Page 227 - Op Amps Design, Applications, and Troubleshooting

Troubleshooting Tips for Oscillator Circuits 209

4.6 TROUBLESHOOTING UPS FOR OSCILLATOR CIRCUITS

The problems with op amp oscillator circuits generally fall into one of three cate-
gories:

1, Completely inoperative (i.e., no output signal)
2, Distorted output waveform
3, Incorrect frequency of oscillation

In many cases, items 2 and 3 occur simultaneously. As always, your best trouble-
shooting tool is your basic electronics theory and your complete understanding of
correct circuit operation.

Oscillator Completely Inoperative. The first thing to check if there is no
output signal is the power supply voltages. A quick check directly on the ±F CC
pins of the op amp will reveal or eliminate this potential problem. When checking
for missing voltages, be certain to measure directly on the pin of the op amp. If
you measure at some other point, you may fail to detect a poor solder joint, a bro-
ken printed circuit trace, and so on.
If the supplies are proper but the oscillator has no output (and assuming it is
correctly designed to oscillate), then measure the DC level of the output pin of the
op amp. Some oscillators (e.g., Wien-bridge) use AC coupling for some of the feed-
back. If the output is near 0 volts DC (i.e., not driven to either saturation level),
suspect an open in the AC feedback path. If the output is driven to V SAT, the circuit
has a EC problem.
If the circuit is found to have a DC problem and the output is at one of the
saturation levels, note the polarity of the output voltage and then measure the two
input pins. Ask yourself if the polarity of the input pins would cause the output to
be at the present saturation limit. If the answer is no, then the op amp is probably
defective.
If the input polarity would indeed cause the present output polarity, then
mentally examine the circuit paths to determine what signal is supposed to cause
the input polarity to change. That is, in order for the output to change (e.g., oscil-
late), the input pins must have a change. Further, since this is an oscillator, the
changing input signal originates at the output So, if the changing input signal is
missing, trace the path between input and output and determine where the signal
is lost,
A useful technique in some cases is to force the oscillator (or input) to a given
state and monitor the effects elsewhere in the circuit. If the op amp you are using
is short-circuit protected, you can directly short the output to ground momentar-
ily while observing the input pins. Ask yourself if the results agree with the
behavior of a properly connected circuit.

Distorted Output Waveform. If the oscillator being analyzed is a new
design, one common cause for output distortion is an improperly selected op
amp. More specifically, if the slew rate of the op amp is not sufficiently high rela-
tive to the demands of the oscillator, the output will be distorted.