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MotorBikeLes · 5th Apr 2021, 8:11 pm

You say setting up for a square waves seems to affect frequency response.
Yes, of course. Think of a square wave as made up of an infinite series of sine waves of increasing frequency. If you have a good square wave, you have a good high frequency response. The steep rise and corner shape reflect the highest frequencies, the slope (if any) of the horizontal part reflects the low frequency response. If the peak of the rising part overshoots too much, your frequency response will likely have a few "ups and downs" at the higher end.
I don't have the CDU150 info here, but all these settings must be done in the correct order. i cut my teeth on this starting with a Telequipment S31, something out of the arc compared with the CDU150. My test gear was VERY limited back then, but I was only working with a scope of 3MHz bandwidth, less than 10% of what you have there. The TQ manuals laid out exactly how to go about the settings, from stating with an internal fast rise signal for your 10Xs probe compensation. Everything must be done in the correct order or you will never succeed. David explained the order a few posts ago, but probably in a way few novices would understand. I can improve on his explanation by completely bypassing it and advising follow the manual's instructions to the letter in the order they are given. Of course if you have no manual, then it is time to understand his advice.
If a twiddler will not do what it should, find out why. The little silvered capacitors can go open circuit, and the little ones with plastic dielectric spacing can get damaged and go short circuit. Both of these can be intermittent. If a "fully open" capacitative twiddler has no effect, is there some parallel capacitance there that should not be there.
Finally, for final high frequency compensation, you must use a "proper" signal generator designed for the purpose, which gives its output into a 50ohm load.
Follow those rules and you will identify each fault (or fault area) one at a time, finishing with a properly calibrated unit.
Les

5th Apr 2021, 8:11 pm	#91
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Cossor CDU150 rescue You say setting up for a square waves seems to affect frequency response. Yes, of course. Think of a square wave as made up of an infinite series of sine waves of increasing frequency. If you have a good square wave, you have a good high frequency response. The steep rise and corner shape reflect the highest frequencies, the slope (if any) of the horizontal part reflects the low frequency response. If the peak of the rising part overshoots too much, your frequency response will likely have a few "ups and downs" at the higher end. I don't have the CDU150 info here, but all these settings must be done in the correct order. i cut my teeth on this starting with a Telequipment S31, something out of the arc compared with the CDU150. My test gear was VERY limited back then, but I was only working with a scope of 3MHz bandwidth, less than 10% of what you have there. The TQ manuals laid out exactly how to go about the settings, from stating with an internal fast rise signal for your 10Xs probe compensation. Everything must be done in the correct order or you will never succeed. David explained the order a few posts ago, but probably in a way few novices would understand. I can improve on his explanation by completely bypassing it and advising follow the manual's instructions to the letter in the order they are given. Of course if you have no manual, then it is time to understand his advice. If a twiddler will not do what it should, find out why. The little silvered capacitors can go open circuit, and the little ones with plastic dielectric spacing can get damaged and go short circuit. Both of these can be intermittent. If a "fully open" capacitative twiddler has no effect, is there some parallel capacitance there that should not be there. Finally, for final high frequency compensation, you must use a "proper" signal generator designed for the purpose, which gives its output into a 50ohm load. Follow those rules and you will identify each fault (or fault area) one at a time, finishing with a properly calibrated unit. Les Last edited by MotorBikeLes; 5th Apr 2021 at 8:15 pm. Reason: Added sentance for clarity.