Page 175 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

FIGURE 12-1E Selecting sampling rate via Winrad.
Should the sampling rate top out at a number lower than 192,000, upgrading the
sound card to a 192-kHz sampling rate is recommended. However, in an SDR, it is
still possible to have very usable tuning results with existing sound cards with

sampling rates of 44.1 kHz, 48 kHz, or 96 kHz. One solution is to obtain a
variable-frequency oscillator such as a direct-digital-synthesis (DDS) generator,
which is available in kit or completed form. The DDS generator can tune to the
stations instead of the Winrad software program.
SDR Front-End Circuits, Filters, and Mixers

As stated previously, the SDR front-end circuits usually require some type of filter
prior to mixing or down-converting to the low-frequency IF signal for the computer

to process. The filter generally must cut off or attenuate low-frequency noise in the
IF band of about 100 kHz and below. Therefore, the simplest filter can consist of a
high-pass filter with a cutoff frequency at about 200 kHz to 100 kHz. With a
high-pass filter, low-frequency noise such as power-line hum can be attenuated