Interfacing and Processing 253frequencies l0,000 times higher. However, special radio transmissions (for submerged
submarines, national clocks, and caving) may use frequencies below 100 kHz and even
those below 20 kHz.
8.2.2 Requirement
All active devices are potentially susceptible to EMI. BJTs, all kinds of fi eld effect
transistors (FETs), and also valves can all act as rectifi ers at RF, demodulating radio
transmissions. However, this is very much more likely with BJTs, as the nonlinearity of a
BJT’s forward biased base-emitter junction that gives rise to rectifi cation is triggered by
considerably lower levels of RF voltage or fi eld strength. All kinds of FETs and valves
are relatively “ RF proof ” in comparison. Oxidized copper, generally dirty contacts,
crystalline soldered joints, or wrong metal-to-metal interfaces can all act as RF detectors
as well, through rectifi cation.
8.3 Balanced Input ......................................................................................................
Balanced inputs, when used properly, can clean up hums, buzzes, RFI, and general
extraneous rubbish. When not used properly, the balanced-input’s object may be partly
defeated, but the connection will probably still improve the amplifi er’s and system’s
effective SNR.
8.3.1 Defi nition
To be truly balanced, a balanced input and the line coming in and the sending device
must all have equal impedances to (signal) ground, to earth, and to everywhere else. Also,
the signal must be exactly opposite in polarity but equal in magnitude, on each conductor.
8.3.2 Real Conditions
In practice, the signal is not of exactly opposite polarity. At high frequencies (and low
frequencies in some poorly designed equipment), phase shifts add or subtract up to 90°
or more, from the ideal 180° polarity difference. Otherwise the requirement for having a
signal of opposite sign on each conductor is usually met. The exception is when one-half
of a ground-referred, balanced source has been shorted to ground. Not surprisingly, this
degrades the benefi ts of balancing.