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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