Page 176 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

effectively so that the demodulated signal does not include this type of

low-frequency noise.
However, ideally, the filter should be a combination of a high-pass filter and a
low-pass filter in which the low-pass filter prevents signals from out of the band of
interest to be demodulated. The low pass filter is especially required if the SDR
front-end circuit includes a switch mode RF mixer. A switch mode RF mixer is also a
harmonic mixer which means that RF signals near a harmonic of the local oscillator
'
or switching frequency will also mix down to the IF. Examples of switch mode
mixers include commuting mixers and sampling circuits. It should be noted that a
band-pass filter has a high-pass and a low-pass filtering characteristic. Thus, at a
minimum, a single-inductor/capacitor (Le) band-pass circuit should be used for
filtering. But a multipo:le band-pass filter may be used for wider bandwidth and

sharper cutoff characteristics.
More elaborate filters usually result in better noise performance and overload
characteristics, though. More filtering restricts the signals to a smaller portion of the
RF spectrum and thus removes interfering signals for "jamming," or overloading,
the mixer.
Figure 12-2 presents examples of simple Le and multistage band-pass filters. The

circuits located at the top portion of the figure illustrate a simple Le band-pass
network, whereas the circuit at the bottom shows a multistage Le band-pass filter
for sharper cutoff to out-of-band signals and noise.