Re: DESIGNING EXTERNAL RETRACE-BLANKING

tomg@xxxxxxxxxxx wrote:
Dear Analog Gurus,

I need some help/sanity-checks/ideas/more pairs of eyes:

I am trying to add a retrace-blanking circuit to my existing
curve tracer design, which uses a separate oscilloscope for
its display. The output of the blanking circuit will connect to
the scope's External Z-Axis Input, which needs >= 5v (into
10K, I think) to blank the trace.

What I have come up with is ALMOST seeming workable.
But I'm running into trouble, trying to make it work robustly
all the way from 60 Hz to 22 kHz.

I've uploaded LT-Spice files for what I have, so far, here:

http://www.fullnet.com/u/tomg/blanking.asc
http://www.fullnet.com/u/tomg/blanking.plt
http://www.fullnet.com/u/tomg/op275.sub
http://www.fullnet.com/u/tomg/op275.asy

[In LT-SPice, open and run the "blanking.asc" file. The other three
are then used automatically, as long as they're in the same folder.]
[Download LT-Spice (AKA SwitcherCad III), free, from
http://www.linear.com.]

The circuit is essentially just a buffered inverting differentiator
followed by a comparator, in order to generate a positive pulse
only during the fall-off at the end of the ramp, and zero elsewhere.

Included are two "artificial" voltage sources that closely approximate
the existing available sawtooth ramp sweep signal, at both 60 Hz
and 22 kHz. (This is actually just the available "raw" sweep signal,
from before it is set to the desired amplitude, offset-zeroed, and
power-amplified, which is used because it will always have
approximately the same amplitudes and offsets.) Just use the
"Move" mode to move the jumper ("X2"), to select one of the
sweep voltage sources.

The main problem it has is that when the differentiator is
made "fast-enough" for good timing at 22 kHz, its output at 60 Hz
has a way-too-low amplitude. It can be made to work, in
simulations, at both 60 Hz and 22 kHz. But when the 22 kHz
response is (almost) "good-enough", the 60 Hz response is
extremely dicey, relying on offsets and amplitudes being
within ranges on the order of a few hundred uV, which
seems unworkable outside the realm of simulation.

I would hate to have to try to squeeze one more 8-DIP onto
the existing board, beyond the two that this circuit will
already require. But, if I HAVE to, I guess I MIGHT be able to find a
way...

Any suggestions would be MOST welcome. I did also try using
a 555 as a one-shot, triggered with an existing end-of-ramp "sync"
pulse (normally used to sync the transitions of the staircase
waveform that's used for Base/Gate drive). But the differentiator
seems like a more straightforward way to get the beginning and
end of the pulse to be in the right places, when dealing with
varying ramp frequencies/fall-times.

Thanks!

Regards,

Tom Gootee, 15MAY2006

tomg at fullnet.com

------------------

OK, I realize that this probably isn't terribly interesting or
exciting,
to most of you. BUT, just in CASE someone might actually
be interested, at least someday, here's a Progress Report::::

---------

I have reworked the External Oscilloscope Blanking-Pulse Generator
design (for an X-Y-Mode (XY) scope's "External Z-Axis Input") (an
add-on for
my existing Curve Tracer, for use only when the Sawtooth Ramp
sweep-waveform-type is selected, i.e. not when either the Sine or
Triangle Ramp
Device-Under-Test (DUT) Sweep Waveform type is selected).

And... I made significant improvements in performance and robustness,
to the point where I THINK it might now be "Not Crap", and,
dare I say it(?!), MAY even be "Good Enough", OR, EVEN, possibly,
(gasp!!)... "Good" !!

(But OH how I more than half-expect to be "corrected", about that!
;-) )

I decided to just "bite the bullet" and "add another opamp", to
boost the differentiator's output (by 101x), before sending it to
the comparator, which did the main trick.

I also "corrected" the differentiator's component-value ratios,
ditched the weird ac-coupling attempt, and used much-faster opamps.
And, I inserted V-sources to simulate worst-case offset-voltages, for
temperatures from 0 to 100C, and then set resistances to make sure
that the comparator's input and output would stay below the proper
thresholds, for all expected sweep/input, temp, and offset conditions,
except for negatively-sloped sweep-voltage portions, which are where
the blanking of the scope's trace is desired.

The new blanking-circuit design's LT-Spice files are (or, will soon be)
at:

http://www.fullnet.com/u/tomg/blankng2.asc
http://www.fullnet.com/u/tomg/blankng2.plt

[Note: No other user-created files are needed, to run the simulation
with
the files given above. You would Open the "blankng2.asc" file, and run
it.
The "blankng2.plt" file would then be automatically used (just to set
up
the "Plot Settings" that I was using, for the waveforms' display
window).]

I will probably try to change at least the first (buffer) opamp (U1)
from an LT1364
back to an OP275 (or maybe something else, actually), to save $.

I am also seriously considering just using a JFET and a couple of
resistors, as
the input buffer, instead of an opamp, especially if it will save
board real estate.

And... I would *LIKE* to find a widely-available opamp that has a
slew-rate that's
SOMEWHAT-similar to the LT1364's 1000V/uS (in a Dual 8-DIP), except
cheaper (by a LOT, I hope). And, I'm guessing, a significantly-slower
opamp
would probably work just about as well, anyway (and maybe better,
especially
considering the 1364's offset voltage drift versus temperature).
(Besides, I just
"threw in" the LT1364, because I wanted to try a faster opamp and a) I
have some on
hand and they are used elswhere in this unit, and, maybe mainly in this
case,
b) just because it was in the built-in LT-Spice component library and I
was
in a bit of a hurry.)

That reminds me: I'd also like to find a cheap, widely-used
comparator, in
8-DIP or smaller (through-hole only), to use in place of the LT1011
comparator,
which I used in this design and simulation mainly just because it was
already
available in the built-in LT-Spice component library (and, because I
was *hoping*
that its performance would be quite similar to that of many
"run-of-the-mill"
comparators that I might eventually specify for this circuit).

Thanks for wading-through all of that! SOrry to have blathered-on for
so long!

If anyone has ANY opinions or comments, suggestions, ideas, etc, about
anything
related to this circuit, I would REALLY appreciate hearing them!!

Thanks, very much.

(On your marks.... Get set... GO!!) :-)

Regards,

Tom

tomg at fullnet.com

http://www.fullnet.com/u/tomg

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