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G8BBZ · 19th Apr 2016, 12:09 pm

The Redifon R408 variable IF bandwidth is realised by the use of two 8kHz wide bandpass filters, one with a nominal centre frequency of 76kHz (the lower filter) and one with a nominal centre frequency of 84kHz (the upper filter) Both of these filters have tunable centre frequencies, the lower filter from 76kHz to 80kHz and the upper filter from 84kHz to 80kHz. Tuning is effected by the IF Bandwidth control and which filter moves and by how much is determined by the type of modulation selected.
The overall passband of the 80kHz IF stage is determined by the extent of the overlap of the passbands of the two filters.
So for AM, the lower filter centre frequency is moved up and the upper filter centre frequency is moved down to provide a symmetrical overlap passband.
For SSB, one filter is set to the nominal centre frequency - which puts one edge of the passband at 80kHz and the other filter is tuned to provide overlap. So for USB, the upper filter is set to the nominal centre frequency, such that the lower edge of the passband is at 80kHz, and the lower filter centre frequency is tuned to, say, 79kHz giving a passband overlap from 80kHz to 83khz.
If the sideband selected is now switched to LSB, then the filters are automatically moved so that the lower filter is at the nominal centre frequency, such that the upper edge of the passband is at 80kHz, and the upper filter is moved to a centre frequency of 81kHz giving a passband overlap from 80kHz to 77kHz.
There are no block filters of the type usually associated with true ISB receivers, and as there is only one 80kHz IF filter and one audio chain reception of both sidebands simultaneously is not possible. The wording used in the literature is confusing in this respect.
I think it must be correct that ships equipped for true ISB working for passenger telephone calls must have worked direct to PTT operated land stations rather than conventional coast stations. As far as I am aware, those ships which did have ISB telephony capability also carried a separate, conventional marine radio installation for normal ship's traffic. As to which land stations were used, I await somebody to let us know!
cheers
Peter G8BBZ

19th Apr 2016, 12:09 pm	#34
G8BBZ Pentode Join Date: Oct 2005 Location: Hebden Bridge, West Yorkshire, UK. Posts: 200	Re: ISB Receivers The Redifon R408 variable IF bandwidth is realised by the use of two 8kHz wide bandpass filters, one with a nominal centre frequency of 76kHz (the lower filter) and one with a nominal centre frequency of 84kHz (the upper filter) Both of these filters have tunable centre frequencies, the lower filter from 76kHz to 80kHz and the upper filter from 84kHz to 80kHz. Tuning is effected by the IF Bandwidth control and which filter moves and by how much is determined by the type of modulation selected. The overall passband of the 80kHz IF stage is determined by the extent of the overlap of the passbands of the two filters. So for AM, the lower filter centre frequency is moved up and the upper filter centre frequency is moved down to provide a symmetrical overlap passband. For SSB, one filter is set to the nominal centre frequency - which puts one edge of the passband at 80kHz and the other filter is tuned to provide overlap. So for USB, the upper filter is set to the nominal centre frequency, such that the lower edge of the passband is at 80kHz, and the lower filter centre frequency is tuned to, say, 79kHz giving a passband overlap from 80kHz to 83khz. If the sideband selected is now switched to LSB, then the filters are automatically moved so that the lower filter is at the nominal centre frequency, such that the upper edge of the passband is at 80kHz, and the upper filter is moved to a centre frequency of 81kHz giving a passband overlap from 80kHz to 77kHz. There are no block filters of the type usually associated with true ISB receivers, and as there is only one 80kHz IF filter and one audio chain reception of both sidebands simultaneously is not possible. The wording used in the literature is confusing in this respect. I think it must be correct that ships equipped for true ISB working for passenger telephone calls must have worked direct to PTT operated land stations rather than conventional coast stations. As far as I am aware, those ships which did have ISB telephony capability also carried a separate, conventional marine radio installation for normal ship's traffic. As to which land stations were used, I await somebody to let us know! cheers Peter G8BBZ __________________ Peter G8BBZ