Figure 1. - automatic temperature compensation circuit schematic
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 automatic temperature compensation built into an oscillator circuit, automatic fine tuning circuitry AFT or, AFC in AM Receivers and frequency synthesisers with phase locked loops.
Assuming you have studied the oscillator drift topic you would know that thermal considerations play a major role in oscillator drift. One possible automatic temperature compensation circuit uses a combination of varactor diodes, thermistors (resistors whose resistance varies with temperature) and a few other components. In a way it works a little like a voltage controlled oscillator except the DC control voltage applied is determined solely by the affect of temperature variation on the thermistors.
A typical automatic temperature compensation circuit for drift correction might look like the schematic in figure 1 below.
Figure 1. - automatic temperature compensation circuit schematic
It is near impossible to give circuit values here because of so many variables involved.
Firstly what type of thermistor are you able to readily purchase? What is it's resistance at room temperature? What is the resistance change per degree temperature variation. What type of varactor diodes are you able to purchase? What is their capacitance at say half V+?
The variables and permutations thereof lead to endless possibilities but, I'll give you some pointers.
1. The general idea of the automatic temperature compensation circuit is to apply about half V+ (this figure is arbitrary) to the point where both cathodes of the varactor diodes meet. This is also denoted as point "X" just in case we are talking about a variable oscillator. While moving frequency you would have a switching system that switches half V+ into this point and at the same time removes V+ from the point to the left of R1. This means that whatever the series capacitance of D1 and D2 also in series with C1 is always in circuit and are counted as part of any normal oscillator parallel capacitance. If it's a fixed tuned oscillator then no switching system is necessary.
The object of the exercise is that at all times, at a stable temperature the voltage is constant and as a consequence the series capacitance of D1, D2, and C1 remains constant and forms part of the oscillator tank circuit.
2. When we have some variation from our previous stable temperature we need that variation to be detected by the thermistors Th1 and Th2, which then alter their resistance and so the voltage divider network of the thermistors, each with R1 and R2 in parallel vary in net resistance value, and apply a different DC voltage to the varactors which in turn means a different net capacitance applied to the oscillator tank circuit. Resistor R2 is simply another portion of the voltage divider network which may or may not be necessary. That's the theory anyway.
Just remember depending on the operating frequency, the net overall variation in capacitance we are looking for is probably a mere fraction of one picofarad.
Unfortunately one big downside of thermistors can be their "slow reaction" time. They don't always respond immediately to variations. How much I don't know, I can't find my old thermistor data book so I'm relying entirely upon memory here - as always. But it can be a problem.
It's really quite an interesting circuit for the avid experimenter. If you intend to experiment then please let me know.
Automatic fine tuning or AFT is also known to some people as automatic frequency correction or AFC
Curiously the origins here relate to an old type of FM detector. I know the circuitry, or something similar was in operation in old time TV receivers where all the video information was AM, only the sound was FM.
See also: fm radio receivers.
This type of detector is called a "frequency discriminator". The IF signal is amplified and passed though a "limiter" to remove AM components of the signal which is then passed to the frequency discriminator.
Figure 2. - frequency discriminator schematic
Because of the phase relationships of the tuned centre tapped secondary any frequency deviation is translated into a DC voltage which then passes through a low pass filter to remove any RF components and then on to a varactor diode in the oscillator. These circuits have largely been superseded by digital phase locked loops (PLL) in frequency synthesisers.
Another of my favourite topics! Basicly, a frequency synthesiser relies on several component parts, firstly there is a highly accurate master crystal oscillator which might be 5 Mhz. This master oscillator is digitally divided down to a suitable reference frequency, let's say for example divide by 500 to get a 10 Khz reference.
Then we have a voltage controlled oscillator which for an AM receiver with a 450 Khz IF would operate at around 990 Khz to 2100 Khz.
Next we have another digital divide by "N" circuit which divides the VCO auxilliary output by 99 to 210. No matter where the VCO frequency is, the divided output which is unlikely to be exactly 10 Khz, is sent to a "phase lock loop" where it is compared with the "master reference" frequency from the precision master crystal oscillator.
An error correction voltage is developed and sent to the VCO to put it exactly on frequency.
An interesting circuit I saw years ago relied upon a frequency counter (similar principles) and the ouput of the least significant digit This output was also compared to a suitable refence frequency in a PLL.
Modern integrated circuits are such that all these functions are often contained within one IC, a typical example is the PLL Frequency Synthesiser family MC145152 to MC145158. This is a comprehensive pdf data sheet.
A comparatively recent development is "direct digital synthesis". Here the sine wave output is developed by digital to analog by means of a look-up table and microprocessor.
If I could be certain that the component parts were readily available to everyone I would like to do a frequency synthesiser or direct digital synthesis project.
Not strictly correct as an oscillator drift correction circuit but worthwhile if you decide to advance forward in this topic. Used extensively in some frequency synthesisers, especially as the phase detector.
This is a rather simplistic explanation of a 74HC4046 phase-locked-loop (PLL) with a vco because whole books have been devoted to phase locked loops, offering varying degrees of mathematical complexity. It is not a subject with which you can safely take a casual approach. A vco stands for "voltage controlled oscillator" which has been dealt with earlier in a discrete form. These devices are frequently used in conjunction with digital dividers and frequency synthesisers.
From my Newsletter May, 2002
Hans Summers told us on G-QRP about the Huff & Puff stabilised VFO.
"You might want to have a look at the Huff & Puff stabilised VFO. An easy circuit which can cancel large amounts of start up and ongoing drift... for as much info as you could want."
On his site Hans says; "The Huff-Puff technique is a method of stabilising the frequency of ordinary L-C VFO's. Most VFO constructors will have experienced great difficulty obtaining a stable frequency, at least without careful attention to temperature compensated capacitors etc. The Huff-Puff approach was pioneered by the late Klaus Spaargaren PA0KSB, and results in a rock-stable VFO effectively locked to a crystal-derived reference frequency. Over the years several magazine articles have appeared describing both the original circuit and subsequent enhancements. Some of the articles are reproduced here, along with details of my own Huff-Puff projects, and an article sent to me by Olivier F5LVG about his Simple Frequency Stabiliser..."
Hans has heaps of downloadable articles.
See:
http://www.hanssummers.com/huffpuff.html
oscillator drift
buffer amplifiers
colpitts oscillators
crystal oscillators
hartley oscillator
oscillator basics
voltage controlled oscillators
74HC4046 VCO and PLL
Huff & Puff stabilised VFO
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Updated 15th May, 2002