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577 curve tracer and low-current looping
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I recently put a long-dormant 577 back into service, got a transistor test fixture for not-too-silly money, and am getting to know the unit. Definitely more flexible than the 7CT1N. But worse in one respect.
When sweeping low-current beta, looping is much worse. See pics of 577 and 7CT1N measuring the same 2N3904 at 1uA/step Ib, 200uA/div and 2V/div display. 7CT1N: ;attach=1411147;image 577 : ;attach=1411141;image (Perhaps not coincidentally, 1uA is the lowest current range on the 7CT1N). I tested several other NPN's, with the same result. Adjusting the 177 "Looping Compensation" control had no affect on the trace. I don't see any adjustment in the cal procedure to normalize it. I do, however, see several entries in the op manual warning "When measuring beta at lower currents, DC mode is necessary to prevent looping." Would like to hear from other 577 users, is this normal behavior in AC mode? At what current does yours show this much looping? I frankly expected it would be better than the 7CT1N. |
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That's pretty much identical to what I see on my 577 with a 2N3904 and the same settings.
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I dimly recall that there was at least a calibration step that verifies operation of the looping compensation control, but as you experienced, it has no effect here. -----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Precaud via groups.io Sent: 13 February 2022 03:41 To: [email protected] Subject: [TekScopes] 577 curve tracer and low-current looping I recently put a long-dormant 577 back into service, got a transistor test fixture for not-too-silly money, and am getting to know the unit. Definitely more flexible than the 7CT1N. But worse in one respect. When sweeping low-current beta, looping is much worse. See pics of 577 and 7CT1N measuring the same 2N3904 at 1uA/step Ib, 200uA/div and 2V/div display. 7CT1N: ;attach=1411147;image 577 : ;attach=1411141;image (Perhaps not coincidentally, 1uA is the lowest current range on the 7CT1N). I tested several other NPN's, with the same result. Adjusting the 177 "Looping Compensation" control had no affect on the trace. I don't see any adjustment in the cal procedure to normalize it. I do, however, see several entries in the op manual warning "When measuring beta at lower currents, DC mode is necessary to prevent looping." Would like to hear from other 577 users, is this normal behavior in AC mode? At what current does yours show this much looping? I frankly expected it would be better than the 7CT1N. |
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I can confirm the same behaviour on my 577 also.
As I recall, the 7CT1N has a triangle-wave generator for the collector voltage, at a higher frequency than the line frequency, sine wave (or near sine wave) that the 577 uses for collector voltage on non-DC settings, so I would expect some behavioural differences, although I agree these artifacts are a little annoying in something otherwise so capable as a 577 is. I have found the effect is worse with flying leads, so I surmise that test fixture inductance plays a part in these artifacts. On Sun, Feb 13, 2022 at 10:59 AM Precaud via groups.io <jbau= [email protected]> wrote: Thanks for checking this on your unit, Dave. Yes, there's a step in the op-- Andy |
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