fcembrola@BTINTERNET.COM wrote:

> Dear Keith,
>
> thank you for your replay.
> You are right, it is a UHF TV tuner module.

Aha, so it is really a demodulator.
And thus you have video out and audio out?

> Someone else replied also to my query.
> And he said, VT should be connected to 33V.
> Can you confirm this?

I am almost certain it is relevant to the tuning voltage,
but what the tuner manufacturer means by this is unclear.

a) The (input) voltage to driving the varicap tuning diode.
This is the case for analogue-controlled tuner modules.
However, I'd guess the I2C bus is for operating
a digitally programmable PLL which controls the tuning voltage.
If this is the case (quite likely), then this pin is not
likely to be an input. You would not want digital _and_
analogue control, nor can I see how you could.

b) The (output) voltage driving the varicap tuning diode.
This is not a particularly useful thing to know
if you've got a digitally programmable PLL.

c) The supply rail for the varicap tuning voltage circuit.
The junction capacitance of a varicap diode changes with the
voltage across it. To get a practical range, you need more
than 0 to 5V across it. Typically up to 28V.
So the variable voltage circuit needs around 33V power rail.
This would be my bet as the most likely function of this pin.

But you can make an informed guess by applying the 5V rail
and measuring the voltage on this pin. If its always < 5V,
its probably not an output.

> what is the most economical way way of generating 33V from 12V?

The _cheapest_ way depends on where your circuit is.

TV manufacturers tend to tap off a high-voltage from their custom-spec
transformer and put it through a big value resistor into a 33V Zener
diode. This is fine, as the varicap circuit needs very little current.

For my teletext project I only had 5V and 12V, so I made a charge pump
from a CMOS hex inverter gate chip, some small diodes, and caps. This
converted 12V to 75V open circuit. After adding a 33V Zener and
connecting the tuner, you get a steady 33V rail. This works, I've tried
it. This should let your project run entire from a simple 12V supply
like a car battery / lighter socket.

Tuners are improving all the while. I thought it was neat when I got
one with all the video/audio built in (Philips FQ916).
Nowadays they come in smaller cans, have 33V chargepumps, and
even FM radios! These fancy ones get used in PCTV/radio cards.

Check out:

http://www.dc-na.comp.philips.com/tuner_product_index.html