In the past, sensitivity was one of the most important receiver performance specifications. Today, instead of sensitivity, we speak of a receiver’s minimum discernible signal, or MDS. This is the weakest signal that a receiver will detect. One parameter that affects a receiver’s MDS is the noise figure. The noise figure of a receiver is the ratio in dB of the noise generated by the receiver compared to the theoretical minimum noise.
QUESTION: What does the MDS of a receiver represent? (E4C07)
ANSWER: The minimum discernible signal
QUESTION: What is the noise figure of a receiver? (E4C04)
ANSWER: The ratio in dB of the noise generated by the receiver to the theoretical minimum noise
A related specification is the noise floor. A noise floor of -174 dBm/Hz, is the theoretical noise at the input of a perfect receiver at room temperature. If a CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz, the level of an unmodulated carrier input to this receiver would have to be -148 dBm to yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth.
QUESTION: What does a receiver noise floor of -174 dBm represent? (E4C05)
ANSWER: The theoretical noise in a 1 Hz bandwidth at the input of a perfect receiver at room temperature
QUESTION: A CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz. What would be the level of an unmodulated carrier input to this receiver that would yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth? (E4C06)
ANSWER: -148 dBm
Modern transceivers use digital signal processing (DSP) to filter out noise and eliminate interference at one of the IF stages. Instead of just a few fixed bandwidths, DSP techniques allow an operator to select from a wide variety of bandwidths, enabling the operator to select a receive bandwidth that’s optimum for the type of signal he or she is receiving. This maximizes the signal to noise ration and minimizes interference.
QUESTION: What is an advantage of having a variety of receiver IF bandwidths from which to select? (E4C10)
ANSWER: Receive bandwidth can be set to match the modulation bandwidth, maximizing signal-to-noise ratio and minimizing interference
Although modern transceivers have sophisticated DSP filters, many also use roofing filters to improve receiver performance. Roofing filters filter incoming signals before the first intermediate frequency (IF) stage, blocking strong signals near the receive frequency that could cause overloading and distortion in the following amplifier stages. In doing so, a narrow-band roofing filter improves a receiver’s dynamic range.
QUESTION: How does a narrow-band roofing filter affect receiver performance? (E4C13)
ANSWER: It improves dynamic range by attenuating strong signals near the receive frequency
Back in the day, when superheterodyne receivers had intermediate frequencies in the 400 – 500 kHz range, image rejection was a problem. If there was a strong signal present on a frequency about two times the IF away from the receive frequency, you might hear that signal. For example, you might hear a strong signal transmitting on 15.210 MHz on a receiver that has a 455 kHz IF frequency and is tuned to 14.300 MHz.
QUESTION: What transmit frequency might generate an image response signal in a receiver tuned to 14.300 MHz and that uses a 455 kHz IF frequency? (E4C14)
ANSWER: 15.210 MHz
One solution to this problem is to select a higher IF frequency. Doing so makes it easier for front-end circuitry, such as IF filters, to eliminate image responses. A front-end filter or pre-selector of a receiver can also be effective in eliminating image signal interference.
QUESTION: Which of the following choices is a good reason for selecting a high frequency for the design of the IF in a superheterodyne HF or VHF communications receiver? (E4C09)
ANSWER: Easier for front-end circuitry to eliminate image responses
QUESTION: Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals? (E4C02)
ANSWER: A front-end filter or pre-selector
Because most modern transceivers use digital techniques to generate a local oscillator signal to tune a receiver, synthesizer phase noise might be a problem. An effect of excessive phase noise in the local oscillator section of a receiver is that it can combine with strong signals on nearby frequencies to generate interference. The process whereby local oscillator phase noise combines with adjacent strong signals to create interference to desired signals is called reciprocal mixing.
QUESTION: What is an effect of excessive phase noise in a receiver’s local oscillator? (E4C01)
ANSWER: It can combine with strong signals on nearby frequencies to generate interference
QUESTION: What is reciprocal mixing? (E4C15)
ANSWER: Local oscillator phase noise mixing with adjacent strong signals to create interference to desired signals
Software-defined radio (SDR) is becoming more popular in amateur radio. It is, therefore, necessary to know something about SDR receiver characteristics. For example, an SDR receiver’s analog-to-digital converter (ADC) sample width in bits has the largest effect on an SDR receiver’s linearity. An SDR receiver is overloaded when input signals exceeds the reference voltage of the analog-to-digital converter. When this happens, the ADC outputs the maximum value no matter what the input value is.
QUESTION: Which of the following has the largest effect on an SDR receiver’s dynamic range? (E4C12)
ANSWER: Analog-to-digital converter sample width in bits
QUESTION: An SDR receiver is overloaded when input signals exceed what level? (E4C08)
ANSWER: The reference voltage of the analog-to-digital converter
Most transceivers have built-in attenuators to reduce receiver overload. Although attenuators reduce the strength of incoming signals, they have little or no impact on signal-to-noise ratio because atmospheric noise is a much bigger contributor to the overall noise level than internally generated noise, and the attenuator attenuates atmospheric noise as much as it does the incoming signal.
QUESTION: Why can an attenuator be used to reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio? (E4C11)
ANSWER: Atmospheric noise is generally greater than internally generated noise even after attenuation
Finally, here is a miscellaneous question on FM receiver performance characteristics.
QUESTION: What is the term for the suppression in an FM receiver of one signal by another stronger signal on the same frequency? (E4C03)
ANSWER: Capture effect
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