Harris Gates One 994 9202 002, Technical Manual

Page: 83 / 110

SECTION F
PDM GENERATOR (A15)
F.1.
Principles Of Operation
The purpose of the PDM Generator board
is to produce the pulse width modulated
signals which set up the carrier power and
to convert the incoming audio to a PDM
signal to modulate the transmitter.
The audio input circuitry starting at pins
J1-5 and J1-7 is a balanced input terminated
in 600 ohms with RFI filtering. Diodes
CR1, CR2, CR3 and CR4 act as clamps to
prevent the audio signals from going more
than +/-15 volts.
Operational amps whose outputs are U1-
7, U1-1 and U1-8 consist of a solid state
transformer which converts the incoming
balanced audio to a single ended signal at
pin U1-8. R21 adjusts the audio input gain
continuously from -10 dBm to +10 dBm.
From there the audio may pass through a
Bessel low pass filter through op amps U2-7
and U2-1. Switch 1 determines whether the
Bessel filter is in the audio circuitry path or
not.
This Bessel filter is switchable and just
slightly rolls off the high frequency audio
to prevent excessive overshoot of the PDM
filter during square wave modulation.
Past S1, multiplier U15 acts as a modula-
tion tracking amplifier and adds the dc from
R105 and R107, and the audio through R31
into the power output control U12.
The analog power control signal from the
Controller comes in on J1-3, and is ampli-
fied by U12. This IC outputs a dc power
control signal with audio level tracking up
and down with it. U12 provides this signal
to the output comparators of the PDM gen-
erator. R111 is adjusted to give 0 pulse
width when the analog power signal is at
minimum.
The triangle wave circuitry of the PDM
generator begins with U11, a 240 kHz os-
cillator whose output is a 5 volt peak-to-
peak square wave. This signal is sent to
divider U4. It divides the 240 kHz by 4 and
produces two 60 kHz square waves whose
phase relationship is 90 degrees apart.
These two square waves drive the Op Amp
integrators, consisting of U7 and U8.
The integrators produce triangle waves
which are connected to the output compa-
rators U9 and U10. These comparators de-
velop the four phases of PDM signals using
the two phases of dc/audio and the two
phases of triangle waves. Each comparator
IC produces outputs which are 180 degrees
out of phase from each other. This occurs
because the two polarities of audio cause
comparator action at different halves of the
triangle wave cycle.
Since one trangle wave is 90 degrees out
of phase to the other, four phases of PDM
are developed.
The 4 PDM signals leave the PDM Gen-
erator board on J2, and go to the PDM
Amplifiers.
The outputs of comparators U9 and U10
are open collector outputs. The supply for
the open collector outputs is derived from
the emitter follower pair of Q4 and Q5. A
HIGH input (about 5 volts) is supplied to
J1-1 by the Controller board which will
cause Q3 to conduct. This turns off Q6 and
allows its collector to rise to +15 volts and
the emitter followers Q4 and Q5 will apply
about 14 volts to the output circuit.
During a PDM Interrupt a logic 0 is ap-
plied to J1-1 by the Controller board and the
process above reverses. Zero volts is ap-
plied to the output circuit. Diodes CR20-
CR23 are used to speed up the discharge of
the capacitance of the coaxial cables from
the PDM Generator to the PDM Amplifiers.
The +15 volt power supply starts with an
RFI filter, L1 and C85. This filter rejects any
RF energy that would be coupled in via the
20 volt supply on J1 pin 17 and 19. Diode
CR14 prevents the input of voltage regula-
tor U3 from going negative. CR16 on the
output of U3 acts as a transient suppressor
and prevents the output of the voltage regu-
lator from going above approximately 15
volts. Diode CR15 protects U3 if there
should be a short on the input. LED DS1
illuminates to indicate that the 15 volt sup-
ply is basically working.
This same discussion applies to the nega-
tive 15 volt supply except all the voltage
polarities are reversed.
F.2.
Replacement/Alignment
If you are replacing the PDM Generator
board, or checking its adjustment, the fol-
lowing procedures should be used.
Set one of the front panel Power Level
pots fully counter clockwise.
Connect an oscilloscope to an output of
the PDM Generator at the right side of R45.
This is the same point as the anode of CR23.
Turn the transmitter on at the power level
that is set to zero.
Adjust R111 (zero power adjust) so that
there is only a very narrow width pulse at
the output of the PDM Generator. Then
adjust R111 clockwise until the pulse just
disappears. There should be no PA current,
PA voltage or power output is observed on
the front panel meters.
Operate the transmitter at a normal power
level with the normal amount of audio ap-
plied. Adjust R21 for the desired amount of
modulation.
F.3.
Troubleshooting PDM Generator
F.3.1.
Symptom: No Pulses At J4, Caus-
ing Zero Power Output from the
Transmitter
Ensure that the REMOTE/LOCAL switch
located on the Controller board is set to the
LOCAL position before performing the
checks outlined below. RF Drive and PDM
are turned off when this switch is in RE-
MOTE position and transmitter is turned
off. After performing these checks, set the
switch back to the REMOTE position.
Possible causes:
F.3.1.1.
Power Setting
Check the PDM Power Level reading on
the Multimeter. An up scale reading should
produce pulses out of the PDM Generator.
F.3.1.2.
Loss of Plus and Minus 15 Volts
Observe the +/-15 volt LEDS (DS1 and
DS2) on the PDM Generator. These should
be lit whenever low voltage is applied.
Measure the +15 volts at the cathode of
CR16 and -15 volts at the anode of CR19.
F.3.1.3.
PDM Interrupt
Check the voltage at the emitter of Q4/Q5
on the PDM Generator. It should be about
15 volts. If it is near zero volts, check for +5
volts at J1-1. This is accessible on R100.
The voltage at J1-1 is about 5 volts for
normal operation and near zero during a
PDM Interrupt. Q5 and/or Q6 may be at
fault if 5 volts is present at J1-1, but no +15
volts is present at the emitter of Q4/Q5. A
continuous PDM Interrupt signal (J1-1 low)
would be the fault of circuitry on the Con-
troller board.
F.3.1.4.
DC/Audio Failure
Measure the voltage at DIP resistor R17
pin 2 and U1-14. Normally this will be 1 to
2 volts depending on PDM level.
Loss Of Triangle Waves. Using an oscil-
loscope, check at U9-9 and U10-9 for a
3.6-4.0 volts peak to peak 60 kHz triangle
wave. If no triangle waves exist at U9 and
U10, check for square waves at U4 pins 2
and 12.
If the square waves exist at U4 pins 2 and
12, but no triangle waves are on U 9 and
Rev.AE: 03-05-2002 888-2314-001 F-1
WARNING: Disconnect primary power prior to servicing.