Superheterodyne Receiver

RF Front End(Amps, Filters)

Local Oscillator

Mixer

IF Strip(Amps, Filters)

Channel SelectivityService Band

Detector(Demodulation)

Multiplication in the Time Domain = Convolution (shifting) in the Frequency Domain

Recovered Information

Image Effect

1

2

33

2

1

1 2 1 2 1 2 1 2

3 2 3 23 2 3 2

Local Oscillator

3 2 = 2 1

Image Effect (cont)

BWIF

fIF fLOfLO + fIF fLO - fIF

Upper Conversion

Band

Lower Conversion

Band

Desired Channel

Desired Channel

Image Band

Image Band

Low Side InjectionHigh Side Injection

Any signals entering the mixer in the Desired Channel and/or the Image Band will be converted down into the IF bandwidth, amplified, and demodulated.

This includes:

1. Desired and interfering (image band) signals entering the antenna

2. kTB noise amplified by the RF amplifiers

Observations

The desired channel and its image band are always separated by twice the IF frequency..The LO frequency is always exactly between the desired channel and its image band,

fLOfLO + fIF fLO - fIF

Upper Conversion

Band

Lower Conversion

Band

Observations (cont)

The higher the IF frequency, the greater the frequency displacement between the desired channel and its image band.

fIF fLOfLO + fIF fLO - fIF

Observations (cont)

Filtering (pre-selector) before the mixer, in the RF front end, can reduce the effects of image band signals and kTB noise.

LO DSUS

Service Band RangeImage Band Range

LO Range

Pre-selector Response

Image Rejection for DS

Worst Case Image Rejection

Pre-selector Math

Image Rejection for DS

Worst Case Image Rejection

fc : Center of Service Band and Pre-selector pass-band

BWS :Pre-selector Passband~ Service Band Range

Image Rejection Shape factor for Specific Channel DS:

Worst Case Image Rejection Shape Factor

2US DS IF

c US CUS

S C US

f f f

f f fSF

BW f f

, 2US s MAX IFf f f

fs,MAX

c US CUS

S C US

f f fSF

BW f f

, 2US s MIN IFf f f

DSUS

2fIF

(Low-side Injection)(High-side Injection)

USDS

2fIFfs,MIN

Image Band Thermal NoiseA pre-selector filter between the antenna and first RF amp limits all out of band interfering signals entering the receiver, including images. . . . . . But does not reduce thermal noise in the image band.

Absent additional mitigation, this will allow thermal noise power equal to kT0B from both the image band and desired channel to enter the mixer and pass through the IF. This effectively doubles the noise bandwidth , and the effective noise power entering the receiver is 2kT0B, where B is the IF bandwidth.

If a second pre-selector is included following the first RF amplifier, the amplified thermal noise in the image band can be reduced to well below the amplified on-channel thermal noise (which we can’t do anything about), effectively eliminating it, as well as providing additional image rejection.

Pre-selectorRF Amp Pre-selector

RF Amp

Mixer

Effect on Noise FigureWhen determining the noise figure/ratio of the receiver, structure and the placement of the pre-selector filters must be taken into consideration.

If a pre-selector is included between the first RF amp and the mixer, there is no appreciable noise power contribution from the image band, and Friis’ formula will provide an accurate noise ratio.

If there is no pre-selector between the first RF amp and the mixer, then the result of Friis’ formula must be doubled (add 3 dB to resultant noise figure) to account for the excess noise contribution from the image band.

Computing Image Rejection

1. Determine the shape factors for the desired frequency and image frequency (mid-band or band edge) for all pre-selectors (they may have different bandwidths).

2. Based on the poles/ripple of each pre-selector, determine the attenuations corresponding to the shape factors determined in step 1.

3. The image rejection for each filter is the difference between the attenuation for the image and the attenuation for the desired frequency.

4. The total image rejection in dB is the sum of the image rejections in dB as determined in step 3.

Pre-selectorA

RF Amp

Pre-selectorB

RF Amp

Mixer

Example

Pre-selector A: fc = 121 Mhz, BW3dB =8 Mhz 2 Poles, 0 dB ripplePre-selector B: fc = 121 Mhz, BW3dB = 7.5 Mhz 3 Poles, 0.1 dB rippleQU = 80

Service Band: 118 – 124 MhzIF: 9.4 Mhz, Low Side Injection

DS: 124 Mhz

US:105.3 Mhz

Worst Case Image Rejection

fc : 121 Mhz

BWS : 6 MhzImage Band

BW3dB

18.8 Mhz

Worst case Image Frequency: 124Mhz – 2(9.4 Mhz) = 105.3 Mhz (FM Broadcast)

Preselector SF@124 Mhz

Att(dB)@124 Mhz

SF@105.3 Mhz

Att(dB)@105.3 Mhz

IRR(dB) Ins Loss(Midband)

Ins Loss@124 Mhz

A 0.74 1.1 -4.22 25 23.9 2.7 3.8

B 0.79 0.4 -4.5 43 42.6 4.4 4.8

Total Worst Case Image Rejection Ratio = 66.5 dB