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PICList Thread
'Help with Digital Modulator'
1998\06\03@135034 by fcembrola

Hello picsters!

My project involves using a PIC 16C84 to
control a UHF digitally programmable
modulator. It uses the I2C BUS to program its
modulated frequency.

The modulator in question has 8 pins labelled
as follows:

1 - SCL (this is the serial clock)
2 - SDA (the serial data)
3 - VT
4 - Video in
5 - Audio in
6 - +5v
7 - GND
8 - Boost

I can understand, and know how to use all the
functions of the pins except for
pin 3 (VT). I have no idea what it is
supposed to do and how to use it. I would
appreciate any help or any hint where I can
get the right info.

The modulator part number is RMBT-UK36L

Francesco Cembrola

1998\06\03@161106 by Mark Meir

-----Original Message-----
From: fcembrola <spam_OUTfcembrolaTakeThisOuTspamBTINTERNET.COM>
Date: ŽŒ’ ¿‡Ž˜Ž, ŽŒÝŽ 03, 1998 08:50
Subject: Help with Digital Modulator

{Quote hidden}

I think the VT is V tune , this goes to the v v c  (voltag variable cap)
diode to tune the frequency of the modulator.
on TV tuners this voltag is 33v and is very stable.

Meir Mark.

1998\06\04@214813 by Dave Miga

fcembrola wrote:
{Quote hidden}

Francesco: The VT (or on some tuners TV) is the tuning voltage. It is an
OUTPUT that varies with the pll frequency chosen by the mcu.   -Dave
Miga, Electronic Design Specialists

1998\06\04@223058 by Nigel Goodwin

picon face
In message <>, fcembrola
<EraseMEfcembrolaspam_OUTspamTakeThisOuTBTINTERNET.COM> writes
{Quote hidden}

VT is the HT feed for the tuning voltage, it's usually 33 volts,
regulated using a 'super' zener such as a ZTX33B or a TAA550. I've just
checked the circuit of a Grundig GRD300 satellite receiver, this uses an
I2C modulator, this actually uses a 28 volt feed (which is also common
to the tuner), and is just a direct feed from the SMPSU, no extra
regulation at all.

BTW, did you manage to get a data sheet for the modulator, I've been
considering something similar, but don't seem to be able to locate data
sheets for the ones used in equipment that I've seen. I think most of
them seem to be made by either Alps or Mitsumi.


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1998\06\05@145922 by Keith Howell

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:

1998\06\05@172232 by David VanHorn

Where did this cute little device come from?

1998\06\06@001355 by Norman Gillaspie

> -----Original Message-----
> From: pic microcontroller discussion list
> [@spam@PICLISTKILLspamspamMITVMA.MIT.EDU]On Behalf Of Keith Howell
> Sent: Friday, June 05, 1998 2:50 AM
> Subject: Re: Help with Digital Modulator
This information (The analog Tuning Voltage) can be used at the factory
for alignment. Some PLL's can force the tuning voltage high or low
the osc frequency can be read and adjusted. In some cases the PLL
phase detector can be tri-stated and an analog volatage can be applied
by external test equipment and parameters measured or the I2C controlled PLL
can be stepped thru various frequencies and the voltage vs frequecny can be
measured. This can then be used to adjust the phase detector sensitivity
to compensate for differences. This can be used to maintain stability or
optmize performance.


*****PC-Sat provides netnews hosting and high speed news streams for ISP and
Commercial acounts. Norman Gillaspie. 650-864-5263 ********

{Quote hidden}

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