PRS 2280 Digital 2nd IF and Demodulator

I worked for a total of 2 years at Plessey Roke Manor. One of the projects I contributed to was a digital IF strip and demodulator for a communications receiver.

In 1984, we replaced a bank of five crystal filters and a deal of demodulation electronics inside one of these PRS2880 receivers. We implemented quadrature digitisation at the point in the chain of the second conversion stage at 10.7 MHz. A pair of original TMS32010's implemented I and Q IF filters and a third such processor implemented a variety of demodulators (USB, USB, DSB, FM, AM and CW/BFO). In addition, some of the DSP operations were implemented in discrete TTL which was recently available in HCT form.

I designed and constructed the main circuit board, wrote all of the microcode (except for the Hilbert phase shifter), got it going and did some measurements. I particularly enjoyed implementing certain minor interface features, such as the digital AFC and tuning indicator. We actually implemented two systems, where one used aliasing in the ADC as the second conversion stage.

The performance of our system was very good, in terms of power consumption, I/Q balance and, surprisingly, image rejection. We wrote a short paper, and as a contribution to an IEE Colloquium at Savoy Place, this became my first publication outside of Wireless World.

This project changed communication receiver design: virtually all subsequent radio systems have used digital IF systems, from the car radio to the battlefield walky-talkies.

(C) DJ Greaves. 1984.