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Front End (TUNER)

Discussion in 'Electronic Design' started by Myauk, Mar 23, 2007.

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  1. Joerg

    Joerg Guest

    I did back in the 90's. Shook my heard and discarded the tuners
    afterwards, then cracked out a Mini-Circuits DBM to build the receiver.
    In the 80's they were better but there I used tuners scrapped out of
    older TVs. Probably the older engineers still knew how to do it. One had
    Ge-Transistors (AF239?) and was, well, somewhat ok. The other one had
    two tubes and was quite excellent.

    And sure enough the TV where that tube tuner came out of was the best
    (until it's flyback XFMR went up in a plume of smoke one fine day). When
    a powerful pager system was installed near our home almost all "modern"
    transistorized TVs fell off the rocker. This signal was even
    out-of-band, BTW. Telecom came out with lots of expensive Rhode&Schwarz
    gear, determined that poor tuner design was to blame so they were not
    legally obliged to do anything about it yada, yada, yada.... Lots of
    negative PR mounted and then they agreed to supply notch filters for
    free. I mounted a lot of these for neighbors because installation was
    not included.

    Besides the transistor versus tube thing there was another major
    difference: The tube tuner had a ganged variable capacitor while the
    transistor version used varicap diodes. I did not investigate the tuners
    in our current sets. Not really worth it. Not just because of lousy RF
    performance but except for an occasional old movie and the evening news
    there ain't much to watch anyhow.

    Even the old tuners were "in a can", except that those cans were the
    size of a small paperback book. Very nicely done. For example, they had
    thin copper foil on a rubber cushion to make sure that the lid was
    RF-tight. They don't make'em like that anymore :-(
  2. Rich Grise

    Rich Grise Guest

    Did you try an attenuator? Every TV I've had had an exquisitely sensitive
    front end - it's almost amazing what an ordinary $39.00 table model can
    pluck out of the noise. I can see how shoving the output of an antenna
    preamp or distribution amp into it could overwhelm it.

  3. Joerg

    Joerg Guest

    That's what I ended up doing, losing a few channels :-(

    Thing is, the old ones didn't need that yet were sensitive enough to
    recognize the noise increase when an antenna was connected. That's how
    it ought to be but is no more.

    BTW it's the same with radios. Two classic examples: My old Becker car
    radio always worked. Good old dial with string and pulleys. Then I
    bought an Audi and it had their, ahem, high-end radio (called "Gamma").
    Horrible. In half of our town all you heard was "deedle dit deeee deedle
    dee". The newly installed paging system, on every FM channel. Yuck.

    The other was here in the office. Nice "modern" stereo fell apart every
    night on the AM band. A cacophony of noise. Out it went. In came ye olde
    tube radio from the 50's and it works just fine, night and day. Oh,
    Phil, you'd like this: The well-performing tube radio was made in
    Australia, it's an Astor BPJ. The enclosure is IMHO ugly but it sure
    works nicely.
  4. Gary Tait

    Gary Tait Guest

    Reverse encgineer the tuner. Of find a datasheet on it.
    It isn't. I2C only controls the PLL IC, which may have GPIO to control
    band selects.
    In a nutshell, it is a couple voltage tuned filters, feeding a mixer
    supplied by a voltage tuned oscillator. The PLL IC, via I2C, tunes the
    voltage tuned oscillator, and by proxy, the tuned filters. The output of
    the mixer goes into an IF strip, and a video detector. There may be a
    number of bands using different coils in the tuned filters, selected by
    PIN diodes, which are controlled by GPIOs on the PLL chip, or discrete
    inputs on the module.
    As said, the 33V is just to tune the varicaps. It requires a higher
    voltage to tune higher frequencies. It only needs to be a few ma for
    that. The 5V can be a standard 5V supply, although if your project has
    sigificant digital, it should be a separate analog 5V.
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