strange output on CRO while measuring output of 45Mhz

Thread Starter

Vindhyachal Takniki

Joined Nov 3, 2014
549
I am measuring output of 45Mhz of programmable oscillator of: http://www.cardinalxtal.com/products/programmable-oscillator-3

1. When I measuring it on gwinstel gds-11028 DSO,.
2. Gnd loop is mimized, like this: https://goo.gl/images/QqHD2d
3. Probes are conencted directly on oscillator output pins
4. Vdd = 3.3V

5. When on 1x probe: Signal is sine wave riding on 1.6Vdc. Peaks of sine wave dont touch the 3.3V & 0V limtis.

6. When on 10x probe, signal is sine wave which seems to ride on dc also, but this time negative peak of sine goes below zero volt

7. Why is that so?
8. Also what is output of this oscillator, we have TTL version, is it sine wave?
I thought it would be square wave.
 

BobaMosfet

Joined Jul 1, 2009
1,169
Remember, your probes are not 'invisible'. The oscilloscope can only see the view as presented by the probe. If a probe is not properly adjusted, or calibrated, it can alter what the scope sees, so verify your probes.
 

Papabravo

Joined Feb 24, 2006
14,199
I am measuring output of 45Mhz of programmable oscillator of: http://www.cardinalxtal.com/products/programmable-oscillator-3

1. When I measuring it on gwinstel gds-11028 DSO,.
2. Gnd loop is mimized, like this: https://goo.gl/images/QqHD2d
3. Probes are conencted directly on oscillator output pins
4. Vdd = 3.3V

5. When on 1x probe: Signal is sine wave riding on 1.6Vdc. Peaks of sine wave dont touch the 3.3V & 0V limtis.

6. When on 10x probe, signal is sine wave which seems to ride on dc also, but this time negative peak of sine goes below zero volt

7. Why is that so?
8. Also what is output of this oscillator, we have TTL version, is it sine wave?
I thought it would be square wave.
As the frequency of a source goes up, it becomes harder and harder to have near vertical edges and flat tops. As you approach the bandwidth limit of the scope, square waves take on a more rounded appearance, and the output of an oscillator does as well. It is actually a combination of effects.
 

nsaspook

Joined Aug 27, 2009
7,464
Remember, your probes are not 'invisible'. The oscilloscope can only see the view as presented by the probe. If a probe is not properly adjusted, or calibrated, it can alter what the scope sees, so verify your probes.
Exactly. 1X probes usually have lower bandwidth than the same type probe in 10X mode causing a reduction in the displayed high frequency component of the measured signal.
http://www.ni.com/white-paper/14825/en/
SP200B Switchable Attenuation Probe Bandwidth
6 MHz 200 MHz
1:1 10:1
 

crutschow

Joined Mar 14, 2008
25,245
A 1X probe tip load sees the full capacitance of the probe coaxial cable plus the oscilloscope input, whose total is likely upwards of 50pF.
This provides a low impedance load to the oscillator output which can significantly affect the output waveform of a 45MHz source.
For example, 50pF of capacitance has an impedance of only 70Ω @ 45MHz.
You generally should not use a 1x probe at frequencies above a MHz or so.

A 10X probe has about a tenth of that capacitance, so will have less effect on the signal.
 
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