Oscillators

OSCILLATORS THIS OSCILLATORS TUTORIALS SITE This section devoted to oscillators is one of the most interesting for newcomers. You need a thorough grasp of oscillators to fully understand the later and much more complicated electronics tutorials on radio receivers and transmitters.

OSCILLATOR BASICS Here we explain a lot of the basics involved. We discuss the principles of oscillator operation, briefly look at two popular types, Hartley Oscillators and Colpitts Oscillators as well as Frequency or Phase Stability of an oscillator. Then we go on and consider reducing Phase Noise in oscillators, discuss the effects of ambient changes on stability in oscillators and finally minimizing frequency drift in oscillators.

CLAPP OSCILLATORS A clapp oscillator is in effect a series tuned version of the colpitts oscillator. Perhaps the simplest Colpitts oscillator to construct and get running is the "series tuned" version, more often referred to as the "Clapp Oscillator". Because there is no load on the inductor a high "Q" circuit results with a high L/C ratio and of course much less circulating current. This aids drift reduction. Because larger inductances are required, stray inductances do not have as much impact as perhaps in other circuits.

COLPITTS OSCILLATORS Colpitts oscillators are somewhat similar to the shunt fed Hartley circuit except the Colpitts oscillator, instead of having a tapped inductor, utilises two series capacitors in its LC circuit. With the Colpitts oscillator the connection between these two capacitors is used as the centre tap for the circuit.

CRYSTAL OSCILLATORS AND CRYSTAL GRINDING Crystal oscillators are oscillators where the primary frequency determining element is a quartz crystal. Because of the inherent characteristics of the quartz crystal the crystal oscillator may be held to extreme accuracy of frequency stability. Temperature compensation may be applied to crystal oscillators to improve thermal stability of the crystal oscillator. What is crystal grinding? There was an exchange of emails asking about crystal grinding on the 'Flying Pigs' list. Karl Kanalz, W8TIF gave his insight and experience on the topic of crystal grinding FT-243 crystal blanks and has graciously consented to it being reprinted here.

HARTLEY OSCILLATORS Hartley oscillator are inductively coupled, variable frequency oscillators where the oscillator may be series or shunt fed. Hartley oscillators have the advantage of having one centre tapped inductor and one tuning capacitor. This arrangement simplifies the construction of a Hartley oscillator circuit.

VOLTAGE CONTROLLED OSCILLATORS

A voltage controlled oscillator or as more commonly known, a vco, is an oscillator where the principal variable or tuning element is a varactor diode. The voltage controlled oscillator is tuned across its band by a "clean" dc voltage applied to the varactor diode to vary the net capacitance applied to the tuned circuit.

OSCILLATOR DRIFT AND DRIFT CORRECTION CIRCUITS 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. An oscillator drift correction circuit is any circuit which automatically brings an oscillator back onto it's assigned or tuned frequency. Such circuits might be frequency synthesisers with phase locked loops, automatic fine tuning circuitry AFT or, AFC in AM Receivers and automatic temperature compensation built into an oscillator circuit.

HARTLEY OSCILLATORS What are Hartley oscillators? Hartley oscillator are inductively coupled, variable frequency oscillators where the oscillator may be series or shunt fed. Hartley oscillators have the advantage of having one centre tapped inductor and one tuning capacitor. This arrangement simplifies the construction of a Hartley oscillator circuit. First off let's look at a schematic of a hartley oscillator.

Figure 1 - outline schematic of a hartley oscillator

Designing a Hartley Oscillator Here I'll present the schematic for my old favourite, together with a buffer stage and an amplifier stage which should deliver about 5V P/P into a 50 ohm load. We'll discuss each relevant stage and produce some rule-ofthumb design info. Because the consensus comes down in favour of FETS and I'm big enough to lay aside my prejudices in the noble cause of advanced education we'll use a FET oscillator. Nothing to do with a few friends who might belt me up!

Figure 2 - schematic of a hartley oscillator

For this design I'm going to say we will be constructing a general purpose VFO to operate at 5000 - 5100 Khz no particular reason, pick anything you like. Now I chose a 2N4416A FET purely because I bought a big bag of them years ago and have them on hand. You could use any general purpose JFET you can readily obtain. Note the 2N4416A is a metal can and the case is grounded. The frequency determining components are L1, Ct (a nominal 10 pf trimmer), C1a, C1b, C2, C3, Cv and C4. Note: I have been asked a number of times the function of C4 in this circuit. Capacitor C4 is to reduce the loading on the tuned circuit components. It may be as small as possible consistent with being able to provide sufficient drive to the succeeding buffer amplifier stage. Often the home constructor will often make C4 a trimmer. The other components are bog standard. The two resistors, silicon diode and zener diode need never change, capacitor C5 is about right for this frequency. C6 can be selected to give higher / lower output to the buffer amplifier. Smaller C6 values give lower output and conversely higher values give larger output. The silicon diode I'll explain later, the zener diode is to give a regulated 6.2 volt supply. Now there is NOTHING sacred about my frequency determining capacitor combination O.K.? Too many people look at these kind of circuits and think they must duplicate everything literally, not so. This is just a typical representation. C1 to C3 plus Cv and Ct are just a combination of parallel and some series capacitors all designed to give us a bit of flexibility with the tuning range. Cv could easily be replaced by two back to back tuning diodes. What you need to do to get the circuit to work is to have an inductive reactance for L1 of around about 180 ohms. At 5 Mhz this works out at about 5.7 uH and, if you don't know how I arrived at that figure I seriously recommend you spend some time on my other tutorials such as Basics and LC Filters. The important aspect is that the feedback point from the source of the JFET connects to about 25% of the windings of L1 from the ground end. Now I've depicted an air cored inductor. It could be, just as one example among a great many, 18 - 19 turns of #20 gauge wire on a 25.4 mm (1") diameter form spread evenly over a length of about 25.4 mm (1"). The tap would be at about 4 1/2 turns. Check that out with the formula's I taught you elsewhere. Alternatively, with degraded performance, you could use a T50-6 toroid and wind say 37 turns of #24 wire (5.48 uH) tapping at 9 turns. The AL factor for a T50-6 is 40. Again do the other tutorials if necessary, I'm not going to repeat old work and it's going to be even harder from here on. I'll thoroughly explain new concepts, not the old. So if we are to have our oscillator working at about 5 Mhz, we know the LC is 1013 and if L is say 5.7 uH then total C for resonance (just like LC Filters eh!) is about 177 pF. We want to be able to tune from 5000 to 5100 Khz a tuning ratio of 1.02 which means a capacitance ratio of 1.04 (min to max). Let's fiddle with some numbers! I have a Jackson Bros. air variable capacitor (very Rolls-Royce) which swings from 10.5 pF to 105 pF, a typical 10:1 ratio in air variables. This I will use for Cv. If the total swing is 1.04 (actually 1.0404:1) and Cmax is 177 pF it follows Cmin is 170 pF. A variation of only 7 pF (roughly). Now we're treading on unsafe ground here with such a large variable capacitor. We could:

A) rip plates off it to reduce capacitance (don't even think about it) B) go to varactor diodes with a small swing. That's O.K. but performance becomes degraded. C) obtain a smaller air variable with Cmax of say 25 pF. Just to prove I'm a glutton for punishment and if you're still here so are you, we will purely for the mathematical exercise, persevere with the 105 pF variable. What if we eliminate C3 and make C2 = 15 pF NPO then the series combination of C2 and Cv swing 6.176 pF to 13.125 pF, a variation of over 6.9 pF - are you lost? Go back to the other tutorials on capacitance. If our Cmax was 177 pF then 177 - 13.125 = 163.875 and the 177 pF was approximate anyway. I'd make Ct a 10 pF air trimmer (if available, if not, a ceramic or whatever the supplier offers but 10 pF max.). That leaves about 154 pF to make up. How about making C1a and C1b into 3 NPO capacitors of say 2 X 47 pF and 1 X 56 pF all NPO types. In total that comes to less than 177 pF max. but don't forget there are stray capacitance's in the circuit. In the final wash-up you could simply use 3 X 47 pF. Here's what we finish up with

Figure 3 - final schematic of a hartley oscillator

Advantages of Hartley Oscillators The frequency is simply varied by the net value of C in the tank circuit. The output amplitude remains constant when tuned over the frequency range. The feedback ratio of L1 to L2 (figure 1) remains constant.

Disadvantages of Hartley Oscillators The output is rich in harmonic content and therefore not suitable where a pure sine wave is required.

Colpitts oscillators What are colpitts oscillators? Colpitts oscillators are somewhat similar to the shunt fed Hartley circuit except the Colpitts oscillator, instead of having a tapped inductor, utilises two series capacitors in its LC circuit. With the Colpitts oscillator the connection between these two capacitors is used as the centre tap for the circuit. The basic Colpitts oscillator circuit look like this and you will see some similarities with the Hartley Oscillator..

Figure 1 - schematic of a collpitts oscillator Perhaps the simplest Colpitts oscillator to construct and get running is the "series tuned" version, more often referred to as the "Clapp Oscillator". Because there is no load on the inductor a high "Q" circuit results with a high L/C ratio and of course much less circulating current. This aids drift reduction. Because larger inductances are required, stray inductances do not have as much impact as perhaps in other circuits.

Figure 2 - schematic of a series tuned colpitts or "clapp" oscillator Rather than present designs for specific frequencies for the Colpitts Oscillator we have submitted a schematic which may be "impedance" scaled to any frequency. Simply convert the suggested reactances back to the required inductor and capacitances at your band of interest.

Component selection for the Colpitts oscillator The Colpitts oscillator inductor should be around 250 - 300 ohms and the "NET" capacitive reactance should total around the same. Feedback capacitors Cfb, both "a" and "b" are each in the region of 45 ohms leading to very large values which is very helpful in swamping out the capacitive effects of the transistor used. The total capacitive reactance of the parallel combination of capacitors depicted as series tuning below the inductor in a series tuned Colpitts oscillator or "Clapp oscillator" should have a total reactance of around 200

ohms. Not all capacitors may be required in your particular application. Pay particular attention to our comments in Oscillator Basics.

Tuning the Colpitts oscillator Perhaps the best approach to values used for tuning a Colpitts oscillator might be to give a practical example. Consider constructing an oscillator which tunes part of the 40M amateur radio band, 7.0 - 7.2 Mhz. Now that is a frquency ratio of 1.02857 requiring a modest net capacitance variation of 1.058. If that is not understandable go back to our basics. Using an inductor of 300 ohms at 7 Mhz for our Colpitts or Clapp oscillator yields a value of about 6.8 uH. Each Cfb at 45 ohms works out at 500 pF so we will try 470 pF. Using an inductor of 6.8 uH requires a total capacitance of 76 pf to resonate at 7.0 Mhz. At 7.2 Mhz this value has dropped down to 71.86 pF a small variation. Effectively all the capacitors are in series in a Colpitts oscillator. That is Cfb-a and Cfb-b are each in series with the total parallel combination below the inductor. Given Cfb are each 470 pF what values are the parallel combination to achieve outcomes of net 76 pF and 71.86 pF? Had you done basics and in particular capacitance you would know the answer. We're not being smart here just pointing out there is no such thing as a "free" lunch, you have to know "the basics". Having done these lectures on the internet for several years, yes I'm Ian Purdie VK2TIP, I don't appreciate email questions from lazy students expecting me to complete or provide their assignments for them. Do some work for yourself!. Back to our Colpitts oscillator, I'll give you a "FREE" clue to the answer with these colourful formulas for the answer in figure 3. I know a shorter method. See this example in capacitance.

Figure 3 - calculations for a series tuned colpitts or "clapp" oscillator Here in our Colpitts oscillator Ctotal-max is the maximum of the parallel combination including the variable capacitor Cv, set at maximum while Ctotal-min is the same combination with Cv set at minimum. Clear on that? Note that the other series capacitor depicted in line with Cv in figure 2 may or may not be required in your particular application. Anyhoo! your calculations should have yielded a Cmax of 112.3 and Cmin of 103.51. Now that is a pretty tiny swing, so you can see the possible need for a series capacitor with Cv if all you have available is a fairly high value Cv. Let's assume Cv is 5 - 25 pf (a variation of 20 pF) and Ct is a 10 pF trimmer. Where do we stand now with our Colpitts oscillator calculations? Allowing for taking strays into account we will take the full value of Ct of 10 pF, therefore the balance of the parallel combination accounts for Cmax of 102.3 and Cmin of 93.51. The net variation of C still remains 102.3 93.51 = 8.79 pF so Cv needs to be seriously reduced. How is largely determined by the same method below (suck and see approach) but I come up with a "starting point" of about 12 pF in series with Cv.

Therefore nearing completion of calculations for our Colpitts oscillator, we find this new series combination with Cv produces a net Cv-max 8.1 pF and Cv-min 3.53 pF. This is obviously far too much reduction using the 12 pF capacitor, so try using some higher values like 27 pF! Using 27 pF in our selection gives Cv-max 12.98 pF and Cv-min 4.22 pF and a net variation of 8.76 pF. Kicked a goal! New Cv is (at max) 13 pF and Ct assumed at 10 pF a total of 23 pf from a required maximum of 112.3 leaving about 89 pf to be made up of fixed capacitors. I'd use at least three fixed capacitors, say 33 pF plus 2 X 27 pF.

Screening your Colpitts oscillator components Ideally your frequency determining components L1 and the parallel capacitors should be screened in an earthed shield.

The reality of your labourious Colpitts oscillator calculations All those sums you did above are just the starting point for your Colpitts oscillator. Lifes realities are that all things "won't go according to plan". Stray inductance and stray capacitance will play havoc with your calculations, but you're right in the "ball park" and you should understand what needs to be done to adjust frequency. Remember no inductor you wind is going to be a perfect 6.8 uH inductor for L1 in figure 2 above.

Taking output from the Colpitts oscillator The output from the Colpitts oscillator is through output capacitor Co, this should be the smallest of values possible, consistent with continued reliable operation into the next buffer amplifier stage. That statement is true for all oscillators. Typical values for Co in a Colpitts oscillator might be 47 pF.

CLAPP OSCILLATORS What are Clapp Oscillators? A clapp oscillator is in effect a series tuned version of the colpitts oscillator. Perhaps the simplest Colpitts oscillator to construct and get running is the "series tuned" version, more often referred to as the "Clapp Oscillator". Because there is no load on the inductor a high "Q" circuit results with a high L/C ratio and of course much less circulating current. This aids drift reduction in what is otherwise a clapp oscillator. Because larger inductances are required, stray inductances do not have as much impact as perhaps in other circuits.

Figure 1 - schematic of a series tuned colpitts or "clapp" oscillator