Page 76 - Troubleshooting Analog Circuits

Consider Your Wire Type 63

a miracle. Often you have to unbundle the wires and separate the offending ones or
the sensitive, delicate signals from the rest. Also, you may end up rewiring some or
all of the wires into shielded cables.
Remember, Teflon is a good insulator, but air is even better. If you have to add
struts, standoffs, or spacers to make sure that the critical wires stay put, go right
ahead. If you have problems, the wire manufacturers can give you some advice.
Conversely, just as you use Teflon or air when you need a superior insulator, you
have to be careful to get your best conductances. A friend who is an amateur radio
operator says that many kinds of problems in RF circuits arise because nuts and bolts
are used to make ground connections. If a lock washer or star washer is not included,
the mechanical connection can loosen, the ground impedances will change with
every little stress or strain, and nasty intermittent electrical problems will result. So. a
major factor in the reliability of these circuits is ensuring the integrity of all bolted
joints by always including star washers. And, make sure that wires and connectors do
not get so loose as to hurt the reliability of your circuit or system. (See also com-
ments on star washers in Chapter 13.)
When you use shielded cable, should you ground the shield at one end or at both
ends? Many cases call for a ground at the receiving end of the cable, but there are
cases in which the shield is the main ground return. Neither way is necessarily bad,
but be consistent. Likewise, in the design and the execution of the design, avoid
ground loops, which can cause weird noise problems. In my systems, I build my
analog ground system completely separate from the digital ground and make sure
that the case or package ground is also strictly divorced. Then, after I use an ohm-
meter to confirm that these grounds are really separate, I add one link from the analog
ground to the digital ground and another link to the case or chassis. This technique
works well for me, and I recommend it.
It is a little-known fact that some coaxial cable can degrade just sitting on the
shelf. (Well, that’s true, but it degradesfaster if the shelf is sitting in the sunshine, or
out in the rain. . . .) Some specialty types of cable whose codes and specifications are
nominally similar can have an outer jacket that is not guaranteed to have good chem-
ical stability. The jacket may be especially resistant to some chemicals, but less resis-
tant to others. Specifically, in the 1950s there was a lot of military-surplus cable
similar to RG-58 and RG-74 that did not have good stability. As the outer jacket
degraded, the inner insulator was chemically degraded and the cable’s UHF attenua-
tion was degraded. In other cases, the outer shield was chemically attacked and cor-
roded, and its conductivity got worse and its UHF attenuation was also impaired.
Most of that old cable has died and you can’t even find it in junk-piles any more. But
there are still specialty cables being made and sold now that do not last as well or age
as gracefully as you would expect a good wire to do. If you select a cable to be espe-
cially resistant to one kind of chemical, it may be less resistant than normal to the
attack of other ordinary chemicals. So, you should be aware that, even in something
as simple as a wire, there may be more problems than meet the eye.
Heck, I just had a couple yards of Teflon-insulated wire sitting outside my kitchen
window, running over to a sensor for an electronic thermometer. The wire would
only get, on the average, 1 hour per day of direct sunlight. After 10 years, the yellow
wire was still in good shape, but the white insulation had just about died utterly. Who
wants to explain that one?
Recently an engineer showed me the results of a study of wire for loudspeakers.
He showed that the inductance of ordinary two-conductor wire (per 20 feet) can
cause a small but noticeable phase shift-perhaps 10 degrees at 20 kHz, even with
the large and ultra-expensive speaker cables ($10 per foot and up). But when he took
flat “ribbon” cable with 40 conductors (which is typically used to bus digital signals