Halfwave lowpass filters
Simple SDR RX

A Simple Software-Defined Radio (Hardware)

The software-defined radio you see above is heavily inspired on Gerald Youngblood’s articles in QEX. The receiver is based on the FST3253 analog multiplexer and uses a standard DIP oscillator (28.322 MHz) as its local oscillator.

A 74HC74 is used to divide the 28.322 MHz LO signal by four and simultaneously generates the necessary 90-degree quadrature signal. Thus, the center frequency is 7.080,5 MHz. The receiver generates two low-frequency signals, I and Q, which need to be processed further by a PC with a soundcard. The I and Q outputs are connected to the soundcard, which samples the signals at 48 kHz or greater.

The tuning range of the receiver is equal to plus and minus the sampling rate divided by two. Thus, at a sampling rate of 48 kHz, the tuning range is 7.080,5 +/- 24 kHz, which gives 7056.5 to 7104.5 MHz.

Because the mixer in the receiver uses switches, the receiver also receives RF signals found at odd harmonics of the tuning frequency. To remove these signals, the receiver needs preselection. The RF preselection filter is shown to the right.

A redesign of the circuit is needed to increase the supply voltage to the opamps, which is only 5V. Furthermore, the opamps load the mixer capacitors asymmetrically. Fixing this also requires a redesign.