Page 175 - Troubleshooting Analog Circuits

I62 13. Letters to Bob

highs on the Preset and Reset lines and toggle in spite of a low-high condition at
clock time. This anomaly happens if the clock is left high and is not pulsed.

J. Koontz
Chief Engineer
Computer Automation
Irvine, CA

Dear Bob:

If an engineer wants to see how to properly control EM1 at its source, he or she
should look at the chassis of any TV and tuner. Chances are, any radiation from inter-
nally generated 15-kHz to 950-MHz signals meets FCC Part 15 rules. Obviously,
most TV receivers do not depend on their plastic cabinets to contain spurious
radiation.
For some time now I have been working with labs that attempt to certify radiating
equipment that was not designed using basic radiation-containment methods. These
methods date back to the time of tube circuits and include using specialized compo-
nents such as feedthrough capacitors, ferrite beads, and toroidal coils. These methods
also include using very light-gauge, low-cost, tin-plated steel cans to enclose radi-
ating components. Soldering the tabs of these 5-sided cans to the PC board forms a
complete 6-sided enclosure around the radiating component. Note that TV receivers’
PC boards have considerable ground-plane area.
If you do not design out EMI, someone will have to design in some Band-Aids to
fix your bad design.
Here are some points to consider:
1. Incorporate as much ground plane as possible on one side of each PC board that
contains digital or analog signals above audio frequencies. Be sure this ground plane
has a low-inductance path to the main chassis-even if this path goes through an
edge connector.
2. Incorporate a T or LRC filter on all input and output lines. The resistor should always
be on the side leading away from the hot circuit. It should be the largest value pos-
sible, as great as 1 kQ. This resistor will damp, or “de-Q,” any resonant circuit that
the interconnect lines form as well as filter the noise. You must choose the resistor
and capacitor carefully so as not to adversely affect the desired signal on the line. The
capacitor should be a ceramic disk with a value between 10 pF and 0.01 pF,
depending on the signal source. Probe each filtered line to confirm that only required
signals are present.
3. Probe each PC board to locate the areas of maximum radiation. Experiment with
metal-foil tape to determine where a metal shield will be most effective. Install a
temporary shield soldered to the ground plane to verify effectiveness.
4. You should examine purchased items such as disks and power supplies for YO-line
filters and radiation containment.
5. A commercially available filter or a suitable substitute should filter your circuit’s
input to check that any conducted interference is at least 20 dB below requirements.
6. Monitor your circuit’s I/O lines during normal operation and track down any unex-