pot R17 on the aural IF synthesizer
board, as needed, for the ±75 kHz
deviation.

5.2 (A4) VHF low-Band Exciter Tray
(1070820; Appendix C) with the 4.5­MHz Composite Input Kit

With the 4.5-MHz composite input kit,
the (A4) VHF ex citer tray is ab le to
operate using either the separate video
and audio baseband inputs or the single

4.5-MHz composite input.

325A, Rev. 0 5-1

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures

The 4.5-MHz composite input kit includes
a composite 4.5-MHz filter board (1227-

1244) and a 4.5-MHz bandpass filter
board (1265-1307).

To align the VHF exciter using baseband
video and audio, refer to the alignment
instructions described in Section 5.1 of
this chapte r. Se lect the baseb and inpu t
operation by a applying a baseband
select, using a jumper or closed contacts,
connected between J7-6 and J7-7 on the
rear of the tray.

To operate the transmitter using the 4.5­MHz composite input, remove the
baseband select command from J7-6 and
J7-7 on the rear of the tray.

Connect a multiburst test signal from an
envelope delay measurement set to the
input of the rear interface panel at J2. On
(A24) the composite 4.5-MHz filter board
(1227-1244), c onnect an oscilloscope
between J7, the center pin, and pin 1 or
3, which are ground. Adjust C21, if
necessary, for the best frequency
response. Adjust R32 for a signal level of
1 Vpk-pk on the oscilloscope. The output,
as measur ed at J6 and J7 of the board,
should be video only with a minimum

4.5-M H z au ra l subcarrie r.
On the (A25) 4.5-MHz bandpass filter

board (1265-1307), adjust the filter with
L2, C3, L4, and C7 for a frequency
response o f no greater than ±.3 dB from

4.4 to 4.6 MHz. Adj ust C 19 for an overall
peak-to-peak variation of less than ±.3
dB from 4. 4 MHz to 4.6 MH z. Rech ec k
the freque nc y res p ons e; it ma y have
changed with the adjustment o f the
envelope delay.

5.3 (A4) VHF Exciter Tray (1070820;
Appendix C) with either Baseband or
the 4.5-MHz Composite I nput

The IF section of the (A4) VH F exciter
tray includes adjustments for automatic
level control (ALC), linearity (amp litude
predistortion), and phase (phase change
vs. level) predistortion for correction of

the nonlinearities of the RF amplifier
trays. The upconverter section also
includes adjustments to the local
oscillator chain tuning and the local
oscillator center frequency tuning. Both
of these were completed at the factory
and should not require adjustments at
this time.

Move the Operate/Standby switch on the
VHF exciter tray to Standby. The setup of
the RF output includes an adjus tm e nt t o
the d rive level of the two V HF amplifier
trays, the adjustment of the linearity and
phase predistortion (which compensate
for any nonlinear responses of the
amplifier trays), and the gain and
phasing adjustments of the two VHF
amplifier trays.

Verify that all of the red LEDs on the ALC
board ar e extinguished. The following list
describes the meaning of each LED when
they are illuminated:

• DS1 (Input Fault) – Indicates that an
abnormally low or no IF is present at
the i nput of the board

• DS2 ( AL C Fault) – Indicates that the
ALC circuit is unable to maintain the
signal level requested by the ALC
reference. This is normally due to
excess ive atten ua tion in the linea r ity
signal path or the IF phase corrector
signal path or because jumper W3 on
J6 is in the Manual ALC Gain position.

• DS3 (Video Loss) – Indicates a loss of
video at the input of the board

• DS4 (Mute) – Indicates that a visual
Mute command is present (not used
in th is con f ig u ration)

• DS5 (Modulator Enable) – I ndicates
that the modulator IF output has
been selected (this is only used if a
receiver tra y is pres e nt i n the
system). DS5 is always on with no
receiver.

325A, Rev. 0 5-2

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures

The ALC is muted when the transmitter is
in Standby. To monitor the ALC, tur n off
the two amplifier on/off circuit breakers
on th e AC input as s em b l y in t he re ar of
the cabinet and switch the transmitter to
Operate. Adjust the power adjust gain
pot on the front panel of the VHF exciter
tray to obtain +0.8 VDC on the front
panel meter in the ALC position. On the
ALC board (1265-1305), move jumper
W3 on J6 to the Manual position,
between pins 2 and 3, and adjust R87 on
the ALC board for +0.8 VDC on the front
panel meter in the ALC position. Move
jumper W3 bac k to Auto (between pins 1
and 2); this is the normal operating
position. The detected IF signal level at
J19-2 of the ALC board is connected to
the transmitter control board that
distributes the level to the two VHF
amplifier trays where it is us ed as a
reference for the automatic gain control
(AGC) in each amplifi er tray.

5.4 IF Phase Corrector Adjustment

equipment to monito r the differential
phase or intermodulation products of the
RF output signal. There are three
corrector stages on the IF phase
corrector board, each with a magnitude
and a threshold adj us tment that are
adjusted, as needed, to correct for any
differential phase or intermodulation
problems. Adjust the R3 threshold for the
cut-in point of the correction and the R7
magnitude for the amount of the
correction that is needed.

Jumper W1 on J8 is set to give the
de sired polarity of the co rrection shape d
by the threshold R11 and the magnitude
R15 adjustments. After setting the
polarity, adjust the R11 threshold for the
cut-in point of the correction and the R15
magnitude for the amount of the
correc tion that is needed. Finally, adjust
the R19 threshold for the cut-in point of
the correction and the R23 magnitude for
the amount of the correction that is
needed.

As shipped, the exciter was preset to
include linearity (gain vs. level) and
phase (phase vs. level) predisto rtion. The
predistortion was adju sted to
approximately compensate the
corresponding non-linear distortions of
the amplif ier tra ys and shou ld not requ i re
additional adjustments.

Locate (A9) the IF phase corrector board
(1227-1250) mount ed in the VHF exciter.
The amplitude correction portion of the
bo ard is no t u t i li zed in th is co nf igur a t io n.
As a result, jumper W3 on J10 should be
in the Disable position, to +6.8 VDC, and
R35 and R31 should be fully counter­clo c kw is e ( CCW). R6 8 is the ra ng e
adjustment and should be set in the
middle of the r ange. T he phase
correction Enable/Disable jumper W2 on
J9 should be in the Enable position, to
ground.

Switch the input video test source to
select an NTSC 3.58-MHz modulated
staircase or ramp test waveform. Set up
the station demodulator and monitoring

Note: Adjusting these pots changes
all visual parameters and should be
done cautiously.

5.5 Linearity Corrector Adjustment

The IF linearity correction function
consists of three non-linear cascaded
stages, each having adjustable
magnitude and threshold, or cut-in
points, on the ALC board. The threshold
adjustment determines at what IF signa l
level the corresponding corrector stage
begins to increase gain. The magnitude
adjustment determines the amount of
gain change for the part of the signal that
exceeds the corresponding threshold
point. Ref er to the VHF exc iter tra y
control locations drawing, ALC board
(1265-1305), to find the adjustments for
the first through third linearity corrector
stages. Because the stages are cascaded,
the order of correction is important. The
first stage should cut in near white level,
with the cut-in point of the next stag e
toward black, and with the last stage
primarily stretching sync.

325A, Rev. 0 5-3

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures

To adjust the linearity correctors from
scratch, ensure that the transmitter is
operating at full power with the desired
A/V ratio. Check that jump er W1 on J4 of
the ALC board is enabled between pins 1
and 2. Make sure that the ALC voltage is
set to +0.8 VDC as monitored on the
front panel meter in the ALC position.

Insert a modulated ramp video test
signal into the transmitter. Demodulate
the output signal of the t ransmitter and
observe the waveform on a waveform
monitor while also looking at the signal
on a spectrum analyzer. On the IF ALC
board (1265-1306), preset pots R34,
R37, and R40 (t h res ho ld) f u lly C CW and
the magnitude adjustments R13, R18,
and R23 fully clockwise (CW). On the IF
phase corrector board (1227-1250),
preset pots R7, R15, R2 3, and R35 fully
CW and R3, R11, R19, and R31 fully
CCW.

Set the wavef orm monitor to differential
step filter and the volts/division scale to
.1 volts. Center the display to
ap p rox im a tely t he blank ing le v e l.

Gradually adjust pots R3, R11, and R19
clockwis e on the IF phase corrector
board, as needed, to minimize the
observed thickness of the
intermodulation as seen on the display.

Adjust pots R34, R37, and R40 clockwise
on the IF ALC board, as needed, to give
correction at sync or at low luminance
levels as viewed at the left-most edge of
the waveform monitor.

The intermodulation beat product s
between the colorburst and the aural
carrier at 920 kHz above visual carrier
should also be observed on the spectrum
analyzer while performing the preceding
adjustments . The frequency will vary for
different video systems . When the
adjustments are performed properly, the
intermodula tion products on the
spectrum analyzer should be at least -52
dB down, with a red field input, from
peak visual carrier. The intermodulation

distortion as displayed on the waveform
monitor should be no more than 1 IRE.
Pot R31 on the IF phase corrector boa rd
is used for any extra intermodulation
correction that may be needed.

Note: Any adjustments to the above
pots affects other visual parameters
and some slight adjustments of all of
the pots may be needed to meet all
specifi ca ti on s simu lta neously.

If the transmitter is being driven very
hard, it may not be possible to get
enough sync stretch while maintaining a
flat differential gain. In this case, some
video sync stretch may be used from the
sync tip clamp/modulator board; the
sync stretch adjustment is R48.

Switch the transmitter to Standby.

5.6 Phase and Gain Adjustment of

the VHF Amplifier Trays

The following procedure was completed
at the factory and should only be
followed if one of the VHF amplifier trays
is replaced.

Preset the phase and ga in potentiometer
on each VHF amplifier tray fully CCW.
Switch the transmitter to Operate and
adjust the gain pot on each tray for 25%
Output Po wer. Adjust the phase control
CW on the left VHF amplifier tray. If the
% Visual Output Power goes up, continue
to adjust the phase control until either
the peak is reached or the end-of-travel
is reached. If the % Output Power goes
down, res et the phase co ntrol on the VHF
amplifier tray fully CCW and repeat the
above procedure with the phase control
of the other amplifier tray.

If the end-of- trave l is reach ed on the
phase adjust, reset the phase cont rol
CCW and add a 2-inch length of cable to
the input of the affected VHF amplifier
tray at J1. Readjust the phase of that
tray until a peak is reached or until the
end-of-travel is achieved.

325A, Rev. 0 5-4

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures

If the end-of-travel is reached, repeat the
above procedure and replac e the 2-inch
length of cable with a 4-inch length of
cable. Once a peak has been reached,
move the phase control that is fully CCW
up two turns and repeak using the phase
control on the other tray. This allows
both trays to have some range of
adjustment.

Adjust the gain of both VHF amplifier
trays for 90% Tray Output Power.
Readjust each phase control to peak the
combined output; the phase should only
have been slightly affected. Although it
may take a few turns to notice a change,
there should be a definite peak that is
achieved while adjusting the phase of
each tray. Raise or low er the output
power of each tray to achieve 100%
Output Power. The output power of each
tray should be 90% to 100%.

5.7 Calibration of the Forward
Output Power Level of the
Transmitter

Note: Only perform the following
procedure if the power calibration is
suspect.

Switch the transmitter to Standby and
preset R51, the aural null pot on the
visual/a ura l metering board (1265-

1309) , fully CCW. Adjust R48, the null
offset pot on the visual/aural metering
board, for 0% Visual Output. Perform the
following adjustments with no aural
present. This is accomplished by
removing jumper cable W1, the aural IF
loop-through, that is connected to J16 on
(A5) the sync tip clamp/modulator board
(1265-1302). Connect a sync and black
test signal to the video input jac k of the
VHF exciter tray. Switch the transmitter
to Operate.

Set up the transmitter for the appropriate
average output power level: sync + black
0 IRE setup/wattmeter=298 watts; sync
+ black 7.5 IRE setup/wattmeter=273
watts.

Note: The transmitter must have 40
IRE units of sync.

Adjust R28, visual calibration, on (A19)
the visual/aural metering board (1265-

1309) for 100% on the front panel meter

in the % Visual Output position.
With the spectrum analyzer set to the

zero span mode, obtain a peak reference
on the screen. Reconnect jumper cable
W1 to J16 on (A5) the sync tip
clamp/modulator board. While in the
Visual Output Power position, adj ust L3
for a minimum visual power reading.
Turn the power adjust pot on the front
panel until the original peak reference
level is attained. Peak L1 and C8 for a
maximum aural power reading and then
also adjust R20 for a 100% Aural Power
reading. Switch the transmitter to the
Visual Output Power position and adjust
R51, th e au ra l nul l pot, fo r 100% V is ua l
Power.

5.8 Calibration of the Reflected

Output Level of the Transmitter

On the meter, in the Visual Power
position, turn the power adjust pot to
20%. Check that the jumper is in Manual
on the VHF filter/amplifier board
(1064251). Reverse the cables on A9-A5,
J3 and J4, and adjust R39 on the
visual/aural metering board (1265-1309)
for a 20% reading in the Reflected Power
position. At this 20% reference power
reading, the VSWR LED mounted on the
front panel of the exciter should be
illuminated. If this LED is not lit, adjust
R22 on the transmitter control board in
the VHF excit er tray unti l the VSWR LED
jus t turns on. Turn the power adjust pot
slightly CCW and the LED should go out.
Turn the pot CW until the LED just turns
on. The reflected output power is now
calibrated.

Switch the transmitter to Standby. Move
the cables on A9-A5, J3 and J4, to their
original positions.

325A, Rev. 0 5-5