Page 35 - Troubleshooting Analog Circuits

22 2. Choosing the Right Equipment

turn off the DVM, the function generator, the soldering iron, and finally even the
power supplies, but the oscillation is still there.
Now you start looking around the lab to see who has started a new oscillator or
switching regulator that is doubling as a medium-power transmitter. Aside from
yelling, “Who has a new circuit oscillating at 87 kHz?” what can you do to solve the
problem? One useful tool is an ordinary AM transistor radio. As we have all learned,
FM radios reject many kinds of noise, but AM radios scoop up noise at repetition
rates and frequencies that would surprise you.
How can a crummy little receiver with an audio bandwidth of perhaps 5 kHz detect
noise in the kilohertz and megahertz regions? Of course, the answer is that many
repetitive noise-pulse trains (whose repetition rates are higher than the audible spec-
trum but below the AM frequency band) have harmonics that extend into the vicinity
of 600 kHz, where the AM receiver is quite sensitive. This sensitivity extends to
signals with amplitudes of just a few microvolts per meter.
If you are skeptical about an AM radio’s ability to detect these signals, tune its dial
down to the low end, between stations. Then, hold it near a DVM or a computer or
computer keyboard, and listen for the hash. Notice, too, that the ferrite stick antenna
has definite directional sensitivity, so you can estimate where the noise is coming
from by using either the null mode or pointing the antenna to get the strongest signal.
So, the humble AM radio may be able to help you as you hike around the lab and
smile pleasantly at your comrades until you find the culprit whose new switching
regulator isn’t working quite right but which he neglected to turn off when he went
out to get a cup of coffee.
The grid-dip meter. On other occasions, the frequency and repetition rate of the
noise are so high that an AM receiver won’t be helpful in detecting the problem.
What’s the tool to use then? Back in the early days of radio, engineers found that if
you ran a vacuum-tube oscillator and immersed it in a field of high-power oscilla-
tions at a comparable frequency, the tube’s grid current would shift or dip when the
frequencies matched. This tool became known as the “grid-dip meter.” I can’t say
that I am an expert in the theory or usage of the grid-dip meter, but I do recall being
impressed in the early days of monolithic ICs: A particular linear circuit was oscil-
lating at 98 MHz, and the grid-dip meter could tickle the apparent rectified output
error as I tuned the frequency dial back and forth.
That was 25 years ago, and, of course, Heathkit’ has discontinued their old Grid
Dip and Tunnel Dip meters in favor of a more modem design. The new one, simply
dubbed HD- 1250 “Dip Meter,” uses transistors and tetrode FETs. At the bargain
price of $89, every lab should have one. They’ll help you ferret out the source of
nasty oscillations as high as 250 MHz. The literature that comes with the HD- 1250
dip-meter kit also lists several troubleshooting tips.
When grid-dip meters first became popular, the fastest oscilloscope you could buy
had a bandwidth of only a few dozen megahertz. These days, it is possible to buy a
scope with a bandwidth of many hundreds of megahertz, so there are fewer occasions
when you might need a grid-dip meter. Still, there are times when it is exactly the
right tool. For example, you can use its oscillator to activate passive tuned circuits
and detect their modes of resonance. Also, in a small company where you can’t af-
ford to shell out the many thousands of dollars for a fast scope, the dip meter is an
inexpensive alternative.

16. A few working circuits, if available. By comparing a bad unit to a good one, you can

1. Heath Company, Benton Harbor, Michigan, 49022; (I-8OO-253-0570),