low frequency micropower vco needed

[John Woodgate]

I read in sci.electronics.design that Genome <[email protected]>

BNA

Have you undergone a major mutation?

 

[Albre]

I looked at the Phillips parts (previously) and didn't see any thing
nearly that low. Your 4 ua sounds more like the current drawn when the
chip is not selected or in standby mode.

I checked the ONSemi MC74HC4046A. It states on the first page that the
quiescent current consumption is 80 ua, and that is with the vco
disabeled.

I did not see any graphs for vco frequency vs supply current, which
would be very useful to have.

If you know of a part that uses 4 ua of supply current with the vco
active, drop me a line.... I'm getting far different numbers.

Thanks,

A

 

[Spehro Pefhany]

I looked at the Phillips parts (previously) and didn't see any thing
nearly that low. Your 4 ua sounds more like the current drawn when the
chip is not selected or in standby mode.

That's what Iq means. If you want it actually do something it draws
more current.

I checked the ONSemi MC74HC4046A. It states on the first page that the
quiescent current consumption is 80 ua, and that is with the vco
disabeled.

You might want to read past the executive summary on the first page.
That 80uA might be a mistake. The closest figure to that in the actual
specs is 160uA max at <125C. It's very temperature dependent, of
course.

I did not see any graphs for vco frequency vs supply current, which
would be very useful to have.

It will be very much dependent on the RC values- the resistors set
currents in current mirrors.

If you know of a part that uses 4 ua of supply current with the vco
active, drop me a line.... I'm getting far different numbers.

Thanks,

Ok.


Best regards,
Spehro Pefhany

 

[Albre]

You might want to read past the executive summary on the first page.
That 80uA might be a mistake. The closest figure to that in the actual
specs is 160uA max at <125C. It's very temperature dependent, of
course.

OK, I'll go back and look att he Phillips parts again and compare.

I find that the 4584 is not as available as the 74HC14. Both are
schmitt trigger inverters. If I base my vco on the 74HC14, will it
still allow me to change the frequency of the oscillator by changing
the Vcc?

The schematic that looks most promissing is at:

http://www.discovercircuits.com/PDF-FILES/4584vco.pdf

I hope to substitute the 74hc14 in place of the 4584.

OK, yes or no?

Thanks all.

A

 

[Albre]

That works because the 4584 has hysteresis. I think an ordinary
inverter will vary only a small amount.

Hey JT, don't all Schmitt triggers have hysterisis, which is what
makes them special???

A

 

[Spehro Pefhany]

OK, I'll go back and look att he Phillips parts again and compare.

I find that the 4584 is not as available as the 74HC14. Both are
schmitt trigger inverters. If I base my vco on the 74HC14, will it
still allow me to change the frequency of the oscillator by changing
the Vcc?
Yes.

The schematic that looks most promissing is at:

http://www.discovercircuits.com/PDF-FILES/4584vco.pdf

I hope to substitute the 74hc14 in place of the 4584.

OK, yes or no?

Not in that exact circuit. You put 15V Vdd on a 74HC14 and it *will*
fail.


Best regards,
Spehro Pefhany

 

[Ken Smith]

I need a low frequency vco (to operate between 55 and 65 Khz) that
draws very little power (microamp supply current if possible). Goggle
turns up uhf and microwave vco's that draw 10's of ma. It needs to be
a square wave, but can live with a triangle or sine wave output.

A single chip solution would be preferred, but can build it from
discrete components if necessary.

I'd like to have it sweep between 55 and 65 Khz with the varying
control voltage.

A cmos r/c oscillator might work just fine, but I can't figure out how
to vary the frequency with a dc input voltage:>:

How about using a CD4053 as the inverters in a CMOS oscillator.


X1
+5V---O
<--------
GND--O !
Y1 .X0 !
GND----O . !
<----/\/\--------+---!!----+
VC------O !
.Y0 !
.---------------------------


Something like this is what I have in mind. The CD4053 has some problems
when you use it in an oscillator. The output tends to have a glitch in it
as it switches. You may have to put an extra cap to ground at the control
input of the X1 section.

The range of VC is fairly restricted because it must be above about 1/2Vcc
to get any oscillation at all.

 

[Albre]

I don't understand the circuit. Can you give a www url or other link
to a more detailed circuit.

Or, post a gif or pdf in abse please.

I do not need a wide frequency shift, so the 1/2 of Vcc limit might be
ok.

Thanks,

A

 

[Mark Zenier]

I need a low frequency vco (to operate between 55 and 65 Khz) that
draws very little power (microamp supply current if possible). Goggle
turns up uhf and microwave vco's that draw 10's of ma. It needs to be
a square wave, but can live with a triangle or sine wave output.

A single chip solution would be preferred, but can build it from
discrete components if necessary.

I'd like to have it sweep between 55 and 65 Khz with the varying
control voltage.

A cmos r/c oscillator might work just fine, but I can't figure out how
to vary the frequency with a dc input voltage:>:

Any suggestions?

The oscillator section of the CD4046, 74HC4046 PLL.

Mark Zenier [email protected] Washington State resident

 

[Zak]

I find that the 4584 is not as available as the 74HC14. Both are
schmitt trigger inverters. If I base my vco on the 74HC14, will it
still allow me to change the frequency of the oscillator by changing
the Vcc?

I'd think a schmitt trigger oscillator would be quite insensitive to
channges in Vcc, as long as the hysteresis is a constant fraction of the
supply voltage. That leaves out any effect of series resistance in teh
output stage.

Thus I would suppose it largely depends on the chip used.

That said, what would it be like to build an oscilaltor and put a FET in
the feedbackm, as a kind of 'variable resistor'? At least that gives you
a defined direction of sensitivity.


Thomas

 

[Albre]

Hi Ken,

Saw your post in abse, thanks for the info.

I'm not sure whether I'll use the shmitt trigger based solution, or
the ring oscillator you posted. Since I need a relatively small
frequency variation, I'll probably go with the one that gives the
smaller frequency excursion.

Thanks to all who participated in the discussion, I learned alot and
appreciated all the suggestions!

Regards,

A

 

[Jim Thompson]

I'd think a schmitt trigger oscillator would be quite insensitive to
changes in Vcc, as long as the hysteresis is a constant fraction of the
supply voltage.

If that were true. However, the hysteresis is determined by the
_strengths_ of MOS devices, so the hysteresis doesn't track Vcc very
well.

That leaves out any effect of series resistance in teh
output stage.

Thus I would suppose it largely depends on the chip used.

That said, what would it be like to build an oscilaltor and put a FET in
the feedbackm, as a kind of 'variable resistor'? At least that gives you
a defined direction of sensitivity.


Thomas

...Jim Thompson

 

[Ken Smith]

I don't understand the circuit. Can you give a www url or other link
to a more detailed circuit.

There is no more detailed circuit.

Or, post a gif or pdf in abse please.

My provider doesn't carry that group.

I do not need a wide frequency shift, so the 1/2 of Vcc limit might be
ok.

My circuit gives a very wide frequency shift so I think you are better off
with other circuits.

 

[Ted Edwards]

I need a low frequency vco (to operate between 55 and 65 Khz) that
draws very little power (microamp supply current if possible). Goggle
...
Any suggestions?

What's the lowest supply current op-amp quad (or??) that will operate
nicely at your frequency? (I'm not up to date on micropower OAs.) If a
suitable one exists, here's a possible.

There's a very simple circuit (one OA + 4 resistors) for an amplifier
whose gain can be switched from +1 to -1. You would apply your control
voltage to its input. A fet could be used for the switch.

A second OA would be used as a simple R-C integrator with the output of
the previous applied to its input.

A two-level detector would be used to detect when the integrator output
reached V1 and V2 and its output used to drive the switch.

The slope of the integrator output is directly proportional to the input
- more voltage, steeper slope, less time to get from V1 to V2, higher
frequency.

Ted

 

[Jim Thompson]

What's the lowest supply current op-amp quad (or??) that will operate
nicely at your frequency? (I'm not up to date on micropower OAs.) If a
suitable one exists, here's a possible.

There's a very simple circuit (one OA + 4 resistors) for an amplifier
whose gain can be switched from +1 to -1. You would apply your control
voltage to its input. A fet could be used for the switch.

A second OA would be used as a simple R-C integrator with the output of
the previous applied to its input.

A two-level detector would be used to detect when the integrator output
reached V1 and V2 and its output used to drive the switch.

The slope of the integrator output is directly proportional to the input
- more voltage, steeper slope, less time to get from V1 to V2, higher
frequency.

Ted

I think the '4046 that someone suggested will be the lowest power, and
certainly simplest to set up.

...Jim Thompson