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From: et472@FreeNet.Carleton.CA (Michael Black)
Subject: Re: 10.7MHz video demodulator
Date: 19 Jan 2003 17:17:53 GMT
Organization: The National Capital FreeNet
Reply-To: et472@FreeNet.Carleton.CA (Michael Black)
NNTP-Posting-Date: 19 Jan 2003 17:17:53 GMT
X-Given-Sender: firstname.lastname@example.org (Michael Black)
John Woodgate (email@example.com) writes:
> I read in sci.electronics.design that Steve Andrew
> wrote (in <6OjW9.2073$m47.10729@news-
> server.bigpond.net.au>) about '10.7MHz video demodulator', on Sat, 18
> Jan 2003:
>>I cannot say what bandwidth your R8500 has at 10.7MHz
>>but my ICOM R7000 bandwidth is around 5-6MHz so you should be able to
>>receive the sound and colour carriers as well.
> It sounds suspiciously like masochism to design a 6 MHz bandwidth IF
> with a centre frequency of 10.7 MHz. The asymmetry problems must be
I suspect it's not designed there, just incidental.
Likely, there is a roofing filter at some higher IF, and that happens
to be 6MHz or so wide. Though, who knows whether that wide an IF
is due to design limitations, or they deliberately made it that wide
for the sake of this TV adaptor. If the latter, it seems pretty silly
since it must impact on the rest of the design, and TV sets are pretty
good for that sort of thing. More likely, if it's deliberate, it's
there for panadaptor type things (a visual representation of the signals
in that 6MHz or so band segment) and the TV adaptor is again incidental.
So the point that being tapped is at the output of the mixer that
brings the signal down to 10.7MHz, before any filtering (or minimal
filtering) comes into the signal path. There may be a good reason in
the overall design which makes the output point at that frequency rather
than at the higher IF.
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