I can't provide a whole lot of info here, but here are some comments.

I GUESS these are ceramic filters. They sound like they are for the sound
section of an NTSC TV receiver. NTSC transmitted sound as an FM carrier
with +/-25kHz deviation 4.5MHz above the video carrier. Since the video
was transmitted using vestigial sideband (lower sideband suppressed below
something like 1.25MHz), the sound carrier appeared to be the same as a
4.5MHz subcarrier on the video. Early TVs used a separate FM receiver for
sound, but, for maybe the last 50 years or so, TVs used "intercarrier"
sound where the audio was considered to be a subcarrier on the video
signal. So, the complete 6MHz channel went through the video IF. An
envelope detector demodulated the VSB AM video. The sound carrier then
appeared as a 4.5MHz signal above the video. This was notched out with a
filter before being sent to the remainder of the video circuitry (color
demodulator, etc.). A 4.5MHz bandpass filter was used to pull out the
sound carrier, which was then sent to the sound IF. Finally, the sound was
demodulated using one of several types of FM demodulators. A common one is
the quadrature detector or discriminator. These used either a "quadrature
coil" (parallel resonant LC) or a ceramic discriminator. That's what you
have. I've used a ceramic discriminator with an SA639 to demodulate FSK
data.

Good luck!

Harold


> Hello all,
>
> in my attempts to learn more about RF in general I've been playing with
> tuners, downmixers, etc.
>
> My next stumbling block has been filters.
>
> The particular circuit I'm working on needs a 4.5MHz filter. Researching
> I've found that they are common in TV/VCR gear, so I figured I could
> scavenge a few and play around with them.
>
> Finding datasheets has been very difficult however. I was wondering if
> someone here could provide some guidance?
>
> The filters I'd like to play with look like this:
> http://www.surplussalespa.com/catalog/product_info.php?cPath=3D&products_=
id=3D440
>
> What's confusing me is the numbers.
>
> I have 4 filters on a board for a portable TV in front of me, they are
> labelled:
> a4.5A (5 pins)
> T4.5B (3 pins)
> F4.5D (3 pins)
> D4.5Z (3 pins)
>
> I understand the difference between the 3 pin and 5 pin parts, but what
> do they suffixes (and perhaps the prefixes) mean?
>
> For example, some datasheets I've found seem to indicate that the only
> difference between A, B, D and Z are pass bandwidth.
>
> Others indicated for example that the B is a notch filter (although the
> surplus site that I linked indicates it's a pass filter?!)
>
> Some describe the filter as a "Ceramic Discriminator", what does that
> mean?
>
> Thanks for any help!
>
> TTYL
>
> --
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