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Sec. 4–17 Software Radios 297
Zero-IF receivers have several advantages. They have no image response. The same
zero-IF receiver hardware can be used in many different applications for manufacturing economy.
Since DSP hardware is used, the effective RF bandpass characteristics and the detector
characteristics are determined by DSP software. (See next section.) The software can
be changed easily to match the desired application. The same zero-IF hardware can be used
for receivers in different VHF and UHF bands by selecting the appropriate LO frequency
(F LO = f c ) and tuning the front-end filter (usually a single-tuned circuit) to f c .
The zero-IF receiver has the disadvantage of possibly leaking LO radiation out of the
antenna input port due to feed-through from the mixer. Also, there will be a DC offset on the
mixer output, if there is LO leakage into the antenna input since a sine wave (LO signal)
multiplied by itself produces a DC term (plus an out-of-band second harmonic). The use of a
high-quality balance mixer and LO shielding will minimize these problems. The receiver can
also have a poor noise figure, since the front end usually is not a high-gain, low-noise stage.
As in any receiver, the hardware has to be carefully designed so that there is sufficient
dynamic range to prevent strong signals from overloading the receiver (producing spurious
signals due to nonlinearities) and yet sufficient gain for detecting weak signals. In spite of
these difficulties, the zero-IF receiver provides an economical, high-performance solution for
many applications. A practical zero-IF receiver with excellent selectivity provided by DSP
filtering is described in QST [Frohne, 1998].
Interference
A discussion of receivers would not be complete without considering some of the causes of
interference. Often the owner of the receiver thinks that a certain signal, such as an amateur
radio signal, is causing the difficulty. This may or may not be the case. The origin of the
interference may be at any of three locations:
• At the interfering signal source, a transmitter may generate out-of-band signal components
(such as harmonics) that fall in the band of the desired signal.
• At the receiver itself, the front end may overload or produce spurious responses. Frontend
overload occurs when the RF or mixer stage of the receiver is driven into the
nonlinear range by the interfering signal and the nonlinearity causes cross-modulation
on the desired signal at the output of the receiver RF amplifier.
• In the channel, a nonlinearity in the transmission medium may cause undesired signal
components in the band of the desired signal.
For more discussion of receiver design and examples of practical receiver circuits, the reader
is referred to the ARRL Handbook [ARRL, 2010].
4–17 SOFTWARE RADIOS
Software radios use DSP hardware, microprocessors, specialized digital ICs, and software to
produce modulated signals for transmission (see Table 4–1 and Fig. 4–28) and to demodulate
signals at the receiver. Ultimately, the ideal software receiver would sample and digitize
received signals at the antenna with analog-to-digital conversion (ADC) and process the