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Screw Up

G

garyr

Jan 1, 1970
0
I'm trying to build a DDS VFO I've designed. Today I started adding parts to
the board, first the voltage regulator. Voltage OK, 3.301 volts. Then I
added a 50 MHz oscillator (CTS CB3LV); 3.3 volt line now measures 1 volt.
Poking around a bit I find that the VCC and ground going to the oscillator
chip are swapped (the pin numbers I assigned to the package drawing I
created were incorrect). I remove the oscillator chip, make a few cuts and
jumpers and reinstall the oscillator. Lo and behold! it works; sort of. My
scope indicates that the frequency is right on but the output signal is not
a square wave as expected and is much too large - it is now a sine wave
having peak amplitude of positive 5 volts and negative 2 volts. A 200 ohm
load reduces the output amplitude only slightly. The chip was obviously
damaged but what damage would produce that kind of results?
 
T

Tim Williams

Jan 1, 1970
0
What bandwidth scope, and what probe and method?

Tim
 
G

garyr

Jan 1, 1970
0
Tim Williams said:
What bandwidth scope, and what probe and method?

Tim

You hit the nail right on the head. I had had a very long ground path
between the probe and the oscillator output. I looked at it again using a
little spring-thingie that slips onto the probe tip, providing a very short
ground path between probe and board, and see a pretty good 50 MHz square
wave but with a little ringing and a peak amplitude of about 3.6 volts. So
apparently the chip wasn't damaged by my screwup.

The scope is a Tek TDS 1012, 100 MHz BW, TEK 10X probe.

Many thanks
Gary
 
T

Tim Williams

Jan 1, 1970
0
garyr said:
You hit the nail right on the head. I had had a very long ground path
between the probe and the oscillator output. I looked at it again using
a
little spring-thingie that slips onto the probe tip, providing a very
short
ground path between probe and board, and see a pretty good 50 MHz square
wave but with a little ringing and a peak amplitude of about 3.6 volts.
So
apparently the chip wasn't damaged by my screwup.

The scope is a Tek TDS 1012, 100 MHz BW, TEK 10X probe.

Ok. And at that bandwidth, all you'll see is something kind of sloppy,
vaguely squareish, and with a touch of overshoot, because scopes don't
have minimum-phase response anymore.

If you had a 1GHz+ scope, you'd probably see a nice sharp waveform, maybe
some bumps corresponding to signal propagation if it's connected to a long
trace. But that'd just be guessing at this point.

Tim
 
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