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1MHz Voltage Controlled Oscillator

J

Jim Thompson

I need a design for 1MHz to 2MHz Voltage controlled oscillator for
square wave which uses op-amp. Any help is appreciated...thank you
Click to expand...

Just buy an MC4024... 3.5:1 tuning range

...Jim Thompson
 
B

[email protected]

I need a design for 1MHz to 2MHz Voltage controlled oscillator for
square wave which uses op-amp. Any help is appreciated...thank you
Click to expand...

Have a look at

http://www.national.com/an/AN/AN-20.pdf

at page 9 fig 27 for the basic idea. The circuit is described as as a
triangular wave generator, but in fact the left-hand amplifier is
producing a square wave.

The LM101 won't do 1MHz to 2MHz, but nowadays you can buy faster parts.

The LM101 won't produce a well-defined square waveat the output of the
left-hand amplifier, but an amplifier delivering a rail-to-rail output
swing would do better. The

http://www.onsemi.com/pub/Collateral/MC33201-D.PDF

comes to mind, but it isn't fast enough for a 1-2MHz oscillator either.

There are a couple of ways of making the circuit into a voltage
controlled oscillator, but since this appears to be a homework
excercise, I won't go into them yet.
 
C

colin

I need a design for 1MHz to 2MHz Voltage controlled oscillator for
square wave which uses op-amp. Any help is appreciated...thank you
Click to expand...

square wave suggests using a comparator rather than op amp,
at least for the output,
a 'relaxation oscillator' is probably the best way to go,
wich is just an inverting schmit trigger with negative feedback.

the control voltage can adjust the current in the negative feedback via a
voltage dependant resistor such as a fet or
current source + current mirror + steering diodes.

its possible to do it with a ne555 timer.

Colin =^.^=
 
J

Jim Thompson

square wave suggests using a comparator rather than op amp,
at least for the output,
a 'relaxation oscillator' is probably the best way to go,
wich is just an inverting schmit trigger with negative feedback.

the control voltage can adjust the current in the negative feedback via a
voltage dependant resistor such as a fet or
current source + current mirror + steering diodes.

its possible to do it with a ne555 timer.

Colin =^.^=
Click to expand...

If you want a linear control curve, beware the delay through the
comparator, 555, or whatever you use.

...Jim Thompson
 
C

colin

Jim Thompson said:
If you want a linear control curve, beware the delay through the
comparator, 555, or whatever you use.
Click to expand...

Theres a technique to use an inductor in series with the feedback capacitor
to make the cycle end earlier at higher frequency and so compensate for the
comparator delay.

Colin =^.^=
 
C

colin

colin said:
Theres a technique to use an inductor in series with the feedback capacitor
to make the cycle end earlier at higher frequency and so compensate for the
comparator delay.
Click to expand...

actualy make that a resistor not an inductor lol.

Colin =^.^=
 
J

Jim Thompson

Theres a technique to use an inductor in series with the feedback capacitor
to make the cycle end earlier at higher frequency and so compensate for the
comparator delay.

Colin =^.^=
Click to expand...

Nope, not an inductor, a resistor, such that tau = R*C = tdelay, and
you charge and discharge the capacitor with currents... it's NOT the
"feedback capacitor".

...Jim Thompson
 
C

colin

Jim Thompson said:
Nope, not an inductor, a resistor, such that tau = R*C = tdelay, and
you charge and discharge the capacitor with currents... it's NOT the
"feedback capacitor".
Click to expand...

I think i just beat you to it with the correction at least on my news server
lol,
with a 555 type its not what you would think of as negative feedback but
with a relaxation type using an inverting schmitt trigger the capacitor is
in the lower half of the neg feedback RC divider,
and the extra resistor would be in series with the capacitor but still in
the lower leg.
of course the upper leg would be variable resistance or preferably variable
current.

using the op amp integrator type, the resistor would be in the output of the
op amp,
the triangle is still taken from the capacitor if thats what you want,
but the comparator is fed from the op amp output directly.

A picture is worth a thousand words, however I got this nugget of info from
a post in here quite some time back wich went into more detail with a nice
schematic.

Colin =^.^=
 
J

Jim Thompson

I think i just beat you to it with the correction at least on my news server
lol,
with a 555 type its not what you would think of as negative feedback but
with a relaxation type using an inverting schmitt trigger the capacitor is
in the lower half of the neg feedback RC divider,
and the extra resistor would be in series with the capacitor but still in
the lower leg.
of course the upper leg would be variable resistance or preferably variable
current.

using the op amp integrator type, the resistor would be in the output of the
op amp,
the triangle is still taken from the capacitor if thats what you want,
but the comparator is fed from the op amp output directly.

A picture is worth a thousand words, however I got this nugget of info from
a post in here quite some time back wich went into more detail with a nice
schematic.

Colin =^.^=
Click to expand...

It was probably my post. I discovered that trick at least 30 years
ago ;-)

...Jim Thompson
 
J

Jim Thompson

It was probably my post. I discovered that trick at least 30 years
ago ;-)

...Jim Thompson
Click to expand...

Here's a post from 2004 where I mention that the technique was
discussed in a previous post of mine (which I still can't locate)...

Path:
archiver1.google.com!news2.google.com!news.maxwell.syr.edu!border1.nntp.ash.giganews.com!border2.nntp.ash.giganews.com!nntp.giganews.com!cyclone1.gnilink.net!gnilink.net!peer01.cox.net!cox.net!p01!fed1read07.POSTED!not-for-mail
From: Jim Thompson <[email protected]>
Newsgroups: sci.electronics.design
Subject: Re: Triangle to sinus for high frequencies - how?
Message-ID: <[email protected]>
References:
<[email protected]>
<[email protected]>
X-Newsreader: Forte Agent 1.93/32.576 English (American)
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Lines: 27
Date: Tue, 30 Mar 2004 09:30:38 -0700
NNTP-Posting-Host: 68.98.49.141
X-Complaints-To: [email protected]
X-Trace: fed1read07 1080664238 68.98.49.141 (Tue, 30 Mar 2004 11:30:38
EST)
NNTP-Posting-Date: Tue, 30 Mar 2004 11:30:38 EST
Organization: Cox Communications


[snip]
The slickest fix I've seen is what HP did in some of their function
generators. You put a resistor in series with the timing cap.
Comparator senses the top of the resistor, but the output still comes
off the cap. As the current increases, the resistor adds more volts to
the sensed triangle, effectively reducing the amplitude of the real
triangle. Tweek the resistor for a first order correction. Helps a LOT.
Click to expand...
[snip]

I discussed this technique here several years ago. It not only fixes
the delay overshoot problem but also linearizes the control curve when
used for a VCO.

...Jim Thompson

...Jim Thompson
 
M

Michael A. Terrell

Jim said:
Just buy an MC4024... 3.5:1 tuning range
Click to expand...


Wasn't that designed by some old curmudgeon?


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
J

Jim Thompson

Wasn't that designed by some old curmudgeon?
Click to expand...

Yep. A REALLY OLD curmudgeon. MRI next week to look at why I've
developed leg pain in my left leg only... sufficient pain to make me
limp :-(

...Jim Thompson
 
C

colin

Yep. That's the thread! Thanks!
Click to expand...

Ah yes I remember it well, but sheesh was it realy 2002 ?
shame the drawings get screwed when viewed via google,
I wonder if theres an unscrew it up button somewhere.

Colin =^.^=
 
C

colin

shame the drawings get screwed when viewed via google,
I wonder if theres an unscrew it up button somewhere.
Click to expand...

ah found it right at the top, I wonder what happened to Win's fig tree he
had to bury ?

Colin =^.^=
 
D

[email protected]

M

Michael A. Terrell

Jim said:
Yep. A REALLY OLD curmudgeon. MRI next week to look at why I've
developed leg pain in my left leg only... sufficient pain to make me
limp :-(
Click to expand...


I'll pray for you, my freind. I have swelling and pain in both legs
too, but you're the one who claims to be older than dirt. ;-)


PS, don't tell them how many parts of the MRI machine you designed.
They don't like it when the patient knows how their toys work. :(
 
J

Jim Thompson

I'll pray for you, my freind. I have swelling and pain in both legs
too, but you're the one who claims to be older than dirt. ;-)


PS, don't tell them how many parts of the MRI machine you designed.
They don't like it when the patient knows how their toys work. :(
Click to expand...

Yep, I enjoy tweaking them. Particularly the ultrasound stuff... NO
BRAND out there I didn't have a hand in the analog front-ends ;-)

...Jim Thompson
 
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