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2016 Extra Class study guide: E4D - Receiver performance characteristics

Posted: 14 Mar 2016 11:01 AM PDT
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E4D Receiver performance characteristics: blocking dynamic range;
intermodulation and cross-modulation interference; 3rd order intercept;
desensitization; preselector

One of the most commonly mentioned HF receiver specifications is blocking
dynamic range. The blocking dynamic range of a receiver is the difference
in dB between the noise floor and the level of an incoming signal which
will cause 1 dB of gain compression. (E4D01) Cross-modulation of the
desired signal and desensitization from strong adjacent signals are two
problems caused by poor dynamic range in a communications receiver. (E4D02)

Another specification commonly bandied about is third-order intercept
level. A third-order intercept level of 40 dBm with respect to receiver
performance means a pair of 40 dBm signals will theoretically generate a
third-order intermodulation product with the same level as the input
signals. (E4D10) Compared to other products, third-order intermodulation
products created within a receiver are of particular interest because the
third-order product of two signals which are in the band of interest is
also likely to be within the band. (E4D11)

The term for the reduction in receiver sensitivity caused by a strong
signal near the received frequency is desensitization. (E4D12) Strong
adjacent-channel signals can cause receiver desensitization. (E4D13) One
way to reduce the likelihood of receiver desensitization is to decrease the
RF bandwidth of the receiver. (E4D14)

A preselector might help in some cases. The purpose of the preselector in a
communications receiver is to increase rejection of unwanted signals.
(E4D09)

When operating a repeater, one thing that can occur is intermodulation
interference, or simply intermod. Intermodulation interference is the term
for unwanted signals generated by the mixing of two or more signals.
(E4D06) Nonlinear circuits or devices cause intermodulation in an
electronic circuit. (E4D08)

Intermodulation interference between two repeaters occurs when the
repeaters are in close proximity and the signals mix in the final amplifier
of one or both transmitters. (E4D03) The transmitter frequencies would
cause an intermodulation-product signal in a receiver tuned to 146.70 MHz
when a nearby station transmits on 146.52 MHz are 146.34 MHz and 146.61
MHz. (E4D05) We get this in the following way:

2 x 146.52 MHz 146.34 MHz = 146.70 MHz and

2 x 146.61 MHz 146.52 MHz = 146.70 MHz

A properly terminated circulator at the output of the transmitter may
reduce or eliminate intermodulation interference in a repeater caused by
another transmitter operating in close proximity. (E4D04) The circulator
reduces intermodulation distortion because it helps to reduce the amount of
energy from nearby transmitters that might get into a repeater’s final
amplifier.

Cross modulation is a form of intermodulation. Cross modulation occurs when
a very strong signal combines with a weaker signal and actually modulates
the weaker signal. The most significant effect of an off-frequency signal
when it is causing cross-modulation interference to a desired signal is
that the off-frequency unwanted signal is heard in addition to the desired
signal. (E4D07)

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characteristics appeared first on KB6NUs Ham Radio Blog.