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Buy some external oscillator from eBay and hook up to 5V DC and RF output to DSO.

Seeing sine wave output from external oscillator but expect square wave output.

EC1100-80.000M

MXO45.40M

xo-54B.24M

I connect lab PSU to breadboard and put DIP14 external oscillator in breadboard and connect 1 feet BNC probe to pin 8 (RF out) and DSO, and see all external oscillators give sine wave but they are all square wave ones.

Also put 10uF and 0.1uF caps between Vcc and GND of osc. just in case but no changing in output waveform.

Is problem with using breadboard that might be introduce stray capacitance that skew square wave output to sine wave?

In that case, how I check what kind of output external oscillator give. I cannot solder them to strip board as it is problem to desolder all pins and take it out after testing.

Sample datasheet:

http://www.vishay.com/docs/35025/xo-52.pdf http://www.ecliptek.com/SpecSheetGenerator/specific.aspx?PartNumber=EC1100-80.000M

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  • \$\begingroup\$ Please advise scope model and brand and MHz (bit manufacturers details allow full understanding of specs.) To connect to pins strip say 2 inches of a length of "PVC" insulated wire . Wrap wire around pin N times. twist open end and wire a few times to hold in place then snip off excess wire. \$\endgroup\$
    – Russell McMahon
    Aug 19, 2011 at 8:17
  • \$\begingroup\$ DSO is Agilent DSO-X 2022A, 200 MHz BW, home.agilent.com/agilent/… \$\endgroup\$
    – sekharan
    Aug 19, 2011 at 15:11
  • \$\begingroup\$ You don't need a flash to take a picture of something that emits light \$\endgroup\$
    – endolith
    Aug 19, 2011 at 16:58
  • \$\begingroup\$ And you don't need to take another picture when the first one illustrates the point. \$\endgroup\$
    – kenny
    Aug 20, 2011 at 10:41

1 Answer 1

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What is the bandwidth of your oscilloscope? I suspect the the frequency of the oscillator may be too high for your oscilloscope to display properly.
As you probably know, a square wave is made up of many sine waves, the fundamental frequency and an infinite number of odd harmonics decreasing in amplitude which give it it's shape. So if you have a square wave at 10MHz, you will have a harmonic at 30MHz at 1/3 the level of the fundamental, one at 50MHz at 1/5 the level of the fundamental and so on. The more harmonics present the more it will look like a square wave. Any square wave can be turned into a sine wave using a low pass filter to remove all harmonics apart from the fundamental, so unless your oscilloscope has the bandwidth to display at least the 3rd and 5th harmonic you will not see something that much resembles a square wave.

Judging from the pictures I am guessing the bandwidth is around 100MHz-200MHz for your scope, the 80MHz looks like an almost perfect sinewave, the other two look to have some of the 3rd harmonic present. If your scope has ETS (equivalent time sampling) select this mode, it will display higher frequencies (providing the analogue bandwidth can handle them) but take longer to refresh. If it doesn't you will need a higher spec scope. If you give more details about your setup (scope, probe, scope input specs, sampling rate, analogue bandwidth) it will be easier to confirm one way or the other. If you are worried about breadboard capacitance (very possibly also causing problems) then maybe set it up so the signal lead is not plugged in (do something like bend it out or hang over the edge so you can get the probe to it) and try it. If you can't, then try and make sure there is no ground adjacent to the signal out.

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  • \$\begingroup\$ DSO is Agilent DSO-X 2022A, 200 MHz BW, home.agilent.com/agilent/…. I confuses as 80MHz pure sine as 2nd harmonic near max BW of DSO, understand, but why 24Mhz and 40Mhz also same? Their 5th harmonic only near 200Mhz, right? \$\endgroup\$
    – sekharan
    Aug 19, 2011 at 15:12
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    \$\begingroup\$ I would expect the 24MHz to look a bit better than it does in the picture. What probe are you using? Did you manage to probe the signal pin without it being plugged into the breadboard? It is quite possible the breadboard is causing problems, depending how it is set up. Also make sure the scope is set to single channel mode (2Gsps rather than 1Gsps with dual channel according to the manual) \$\endgroup\$
    – Oli Glaser
    Aug 19, 2011 at 16:17
  • \$\begingroup\$ Should such oscillator be having 10uF and 0.1uF caps between Vcc and GND any time or not ever reqd? \$\endgroup\$
    – sekharan
    Aug 19, 2011 at 20:07
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    \$\begingroup\$ I would add a 0.01uF ceramic cap too, as recommended in the datasheet ecliptek. Place both the 0.01 and 0.01uF as close to the pins as possible with short leads. You could experiment by adding/removing the bypass caps whilst viewing the signal to see what difference it makes. Please can you confirm your probe model? What setting are you using it on? Check the ecliptic datasheet (page 3) for information on maximum load capacitance and suitable probe specs. You will need everything just right with frequencies this high. \$\endgroup\$
    – Oli Glaser
    Aug 20, 2011 at 0:44
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    \$\begingroup\$ I forgot to mention - also refer to the Vishay datasheet, in fact I would concentrate on the lower speed oscillators rather than the 80MHz Ecliptek one. The advice remains similar though, on page 1 (marked 27 in datasheet) there is test information. Bear in mind the rise/fall times (vishay datasheet only gives max values of 10ns for 1-100MHz, ecliptek max 3ns for 80MHz) may be a significant percentage of the waveform, so even probed correctly it won't look like a "perfect" square wave. \$\endgroup\$
    – Oli Glaser
    Aug 20, 2011 at 1:09

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