February 2, 1998

Why so expensive?

Regarding "Demystifying ADCs" (EDN, March 27, 1997, pg 26) and "Remystifying ADCs" (Oct 9, 1997, pg 87):

Why are the solutions and test procedures limited only to those that involved expensive test equipment, especially when the article and contributors acknowledge that many users may not have access to such equipment?

Although the exact frequency of 1.91 MHz cannot easily be achieved without expensive synthesizers, it is possible to use other frequencies in a phase-locked fashion for less than $1000 using a little ingenuity. A square wave is an infinite sum of all the odd harmonics. By starting with a simple square-wave oscillator, such as 40 MHz, the 10-MHz sampling frequency can be derived by dividing by 4. Changing the oscillator frequency can take care of hitting specific or exact frequencies. I use programmable logic for the dividers, so by replacing the PAL, I can get various frequencies for the clock and input, but simple 7474-style flip-flops or binary counters work fine.

Next, to get the input frequency, the 40 MHz can be divided by 20 to get 2 MHz or by 21 to get 1.90 MHz. I have yet to get anyone to explain why a frequency such as 1.91 MHz is sacred for any reason other than that that was the bandpass-filter frequency available at the time. By using a clock driver or buffer, the 1.90-MHz output can be driven into a bandpass or lowpass filter. I use an eight-pole lowpass or an 11-pole bandpass filter. Any overshooting in the output of the buffer or driver is typically composed of high-frequency components and gets filtered out. The main issue is the duty cycle of the square wave and, to a slightly lesser degree, the difference between rise and fall times.

The filter costs about $300; the oscillator, less than $5; the PAL, about $10. The buffer is another $5 (all single-piece prices from catalog houses). The price of a breadboard for these components varies. Using a kit (from Radio Shack), which uses iron-on imaging from a laser printer, you can build the clock and signal source for less than $1000. The results are as good as those from the best test instruments and take only a few hours. I have done tests both ways and have compared results.

One point was also made about the PC being open and unshielded. I have noticed that I have to turn off my PC and especially the monitor when making such measurements. Because I use the PC to download and control the test setup, that does cause some inconvenience.

Scott Taylor
via the Internet

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