[opendtv] Re: opendtv Digest V2 #24

S J Birkill wrote:


> 
> Conventional DTT tuners (i.e all the others, I believe) in the UK use
> single conversion with narrow tracking filters, derived from standard
> analog tradition, but with improved-phase-noise converter chips (Philips,
> Infineon) for digital. These, as with all consumer analog TV tuners
> worldwide, incorporate a simple tracking filter of nominally 12 to 15 MHz
> bandwidth, which is production-line tuned by bending coils or 'goalpost'
> wire loops until a response template is met. Inevitably tracking is
> sub-optimal, and in-channel slope, as well as NF, can be quite high, which
> limits ACI protection as well as sensitivity.


Do these filters use varactor tuning diodes? How do they
avoid overload problems with intrinsically nonlinear
devices (short of dual balanced designs to remove
even order nonlinearities of course). Where I live
one FM station drives about 0.3 volt peak of signal into 75 ohms.

But with varactor diodes doing the tuning, one should use
feedback at assure that the RF tuning was centered on teh
channel, thus assuring no problem in that regard.

Doug McDonald
 
 
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