| Author |
Message |
John Fields
Guest
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 Posted:
Sun Mar 06, 2005 12:18 am Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
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On 5 Mar 2005 08:08:43 -0800, "royalmp2001" <royalmp2001@hotpop.com>
wrote:
| Quote: | John Fields,
The answers to your further questions are
1. The fixed frequency is 30KHz
2. The two frequency ideally would be 10Hz to 30KHZ. Sweep rate not
critical, maybe one cycle up and down every 2 seconds. And no it does
not have to be a linear sweep.
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---
OK. Three more questions: Do you want the sweep to (A) go from 10Hz to
30kHz and then back to 10Hz and then repeat continuously or (B) do you
want the sweep to start at 10Hz, go to 30kHz, then start over again at
10Hz abruptly, and what kind of frequency accuracy are you looking
for?
30kHz
.
A . .
. .
. .
10Hz 10Hz
30kHz 30kHz
. .
B . . . .
. . . .
. . .
10Hz 10Hz
--
John Fields
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royalmp2001
Guest
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| Posted:
Mon Mar 07, 2005 9:43 pm Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
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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 |
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Rich Grise
Guest
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| Posted:
Tue Mar 08, 2005 4:53 am Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
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On Mon, 07 Mar 2005 07:43:09 -0800, royalmp2001 wrote:
| Quote: | 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
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Bob Masta
Guest
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| Posted:
Tue Mar 08, 2005 3:52 pm Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
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On 7 Mar 2005 07:43:09 -0800, "royalmp2001" <royalmp2001@hotpop.com>
wrote:
| Quote: | 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
|
If you are not limited to the 555, the standard way to do this
in old hardware music synths was a voltage controlled
oscillator (VCO) consisting of a current source that charged
a capacitor, a threshold detector, that triggered a short one-shot,
and a transistor to dump the capacitor. The control voltage
set the charging current, hence the frequency. These VCOs can
be made incredibly linear over wide ranges. (You need to add a
small resistor in series with the cap, below the threshold detector
junction, to compensate for the one-shot time.)
Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com |
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Robert Monsen
Guest
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| Posted:
Wed Mar 09, 2005 12:44 am Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
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Bob Masta wrote:
| Quote: |
If you are not limited to the 555, the standard way to do this
in old hardware music synths was a voltage controlled
oscillator (VCO) consisting of a current source that charged
a capacitor, a threshold detector, that triggered a short one-shot,
and a transistor to dump the capacitor. The control voltage
set the charging current, hence the frequency. These VCOs can
be made incredibly linear over wide ranges. (You need to add a
small resistor in series with the cap, below the threshold detector
junction, to compensate for the one-shot time.)
|
The national LF155 datasheet has a schematic for a 3 decade VCO made out
of an LF356 and an LM319. I don't know how linear it is, or how fast
it'll track the input frequency.
Datasheets are a wonderful resource. It would be nice if there was a
catalog of snippets of circuits in datasheets that could be searched for
building blocks. Some industrious web designer should take that on.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God. |
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Roger Johansson
Guest
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| Posted:
Wed Mar 09, 2005 1:52 am Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
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Robert Monsen <rcsurname@comcast.net> wrote:
| Quote: | The national LF155 datasheet has a schematic for a 3 decade VCO made
out of an LF356 and an LM319. I don't know how linear it is, or how
fast it'll track the input frequency.
Datasheets are a wonderful resource. It would be nice if there was a
catalog of snippets of circuits in datasheets that could be searched
for building blocks. Some industrious web designer should take that on.
|
Another keyword to search for is "application notes", with the quote
marks included. For many chips the datasheet and the application notes
are different documents.
For example if we want to find data and application examples for an IC
like the 555 we can try searching for
datasheet 555
and
555 "application notes"
Another good one:
"555 circuits"
gives 350 hits on google, most of them very useful.
The "555 circuits" alternative works best for a very well known chip like
555. For most chips you get better results with the first two alternatives.
--
Roger J. |
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John Fields
Guest
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| Posted:
Wed Mar 09, 2005 6:10 am Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
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On Tue, 08 Mar 2005 10:44:05 -0800, Robert Monsen
<rcsurname@comcast.net> wrote:
| Quote: | Bob Masta wrote:
If you are not limited to the 555, the standard way to do this
in old hardware music synths was a voltage controlled
oscillator (VCO) consisting of a current source that charged
a capacitor, a threshold detector, that triggered a short one-shot,
and a transistor to dump the capacitor. The control voltage
set the charging current, hence the frequency. These VCOs can
be made incredibly linear over wide ranges. (You need to add a
small resistor in series with the cap, below the threshold detector
junction, to compensate for the one-shot time.)
The national LF155 datasheet has a schematic for a 3 decade VCO made out
of an LF356 and an LM319. I don't know how linear it is, or how fast
it'll track the input frequency.
|
---
Nice one but, unfortunately, the OP's asking for from 10Hz to 30kHz,
and the one on the data sheet goes from 10Hz to 10kHz with a control
voltage of zero to 30VDC!
He only needs a period of 2 seconds for his tuning voltage, so that
shouldn't be too hard for the VCO to follow, and I'm thinking along
the lines of a two opamp triangle wave generator to generate the sweep
and a 7555 for the VCO.
Feeding the control voltage input with the sweep voltage and wiring
the chip up as an astable with a 50% duty cycle output would be kind
of interesting in that, leaving out the output buffer and the MOSFET,
the 7555 looks essentially like this:
Vcc>---------+
|
[R1] +--------[Rt]-------+
| | |
TH>----------------+--|+\ +------+ |
| | | >---|R Q|--+--->OUT
Vc>----------+-----|--|-/ | |
| | | |
[R2] | | |
| | | |
+-----|--|+\ | _|
__ | | | >---|S Q|
TR>----------------+--|-/ +------+
| |
[R3] [Ct]
| |
GND>---------+-----+
The input divider resistors, R1,R2,R3, are all the same value, so with
no external voltage on Vc the circuit will oscillate between 1/3 and
2/3 Vcc with an output period of about 1.4RtCt.
If we were now to connect Vc to a voltage source (the sweep voltage
generator) which varied between close to 0V and close to 5V, we would
be able to vary the output frequency over a very wide range, the
highest frequency occurring with Vc close to 0V and the lowest
frequency occurring with Vc close to +5V.
That's scheme 1, but I don't have any numbers yet.
Scheme 2 leaves the the input divider alone, but varies the timing
resistor resistance 3000:1 with the sweep voltage input. That's not
as far-fetched as it sounds, since with the saming timing cap, if we
could get 30kHz with 1000 ohms we ought to be able to get 10Hz with
3 megohms. Since
T = 1.4RC,
For 30kHz and 1000 ohms we'd need
T 3.3E-5
C = ------ = -------- ~ 23.8nF
1.4R 1400
and just to check 10Hz:
T 0.1s
R = ------ = -------------- = 3 megohms
1.4C 1.4 * 23.8nF
Implementation might be something as simple as this:
+5
|
[R1]
|
+----+
E |
Vc---[R]----B Q1 [R2]
C |
+----+
|
[R3]
|
+--->TO 7555 PINS 2 AND 6
|
[Ct]
|
GND
Where Q1 is an N-Channel FET or a PNP bipolar, R1 and R3 make up the
1000 ohm high-freq resistance and R1, R2, and R3 make up the 3M low
frequency resistance with the transistor cut off. Kind of iffy,
though. Maybe an LDR and an LED or an opto? Dunno yet.
Scheme three is a high side voltage controlled current source with a
3000:1 current range feeding the timing cap. 1ľA low freq current out
and 3mA high freq current out? Or 10ľA -> 30mA? Or 5 and 15? Sounds
pretty good and less klunky than scheme 1, and _lots_ less klunky than
scheme 2...
Time will tell. :-)
--
John Fields |
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John Fields
Guest
|
| Posted:
Fri Mar 11, 2005 2:09 am Post subject:
Re: Sweep Generator possible from 555 Astable circuit? |
|
|
On Tue, 08 Mar 2005 19:01:56 -0600, John Fields
<jfields@austininstruments.com> wrote:
| Quote: | On Tue, 08 Mar 2005 10:44:05 -0800, Robert Monsen
rcsurname@comcast.net> wrote:
Bob Masta wrote:
If you are not limited to the 555, the standard way to do this
in old hardware music synths was a voltage controlled
oscillator (VCO) consisting of a current source that charged
a capacitor, a threshold detector, that triggered a short one-shot,
and a transistor to dump the capacitor. The control voltage
set the charging current, hence the frequency. These VCOs can
be made incredibly linear over wide ranges. (You need to add a
small resistor in series with the cap, below the threshold detector
junction, to compensate for the one-shot time.)
The national LF155 datasheet has a schematic for a 3 decade VCO made out
of an LF356 and an LM319. I don't know how linear it is, or how fast
it'll track the input frequency.
---
Nice one but, unfortunately, the OP's asking for from 10Hz to 30kHz,
and the one on the data sheet goes from 10Hz to 10kHz with a control
voltage of zero to 30VDC!
He only needs a period of 2 seconds for his tuning voltage, so that
shouldn't be too hard for the VCO to follow, and I'm thinking along
the lines of a two opamp triangle wave generator to generate the sweep
and a 7555 for the VCO.
Feeding the control voltage input with the sweep voltage and wiring
the chip up as an astable with a 50% duty cycle output would be kind
of interesting in that, leaving out the output buffer and the MOSFET,
the 7555 looks essentially like this:
Vcc>---------+
|
[R1] +--------[Rt]-------+
| | |
TH>----------------+--|+\ +------+ |
| | | >---|R Q|--+--->OUT
Vc>----------+-----|--|-/ | |
| | | |
[R2] | | |
| | | |
+-----|--|+\ | _|
__ | | | >---|S Q|
TR>----------------+--|-/ +------+
| |
[R3] [Ct]
| |
GND>---------+-----+
The input divider resistors, R1,R2,R3, are all the same value, so with
no external voltage on Vc the circuit will oscillate between 1/3 and
2/3 Vcc with an output period of about 1.4RtCt.
If we were now to connect Vc to a voltage source (the sweep voltage
generator) which varied between close to 0V and close to 5V, we would
be able to vary the output frequency over a very wide range, the
highest frequency occurring with Vc close to 0V and the lowest
frequency occurring with Vc close to +5V.
That's scheme 1, but I don't have any numbers yet.
Scheme 2 leaves the the input divider alone, but varies the timing
resistor resistance 3000:1 with the sweep voltage input. That's not
as far-fetched as it sounds, since with the saming timing cap, if we
could get 30kHz with 1000 ohms we ought to be able to get 10Hz with
3 megohms. Since
T = 1.4RC,
For 30kHz and 1000 ohms we'd need
T 3.3E-5
C = ------ = -------- ~ 23.8nF
1.4R 1400
and just to check 10Hz:
T 0.1s
R = ------ = -------------- = 3 megohms
1.4C 1.4 * 23.8nF
Implementation might be something as simple as this:
+5
|
[R1]
|
+----+
E |
Vc---[R]----B Q1 [R2]
C |
+----+
|
[R3]
|
+--->TO 7555 PINS 2 AND 6
|
[Ct]
|
GND
Where Q1 is an N-Channel FET or a PNP bipolar, R1 and R3 make up the
1000 ohm high-freq resistance and R1, R2, and R3 make up the 3M low
frequency resistance with the transistor cut off. Kind of iffy,
though. Maybe an LDR and an LED or an opto? Dunno yet.
Scheme three is a high side voltage controlled current source with a
3000:1 current range feeding the timing cap. 1ľA low freq current out
and 3mA high freq current out? Or 10ľA -> 30mA? Or 5 and 15? Sounds
pretty good and less klunky than scheme 1, and _lots_ less klunky than
scheme 2...
Time will tell. :-)
|
---
This has turned out to be more difficult than I thought it would be
and I've taken on some full-time work for a new client, so I'm going
to have to put it on the back burner and get outta here for a while.
Sorry...
--
John Fields |
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