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Feature Article
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Issue #209 December 2007
THE DARKER SIDE
Are You Locked?
A PLL Primer
by Robert Lacoste
Start | VCO Basics | PLL Basics | Integer or Fractional? | PLL Design | Silicon Trends | It's Your Turn! | Sources & PDF
SILICON TRENDS
Let’s talk silicon for a minute. The first integrated PLL was probably the NE56x series, originally from Signetics, but they are a little obsolete in comparison to today’s standards. Nearly all manufacturers have PLLs in their catalog, so I will not give you a list of all the chips on the planet. For an example, go to www.analog.com/pll to see the PLL product line from Analog Devices. It will give you an idea of the PLL performance you can expect to find on the market, even if you can also find similar products from Texas Instruments, Maxim Integrated Products, and others.
A couple of years ago, most PLL chips were integrating the dividers and PFD, but they were designed to drive an external VCO through an external loop filter. This is still the case for high-performance chips. But, solutions are now available that integrate the VCO too. For example, chips like the Texas Instruments TR3761 and the Analog Devices ADF4360 integrate everything but the loop filter, and of course, the simulation tools of their respective suppliers support the parts.
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| Photo 2—Cypress Semiconductor proposed a low-cost evaluation kit for its CY22393 multiclock generator. All the required components to build a five-output clock generator are visible in the picture. A USB interface chip is on the bottom face. |
Another very interesting trend is mixing PLL technology and direct digital synthesis (DDS). What is DDS? Well, I’m afraid I’ll need another column to cover the specifics. Basically, it’s a digital way to generate variable frequency signals. PLLs are great in high frequencies, but they have difficulties with small frequency steps. It is exactly the opposite with DSS. So, a wedding between the two technologies can give decent results. And the good news is that those solutions are already available in silicon. An example? Chips like the Analog Devices AD9956 can generate frequencies up to 2.7 GHz with sub-hertz resolution. You can’t use it for everyday applications, but it can be useful.
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| Photo 3—The CyberClocksRT software enables you to easily configure the CY22393 chip and download the parameters to the evaluation board through a USB link. Of course, not all frequencies can be generated; however, the tool shows you the frequencies that are not fully exact and the error in ppm. |
On the other extreme, you can find medium-performance PLL chips that are easy to use (requiring nearly no external components) and impressively flexible. For example, assume that you are designing a new multimedia digital device. You have done a system-level design, and you discover that you need four different clock frequencies: 10, 25.55, 8.42, and 5.44 MHz. Are you going to buy four custom crystals? It would be the wrong decision because Cypress Semiconductor has chips like the CY22393, which includes no less than three full PLLs, their associated VCOs, a 4 × 4 crosspoint switch, five programmable extra dividers, and file clock output drivers, all in a space-saving TSSOP16 package for $4 or $5 each in low volumes! You don’t trust me? Have a look at Photo 2, which shows the low-cost Peppermint evaluation kit. You can configure it with the easy-to-use PC-based cyberclocksRT software and a USB cable (see Photo 3). Lastly, Photo 4 shows you the actual output waveform, which is not too bad with a single crystal.
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| Photo 4—I hooked my Hewlett-Packard Logic-Dart pocket analyzer on three outputs of the CY22393, and yes, I got the desired 5.44, 8.42, and 25.55 MHz frequencies from top to bottom, respectively. |
