TROUBLESHOOTING AND REPAIR
INTERNAL MODULATION OSCILLATOR
MOD OSCILLATOR TROUBLESHOOTING AND ADJUSTMENTS 6F-6.
Since both the direct digital synthesizer (DDS) and the pulse generator sections are
clocked by the same clock, the first signal to verify is the input 20 MHz. In the absence
of this clock, no function on the assembly will operate. The amplitude of this wave
should be at least 300 mV p-p.
Direct Digital Synthesizer Troubleshooting 6F-7.
To troubleshoot the direct digital synthesizer, proceed as follows:
SETTING UP:
1. Put the UUT into the preset default state by selecting SPCL 909. This sets the DDS
to generate a sine wave.
2. Enable INT AM modulation.
3. Set MOD LEV to a modulation output level of 4V pk.
4. Enter MOD FREQ of 1 kHz.
5. Connect a 600-ohm load at the MOD OUTPUT connector.
TEST PROCEDURE:
1. Check U1 output clock CLKO at TP10. This logic level signal should be at 3.33
MHz. If there is no signal at this point or the frequency is wrong, either U1 is faulty,
wrong data is written to it, or the 20 MHz signal is inadequate. With the absence of
this signal the DDS sections will not operate.
2. Using an oscilloscope, verify that the most significant bit (MSB) of the phase
accumulator (TP8) is at TTL level and is at 1 kHz (the set modulation frequency).
If the MSB is not as indicated, it is probable that the most significant lookup table
(U2) is faulty. Another possibility is that U1 phase accumulator section does not
function correctly.
3. Next use the oscilloscope to verify the presence of a 10V p-p (±5%) sine wave at the
output of the wave reconstruction DAC (TP2). If not, suspect the DAC (U7) and
the DAC output amplifier (U9A), or the wave tables (U2, U3). If the signal is not
zero centered or the amplitude is in error, check R4 and R5, also verify 10V ±2% at
pin 4 of
(U7).
4. Verify that a 4.77V p-p ±5% sine wave is present at TP3. If the sine wave is not
present or is of the wrong amplitude, check the 3-pole low-pass filter components
(R6, C13, C14,
L1 and
R7).
5. Enter MOD FREQ of 100 kHz.
6. Repeat step 4 above.
7. Enter MOD FREQ of 1 kHz.
6F-4
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