View Single Post
Old 27th Oct 2021, 9:43 am   #16
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 18,809
Default Re: Racal Preselection & Protection unit

The target market for the RA17 were military and governmental organisations. At the time, these people used CW Morse code, FSK for teleprinters, and AM for speech. There wasn't any significant interest in spending money on SSB capability.

So the RA17 got narrow filters and a BFO for Morse, that 1.2kHz filter and the BFO for FSK to feed a teleprinter via a 'terminal unit' that FSK de-modulated audio tones, and the wider bandwidths with the BFO off to do AM.

But SSB was a growing thing. It's spectral efficiency and its power efficiency were attractive, so Racal brought out SSB adaptor boxes with a whole duplicate final IF in them, with a balanced 'product' detector and a stable BFO preset to the right frequencies.

So the RA17 straddles the rise of SSB in military/governmental radio systems.

SSB was invented as a method of cramming multiple conversations onto a single phone line and dates back to the 1920s, but it took a while from invention to getting rolled out in any quantity. These systems were called FDM, frequency division multiplex, and eventually were cramming up to 2600 channels onto a single coax line in the 1970s. They ended as digital techniques replaced them.

It was some radio amateurs who tried using SSB techniques over the air instead of over wires. One prominent exponent of the time was called Arthur (Art) Collins. Yes, that Collins, he went on to found Collins radio. Bell had got what they wanted for their phone lines, they had much less interest in radio things.

So, Collins receivers have SSB capability from early on, It's where they came from.

Racal sets after the RA17/RA117 have SSB facilities built in. When they did the cute little RA1217 and RA217, their first transistor sets, they got full-blooded SSB filters product detectors and accurate oscillators for carrier reinsertion. Filters became optional an so you could order all sorts of combinations, you didn't have to have the SSB filters. As early transistor jobs, the fornt ends of these sets are disappointingly easy to overload.

The RA1772 came along and the front end trouble was definitely fixed. It was an abrupt and large jump in performance.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote