Software-defined radio (SDR) is all the rage. The idea is this: a very simple radio receiver is given topof- the-range performance with the aid of a little software. Even newer is SDVR (softwaredefined valve radio), where a single- valve radio is turned into a world receiver with some help from a PC. Power comes from four AA cells for the heaters, and a 9 V battery provides the anode supply. The circuit is very simple: a PC900 (EC900) triode is used in a homodyne regenerative (Audion) configuration. Adjustment of the feedback is not necessary as the receiver always oscillates at high amplitude. A tuning capacitor can also be dispensed with as fine tuning is done in software. Coarse adjustment of the received band is possible by screwing the inductor core in and out. The receiver works in the 49 m band using a 30-turn coil on an 8 mm former.
The SDRadio program by Alberto (http://digilander.libero.it/i2phd/ sdradio) is used as the decoder. The illustration shows an AM station being received. The sound card used (a USB Sound Blaster) has a sample rate of 96 kHz, giving a tuneable range of 48 kHz. In the illustration we can see three further transmissions. A weakness of the receiver is that it only has one output channel. This means that each transmitter can be seen twice in the spectrum display, and there is no suppression of image frequencies as would be expected in a fully-featured SDR. Sometimes this can result in audible interference, in which case the only remedy is to tune to another transmitter. And if none of the channels appeals, you can simply move to another band with a twist of the screwdriver.