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Sinusoidal oscillator configuration

N

Nick Zalutskiy

Jan 1, 1970
0
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.

I was thinking of using this [ http://www.ecsxtal.com/store/pdf/ecs-31.pdf
] crystal in a Pierce Oscillator configuration. However I am not sure
if this crystal will work in that configuration. Also, I can't seem to
figure out what the output would look like. Would it be a square wave
or a sinusoidal? I came across a diagram that pictured a sinusoidal at
the inverter input and a square wave at the output for the Pierce
configuration. Is that correct?

I would greatly appreciate any pointers on this.

Thank you!
 
D

David L. Jones

Jan 1, 1970
0
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.

A minimal hardware circuit could be a tiny PIC micro with an RC filter
- 3 SMD parts.
Depends on the specs you actually need of course.

Dave.
 
K

Ken S. Tucker

Jan 1, 1970
0
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.

I was thinking of using this [http://www.ecsxtal.com/store/pdf/ecs-31.pdf
] crystal in a Pierce Oscillator configuration. However I am not sure
if this crystal will work in that configuration. Also, I can't seem to
figure out what the output would look like. Would it be a square wave
or a sinusoidal? I came across a diagram that pictured a sinusoidal at
the inverter input and a square wave at the output for the Pierce
configuration. Is that correct?

I would greatly appreciate any pointers on this.

Thank you!

Try a Phase-Shift oscillator. I use them easily for
audio frequencies, not sure I've gone up to 125khz
though.
Ken
 
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.
I was thinking of using this [http://www.ecsxtal.com/store/pdf/ecs-31.pdf
] crystal in a Pierce Oscillator configuration. However I am not sure
if this crystal will work in that configuration. Also, I can't seem to
figure out what the output would look like. Would it be a square wave
or a sinusoidal? I came across a diagram that pictured a sinusoidal at
the inverter input and a square wave at the output for the Pierce
configuration. Is that correct?
I would greatly appreciate any pointers on this.
Thank you!

Try a Phase-Shift oscillator. I use them easily for
audio frequencies, not sure I've gone up to 125khz
though.
Ken- Hide quoted text -

- Show quoted text -



The 125khz crystal is the most difficuot item.

Use bipolar autobiassed amplifier for crystal 100KHZ or so . collector
load of 100kohm. 1M from base to collector.
capacitors. of 470pf from collector to ground and base to ground
crystal from collector to base.Oscillation will build till it clips
( during clipped part of the cycle gain is zero.)
.
Detect signal at the collector with high impedance and reduce the
supply to the collector resistor when a suitable non clippng amplitude
is reached. Should be able to limit harmonics to -30db when a simple
lpf will get you to -40db.
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
maxim makes an oscillator chip $5 that generates up to 50mhz sine, square, triangle. you can vary the frequency by a pot from 10 hz to 50 mhz. it actualy makes a very good lab oscillator. runs on +5v and with an out amp into 50ohm load. it has being many years since i biuld it and still runs ok.
 
N

Nick Zalutskiy

Jan 1, 1970
0
Using a "cheesy" DDS sounds like the solution I am trying to replace
actually. There is a 4 MHz crystal going through a 4060 counter, which
brings down the frequency to 125 kHz (square wave at that point), and
is then fed into an RLC network to filter out the harmonics, giving a
pretty clean sinusoidal. My new design doesn't require a 4 MHz clock
and the counter is pretty bulky and generate a square wave incurring
the need for the RLC network. This solution isnt bad by any means, I
am just investigating different ways of accomplishing the task with
the goal of minimizing the hardware.

This is a learning process for me, so I decided to inverstigate the
Pierce configuration further. I have a 125 kHz crystal, so thats not a
problem. However, I cant get it to oscillate in this configuration.

Schematic: http://en.wikipedia.org/wiki/Image:Pierce_oscillator.svg
I am using the following crystal: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=XC999-ND
with 4.7 pF caps at C1 and C2 (also tried 12 pF) and 10M for R1 (also
tried 1M) and its just not working. The circuit is very simple, so I
am completely lost at what I am doin wrong here. Are my C1 and C2
values wrong? Do I need a series resistor between the crystal and
inverter output?

Thanks everyone for the very informative replies!

-Nick

Nick said:
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.
I was thinking of using this [http://www.ecsxtal.com/store/pdf/ecs-31.pdf
] crystal in a Pierce Oscillator configuration. However I am not sure
if this crystal will work in that configuration. Also, I can't seem to
figure out what the output would look like. Would it be a square wave
or a sinusoidal? I came across a diagram that pictured a sinusoidal at
the inverter input and a square wave at the output for the Pierce
configuration. Is that correct?
I would greatly appreciate any pointers on this.
Thank you!

If they don't have 125kHz as a standard frequency, you may have to get a
bazillion of them before you can get them at a reasonable price.

Assuming that you can get them* the traditional Pierce-ish inverter and
two caps oscillator could, indeed, be expected to give you something
like a sine wave at the input and something more like a square wave at
the output, with the crystal filtering things as they went through from
output back to input.  You'd have to try this out on your circuit to see
how reliably it generated a nice sine wave.

Alternately, you could use (gasp!) discrete components to build a BJT or
JFET Colpitts oscillator.  This gives you a heck of a lot more control
over the critical oscillator parameters, so if you're interested in
wringing the most out of your oscillator this may be the way to go.
With one of these oscillators you can use very light capacitive coupling
from the hot end of the crystal to get a nice sinusoid.

Whatever you do you'll have to make a tradeoff between power output,
circuit complexity, component sensitivity and the pureness of the
sinusoid.  Anything that puts the oscillator pick-off at a point where
the signal is a nice pure sinusoid requires that you take little power
from the oscillator, which in turn means that you must follow your
oscillator with a healthy amplifier, which adds to the circuit complexity..

Were I doing this I would investigate using a cheesy DDS.  This sounds
fancy, but all you do is use an oscillator running at some multiple of
125kHz followed by a divider driving a resistor network to make a
sorta-sinewave that drives an op-amp filter.  Something like a 1.25MHz
oscillator driving a 4017 would let you hand-pick your ten resistor
values to really stomp down on the first five harmonics, which would
leave you only needing to filter above 500kHz or so.

* Check Digikey -- many folks have the rule of thumb that if Digikey
stocks it then it's probably a standard item.  Beware of Digikey part
numbers that have no stock behind them -- those are either special buys
for someone, or they've been discontinued.

--

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says..
See details athttp://www.wescottdesign.com/actfes/actfes.html- Hide quoted text -

- Show quoted text -
 
G

Grumpy The Mule

Jan 1, 1970
0
Howdy,

I think your capacitor values need adjustment. You may find this
article helpful http://www.fairchildsemi.com/an/AN/AN-340.pdf

Note that the series combination of the two capacitors should equal
the required load capacitance of the crystal and that increasing
the value of the capacitor on the gate's output increases feedback.
If the crystal is a bit sluggish you'll want more feedback.

If you have a jfet handy, like the mpf102, you'll find it works
much better than a gate. Though when a spare gate is handy I'm
likely to use it.

Also see,
http://www.fairchildsemi.com/an/AN/AN-118.pdf

This is excellent,
http://ww1.microchip.com/downloads/en/AppNotes/00943A.pdf

Armed with these I'm sure you can get it working if the
crystal is functional.
 
N

Nick Zalutskiy

Jan 1, 1970
0
I've looked at the Fairchild paper before and overlooked the series
combination part, in turn making an error in my load capacitance
calculations.

The load capacitance for the crystal is 12.5 pF. So C1 and C2 should
be 25 pF. I tried 24 pF (two 12 pF in parallel) for C1 and C2, no
luck. I tried giving it more feedback, as you have suggested, by
putting ~27 pF at the output (22 pF in parallel with 4.7 pF), no luck.

I also tried to take into account the gate input and output
capacitance (assumed to be around 5 pF) by placing 17 pF (a 12 and a
4.7 in parallel) as C1 and C2, still no luck.

I also have a JFET, a 5457, so I attemped to put the crystal in the
following configuration: http://www.aldinc.com/pdf/fet_11120.0.pdf
with 10k for Rd, 10M for Rf, no Rl, no buffer at the output, and 24 pF
as C1 and C2, no signs of life. I also tried, J201 and a PN4392 jfets,
still no luck.

At this point it seems like I am playing trial and error, and thats
pretty disturbing in my mind. =(

Thanks,
-Nick
 
D

David L. Jones

Jan 1, 1970
0
Using a "cheesy" DDS sounds like the solution I am trying to replace
actually. There is a 4 MHz crystal going through a 4060 counter, which
brings down the frequency to 125 kHz (square wave at that point), and
is then fed into an RLC network to filter out the harmonics, giving a
pretty clean sinusoidal. My new design doesn't require a 4 MHz clock
and the counter is pretty bulky and generate a square wave incurring
the need for the RLC network. This solution isnt bad by any means, I
am just investigating different ways of accomplishing the task with
the goal of minimizing the hardware.

If that is good enough performance-wise for the existing circuit then
simply replacing the counter and crystal with a 125KHz square wave
output from a tiny 50 cent 5 or 8 pin PIC micro is a good solution to
minimise your hardware. Just one tiny chip and the existing RLC
filter.
If you use the smallest SMD parts you can get then the circuit
footprint will be very small indeed. Double sided component load would
make it even smaller.
I doubt you'll get a smaller solution using more traditional means.

Dave.
 
G

Grumpy The Mule

Jan 1, 1970
0
Howdy,

Do you have a URL for the crystal specification?
I'll look for my gate oscillator spreadsheet and
drop the values into it.

Have you tried more than one crystal?

Remember that the gate should be biased into the linear
operating range to get things started. Sometimes a small
resistance in series with the gate output is necessary in
addition to the 10M-20M feedback resistor.

Then the pi section (the two caps and the crystal) provides
180 degrees of phase shift, which when added 180 degrees from
the inverting gate satisfies the 360 degree round the loop
requirement. The other requirement is loop gain greater
than unity, but the gate has more than enough gain to
compensate for the loss across the pi network.

With low frequency crystals (~100KHz and below) the gate
oscillator usually works, so this is a puzzle. If you have
a 4049 inverter I'd try that. They make vigorous LF oscillators.
With the 4049 use 10M feedback 49pf and 56pf on either side of
the crystal (highest value on the gate's output side) Also use
39pF between the gate output and the pi section (where some
application notes show a few K-Ohms of resistance.) Of course the
10M bias resistor must be directly connected from input to output
of the gate to force it into linear operation.

I have a pierce circuit with an inductor insted of a resistor
in the drain(2.5mH because it's what was handy.) Every LF
crystal or ceramic resonator that I've tried, mostly salvaged
from computer junk, has oscillated if it was ever going to
oscillate. I don't like the app note oscillator for LF crystals.
The pi section capacitors and feedback resistor are too low in
value and the drain resistor would be better replaced by an
inductor. But I can't say it wouldn't work.

Don't be too hard on cut and try (trial and error) somtimes a
person stumbles onto something wonderful via that path.
 
Using a "cheesy" DDS sounds like the solution I am trying to replace
actually. There is a 4 MHz crystal going through a 4060 counter, which
brings down the frequency to 125 kHz (square wave at that point), and
is then fed into an RLC network to filter out the harmonics, giving a
pretty clean sinusoidal. My new design doesn't require a 4 MHz clock
and the counter is pretty bulky and generate a square wave incurring
the need for the RLC network. This solution isnt bad by any means, I
am just investigating different ways of accomplishing the task with
the goal of minimizing the hardware.

This is a learning process for me, so I decided to inverstigate the
Pierce configuration further. I have a 125 kHz crystal, so thats not a
problem. However, I cant get it to oscillate in this configuration.

Schematic:http://en.wikipedia.org/wiki/Image:Pierce_oscillator.svg
I am using the following crystal:http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=XC99...
with 4.7 pF caps at C1 and C2 (also tried 12 pF) and 10M for R1 (also
tried 1M) and its just not working. The circuit is very simple, so I
am completely lost at what I am doin wrong here. Are my C1 and C2
values wrong? Do I need a series resistor between the crystal and
inverter output?

Thanks everyone for the very informative replies!

-Nick

Nick said:
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.
I was thinking of using this [http://www.ecsxtal.com/store/pdf/ecs-31..pdf
] crystal in a Pierce Oscillator configuration. However I am not sure
if this crystal will work in that configuration. Also, I can't seem to
figure out what the output would look like. Would it be a square wave
or a sinusoidal? I came across a diagram that pictured a sinusoidal at
the inverter input and a square wave at the output for the Pierce
configuration. Is that correct?
I would greatly appreciate any pointers on this.
Thank you!
If they don't have 125kHz as a standard frequency, you may have to get a
bazillion of them before you can get them at a reasonable price.
Assuming that you can get them* the traditional Pierce-ish inverter and
two caps oscillator could, indeed, be expected to give you something
like a sine wave at the input and something more like a square wave at
the output, with the crystal filtering things as they went through from
output back to input.  You'd have to try this out on your circuit to see
how reliably it generated a nice sine wave.
Alternately, you could use (gasp!) discrete components to build a BJT or
JFET Colpitts oscillator.  This gives you a heck of a lot more control
over the critical oscillator parameters, so if you're interested in
wringing the most out of your oscillator this may be the way to go.
With one of these oscillators you can use very light capacitive coupling
from the hot end of the crystal to get a nice sinusoid.
Whatever you do you'll have to make a tradeoff between power output,
circuit complexity, component sensitivity and the pureness of the
sinusoid.  Anything that puts the oscillator pick-off at a point where
the signal is a nice pure sinusoid requires that you take little power
from the oscillator, which in turn means that you must follow your
oscillator with a healthy amplifier, which adds to the circuit complexity.
Were I doing this I would investigate using a cheesy DDS.  This sounds
fancy, but all you do is use an oscillator running at some multiple of
125kHz followed by a divider driving a resistor network to make a
sorta-sinewave that drives an op-amp filter.  Something like a 1.25MHz
oscillator driving a 4017 would let you hand-pick your ten resistor
values to really stomp down on the first five harmonics, which would
leave you only needing to filter above 500kHz or so.
* Check Digikey -- many folks have the rule of thumb that if Digikey
stocks it then it's probably a standard item.  Beware of Digikey part
numbers that have no stock behind them -- those are either special buys
for someone, or they've been discontinued.

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details athttp://www.wescottdesign.com/actfes/actfes.html-Hide quoted text -
- Show quoted text -

I still think Tim's solution is the way to go. Isn't such a low
frequency crystal expensive? In any event, he is not suggesting
creating a square wave, but rather a pseudo-sine-wave that will have
much reduced harmonics. I did a sine wave generator for telecom apps
by simply creating a square wave with a flat spot in the middle (i.e.
reduce the size of the jump), then filtering the signal with a
switched capacitor filter. There are Walsh based circuits to make
pseudo sine waves, but you would need to watch out for patent issues.

If you want to use a dac and can make the sample rate a multiple of
the desired frequency, then you just need a simple look-up table. If
that timing cannot be achieved, a coordic can be used to compute the
sine values. That scheme is stone age enough that there should be any
patent issues. The coordic would have the advantage of being able to
change the frequency independent of the clock, i.e. you could make
125KHz and 13KHz with the same circuit.

In any event, if you want a clean signal, it is better not to generate
the harmonics in the first place rather than to filter off the
harmonics.
 
J

Jan Panteltje

Jan 1, 1970
0
I also have a JFET, a 5457, so I attemped to put the crystal in the
following configuration: http://www.aldinc.com/pdf/fet_11120.0.pdf
with 10k for Rd, 10M for Rf, no Rl, no buffer at the output, and 24 pF
as C1 and C2, no signs of life. I also tried, J201 and a PN4392 jfets,
still no luck.

At this point it seems like I am playing trial and error, and thats
pretty disturbing in my mind. =(

Thanks,
-Nick

I have used 100kHz xtals in circuits like this:
ftp://panteltje.com/pub/xtal_osc_125_khz.gif

The fet I used was likely a BF245 (high gain).
I cannot run simulation on this diagram without more info on the xtal.
Your solutions use the xtal in series resonant mode.
This one uses parallel resonant mode.
Give it a try perhaps.
The waveform at the source should be almost a sine already, with some
flattening at the bottom.
In my applicaton startup was really slow...

You can also add a drain resistor of a few hundred ohms, and take the signal from there.
Adapt the capacitors perhaps, ratio 1:2 is normally good.
 
If that is good enough performance-wise for the existing circuit then
simply replacing the counter and crystal with a 125KHz square wave
output from a tiny 50 cent 5 or 8 pin PIC micro is a good solution to
minimise your hardware. Just one tiny chip and the existing RLC
filter.
If you use the smallest SMD parts you can get then the circuit
footprint will be very small indeed. Double sided component load would
make it even smaller.
I doubt you'll get a smaller solution using more traditional means.

Dave.

might help on the filter requirements to use two pins and two
resistors go get a three
step wave form:

_____ _____ _____
pin1 |_| |_| |_
_ _ _
pin2 _| |_____| |_____| |___

_ _ _
combo _| |_ _| |_ _| |_
|_| |_| |_


adjust overlaps to get minimum harmonics

-Lasse
 
D

David L. Jones

Jan 1, 1970
0
might help on the filter requirements to use two pins and two
resistors go get a three
step wave form:

_____ _____ _____
pin1 |_| |_| |_
_ _ _
pin2 _| |_____| |_____| |___

_ _ _
combo _| |_ _| |_ _| |_
|_| |_| |_

adjust overlaps to get minimum harmonics

-Lasse

Nice idea!

Dave.
 
M

MooseFET

Jan 1, 1970
0
Nice idea!

The magic number is 60 degrees to get no 3rd.

If your micro has tri-stated pins, you can do much the same by using
the tristate condition to make the flat spot in the edges.
 
F

Frank Raffaeli

Jan 1, 1970
0
I am looking for a minimal hardware circuit to generate a 125 kHz
sinusoidal wave to be fed into the power stage that is in turn feeding
an antenna. The wave is used to generate a carrier frequency for an
RFID application.

I was thinking of using this [http://www.ecsxtal.com/store/pdf/ecs-31.pdf
] crystal in a Pierce Oscillator configuration. However I am not sure
if this crystal will work in that configuration. Also, I can't seem to
figure out what the output would look like. Would it be a square wave
or a sinusoidal? I came across a diagram that pictured a sinusoidal at
the inverter input and a square wave at the output for the Pierce
configuration. Is that correct?

I would greatly appreciate any pointers on this.
What is your field requirement in A/m at __ distance?
The antenna usually means a coil around a big piece of ferrite.
Therefore, it is narrow band if you can't afford too many losses. What
is your input power requirement? If efficiency is important, then you
may have a problem: The antenna is an R-L-C where R is small and Q is
high. Look at the variability of the inductance and resonant
frequency. Also saturation of the inductor will change the frequency.
You will find it is the antenna setting the frequency requirement and
not the crystal.

I would recommend either a free-running power oscillator at the R-L-C
frequency or phase locking it to a crystal. Be careful when selecting
the capacitor that resonates with the coil - the AC voltage can be
hundreds of volts, depending upon your power output.

Kaschke makes some antennas of varying power handling for these 125
kHz applications. Some have the capacitor built-in and they are pre-
tuned.

Frank
 
The magic number is 60 degrees to get no 3rd.

If your micro has tri-stated pins, you can do much the same by using
the tristate condition to make the flat spot in the edges.

yes,
but using two pins might make it possible to setup two timers to
generate the two waveforms autonomously. Don't think I have ever
seen a timer that could tristate a pin on a compare

-Lasse
 
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