The "kitchen SDR"

Pieter-Tjerk de Boer, PA3FWM web@pa3fwm.nl

(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, April 2017.)

Recently I bought a very cheap kitchen radio (Tesco type 113B, 9 pounds sterling), after someone said on a mailling list that this radio was surprisingly insensitive to local QRM from domestic appliances. The radio looked like a typical simple analog radio, so I wanted to know how they achieved this.

The interior was a surprise to me: it was not, as expected, a simple analog superheterodyne receiver, with coils, MF transformers and a tuning capacitor, but in essence a single IC with only a small quartz crystal, some resistors and (de)coupling capacitors around it: antenna signal in, loudspeaker signal out. Tuning is done using a DC voltage set by a variable resistor. The datasheet of the IC, an AKC6952, is in Chinese and doesn't say much about how it works, but there are similar ICs about which more is known, such as the Si4835 from SiLabs.

Radio amateurs mostly know SiLabs from their Si570, a synthesizer chip for frequencies between 10 and 160 MHz, with a built-in crystal and tuning in steps of less than 1 Hz, and whose phase noise performance is good enough for many radio applications. Among others, it is often used in simple SDRs with a quadrature mixer.

The Si4835 datasheet is a bit more informative than the ACK6952's, and gives a block diagram. We see that it is in fact a complete SDR: a synthesizer, a quadrature mixer, A/D converters, digital signal processing (DSP), and D/A onverters to drive loudspeakers.

I can't know for sure whether the AKC6952 also has this structure. But measurements on the kitchen radio show that it tunes in 3 kHz steps, also does AFC (Automatic Frequency Control) in such steps, and has sharp filters: all of that hints at a digital implementation.

I had heard of such ICs before, but was surprised to find it in such a cheap kitchen radio. But in fact it makes sense that one such a mass-fabricated chip is cheaper then the assembly and alignment costs of an analog receiver. Still, it is somewhat bizarre that although there's a precise frequency synthesizer inside, an imprecise analog mechanism is used for tuning, apparently because that's how a kitchen radio is supposed to look. According to the datasheet, this is even a feature: "mechanical tuning".

It would be nice if we as amateurs could write new software for that DSP, for example with narrower filters and a demodulator suitable for SSB and CW. But unfortunately that is not possible. About similar ICs, like the Si4735, it is known that new software can be loaded, but that software is encrypted, so only the manufacturer can make it.