Problem with this sawtooth generator...

John Popelish wrote:

10 uf is about as big as I have seen that are easily mounted by their
leads. Digikey has the Panasonic E series available up to 10 uf. And
you can raise the value of the input resistor to the second opamp
quite a bit as long as you use a low bias current opamp. Something up
to 10 meg should be no problem. This is a lot cheaper then
accumulating 220 uf. You can also reduce the voltage swing of the
first opamp by several different means, and reduce the current through
the second stage input resistor to slow the charging.
Click to expand...
| \
+------|+ \ / \ / \
| | >--+--> / \ / \
| +-|- / | / \ / \
| | | / |
| +--------+
|
|
V+ | RT
| +----/\/\--------+
/ | |
R | |
/ | |
\ | | \ |
| +---------|- \ |
| | | >--+
+-------|-----+---|+ / |
| | | | / |
| | | |
/ | | R |
R | +---/\/\---+
/ ---
\ --- CT
| |
--------+
|
---
 
On Sun, 11 Jan 2004 17:25:35 GMT, James Meyer <jmeyer@nowhere.com>
wrote:

On Sun, 11 Jan 2004 13:47:16 +0000, Paul Burridge <pb@osiris1.notthisbit.co.uk
wroth:


Why not generate it digitally? All you need is a suitable MCU and DAC.
The 'steps' in the waveform probably won't matter.

Yes, even relatively large steps wouldn't matter in this case, but I'm
hoping for a solution with the existing circuit for the sake of
expediency. I've had some useful feedback and will try a few things...

Did you really need a sawtooth wave? If so, I think I can come up with
a very simple, slow as you want, circuit.
Click to expand...
The shape's not that important, James. It could be sine, square,
triangle, saw, whatever so long as it sweeps reliably between about 1
and 10V out at one Hertz or less. What have you in mind?
--

My opinion is worth what you've paid for it.
 
On Sat, 10 Jan 2004 22:43:28 GMT, John Popelish <jpopelish@rica.net>
wrote:

Try this to prove me wrong. Build a second reference voltage by
putting a third resistor between the first two (R2 and 3) of say, 470
ohms. Use the top end for the - input of first amp, and the bottom
for the + input of the second (or vice versa). This will produce a
small voltage for the integrator even at the start, getting things
rolling much earlier.
Click to expand...
I've tried this suggestion, but just get a constant 12V output. I
believe I have your modification implemented correctly? See:
http://www.burridge8333.fsbusiness.co.uk/triangle.gif

and it should conform to your suggestion. No squiggles, though! :-(
--

My opinion is worth what you've paid for it.
 
Paul Burridge wrote:
On Sat, 10 Jan 2004 22:43:28 GMT, John Popelish <jpopelish@rica.net
wrote:

Try this to prove me wrong. Build a second reference voltage by
putting a third resistor between the first two (R2 and 3) of say, 470
ohms. Use the top end for the - input of first amp, and the bottom
for the + input of the second (or vice versa). This will produce a
small voltage for the integrator even at the start, getting things
rolling much earlier.

I've tried this suggestion, but just get a constant 12V output. I
believe I have your modification implemented correctly? See:
http://www.burridge8333.fsbusiness.co.uk/triangle.gif

and it should conform to your suggestion. No squiggles, though! :-(
Click to expand...
That is what I was talking about. It simulates some offset voltage on
the opamps that should get things latched up a lot faster than having
perfect opamps. Now I am just mystified.

Post the .asc file, and I will try to find out what I missed.
--
John Popelish
 
Paul Burridge wrote:
On Sun, 11 Jan 2004 17:25:35 GMT, James Meyer <jmeyer@nowhere.com
wrote:


On Sun, 11 Jan 2004 13:47:16 +0000, Paul Burridge <pb@osiris1.notthisbit.co.uk
wroth:


Why not generate it digitally? All you need is a suitable MCU and DAC.
The 'steps' in the waveform probably won't matter.

Yes, even relatively large steps wouldn't matter in this case, but I'm
hoping for a solution with the existing circuit for the sake of
expediency. I've had some useful feedback and will try a few things...

Did you really need a sawtooth wave? If so, I think I can come up with
a very simple, slow as you want, circuit.


The shape's not that important, James. It could be sine, square,
triangle, saw, whatever so long as it sweeps reliably between about 1
and 10V out at one Hertz or less. What have you in mind?
Click to expand...
It makes no difference- you NEVER build anything- you are a
pseudo-intellectual and loiterer taking up disk space.
 
On Sun, 11 Jan 2004 21:29:04 +0000, Paul Burridge
<pb@osiris1.notthisbit.co.uk> wrote:

On Sun, 11 Jan 2004 17:25:35 GMT, James Meyer <jmeyer@nowhere.com
wrote:

On Sun, 11 Jan 2004 13:47:16 +0000, Paul Burridge <pb@osiris1.notthisbit.co.uk
wroth:


Why not generate it digitally? All you need is a suitable MCU and DAC.
The 'steps' in the waveform probably won't matter.

Yes, even relatively large steps wouldn't matter in this case, but I'm
hoping for a solution with the existing circuit for the sake of
expediency. I've had some useful feedback and will try a few things...

Did you really need a sawtooth wave? If so, I think I can come up with
a very simple, slow as you want, circuit.

The shape's not that important, James. It could be sine, square,
triangle, saw, whatever so long as it sweeps reliably between about 1
and 10V out at one Hertz or less. What have you in mind?
Click to expand...
Sorry! *NOT* square. No, no, no!

--

My opinion is worth what you've paid for it.
 
Paul Burridge wrote:
On Sun, 11 Jan 2004 21:29:04 +0000, Paul Burridge
pb@osiris1.notthisbit.co.uk> wrote:


On Sun, 11 Jan 2004 17:25:35 GMT, James Meyer <jmeyer@nowhere.com
wrote:


On Sun, 11 Jan 2004 13:47:16 +0000, Paul Burridge <pb@osiris1.notthisbit.co.uk
wroth:


Why not generate it digitally? All you need is a suitable MCU and DAC.
The 'steps' in the waveform probably won't matter.

Yes, even relatively large steps wouldn't matter in this case, but I'm
hoping for a solution with the existing circuit for the sake of
expediency. I've had some useful feedback and will try a few things...

Did you really need a sawtooth wave? If so, I think I can come up with
a very simple, slow as you want, circuit.

The shape's not that important, James. It could be sine, square,
triangle, saw, whatever so long as it sweeps reliably between about 1
and 10V out at one Hertz or less. What have you in mind?


Sorry! *NOT* square. No, no, no!
Click to expand...
Shouldn't you be working on the FSM, troll?!! Let's see how much of a
Mr. Popularity you remain when people realize you are a damned troll
dilettante who does NOTHING but run his fingers across a keyboard.
Anyone who answers your appeals for help is a **MORON**- you learn
NOTHING- you understand NOTHING- you do NOTHING!
 
On Sun, 11 Jan 2004 22:33:47 GMT, John Popelish <jpopelish@rica.net>
wrote:

That is what I was talking about. It simulates some offset voltage on
the opamps that should get things latched up a lot faster than having
perfect opamps. Now I am just mystified.

Post the .asc file, and I will try to find out what I missed.
Click to expand...
Here goes...

Version 4
SHEET 1 880 680
WIRE 64 272 112 272
WIRE 208 272 224 272
WIRE -112 256 0 256
WIRE -112 256 -112 160
WIRE -112 432 -112 512
WIRE -112 512 144 512
WIRE -112 352 240 352
WIRE 240 352 240 304
WIRE 240 304 256 304
WIRE 80 400 112 400
WIRE 112 400 112 272
WIRE 112 272 128 272
WIRE 0 400 -32 400
WIRE -32 400 -32 288
WIRE -32 288 0 288
WIRE 224 272 224 144
WIRE 224 272 256 272
WIRE 224 144 368 144
WIRE 432 144 464 144
WIRE 464 144 464 288
WIRE 464 288 320 288
WIRE 32 240 32 32
WIRE 32 32 288 32
WIRE 288 32 288 256
WIRE 32 32 -112 32
WIRE -112 32 -112 80
WIRE 32 304 32 336
WIRE 32 336 144 336
WIRE 144 336 144 512
WIRE 144 512 288 512
WIRE 288 320 288 512
WIRE 288 512 352 512
WIRE 256 432 464 432
WIRE 464 432 464 288
WIRE 176 432 -32 432
WIRE -32 432 -32 400
WIRE 288 32 544 32
WIRE 544 32 544 240
WIRE 544 320 544 464
WIRE 544 464 352 464
WIRE 352 464 352 512
WIRE 352 512 496 512
WIRE -112 272 -112 256
FLAG 496 512 0
SYMBOL Opamps\\LT1006 32 208 R0
SYMATTR InstName U1
SYMBOL Opamps\\LT1006 288 224 R0
SYMATTR InstName U2
SYMBOL res 224 256 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 100k
SYMBOL res -128 64 R0
SYMATTR InstName R2
SYMATTR Value 47k
SYMBOL res -128 336 R0
SYMATTR InstName R3
SYMATTR Value 47k
SYMBOL res 96 384 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap 432 128 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 220ľ
SYMBOL res 272 416 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 84k
SYMBOL voltage 544 224 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL res -128 256 R0
SYMATTR InstName R6
SYMATTR Value 470
TEXT -112 0 Left 0 !.tran 0 600s 20s 100m

--

My opinion is worth what you've paid for it.
 
On Sun, 11 Jan 2004 21:29:04 +0000, Paul Burridge <pb@osiris1.notthisbit.co.uk>
wroth:

On Sun, 11 Jan 2004 17:25:35 GMT, James Meyer <jmeyer@nowhere.com
wrote:

Did you really need a sawtooth wave? If so, I think I can come up with
a very simple, slow as you want, circuit.

The shape's not that important, James. It could be sine, square,
triangle, saw, whatever so long as it sweeps reliably between about 1
and 10V out at one Hertz or less. What have you in mind?
Click to expand...
A solid-state switch that is regularly closed at a very low duty cycle
to inject a constant current into a modestly sized capacitor will give you a
"staircase" voltage waveform output. It can either be rapidly reset to zero at
the peak or slowly stepped back down. The steps can be small enough to
aproximate a smooth ramp. The slope of the ramp is the product of the duty
cycle and the current so you don't have to have pico amp charge currents or
multi farad capacitors.

Jim
 
"Paul Burridge" <pb@osiris1.notthisbit.co.uk> schreef in bericht
news:kcm3005sk01ov9me1gvek26eupatcgaioo@4ax.com...
On Sun, 11 Jan 2004 22:33:47 GMT, John Popelish <jpopelish@rica.net
wrote:

That is what I was talking about. It simulates some offset voltage on
the opamps that should get things latched up a lot faster than having
perfect opamps. Now I am just mystified.

Post the .asc file, and I will try to find out what I missed.

Here goes...

Version 4
SHEET 1 880 680
WIRE 64 272 112 272
WIRE 208 272 224 272
Click to expand...
[snip]


LTspice -> simulate -> Edit Simulation Command -> Tab Transient -> tick
'start external DC supply voltages at 0'.

remove resistor.


--
Thanks, Frank.
(remove 'x' and 'invalid' when replying by email)
 
Paul Burridge wrote:
On Sun, 11 Jan 2004 22:33:47 GMT, John Popelish <jpopelish@rica.net
wrote:

That is what I was talking about. It simulates some offset voltage on
the opamps that should get things latched up a lot faster than having
perfect opamps. Now I am just mystified.

Post the .asc file, and I will try to find out what I missed.

Here goes...
(snip)
Click to expand...

Found it. The simulation starts out with both opamp outputs
centered. So there is no voltage on the input of the integrator,
except for opamp offset voltage (or its substitute as I specified). I
don't know if you remember one of the neat things about taking the
derivative of an exponential, but the result is also an exponential.
Likewise, if you integrate an exponential, the result is an
exponential.

Well, this circuit starts out summing two exponentials (one from each
opamp output), one positive and one negative, one being the negative
of the integral of the other. And since they are just about equal, as
long as both outputs are unsaturated, they sum to essentially zero
(the positive feedback around U1 is almost perfectly canceled by the
signal through R5).
Display both outputs for the first 420 seconds to see the matched but
inverse exponentials being summed at U1's + input.

So there is no net input voltage for U1 till one of the outputs
saturates (and the two exponential voltages quit tracking). Ever
after, U1 is always saturates up or down, so things work correctly.

I see no way around this start up problem, except to change the design
to eliminate summing the outputs of both amplifiers. This is a pretty
subtle gotcha.

--
John Popelish
 
Fred Bloggs wrote:
Shouldn't you be working on the FSM, troll?!! Let's see how much of a
Mr. Popularity you remain when people realize you are a damned troll
dilettante who does NOTHING but run his fingers across a keyboard.
Anyone who answers your appeals for help is a **MORON**- you learn
NOTHING- you understand NOTHING- you do NOTHING!
Click to expand...
He could have saved everyone a lot of aggravation if he had just
bought a cheap kit: http://www.alltronics.com/kits.htm

RF FIELD STRENGTH METER KIT

This useful kit is a wideband RF detector that indicates RF field
strength in the 100KHz to 500MHz range at low levels. The kit includes
an etched and drilled circuit board and all necessary components, except
9V battery. It also comes with a plastic case, antenna and an attractive
analog meter. The RF power reading is not linear and simply indicates
the presence of RF and its relative level. However, it makes a great BUG
detector or you can use it to test the output of any RF transmitter,
including ham radios, cordless telephones and even car alarm
transmitters! The circuit has an adjustable attenuator to handle
everything from low to high power levels. Use during RFI/EMI
precompliance testing. The kit is easy to assemble and comes with
schematic diagrams and complete instructions.

97K005 $29.00 each

--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida
 
John Popelish wrote:

I see no way around this start up problem, except to change the design
to eliminate summing the outputs of both amplifiers. This is a pretty
subtle gotcha.
Click to expand...
Well- assuming there is no such thing as node initialization in LTSpice,
you can place two anti-parallel diodes in series with the integrator
output and feedback resistor- this means you have a dead-zone of about
+/-0.4V around 6V output where integrator has no effect and comparator
drives itself into saturation quite nicely.
 
Michael A. Terrell wrote:
Fred Bloggs wrote:

Shouldn't you be working on the FSM, troll?!! Let's see how much of a
Mr. Popularity you remain when people realize you are a damned troll
dilettante who does NOTHING but run his fingers across a keyboard.
Anyone who answers your appeals for help is a **MORON**- you learn
NOTHING- you understand NOTHING- you do NOTHING!


He could have saved everyone a lot of aggravation if he had just
bought a cheap kit: http://www.alltronics.com/kits.htm

RF FIELD STRENGTH METER KIT

This useful kit is a wideband RF detector that indicates RF field
strength in the 100KHz to 500MHz range at low levels. The kit includes
an etched and drilled circuit board and all necessary components, except
9V battery. It also comes with a plastic case, antenna and an attractive
analog meter. The RF power reading is not linear and simply indicates
the presence of RF and its relative level. However, it makes a great BUG
detector or you can use it to test the output of any RF transmitter,
including ham radios, cordless telephones and even car alarm
transmitters! The circuit has an adjustable attenuator to handle
everything from low to high power levels. Use during RFI/EMI
precompliance testing. The kit is easy to assemble and comes with
schematic diagrams and complete instructions.

97K005 $29.00 each
Click to expand...
I know, there are some excellent kits available- I found an excellent
one available in limited quantity from an ARRL club in Pennsylvania for
about the same price. They had two board options- Manhattan style and
Pittsburgh style- for assembling components- never heard of that before.
The picture of Manhattan style looked they milled component pads out of
solid copper plane and wire interconnected- but not sure.
 
Frank Bemelman wrote:

LTspice -> simulate -> Edit Simulation Command -> Tab Transient -> tick
'start external DC supply voltages at 0'.

remove resistor.
Click to expand...
You have the same effect with the more reasonable "skip initial
operating point solution"- it is ridiculous to ask for something that
doesn't exist- but if you do, it gives you a ridiculous answer, making
that comparator output exactly what it needs to be to maintain
0.0000000000000000000000000V input differential. I proved this by adding
the diodes which kill any integrator feedback whatsoever- and the
comparator hangs at 6V indefinitely in that noiseless perfectly
mathematical world called SPICE.
 
On Mon, 12 Jan 2004 00:43:57 +0100, "Frank Bemelman"
<fbemelx@euronet.invalid.nl> wroth:

LTspice -> simulate -> Edit Simulation Command -> Tab Transient -> tick
'start external DC supply voltages at 0'.

remove resistor.
Click to expand...
"Skip initial operating point solution" also works minus R6.

Just another case of SPICE appearing to lie to you if you ask it the
wrong question.

Jim
 
Fred Bloggs wrote:
I know, there are some excellent kits available- I found an excellent
one available in limited quantity from an ARRL club in Pennsylvania for
about the same price. They had two board options- Manhattan style and
Pittsburgh style- for assembling components- never heard of that before.
The picture of Manhattan style looked they milled component pads out of
solid copper plane and wire interconnected- but not sure.
Click to expand...
That is where they cut little squares of PC board material and
solder, or glue it over a solid sheet of copper clad board, rather than
design and lay out a real PC board. I guess its ok for prototypes and
homebrew, but I just can't get into that method after three decades of
working with etched boards.


--
We now return you to our normally scheduled programming.

Take a look at this little cutie! ;-)
http://home.earthlink.net/~mike.terrell/photos.html

Michael A. Terrell
Central Florida
 
On Mon, 12 Jan 2004 01:00:05 GMT, Fred Bloggs <nospam@nospam.com> wrote:

They had two board options- Manhattan style and
Pittsburgh style- for assembling components- never heard of that before.
Click to expand...
Manhattan I have heard of, but what is Pittsburgh style? I grew up there
but never have heard the term for a wiring style. Its not the same as
dead bug is it?
 
Rex wrote:
On Mon, 12 Jan 2004 01:00:05 GMT, Fred Bloggs <nospam@nospam.com> wrote:


They had two board options- Manhattan style and
Pittsburgh style- for assembling components- never heard of that before.


Manhattan I have heard of, but what is Pittsburgh style? I grew up there
but never have heard the term for a wiring style. Its not the same as
dead bug is it?
Click to expand...
Pittsburgh construction is like Manhattan, but with the pads etched on a
PCB instead of being cut out and glued in place.

Leon
--
Leon Heller, G1HSM
Email: aqzf13@dsl.pipex.com
My low-cost Philips LPC210x ARM development system:
http://www.geocities.com/leon_heller/lpc2104.html
 
James Meyer wrote:

"Skip initial operating point solution" also works minus R6.

Just another case of SPICE appearing to lie to you if you ask it the
wrong question.
Click to expand...
Actually it is not a lie- it is a valid arithmetic solution- the problem
comes in when SPICE fails to recognize that a positive feedback loop
gain of a million or so makes this a very unstable candidate for a
solution.
 
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