Oscillator drift can be related directly to frequency stability. Drift is the unwanted and unwarrented change in frequency measured over seconds, minutes or hours. Just how stable should an oscillator be? Your oscillator or any electronic project should be as state-of-the-art as is possible, consistent with your design goals.

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OSCILLATOR DRIFT


 

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What is oscillator drift?

Oscillator drift can be related directly to frequency stability. Drift is the unwanted and unwarrented change in frequency measured over seconds, minutes or hours. Just how stable should an oscillator be?

My own belief is the answer to this is a similar answer to many questions related to electronics, your oscillator or any electronic project should be as state-of-the-art as is possible, consistent with your design goals.

While it is possible, given enough resources, financial and otherwise, to construct a state-of-the-art oscillator accurate to within one part in ten to the 14th power for as one example, an amateur radio transceiver, you generally couldn't justify those extreme lengths. On the other hand a similar CW (morse code) transceiver operating at 7 Mhz which drifted around 100 Hz or more every few seconds would be consider very poor and unacceptable.

A cheap portable AM radio receiver which drifted plus and minus 50 Hz over seconds would not necessarily be noticeable. On the other hand if such a receiver kept drifting over time to the point it required constant retuning then it also would be totally unacceptable (reminds me, I must look into my entertainment FM receiver).

Sources of oscillator drift

Assuming quality components are used throughout the oscillator the principal cause would most likely be thermal considerations. Voltage regulation is another consideration along with mechanical stability. Before considering other possibilities of oscillator drift let's look at the causes listed here.

Thermal instability causing oscillator drift

As I said thermal considerations play a major part in oscillator drift.

Consider just one example in close detail using nominal yet highly accurate figures, an oscillator designed for operation at precisely 7 Mhz. Of necessity I must use figures to ludicrous decimal places (and within the limits of my scientific calculator) to illustrate these points so please bear with me.

We will assume a colpitts oscillator where both reactances, Xc and XL = 250 ohms.

At precisely 7 Mhz (7,000,000 Hz) Xc = 250 ohms = 90.94568177 pF

At precisely 7 Mhz (7,000,000 Hz) XL = 250 ohms = 5.68410511 uH

We will assume that the capacitor is made up of a fixed 82 pF NPO capacitor with the balance (90.94568177 pF - 82 pF = 8.94568177 pF) made up by a quality air variable trimmer. For this part we will assume neither the trimmer nor the inductor vary in value. Also there are no other influences on stability and our room temperature is 25 degrees C (77 deg F).

Re-read all that if necessary, it's a mouthful.

For our illustation we are saying the only influence on stability is our 82 pF fixed NPO capacitor when our temperature varies around 25 degrees.

O.K. what's a 82 pF NPO capacitor. It's a capacitor where it's value theoretically should NOT change with varying temperature. NPO means Negative, Positive temperature co-efficient = 0 (zero). Assume in this example it is slightly defective and in fact exhibits a positive drift of 100 ppm (parts-per-million) per degree temperature variation.

Now the temperature after power turn on of our oscillator rises to 28 deg C inside our enclosure. It follows with a temperature rise of 3 degrees our capacitor is going to vary positive 3 X 100 ppm. Applied to a 82 pF capacitor this results in a net variation of +0.0246 pF. This leads to a new value of 82.0246 pF added to our trimmer value we get a new overall capacitance of 90.97028177 pF which when multiplied by our fixed inductor value above leads to a new LC value of 517.0846435 and a new resonance of 6.999053473 Mhz (6999053.473 Hz).

Here our frequency has already varied down by  7,000,000 Hz - 6999053.473 Hz = 946.527 Hz!

Wow, 946.527 Hz! That's almost a full kilohertz. You've just gone across several CW stations and an AM receiver probably needs retuning. O.K. it's a bit extreme but certainly highlights my point.

I deliberately picked the NPO capacitor for good reason. Here I quote from my Philips Capacitor data book, BC06 - 1999 Page 166 "Precision Capacitors NP0" - the temperature co-efficient is zero per degrees kelvin. HOWEVER the tolerance on that temperature co-efficient is plus / minus 30 ppm.

Simply put, the tolerance alone is about 30% of the example I used above. And, remember this is state-of-the-art and high quality manufacturing.

Why did I pick on the NPO precision capacitor? Because it is the one capacitor LEAST likely to cause you problems.

Why did the temperature rise a modest 3 degrees? Simple, after turning the oscillator power on, current flows through all parts of the circuit, power is consumed, POWER = HEAT. A small piece of wire has finite resistance, even a small current flow must by ohms law produce heat losses.

Thermal instability of inductors

If you thought the NPO Capacitor could give problems with temperature change what about the weak link in the chain?

If you use a toroid inductor, say a popular T50-2 type, what's it's temperature co-efficient? According to the Amidon Data Book the 2 type material has a temperature co-efficient of 95 ppm. Gee isn't that awfully close to the defective NPO example I gave above? What a coincidence?

How about NO toroid? Go for an air wound inductor instead? Yes an improvement! BUT copper wire expands and contracts with temperature variations. Fix it to a former, say a piece of plexiglass or similar so chances of mechanical variation by temperature affects are minimised. More improvement!

Other causes of oscillator drift

Another leading cause is poor power supply regulation and often using too high a voltage can cause problems. Similarly trying to take power from the ocillator will contribute to oscillator drift.

Use the best regulated (locally) and well bypassed power supply at the lowest voltage which will ensure reliable operation is a must.

Mechanical stability is another consideration. Use rigid larger diameter wires for interconnections where necessary between components.

Is all lost with oscillator drift?

Certainly not!

Over the years I have seen some remarkable designs for overcoming oscillator drift, then again I have spent days trying to cure the problem also. Nothing is more frustrating when working on an oscillator connected to an accurate frequency counter, and near an open window on a fine day, watching the frequency gently go up and down in perfect harmony with the prevailing breeze as the curtains wave about like synchronised swimmers to the beat of the frequency display.

At times like that, great white shark wrestling, training crocodiles or playing with the local brown snakes probably seem less demanding hobbies.

What do you do to cope with the problem? Follow this check list to minimise oscillator drift. Note I said "minimise" - you will never eliminate it entirely.

Suggested cures for oscillator drift

Use quality components for the application

With capacitors never use one larger value. Try and use a mix of values. Here VK2TIP is going to give you a good "tip". From time to time you will probably encounter opportunities to buy surplus "quality" capacitors. Note I said "quality", avoid sales of below spec merchandise. Among those quality capacitors and, here I'm talking modern ceramic types, you will find different colour codings.

Among the miniature Philips ceramic plate capacitors, as one example only, you will find the following colour code.

P100 = red-violet,  NP0 = black,  N075 = red,  N150 = orange,  N220 = yellow,  N330 = green

When and if the opportunity arises, purchase a mix of various values of these types. Values 47 pF and below. Why?

Because with time and experience, you will learn how to substitute different temperature co-efficient capacitors to compensate for oscillator drift.

Capacitors to avoid are of course low tolerance ceramics (fine for coupling and bypassing though), mica and silvered mica types which are over rated by many people.

Use the best inductor possible

An air wound inductor of larger diameter wire, firmly fixed to an inert former is probably the best. However the main problem here is that usually such an inductor assumes a large physical size which may create other problems. This was the reason for the introduction of iron powder and ferrite cores and toroids - minimise size.

If size is a big consideration then use a toroid of the lowest possible temperature co-efficient consistent with the frequency range of interest. Fix the coil turns with quality "Q" dope or similar.

Use the best regulated power supply at the lowest voltage possible

Here I would review the various tutorials on power supplies. To minimise oscillator drift I would go for an on board regulator capable of adjustment. You don't need a high powered job, we're only talking milliamps here so something like the TO-92 version of an LM317 would do the trick.

The power supply feeding this should ideally, also be well regulated, probably 12V.

The trouble is these TO-92 regulators are often hard to come by so you may have to go to a fixed regulator like a 5V or, 8V type. It all depends on what is available to you. At worst you go to a zener diode. In any event experiment BUT regulate.

Make sure your oscillator is "well buffered" to minimise oscillator drift

Good buffering is absolutely essential to any oscillator to minimise drift. Taking power directly from an oscillator is a sure recipe for problems in many cases. The components in a buffer are relatively cheap.

Mechanical stability has an effect on oscillator drift

Rigid construction is of course best. Components and wiring capable of mechanical vibration are a source of problems in oscillator drift. Also a source of "microphonics".

Just how rigid? Well like all things here, aim for the best but live with what you can achieve in the real world.

I know amateur radio operators who regularly drop projects from a height of 300 mm (12") over a table as a "mechanical drift" test. The project usually passes!

Shielding and heat sinking

I've seen oscillators constructed on a base of heavy sheet brass. Why? Mechanical reasons and to conduct heat away as much as possible. Depending on your circumstances that's probably over the top but you get the idea, ("drift?") <G>.

I would most certainly shield the "frequency determining components" with something like scrap PCB, thin copper or brass sheet. I've even then insulated it by gluing pieces of styrofoam to it. O.K. sometimes I'm too obsessive.

Other solutions to oscillator drift

In precision applications I've solved oscillator drift by accepting temperature variation as a fact of life and met it head on!

Simple, what's the highest temperature you expect to encounter? Add 10 degrees and insert a stable heater inside your shield set to that temperature.

I built, in this case, a precision crystal oscillator operating at 10 Mhz which was the time base for a precision frequency counter. The precision 10 Mhz crystal and associated capacitors were inserted inside an insulated can in which I had placed an "electronic thermometer" along with a current controlled 5 watt power resistor which served as the heater element. This is called a "crystal oven". I used two LM3911 temperature controllers which I think are now obsolete.

From memory the temperature was about 45 deg C (113F) but I wouldn't swear to that. Anyway it was in accordance with the manufacturers recommendation as to the "turning point" for that particular crystal and to that particular specification.

The whole point is that you can solve problems, if you can seriously justify the effort, by raising and constantly maintaining the frequency determining components at a constant temperature somewhat higher than you expect to encounter. Warm up from a "cold" start can be anything from 30 minutes to several days. YES! Several days!

This gives rise to another important point. Usually any oscillator will take a few minutes to temperature stabilise after turn on. If you have just been soldering components then allow at least 15 - 30 minutes before taking oscillator drift measurements. Remember to write your results down in a book after each adjustment. Marvellous what you will learn from this. Especially when swapping low value capacitors of different temperature co-efficients. Record the results comprehensively.

Summary of oscillator drift

Aim for the best you can sensibly achieve in the "real world". Don't "penny pinch" on quality components. Use multiple NPO capacitors in frequency determining areas and if necessary substitute low value capacitors of different temperature co-efficients to compensate for oscillator drift caused by temperature variations.

Use an onboard regulator of no more than 9V.

Aim for the best practical inductor.

Temperature shield the inductor and capacitors BUT not the transistor, regulator and other heat generating devices.

Try for good mechanical stability.

Always use a good buffer after the oscillator.

Give the oscillator time to "temperature stabilise" before taking oscillator drift measurements.

Pray a lot <G>.

 

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RELATED TOPICS on oscillator drift

drift correction circuits

buffer amplifiers

colpitts oscillators

crystal oscillators

hartley oscillator

oscillator basics

voltage controlled oscillators

 
 
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Updated 11th November, 2000

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