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Looking at figure 1 below can give you a somewhat better idea. This is a relatively simplistic explanation of a 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.
Turning our attention to figure 1 we first see a fixed 1 Mhz oscillator, depending upon our application, we try to make this as precision as possible consistent with our goals. This oscillator usually comprises TTL gates. This is "the reference", the accuracy of the whole system is no greater than this reference.
For the purposes of this example, we then pass this reference to a digital divider which divides the reference by ten. It could be "any" number which suits your purposes, same comments apply to the reference crystal frequency.
Figure 1 - a phase-locked-loop with vco
Our divided reference frequency, in this example 100 Khz (could be anything convenient), then passes to the heart of our device, the 74HC4046 phase-locked-loop with vco, the "phase comparator" section.
This device, the 74HC4046, has regularly featured in numerous projects over the last nearly 30 years in various incarnations, CD4046, HEF4046 etc. purely to take advantage of the superior phase comparator sections. The other features were often ignored.
I will come back to the phase comparator section later.
As I just stated, in a lot of projects this part of the 4046 was often ignored in favour of the conventional voltage controlled oscillator. Often some projects had quite a number of these VCO's digitally bandswitched by front panel controls.
I don't immediately have the references available but Dr. Ulrich Rhode published a number of excellent projects for "frequency synthesisers" in the now defunct magazine "Ham Radio" as did a number of other authors. These would have been published throughout the mid to late 1970's.
Back to the 74HC4046 VCO, its tuning range is determined by one external capacitor C1 (between C1A and C1B) and one external resistor R1 (between R1 and GND) or two external resistors R1 and R2 (between R1 and GND, and R2 and GND). Resistor R1 and capacitor C1 determine the frequency range of the VCO. Resistor R2 enables the VCO to have a frequency
offset if required. These are depicted in figure 2 "functional diagram of the 4046 phase-locked-loop with vco" below.
Figure 2 - functional diagram of the 4046 phase-locked-loop with vco
The exact ranges and component values are determined by extensive charts included in the 4046 data sheet - (443K) in PDF format.
This gives us a very flexible VCO capable of operating anywhere up to 17 Mhz, something the early CMOS versions were incapable of doing.
Back to our phase-locked-loop with vco in figure 1.
The output of the VCO which is our "frequency" we use for our project, also goes back via a digital divider to the phase comparator PC2 via pin 3 on the 74HC4046AN
Here we have another area where suitable devices abound. You could have perhaps a fixed division ratio 10, 100, 1000, etc. which in the old days would have been accomplished with TTL 7490 decade dividers. You could have some fixed odd number 73, 99, 256, 987654321, who knows? The limits are your application requirements, your imagination and, the upper limits of the devices you choose. Always consult a data sheet and look for operating limits.
My own personal favourite was always the presettable synchronous BCD decade up/down counter 74192. Here I could stack them up for every decade I wanted in my application. Want to divide by any number between 1 and 9999? Use four 74192's and connect the BCD inputs to BCD switches. You could, and probably still can buy 0 - 9 rotary switches which had the 1-2-4-8 binary outputs. Confused? Look here for more info on 1-2-4-8 binary outputs. Look at the BCD table but only concern yourself with the numbers 0 - 9.
The more modern version of the TTL 74192, is the 74HC/HCT192 and this is the link to the appropriate Philips data sheet. Fairchild continue to manufacture the faster version 74F192 and here is the appropriate Fairchild data sheet. It's typical upper frequency limit was 125 Mhz.
Many of these devices are now becoming obsolete.
Another matter which needs to be considered is that because microcontrollers have become so relatively cheap they can often be used in these applications. Again you are only limited by the device and your imagination. There are also many microcontroller compatible digital divider devices on the market.
BE AWARE there are a lot of "gotcha's along the way to using these devices. Decoupling, pull up or pull down, upper frequency limits are just a few. Even filtering the BCD inputs can produce headaches.
Here as I said is the "heart" of the device. There are actually three on board phase comparators PC1, PC2, and naturally PC3.
Firstly what is a phase comparator?. Consider this, for every frequency there is a discrete period in time. If we take a frequency of 1 Hz then it is of one second duration to complete a full 360o cycle. Like wise 10 Hz would take 0.1 second (100 mS) and 1 Mhz would take one microsecond. The time period is always the reciprocal of the frequency i.e. the number one divided by the frequency.
Now, without getting too deeply into phase relationships and following the KISS principle, it follows that the time period of a frequency of 7105.6 Khz MUST be different from the time period of a frequency, slightly different at 7102.4 Khz.
This is what a phase comparator does. It compares the phase relationship between the reference signal on pin 14 of the 74HC4046 "Sig In" pin with the input frequency on pin 3 "Comp In".
Depending upon which comparator we choose to use (pins 1, 2, 13 or 15) and our method of output filtering (a science in itself) we will get a DC correction voltage. In figure 1 above this correction voltage was applied to pin 9, the VCO driving voltage.
Consider this: If we had an extremely accurate crystal oscillator running at 10 Mhz. Here I mean "really accurate" not a ready purchased crystal oscillator pack of the clock crystal variety but a temperature compensated type held to say one part per million. At 10 Mhz it's possible degree of inaccuracy would be +/- 10 Hz. Right...
We divide that down with digital dividers by 10,000 to achieve a final reference of 1,000 Hz or 1 Khz, as depicted in figure 1. This is applied to pin 14 of the 74HC4046.
At the same time we have a discrete VCO running between 100 - 110 Mhz, this is fed into a digital divider capable of operating at those frequencies. Our digital divider is set up and programmed to divide between the numbers 100,000 and 110,000. The resulting output lying somewhere between 1000 Hz and 1100 Hz (1 Khz and 1.1 Khz) is fed to pin 14 of the 74HC4046.
Within the phase detector it will detect a difference in phase relationship (assuming we weren't exactly on frequency to start with). Remember our highly accurate reference frequency has a phase duration for a full cycle of exactly one milli-second. On the other hand the time duration is somewhere between 1 / 1000 = 0.001 or one milli-second and 1 / 1100 = 0.00090909' or 909.0909' milli-seconds.
Depending upon the variance (phase relationship) up or down, the output filtering and other factors, there will be generated a DC control votage which we apply to the varactor diode of our VCO to bring it precisely "on frequency" with our digital divider settings.
For reasons of practical expanation I have grossly oversimplifed much of that. For example some phase comparators only need 90o of the cycle. That's another story.
The example I have illustrated or at least the theory is the very basis of tuning modern day receivers. Be it a cell phone, AM / FM receiver, communications receiver or TV set. If it's of modern manufacture that's how the local osciilator and consequently the receiver is tuned!
And that was only one example or application. If there is sufficent interest I will produce a line of interesting applications for the 74HC4046.
DATA SHEET - 74HC4046 PHASE-LOCKED-LOOP WITH VCO: 74HC/HCT4046 data sheet - (443K) in PDF format.
Reference books on phase locked loops I've used and/or referred to in the past include:
Phase-Locked Loops: Design, Simulation, and Applications by Roland E. Best
Phase-Locked Loop Circuit Design (Prentice Hall Advanced Reference Series) by Dan H. Wolaver
Frequency Synthesis by Phase Lock by William F. Egan
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Created 17th March, 2002
Updated 17th March, 2002