Stick to the schematic
By Barry Harvey, Intersil Inc - October 30, 2008
I recently needed to re-create a settling-time-measurement jig I built 25 years ago. It transmits and receives 10V swings and is flat enough and magnifies enough to see 0.01%/division within 35 nsec on an oscilloscope. You can’t buy such a step generator, and you can’t use an oscilloscope unassisted to see 0.01% absolute-settling features.
I instructed our intern to build it. I made sure that he stuffed only the first stage, which is a wideband differential amplifier employing a 35-year-old CA3127 transistor array from RCA Semiconductor, which General Electric Semiconductor and then Harris Semiconductor absorbed. Now, Intersil has absorbed it, so we have the part in inventory.
He proudly showed it working in the lab. Square waves at the input came out square. He commented, though, that the differential outputs seemed to have a large offset. I checked the operating currents, and they were correct. I then applied a triangle wave to the input. Holy schmaltz! The output was curved! The other of the differential outputs was simply railed—not good. I inadvertently rubbed the board with a finger and saw the entire waveform warp back to triangle shape and shift its dc level.
OK, we had an oscillation there, one that was too fast for our 400-MHz oscilloscope to see. I discovered that I could get any manner of triangle distortion or offset depending on where I touched. A finger presents an impedance of approximately 100 pF in series with approximately 100Ω to ground, and this condition loads and modifies oscillations. A couple of things happen during oscillation. The oscillation grows in amplitude until it overloads devices, causing dc rectification and operating-point shifts. Thus, the offset varies with where you touch your finger. A consequence of this variation is that low-frequency signals variably encounter the dc shift, so gain and offset variations and even nonlinearities on the signal can occur.
The other result of oscillation is radiation, meaning that you can find the oscillation using an antenna and a very-high-frequency-sensitive lab instrument, such as a spectrum analyzer. So, I made a small antenna from a coaxial cable cut and a small loop connecting from the center conductor to the shield. This small transformer secondary coupled energy efficiently enough for a spectrum analyzer to show.
So, holding my loop near the part, I saw a 1.6-GHz spectral spike that disappeared when I powered the system off. I then found that I could put my finger anywhere, even in ground regions, and influence the spectrum. Unfortunately, I couldn’t find a place that killed the oscillation. I used a metal probe, fingering down to keep the probe length and inductance low.
The grounds were hot everywhere! I had our intern drill holes in a grid every ¾ in. between top foil and bottom grounds, shorting them all over.
After three sessions, I made no progress, so I looked at the data sheet of the CA3127. The peak gain-bandwidth product was 1.2 GHz. How do you get a 1.6-GHz oscillation from a 1.2-GHz part?
At this point, I didn’t trust the part. I asked what part it was, and the intern assured me that it was a 3127—the HFA3127! Looking at that part’s data sheet, I noticed an improved 8-GHz array at the gain-bandwidth-product curve, and, although an 8-GHz part can oscillate at 1.6 GHz, our board layout couldn’t handle such fast transistors.
He replaced the array with a CA3127, and all was fine—no offset, no distortions. I didn’t recommend the HFA devices, so I blamed the intern.
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