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Communication Systems. Prof. Chungming Kuo. Chapter 5. Superheterodyne Receivers (cont.). Superheterodyne Receivers. A communications receiver is an electronic unit that responds to signals. It separates the desired signal from all others.
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Communication Systems Prof. Chungming Kuo
Chapter 5 Superheterodyne Receivers (cont.)
Superheterodyne Receivers • A communications receiver is an electronic unit that responds to signals. • It separates the desired signal from all others. • It performs the demodulation and other signal processing operations required to produce the desired output.
Superheterodyne Receivers (cont.) • While there are several types of receivers, by far the most common is the superheterodyne receiver. • It is based on the principle that all signals are down-converted to a common intermediate frequency (IF) for proper filtering.
Receiver Terminology • Selectivity - A measure of how well the receiver can separate the desired signal from all others present at the input. • Sensitivity - A measure of how well the receiver can respond to very weak signals.
Limitations of TRF Receiver • Selectivity must be established in the RF amplifier stage. • The capability of designing highly selective stages over a broad frequency range is a difficult task. • Moreover, if tuning is required, the task is even more difficult.
Superheterodyne Concept • The discussion at this point will be limited to the single-conversion superheterodyne receiver. • The concept is to establish an intermediate frequency (IF) amplifier stage in which very high selectivity can be achieved.
Superheterodyne Concept (cont.) • All signals are then down-converted to this frequency range for filtering.
IF and LO Frequency Relationships • The LO frequency may be higher or lower than the incoming signal frequency. When it is higher, • When it is lower,
Image Frequency • There is a potential problem with respect to an image frequency. A frequency on the opposite side of the LO frequency by the amount of IF frequency can mix with the LO frequency to cause a spurious output.
Image Frequency (cont.) • For LO above input frequency, • For LO below input frequency,
Design Considerations • Image interference can be minimized by having enough selectivity in the RF stage to reject the image component. • The higher the IF frequency, the easier it is to reject the image component.
Design Considerations (cont.) • However, selectivity at higher frequencies is more difficult so there is an engineering tradeoff. • More sophisticated receivers employ dual-conversion forms. Image rejection is optimized in the first stage and selectivity is optimized in the second stage.
Example 1 • A single conversion receiver is tuned to 40 MHz and IF frequency is 5 MHz. • Determine LO and image frequencies if LO frequency is higher than signal frequency.
Example 2 • Repeat analysis of Example 1 if LO frequency is lower than signal frequency.
Summary • The superheterodyne receiver concept is the basis for most modern receiver designs. • The selectivity of a superheterodyne receiver is established in the IF stage. • Tuning of a superheterodyne receiver consists of tuning the LO such that the desired mixer output falls in the center of the IF band.
