3.2.4 Image Signals and Image Reject Filtering

Chapter 3.2.4 Image Signals and Image Reject Filtering

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 56 System Level Architecture and Design Considerations3.2.4  Image Signals and Image Reject FilteringAt the RF frequency, there are filters to remove out of band signals that may be picked up by the antenna. Any filter in the RF section of the radio must be wide enough to pass the entire receive band, and therefore can do little about any in-band interferers. In a superheterodyne receiver, the filters in the RF section also have the added task of removing the image frequency and are thus sometimes called image reject filters. A superheterodyne receiver takes the desired RF input signal and mixes it with some reference signal to extract the difference frequency. The LO reference is mixed with the input to produce a signal at the difference frequency of the LO and RF as shown in Figure 3.2. As mentioned earlier, a signal on the other side of the LO at the same distance from the LO will also mix down “on top” of the desired frequency. Thus, before mixing can take place, this unwanted image frequency must be removed. Thus, another important specification in a receiver is how much image rejection it has. Image rejection is defined as the ratio of the gain Gsig of the desired signal to the gain of the image signal Gim. æö=ç÷èøIR 10logsigimGG (3.27)In general a receiver must have an image rejection large enough so that in the case of the largest possible image signal and the weakest receive channel power, the ratio of the channel power to the image power, once downconverted, is still larger than the minimum required SNR for the radio. The amount of filtering can be calculated by knowing the undesired frequency with respect to the filter center frequency, the filter bandwidth, and filter order. The following equation can be used for this calculation: æöæö-D= ´= ´ç÷ç÷-èøèødBBW20log20log 222udcbecnffnfAfff (3.28)where AdB is the attenuation in decibels, n is the filter order, and thus n/2 is the ef-fective order on each edge, fud is the frequency of the undesired signal, fc is the filter center frequency, and fbe is the filter band edge.Example 3.5: Image Reject FilteringA system has an RF band from 902–928 MHz, a 200-kHz channel bandwidth, and channel spacing. The first IF is at 70 MHz. Determine the required order of the 26-MHz filter to get a worst-case image rejection of better than 50 dB. If the received image signal power is -40 dBm, the minimum input signal power is -75 dBm, and the required SNR for this modulation is 9.5 dB will this be enough image rejection? Solution: The frequency spectrum is shown in Figure 3.12. At RF, the local oscillator fre-quency fLO is tuned to be 70 MHz above the desired RF signal so that the desired