Sweep Generator possible from 555 Astable circuit?

[John Fields]

On Thu, 3 Mar 2005 21:48:07 -0800, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

"royalmp2001" <royalmp2001@hotpop.com> wrote in message
news:1109907052.577771.251830@f14g2000cwb.googlegroups.com...
Can anyone tell me if it is possible to make a square wave sweep
generator from a standard 555 astable configuration. What mods are
needed? If not what would be the easiest, simplest, cheapest way?

I've been in the electronics field for a good while
and managed to never come across a "square
wave sweep generator"

---
Any function generator (or function generator chip) with a square wave
out and a VCF input can be used to generate a square wave which can be
swept in frequency.
---

and I remain confounded
as to what it might do. Could you elaborate?

What gets swept?

---
The output frequency
---

What waveforms result?

---
A rectangular waveform with a 50% duty cycle.
---

I'm sure a 555 could be made to do something
that might be given such a name, but there are
too many possibilities without more details.

---
If a swept square wave is the goal, There are only four possibilities
using only the 555 or its CMOS brethren, three of which are varying
the resistance of the timing resistor, the capacitance of the timing
capacitor, or both, with the modulating voltage. In addition, the
timer would have to be configured like this:




+-------+
+-O|T- OUT|--+--->OUT
| | | |
+-O|TH | |
| +-------+ |
+----[Rt]-----+
|
[Ct]
|
GND

and, if the 555 is used, note must be taken of the fact that its
output _isn't_ rail to rail.

The fourth, and simplest, possibility (albeit not the one which
provide the greatest deviation) would be to use the CONTROL VOLTAGE
input to sweep the output frequency.

--
John Fields

 

[John Popelish]

royalmp2001 wrote:

Can anyone tell me if it is possible to make a square wave sweep
generator from a standard 555 astable configuration. What mods are
needed? If not what would be the easiest, simplest, cheapest way?
Thanks

The 555 switches output states as the timing inputs (trigger and
threshold) pass through 1/3 and 2/3 of its supply voltage. If you
connect a capacitor to its timing inputs and want that capacitor to
charge up and down at various rated (and change direction of charge
each time the 555 output changes states,) you need a bi-directional
(direction switchable) source of current for the capacitor that also
varies the value of current over time to produce the sweep. A CA3080
variable transconductance amplifier could perform both these tasks.
Its differential input would monitor the output state of the 555 to
switch current directions each time the trigger or threshold voltage
passes through their respective boundary voltage, and its current set
input would be used ot vary the magnitude if its output current. As a
bonus, the timing capacitor voltage is a linear swept triangle wave.
The only remaining problem is to come up with a circuit addition to
sweep the current control pin on the 3080. A second 555 could do
that.

http://www.intersil.com/data/fn/fn475.pdf

--
John Popelish

 

[Rich Grise]

On Mon, 07 Mar 2005 07:43:09 -0800, royalmp2001 wrote:

Thanks for the questions, John Fields.

I need the frequency to cycle from 10Hz upto 30KHz then down to 10 then
back up to 30K, etc
Accuracy is not critical at all, I'll take whatever is feasible with
this kind of circuit, even if it can't go all the way down to 10Hz.
Thanks, John

Can you still get these?

http://www.chipcatalog.com/Intersil/ICL8038.htm

I did one of them once, although I didn't do the whole 1000X sweep range -
the thing that stands out in my mind was tweaking R11 and R12 to try to
get that little point off the top of the "sine wave".

You could probably extend the sweep range with some clever current-source
design.

Good Luck!
Rich