1Many receivers must be capable of handling a very wide range of signal powers at the input while still producing the correct output. This must be done in the presence of noise and interference which occasionally can be much stronger than the desired signal.Noise sets the threshold for minimum detectable signal power - MDSDistortion sets the maximum signal power level. The third order input intercept (IIP3) is a figure of merit that is directly related to the intermodulation distortion produced by a particular design.23Why frequency translation? The original concept in 1917 addressed current technology. The vacuum tubes of that day were not capable of providing any gain above 1 or 2 MHz. By using the nonlinearity of a vacuum tube along with gain at low frequencies (a few hundred kHz typically), receivers could be built that were sensitive in the MHz range. This enabled the power level of radio transmitters to be greatly reduced.Today, gain is cheap, but the superhet architecture has lived on and has much broader use. It allows the designer to optimize the receiver performance through clever choice of intermediate frequencies and filtering.Direct conversion is less common but has become recently more popular in single chip radios. It can eliminate off-chip bandpass filters, replacing them with on-chip DSP lowpass filters.4The front end of the receiver performs the frequency translation, channel selection and amplification of the signal.5The superheterodyne or superhet architecture uses an intermediate (IF) frequency following the mixer. This is selected such that amplifiers and channel selection filters are available with suitable performance. Image rejection also plays a role as will be seen later.The direct conversion mixes down to DC. The advantage is that filters can be integrated on chip using active or digital filter design approaches. But, LO leakage causes a DC offset. Also, the mixer in most cases must be a complex image rejecting design because the signal and image fold over onto the same frequency.6A mixer doesn’t really “mix” or sum signals; it multiplies them.Note that both sum and difference frequencies are obtained by the multiplication of the two input sinusoidal signals. A mixer can be used to either downconvert or upconvert the RF input signal, A. The designer must provide a way to remove the undesired output, usually by filtering.ttABtBtA )cos()cos(2)sin)(sin(2121217Even in an ideal multiplier, there are two RF input frequencies (FRFand FIM) whose second-order product has the same difference IF frequency. FRF- FLO= FLO- FIM= FIFThe two results are equally valid. One is generally referred to as the “image” and is undesired. In the example above, the lower input frequency is designated the image.8A bandpass preselection filter is often used ahead of the mixer to suppress the image signal. The IF and LO frequencies must be carefully selected to avoid image frequencies that are too close to the desired RF frequency to be effectively filtered. In a receiver front end, out-of-band inputs at the image frequency could cause interference when mixed to the same IF frequency. Also, the noise present at the image would also be translated to the IF band, degrading signal-to-noise ratio. Alternatively, an image-rejection mixer could be designed which suppresses one of the input sidebands by phase and amplitude cancellation. This approach requires two mixers and some phase-shifting networks.So far, the spectrum exhibited by the ideal multiplier is free of harmonics and other spurious outputs (spurs). The RF and LO inputs do not show up in the output. While accurate analog multiplier circuits can be designed, they do not provide high dynamic range mixers since noise and bandwidth often are sacrificed for accuracy.9A narrow band, fixed frequency filter (crystal, SAW, ceramic) is often used for channel selection. It is easier to build a high Q narrowband fixed frequency filter at a lower frequency than to build a tunable high Q high frequency filter.The local oscillator tunes the front end to select the input frequency. fIF= fRF– fLOThe example shown above downconverts to a lower intermediate frequency. This is the superhetrodyne approach invented by Armstrong. Another choice, the direct conversion architecture, downconverts directly to baseband (zero IF). Then, a simple lowpass filter is used for anti-aliasing, an A/D converter and DSP is used for demodulation.10There are two cases that apply with downconversion – IF freq. lower than RF.Case 1. LO frequency is higher than RF frequency. This places the image frequency 2 x fIF above the RF frequency. A sharp cutoff lowpass filter (LPF) or bandpass filter ( BPF) could be used to attenuate the image.fIF= fRF– fLOCase 2. RF frequency is higher than LO frequency. This places the image frequency 2 x fIF below the RF frequency – now inband for a LPF. A sharp cutoff bandpass filter ( BPF) must be used to attenuate the image.fIF= fLO– fRF11The upconversion cases often can use a LPF for image rejection. In fact, the whole reason for upconverting in a receiver is to make image rejection more effective. But, we see that for the same fRF, the two cases give much different results.Case 1: Here the LO is higher than RF. Two input frequencies produce the same IFfRF+ fLO= fIFfIM– fLO= fIFThe image frequency is much higher than the RF frequency. This makes it easy to use a simple LPF to get significant image rejection.Case 2: Same equations, but now the LO is lower than RF. This places the IF and IM frequencies lower, making it more demanding for the LPF to provide significant image rejection.An IF filter is often used here to block potentially interfering spectral inputs from creating distortion downstream in the receiver where amplification is provided. This function is often called a “roofing filter”.12Or, alternatively, if we chose to keep the same IF frequency, probably a common choice since IF filters are available at only certain frequencies, the picture changes slightly from cases 1 and 2.Case 3: fRF+ fLO= fIFfIM– fLO= fIFCase 4:fLO- fRF= fIFfIM– fLO= fIFOnce again, the high LO injection leads to a higher image frequency and better image rejection.13Having said the above, we also have a cost consideration. IF filters are available only at certain frequencies if we want inexpensive mass-produced filters. Here are some common ones: 455 kHz, 10.7 MHz, 21.4 MHz, 45