Communication Systems Prof. Chungming Kuo

Superheterodyne Receivers (cont.) 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.

Classical Tuned Radio Frequency (TRF) Receiver

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.

Superheterodyne Block Diagram

Superheterodyne Spectral Chart

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.

Double-Conversion Receiver

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.