Eliminating a specific voltage from a sawtooth waveform

[Paul Burridge]

Hi chaps,

Let's say I have a sawtooth wave generator. It cycles from 0 to 7V
over 30 seconds and then resets to 0 and so on and so on. Now let's
say I want to eliminate, for example, the part of the ramp from 3.3V
to 3.8V. Doesn't matter whether the output's held temporarily at 3.3V
for a spell and then jumps straight to 3.8V and continues to rise, or
else is grounded (or supply railed even) for that same short interval.
I just don't want that particular voltage span appearing at the
output. What's the best way to go about it?

Thanks,p.

 

[Roger Johansson]

Let's say I have a sawtooth wave generator. It cycles from 0 to 7V
over 30 seconds and then resets to 0 and so on and so on. Now let's
say I want to eliminate, for example, the part of the ramp from 3.3V
to 3.8V. Doesn't matter whether the output's held temporarily at 3.3V
for a spell and then jumps straight to 3.8V and continues to rise, or
else is grounded (or supply railed even) for that same short interval.
I just don't want that particular voltage span appearing at the
output. What's the best way to go about it?

A window comparator detects that interval and grounds the output.

Details: Two comparators detect when the voltage is over level1 and
under level2. The outputs from these comparators are used in a logic
gate so when both are on the output from the logic gate turns on the
transistor which grounds the output.

 

[George R. Gonzalez]

Hi chaps,

Let's say I have a sawtooth wave generator. It cycles from 0 to 7V
over 30 seconds and then resets to 0 and so on and so on. Now let's
say I want to eliminate, for example, the part of the ramp from 3.3V
to 3.8V. Doesn't matter whether the output's held temporarily at 3.3V
for a spell and then jumps straight to 3.8V and continues to rise, or
else is grounded (or supply railed even) for that same short interval.
I just don't want that particular voltage span appearing at the
output. What's the best way to go about it?

Thanks,p.

If you want it to be really simple, you could make a mickey-mouse
window gadget with three resistors and two diodes.

The three resistors make up a voltage divider with two taps.

The bottom tap biases a diode into conduction during the bottom 0 to 3.3V
part. The top tap biases another diode into conduction from 3.8 on up.

It will of course have the standard diode drops, use shottky diodes to
minimize this,
or you can partially adjust these drops out.
It will also have the standard diode temperature coefficients, but what do
you want from
five cheap passive components?

 

[George R. Gonzalez]

oops, never mind, my simple solution is too simple, it gives you the
dead-band, but doesnt skip over it.

 

[Don Lancaster]

Hi chaps,

Let's say I have a sawtooth wave generator. It cycles from 0 to 7V
over 30 seconds and then resets to 0 and so on and so on. Now let's
say I want to eliminate, for example, the part of the ramp from 3.3V
to 3.8V. Doesn't matter whether the output's held temporarily at 3.3V
for a spell and then jumps straight to 3.8V and continues to rise, or
else is grounded (or supply railed even) for that same short interval.
I just don't want that particular voltage span appearing at the
output. What's the best way to go about it?

Thanks,p.

Basic Stamp, of course.
--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected] fax 847-574-1462

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Paul Burridge]

A window comparator detects that interval and grounds the output.

Sounds promising. Are they programmable/variable? I'd like to make the
notch adjustable if at all possible.

 

[John Popelish]

<ahem> Either zero volts or supply rail will do (see above).

Sorry I buzzed through so fast I missed the details. I hope you
realize that you cannot get the output from 3 to 3.8 volts without
passing through the forbidden zone, however quickly. If you want ot
substitute a fixed value while the input ibs in the dead zone, I think
the most obvious solution is a window comparAtor driving a spdt cmos
analog switch, with the input going to one throw and the chosen fixed
replacement going to the other throw. The window comparator has very
nice adjustability.

 

[Roger Johansson]

Sounds promising. Are they programmable/variable? I'd like to make the
notch adjustable if at all possible.

Spehro Pefhany gave you a circuit schematic of exactly that circuit a
few posts later in the thread.

The two comparators are to the left, comparing the input voltage with
the two levels you specified.
He used a trick to avoid the need for a separate logic gate in the
middle, using a pull up resistor instead.
But it works as a logic function, when either or both comparators can
influence the control input to the cmos analog gate at the right side
of the circuit.
When none of them drags down the voltage the control input gets a
certain input voltage.

Study his schematic to understand what I described.

You can put pots at the inputs of the comparators to make the input
levels separately adjustable.

 

[Roger Johansson]

A window comparator detects that interval and grounds the output.

I can't sleep, so I can give you some more on this.

A window comparator detects when a voltage is within a certain range
of voltages, a window.

A window comparator is not a single component you buy, it is a circuit
you build from a few components.

You use two comparators, a voltage divider chain at the inputs of the
two comparators, to give them the reference voltages.

At the outputs of the two comparators we put a logic function, which
reacts to when both comparators signal that the input signal is within
the specified range.

The logic function can be an OR gate, or a NOR, or a NAND or and AND,
we can usually fix the polarities so it works no matter what logic
gate we use.

That is what makes up a window comparator circuit, two comparators and
a logic function.

In Spehro's circuit he used a trick which means we do not need to buy
a logic gate chip, he performed the logic gate function in a smarter
way. I don't have the circuit in front of me now, so I just describe
it in general. He used a pull-up resistor at the point where the
outputs from the two comparators come together.
And it probably works so that when none of the comparators drag down
the voltage in that point the resistor pulls it up.

The logic function lies in the expression "when none of the
comparators drag down the voltage".

The "none of the two" is a logic function. It can also be expressed as
one or the other, an OR function.

The output from this logic function steers the analog gate to the
right in the schematic, which lets the input signal through or not.

You can put pots at the inputs of the comparators to make the input
levels separately adjustable.

Insert trim pots in the voltage divider chain at the far left in the
schematic to give you adjustable voltage levels to the inputs of the
two comparators, which they compare the input voltage with.

 

[Paul Burridge]

Here's an easy way with just a few parts. The voltage Vx is where it
sits suring the dead spot.

[circuit snipped]

Thanks very much, Speff. Marrying this up with Roger's explanation has
enabled me to determine this does appear to be a very workable idea
for a satisfactory solution. Thanks again!

 

[Paul Burridge]

I can't sleep, so I can give you some more on this.

Thanks Roger. Very enlightening, I must say. Sorry you couldn't sleep
but at least I've benefited from your insomnia.