strange waveforms at CD player oscillator - diyAudio
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Old 20th June 2007, 05:54 AM   #1
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Default strange waveforms at CD player oscillator

Not having the data sheet for the CDX2587Q (decoder/DAC) used in my Kenwood DPF-J6030, I measured the waveforms with a 100 MHz Tek scope.

Result:
- At the left hand pin of the crystal, I get roughly 17 MHz (presumably 16.9344 - I have not yet taken out the PCB to look at the marking of the crystal) at about 90 mV pk-pk. The wavefront looks really nasty. There is one full, slightly asymmetrical sine every 60 ns, but the duration of both half waves together is more like 15 ns. In between those larger things, there are three smaller sines of about 40 mV.
If the crystal is swinging at its fundamental + 4th harmonic, I would expect a sine with 60 ns period and a higher frequency sine riding on top, but this is more like a 68 MHz sine that has every fourth period amplified.

- At the right hand pin, I get only a remotely saw-tooth-shaped signal of 40 mV pk-pk amplitude that is at roughly 4x the presumed fundamental.


I used to do clock injection quite freuently years ago but I cannot recall ever seeing a strange waveform like this.

PS:
Looking at my notes, the right hand side has four periods per 50 ns, so this would be the 5th harmonic. I will need to go back and remeasure.
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Old 20th June 2007, 06:23 AM   #2
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Crystal oscillators are very sensitive to the capacitance at the terminals of the crystal. Your scope probe is a low impedance, with a finite amount of capacitance. As soon as you touch it to the crystal pin, you change the loading on the crystal and it doesn't work normally. You would need to use a high impedance FET probe to look at the waveforms at the crystal pins without disturbing the operation of the circuit too much.

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Old 20th June 2007, 07:43 AM   #3
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You might be right, but I am not convinced. I was using a 500 MHz 10:1 Tektronics probe that has a capacitance in the very low pF range (will look it up tonight), and one of the pins of the crystal should be connected to the output of the gate that makes up teh gain of the oscillator.
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Old 20th June 2007, 03:41 PM   #4
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What are you expecting to see? The waveform from the crystal probably isn't a nice, clean squarewave. There are circuits after the oscillator that clean up the waveform.

I_F
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Old 20th June 2007, 06:55 PM   #5
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I'll wager a euro to a centime that you'll have much more luck finding the datasheet for the CXD2587Q.
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Old 21st June 2007, 04:42 AM   #6
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Right you are, thanks!
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Old 21st June 2007, 06:28 AM   #7
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Quote:
Originally posted by rfbrw
I'll wager a euro to a centime that you'll have much more luck finding the datasheet for the CXD2587Q.
Your signature is interesting. I started working at Motorola at corporate HQ in Schaumburg in 1981. At that time the motto of the day was "work smarter, not harder". We used to actually get paid for overtime for a while. As time passed, we stopped getting paid more, but got time off in compensation for long working hours. It was great. As time passed, the mott became "work smarter". Eventually they stopped compensating long hours at all and the motto became "WORK!" God, that place sucked!

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