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'[EE]: sync separator'
2002\09\18@140902 by -8859-1?Q?Hern=E1n_Freschi?=

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face
I've found a sync detector circuit on an old TV book, it's really simple
and works beautifully, the pulses are excellent, just that too low for
the pic to detect, around 2,4v (and even if it was, I still want it to
be 0 or 5v).

The circuit is pretty simple a BC548 with a 150k between base-emitter,
and 27k between base-collector. Input goes to the emitter and output is
on the emitter through a 0,1uF. Base goes to ground through a 10uF, and
power (6-12v) to emitter through 10k.

                        |----- 6-12v
                        _
                       | |10k
                       |_|
IN      E   C     |    .1uF
---------\   /-----++----||--- c_out
 |      -----      |
 | 150k   | B  27k |
 +/\/\/\--+--/\/\/\+
          | +
            = 10uF
            |
           gnd

(Circuit borrowed from a Sony TV-900U. I don't know if the original
transistor was germanium, but replacement is BC547, according to Google
;)

The problem is, that I want a 0V-or-5v square output. I Tried with a
Schmitt trigger and didn't work, so I tried with a LM393, and didn't
work. It works with a uA741, but for some reason the output is low, and
when I connect vref, works for a while and then stays high, looks like a
capacitor charging or something.

So I checked the comparator input with the scope, and notice that, even
though the size of the pulses is the same, the vertical position of the
waveform goes a little lower, like if I turned down the input voltage.

The circuit is like this:


vref---|\
      |+\___ out
      |-/
c_out--|/

What could be wrong?? It's too simple to not to work!!!

Hernan Freschi

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2002\09\18@145758 by Byron A Jeff

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On Wed, Sep 18, 2002 at 03:07:45PM -0300, Hernán Freschi wrote:
> I've found a sync detector circuit on an old TV book, it's really simple
> and works beautifully, the pulses are excellent, just that too low for
> the pic to detect, around 2,4v (and even if it was, I still want it to
> be 0 or 5v).

So why not simply follow it by a simple comparator set to 2.2V or so?

BAJ

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2002\09\18@152221 by Olin Lathrop

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> So why not simply follow it by a simple comparator set to 2.2V or so?

Or even run it straight into a comparator in the PIC.

On the other hand modifying the detector cicuit is probably easy too.  The
original poster of the circuit must have composed the ASCII art in
proportionally spaced font (DUH!), so it wasn't worth the trouble to follow.


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2002\09\18@155706 by Peter L. Peres

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part 1 117 bytes content-type:TEXT/PLAIN; charset=US-ASCII
I do what I preach. Remember when I wrote about a clamping circuit and an
amplifier ? Here goes (attached).

Peter


part 2 3604 bytes content-type:APPLICATION/PDF; name="syncsepa.pdf" (decode)

part 3 105 bytes
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2002\09\18@160829 by andy n1yew

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why not just use another 2n222 or bc458 transistor as a switch? 0.7v is the
saturating point of most 2n222 transistors.. a current limiting resistor to
the base from c-out, emitter grounded and the collector to vcc through a
resistor(10k or whatever for a pullup to 5v) note that this inverts the
voltage, if it is NPN, and you get the output 0 and 5v..

am i right?

andy n1yew

{Original Message removed}

2002\09\18@174941 by Olin Lathrop

face picon face
> I do what I preach. Remember when I wrote about a clamping circuit and an
> amplifier ? Here goes (attached).

I think this circuit would actually work, but it looks like a rather
hairbrain scheme.  Q1 and Q2 look like a sortof confused current mirror.
Since the Q1 emitter is grounded, its sole purpose seems to be to bias the
base of Q2 so that its emitter is close to 0V at a current of about 29uA.
The net effect is for the emitter of Q2 to present a low (a few KOhm,
dependent on the gain of Q2) impedence to signal levels below ground and a
high impedence (47KOhm) to signal levels above ground.  After several
cycles, this will bias C1 so that the sync tips are just a bit below 0V and
the rest of the signal above 0V.  OK, that's useful, although it could have
been done with a cap, diode, and resistor (Just like in an old fashioned TV.
Gee, maybe those old geesers did know something.  Hmm.)

Now comes the really strange part.  An LM393 is a low offset comparator with
common mode input range to ground and open collector output (hence the need
for R5).  R4 and R3 attempt to provide negative feedback around the
comparator for reasons I can only begin to guess at.  I doubt it will
actually operate nice and linear with a gain of about 10 as implied by R3
and R4.  Maybe this was an attempt to kludge around the fact that the
previous stage provides a signal that needs to be thresholded at 0V.  I
think there is a serious chance of hash on the output with large variations
in the video signal during the visible scan line phase.  Positive feedback
would have been the right thing here, but there's that problem of the input
signal already at the limits of the common mode range...

I would replace this whole message with a cap, two resistors, and a PNP
transistor, although the output would be inverted.


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2002\09\19@162337 by Peter L. Peres

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On Wed, 18 Sep 2002, Olin Lathrop wrote:

*>> I do what I preach. Remember when I wrote about a clamping circuit and an
*>> amplifier ? Here goes (attached).
*>
*>I think this circuit would actually work, but it looks like a rather
*>hairbrain scheme.  Q1 and Q2 look like a sortof confused current mirror.

The circuit is tested and works perfectly as shown. Output is composite
sync with Voh >= 3V, Vol < 0.3V with 10pF load, with flanks shorter than
300nsec (and some compressed video above Voh). By changing the feedback
resistor to a higher value the output becomes rail to rail into 10pF load.

*>Since the Q1 emitter is grounded, its sole purpose seems to be to bias the
*>base of Q2 so that its emitter is close to 0V at a current of about 29uA.
*>The net effect is for the emitter of Q2 to present a low (a few KOhm,
*>dependent on the gain of Q2) impedence to signal levels below ground and a
*>high impedence (47KOhm) to signal levels above ground.  After several

One cycle, max two. The current is 29uA when VeQ2 == VeQ1. When the
emitter of Q2 is taken 10-50mV below that voltage the current becomes very
significant. You can consider that C1 is being held to 0V though 2 ohms or
so for low enough currents (<100 mA). I suggest a modeling with SPICE,
using a PWL input source that mimics a few (2-3) TV line signals. I used
BC182 transistors with beta about 250 at Ib = 100uA.

*>cycles, this will bias C1 so that the sync tips are just a bit below 0V and
*>the rest of the signal above 0V.  OK, that's useful, although it could have
*>been done with a cap, diode, and resistor (Just like in an old fashioned TV.
*>Gee, maybe those old geesers did know something.  Hmm.)

The trick is to clamp to 0V so a dc amplifier referenced to GND can do the
rest. If a transistor amplifier would be used it would be useful to clamp
to some other voltage, perhaps 0.5V.

*>Now comes the really strange part.  An LM393 is a low offset comparator with
*>common mode input range to ground and open collector output (hence the need
*>for R5).  R4 and R3 attempt to provide negative feedback around the
*>comparator for reasons I can only begin to guess at.  I doubt it will
*>actually operate nice and linear with a gain of about 10 as implied by R3
*>and R4.  Maybe this was an attempt to kludge around the fact that the
*>previous stage provides a signal that needs to be thresholded at 0V.  I
*>think there is a serious chance of hash on the output with large variations
*>in the video signal during the visible scan line phase.  Positive feedback
*>would have been the right thing here, but there's that problem of the input
*>signal already at the limits of the common mode range...

The circuit was tested and did not show problems with violent changes in
video level (black/white alternating fields from generator).

*>I would replace this whole message with a cap, two resistors, and a PNP
*>transistor, although the output would be inverted.

You can use whatever you want but this circuit was 'designed' by me
precisely to obtain a clean rail to rail output with usual components and
no adjustments (I already used the other half of the 393 in my circuit).
The output is rail to rail and clean regardless of the supply (3V to 12V
was tested) with the higher gain option. For some strange reason the
output was still ok at 1.4V. Hm, LM 393 is not specced to 1.4V. It is also
stable over temperature (the current mirror does that if Q1 and Q2 are
mounted together as they should be).

I understand that you analysed the circuit 'cold' or with SPICE. I did
some testing with SPICE and then built the circuit on a breadboard and
tested it.

The opamp need not be 393, with a 358 one obtains vertical sync pulses
(the 358 is not fast enough to follow the hsync), a 311 behaved much like
the 393, etc. The opamp must include ground in the input voltage range.

Peter

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2002\09\19@182608 by Olin Lathrop

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> When the
> emitter of Q2 is taken 10-50mV below that voltage the current becomes very
> significant. You can consider that C1 is being held to 0V though 2 ohms or
> so for low enough currents (<100 mA). I suggest a modeling with SPICE,
> using a PWL input source that mimics a few (2-3) TV line signals. I used
> BC182 transistors with beta about 250 at Ib = 100uA.

There is no way you will get 2 ohms effective impedence for negative
voltages.  You will get much lower impedences than for positive voltages,
which is why this circuit works.  The 150KOhms of R1 will appear on the
emitter of Q2 roughly divided by the current gain of Q2.  You say this is
250, so the impedence will appear about 600 ohms.

> I understand that you analysed the circuit 'cold' or with SPICE. I did
> some testing with SPICE and then built the circuit on a breadboard and
> tested it.

I agree that it works.  My point was that it could have been done much
simpler.


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2002\09\20@032541 by Peter L. Peres

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On Thu, 19 Sep 2002, Olin Lathrop wrote:

*>There is no way you will get 2 ohms effective impedence for negative
*>voltages.  You will get much lower impedences than for positive voltages,

You are right here, anyway it clamps the video reliably and quickly. I had
used a much lower bias resistor in the SPICE simulation, that's where the
2R came from, it was actually 20R I think, with 10K bias resistor.

Peter

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