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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
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