One of the most commonly mentioned HF receiver specifications is blocking dynamic range. It is a measure of how strong an interfering signal has to be, referenced to the noise floor of the receiver, to reduce the strength of an incoming signal by 1 dB. The blocking dynamic range of a receiver is the difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression.
QUESTION: What is meant by the blocking dynamic range of a receiver? (E4D01)
ANSWER: The difference in dB between the noise floor and the level of an incoming signal that will cause 1 dB of gain compression
Intermodulation is the term for unwanted signals generated by the mixing of two or more signals and is the result of circuits or devices being nonlinear. A measure of the nonlinearity of a receiver is the third-order intercept level. The third-order intercept level is the level at which a third-order intermodulation product will reach the level of the signal being received. So, for example, if a receiver is said to have a third-order intercept level of 40 dBm it means that a pair of 40 dBm input signals will theoretically generate a third-order intermodulation product with the same level as the input signals. The higher the third-order intercept level, the better the receiver. Compared to other intermodulation products, odd-order intermodulation products created within a receiver are of particular interest because the odd-order product of two signals in the band of interest is also likely to be within the band.
QUESTION: What is the term for spurious signals generated by the combination of two or more signals in a non-linear device or circuit? (E4D06)
ANSWER: Intermodulation
QUESTION: What causes intermodulation in an electronic circuit? (E4D08)
ANSWER: Nonlinear circuits or devices
QUESTION: What does a third-order intercept level of 40 dBm mean with respect to receiver performance? (E4D10)
ANSWER: A pair of 40 dBm input signals will theoretically generate a third-order intermodulation product that has the same output amplitude as either of the input signals
QUESTION: Why are odd-order intermodulation products, created within a receiver, of particular interest compared to other products? (E4D11)
ANSWER: Odd-order products of two signals in the band of interest are also likely to be within the band
Strong nearby signals can also reduce the sensitivity of a receiver. This reduction in receiver sensitivity is called desensitization. Another cause of desensitization is poor dynamic range. Poor dynamic range may also cause the receiver to produce spurious signals by cross modulation. One way to reduce the likelihood of receiver desensitization is to decrease the RF bandwidth of the receiver. This reduces the likelihood of a strong signal appearing in the receiver’s passband.
QUESTION: What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency? (E4D12)
ANSWER: Desensitization
QUESTION: Which of the following describes problems caused by poor dynamic range in a receiver? (E4D02)
ANSWER: Spurious signals caused by cross-modulation and desensitization from strong adjacent signals
QUESTION: Which of the following reduces the likelihood of receiver desensitization? (E4D07)
ANSWER: Decrease the RF bandwidth of the receiver
A preselector might help in some cases. The purpose of the preselector in a communications receiver is to increase rejection of unwanted signals outside of a desired band.
QUESTION: What is the purpose of the preselector in a communications receiver? (E4D09)
ANSWER: To increase rejection of signals outside the desired band
Intermodulation can also happen in the final amplifiers of amateur radio transmitters. For example, intermodulation interference between two repeaters often occurs when the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters. For example, if you had a receiver tuned to 146.70 MHz and a nearby station transmits on 146.52 MHz, you might hear signals on 146.34 MHz and 146.61 MHz as well. Here’s how the signals might combine:
2 × 146.52 MHz – 146.34 MHz = 146.70 MHz and
2 × 146.61 MHz – 146.52 MHz = 146.70 MHz
QUESTION: How can intermodulation interference between two repeaters occur? (E4D03)
ANSWER: When the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters
QUESTION: What 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? (E4D05)
ANSWER: 146.34 MHz and 146.61 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. 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.
QUESTION: Which of the following may reduce or eliminate intermodulation interference in a repeater caused by another transmitter operating in close proximity? (E4D04)
ANSWER: A properly terminated circulator at the output of the repeater’s transmitter
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