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.

Classical Tuned Radio Frequency (TRF) Receiver

RFAMPLIFIER

POSTDETECTIONAMPLIFIER

DETECTOR

ANTENNA

SPEAKER

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

SPEAKER

RFAMP

ANTENNA

X IFAMP

DEMODAF

AMP

MIXER

cf cf

LO c

LO c

f f

f f

IFf

LOfRFB B

IFB BAVC

LO

BASEBAND

(a)

Superheterodyne Spectral Chart

IFf

IFB B

cf LOf imagef f

IFf IFf

RFB

SPECTRALCOMPONENTS

RF AMPLIFIERAMPLITUDERESPONSE

(b)

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,

fIF fLO fc

fIF fc fLO

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,

f image fLO fIF fc 2 fIF

f image fLO fIF fc 2 fIF

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

RFAMP

IFAMP

1

IFAMP

2DEMOD

AFAMPXX

LO1 LO2

MIXER1

MIXER2

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.

fLO fc fIF 405 45 MHz

f image fLO fIF 455 50 MHz

Example 2

• Repeat analysis of Example 1 if LO frequency is lower than signal frequency.

fLO fc fIF 40 5 35 MHz

f image fLO fIF 35 5 30 MHz

Summary The superheterodyne receiver concept is the b

asis 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.