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Old 29th Nov 2017, 9:20 am   #19
Synchrodyne
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Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Introduction PCC189 in Mullard tuners?

Hi Pieter, thanks for the additional background.

Something that particularly caught my attention in Part 1 was your discussion of the IF choices for the French 819-line system. It hadn’t occurred to me previously that any oscillator frequency that fell within Band III also had to fall on the vision or sound carrier for another channel. But now knowing that, the rest of the calculation falls into place. Of course, even if there were any apparent gaps in which local oscillators could fall safely, with the tête-bêche channelling system, they would not align between the even and odd channels. Also, to achieve 13.15 MHz channel spacing for the tête-bêche system as compared with the original 14 MHz, the outer guard bands – 0.1 MHz outside the sound carrier and 0.75 MHz outside the vision vestigial sideband – were discarded. Then the even channels were moved up 0.25 MHz relative to the odd channels (which were “hard left” against the 162 MHz lower limit of the band), I think for the best (and equal) disposition of the both the even and odd sound carriers relative to the vision sidebands. Thus, with the LO frequencies effectively determined by the channel plan, so in turn were the IFs. For an IF channel whose upper edge was just below 40 MHz, the vision IF had to be a multiple of 13.15 plus an offset determined by the relationship between the even and odd channels. For an IF channel with vision carrier at the low end, it was 28.05 MHz, as twice 13.15 MHz plus 1.75 MHz, the latter being the separation between the sound carriers of corresponding even and odd channels. And for the vision carrier at the high end, it was 35.7 MHz, as twice 13.15 MHz plus 9.4 MHz, the latter being the separation between the vision carriers of corresponding even and odd channels.

The decision to standardize on 28.05 MHz vision, 39.2 MHz sound, with oscillator high for the even channels and oscillator low for the odd channels effectively precluded the use of channel F3, for which one transmitter had been shown in the ITU Stockholm 1952 list. It would have required a local oscillator frequency of 28.1 MHz.

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With the oscillator frequency jumping from high side to low side channel-by-channel, it may have restricted the tune choice to the turret type. Possibly it could have been done with a switched incremental inductance tuner, but with a resultant awkward channel sequence.

The 36.15 MHz (vision) IF shown for the earliest 819-line receivers was, I imagine, worked out for the original channelling plan, of which only the first channel was ever used, and which survived as F8A.

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Cheers,
Synchrodyne is offline