Thread: ISB Receivers
View Single Post
Old 4th Feb 2014, 11:55 am   #2
G8BBZ
Pentode
 
G8BBZ's Avatar
 
Join Date: Oct 2005
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 199
Default Re: ISB Receivers

You raise an interesting question. My own experience at the time was that the need for the "classic" 3-channel ISB receiver declined sharply around the time that commercial point-to-point receivers went solid state, because of other, related, developments in technology.
The need for the "carrier" recovery channel faded as the stability of receiver local oscillators improved. As virtually all commercial solid state receivers were designed as SSB receivers from the outset, they were sufficiently stable to resolve either sideband of an ISB transmission without the need for AFC. There were exceptions to this - so links carrying Lincompex circuits may well have still needed carrier based AFC systems to meet the extremely stringent requirements of the Lincompex control channel. By the time of the introduction of frequency synthesizers locked to high stability reference oscillators, the need for carrier AFC had really gone for good.
With the general reduction in size of equipment with the introduction of solid state circuitry, the provision of a second IF chain to support resolution of both sidebands of an ISB transmission simultaneously became less of a mechanical issue. A second IF channel could now be accomodated in a relatively small add-on unit or even as an additional module within the receiver enclosure. The improvement in performance and reduction in size and cost of the block crystal filters needed for sideband selection also made it easier to design ISB receivers, as did the improved performance and sophistication of the AGC systems used in solid state receivers.
Point-to-point circuits are invariably extended from the transmitting and receiving stations to some central traffic office over land lines. These tend to be relatively intolerant of large variations in signal amplitude, due to the prescence of balancing networks, line amplifiers and the like. Receivers designed for use in such applications typically had more "powerful" AGC systems than a receiver which would be used by an operator who could reach the volume control.
So the days of the 7ft. rack full of equipment came to an end. Motor-driven AFC loops no longer chase drifting transmitters up and down the band. Changing from the day to the night frequency is no longer a 20 minute exercise with many opportunities to get it wrong. Setting up an ISB circuit became a simple exercise of entering the frequency on the front panel, selecting ISB on the mode switch and making sure that both 600ohm lines are connected. The receiver was probably being used on a RTTY circuit earlier in the day, and may well be used on a simplex SSB circuit later on. Such was the flexibility and ease of operation which was the real benefit of the transition to solid state electronics.
cheers
Peter G8BBZ
__________________
Peter G8BBZ
G8BBZ is offline