PROPELLER SYSTEMS
Propeller systems, whether hydraulically or
electrically operated, are potent generators of radio
interference. The sources of interference include
propeller pitch control motors and solenoids,
governors and associated relays, synchronizers and
associated relays, deicing timers and relays, and
inverters for synchro operation.
Propeller control equipment generates clicks and
transients as often as 10 per second. The audio
frequency envelope of commutator interference
varies from about 20 to 1000 Hz. The propeller
deicing timer generates intense impulses at a
maximum rate of about 4 impulses per minute.
Values of current in the propeller system are
relatively high; consequently, the interference
voltages generated are severe. They are capable of
producing moderate interference at frequencies below
100 kHz and at frequencies above 1 MHz. However,
the interference voltages can cause severe
interference at intermediate frequencies.
RECEIVER OSCILLATORS
Either directly or through frequency multipliers
or synthesizers, the local oscillator in a
superheterodyne receiver generates an RF signal at a
given frequency. The local oscillator signal is mixed
with another RF signal to produce an intermediate
frequency (IF) signal. Depending on receiver design,
the frequency of the local oscillator signal is either
above or below the frequency of the RF signal by a
frequency equal to the IF.
The amount of interference leaving the receiver
through its antenna is roughly proportional to the ratio
of the tuned input frequency to the intermediate
frequency. For any tuning band on the receiver,
oscillator leakage is highest at the low end of the
band. Also, the lower the intermediate frequency, the
greater the leakage probability.
Although the receiver antenna is the principal
outlet of oscillator leakage, leakage can occur from
other points. Any path capable of introducing
interference into a receiver is also capable of carrying
internally generated interference out of the receiver.
The paths of entry are discussed in more detail later in
this chapter.
Oscillator leakage from a single communications
receiver in an aircraft is not likely to be a direct source
of interference, except in a very large aircraft where
two or more frequencies in the same band are used
simultaneously.
However, high-order harmonics of
the oscillator frequency can become troublesome in
the VHF band and above.
Oscillator leakage from a swept-tuning receiver
can produce interference in any receiver aboard the
aircraft. This is done directly (on harmonics) or by
nonlinear mixing, as shown in the following example:
. Receiver A, operating at a frequency of 2100
kHz, with an IF of 500 kHz, has oscillator leakage at
2600 kHz (or 1600 kHz).
. Receiver B, operating at 150 MHz, with an IF
of 10 MHz, has oscillator leakage at 160 MHz (or 140
MHz).
. Receiver C, sweeping a frequency band from
200 to 300 MHz, with an IF of 30 MHz, has oscillator
leakage across the band 170 to 270 MHz (or 230 to
330 MHz).
Each receiver is capable of interfering with the
other receivers at the oscillator frequency and its
harmonics. In addition, with the presence of a
nonlinear detector, the leakage signals from the three
receivers can be mixed and interfere with the
following frequencies:
l Receivers A and B, after nonlinear mixing, can
produce interference at 160±2.6 MHz.
. Receivers A and C can similarly produce
interference at any frequency from 200±2.6 to 300
±2.6 MHz; receivers B and C between 200 ±160 to
300±160 MHz.
NONLINEAR ELEMENTS
A nonlinear element is a conductor,
semiconductor, or solid-state device whose resistance
or impedance varies with the voltage applied across it.
Consequently, the resultant voltage is not proportional
to the original applied voltage. Typical examples of
nonlinear elements are metallic oxides, certain
nonconducting crystal structures, semiconductor
devices, and electron tubes. Nonlinear elements that
could cause radio interference in aircraft systems are
overdriven semiconductors and vacuum tubes,
oxidized or corroded joints, cold-solder joints, and
unsound welds.
In the presence of a strong signal, a nonlinear
element acts like a detector or mixer. It produces sum
and difference frequencies and any harmonics from
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