3.13: Exercises
- Page ID
- 41199
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- A superheterodyne receiver has, in order, an antenna, a low-noise amplifier, a bandpass filter, a mixer, a second bandpass filter, a second mixer, a lowpass filter, an ADC, and a DSP that will implement quadrature demodulation. Develop the frequency plan of the receiver if the RF input is at \(2\text{ GHz}\) and has a \(200\text{ kHz}\) single-channel bandwidth. The final signal applied to the ADC must be between DC and \(400\text{ kHz}\) so that \(I/Q\) demodulation can be done in the DSP unit. Noise considerations mandate that the LO of the first mixer must be more than \(10\text{ MHz}\) away from the input RF. Also, for a bandpass filter to have minimum size, the center frequency of the filter should be as high as possible. It has been determined that the appropriate trade-off of physical size and cost is to have a \(100\text{ MHz}\) bandpass filter between the two mixers. (Note: \(100\text{ MHz}\) is the center frequency of the bandpass filter.)
- Draw a block diagram of the receiver and annotate it with symbols for the frequencies of the LOs and the RF and IF signals.
- What is the LO frequency \(f_{\text{LO1}}\) of the first mixer?
- What is the LO frequency \(f_{\text{LO2}}\) of the second mixer?
- Specify the cutoff frequency of the lowpass filter following the second mixer.
- Discuss in less than \(\frac{1}{2}\) a page other design considerations relating to frequency plan, filter size, and filter specification.
- Short answer questions. Each part requires a short paragraph of about five lines and a figure where appropriate to illustrate your understanding.
- Explain the operation of a superheterodyne receiver.
- Compare zero-IF and low-IF receivers.