1972 · Single-Chip Radio Receiver · Work 863 of 1,871
A small home-built radio lying open on a pine kitchen table in England in the evening, on a board of brown pressboard six centimetres long a short black ferrite rod five centimetres long wound with fine copper wire, a small variable capacitor with a plain black knob, a single plain unmarked silver-grey button cell in a wire clip, and standing on three thin wire legs a tiny round silver metal can five millimetres across, a cream plastic crystal earpiece on a thin cable lifted by a hand in a knitted sleeve, a soldering iron on a folded cloth, a brown teapot soft behind, warm lamplight from the left.
What it was used for
The Ferranti ZN414, a complete AM radio circuit on one chip, was sold through the hobby trade to private buyers: in January 1973 Practical Wireless names Bywood Electronics in Hemel Hempstead, Henry's Radio in the Edgware Road in London and Davian Electronics in Oldham, the price the same from all three at 1.25 pounds including postage. The first documented project is Halvor Moorshead's Mighty Midget, a medium-wave receiver in a snuff box for about 2 pounds in all; the occasion was a demonstration of the part on the BBC programme Tomorrow's World at the beginning of November 1972. Externally it needed a ferrite rod aerial of 90 turns, a 250 pF trimmer, 2 resistors, 2 capacitors, a crystal earpiece and a 1.4 V mercury cell, which lasted over 400 hours for about 16 pence.
What was new
What characterised the part was that it needed no intermediate-frequency coils and no filter block, while the competing AM radio ICs were designed for IF filter blocks: a dealer's 1973 selector table lists the TAD100, TBA651 and CA3088E under the heading that they are all compatible with Toko filter blocks; Watford Electronics advertised the ZN414 in November 1973 as a complete receiver requiring no IF coils for tuning. The Mullard TAD100 was already on sale in Britain in March 1972 at 1.50 to 1.97 pounds. Technically the ZN414 was not a superhet but a tuned-radio-frequency receiver - a type discontinued in the thirties. In Practical Wireless L. A. J. Ireland explains why: an integrated IF stage would have to pass the signal several times between the chip and external components, each time incurring the risk of the instability the superhet was meant to avoid. Ferranti's own claim is narrow: the smallest radio IC in the world that offers a satisfactory alternative to the superhet.
What the sources leave open: The year 1972 is documented only indirectly: Ferranti gives the design start as November 1970 and the first working device as July 1971, and Practical Wireless dates the television demonstration to early November 1972. No product announcement from 1972 was found in the trade press; the first records are January and February 1973. On the CDI process, Ferranti's author and Wireless World agree: discovered at Bell Laboratories, developed further by Ferranti. Unit numbers are missing.