On Tue, 15 Mar 2011 16:47 +1300, "Richard Prosser" wrote: > Yes, I do have typical and max ESR values (1k4 and 3k5) for the > crystal but have had difficulty in the past getting correlation > between differential voltage measurements and the recommended current > probe measurements. The ESR resistance is in series with the reactive > components of the crystal and so there is a significant phase shift. > It might work if I had an equivalent parallel resistance value but I > don't, and the crystal equivalent circuit lumped component values are > not readily available either. >=20 > Also the drive voltage is close to a square wave while the output > voltage is nearly sinewave so I'm not sure a direct rms difference (or > rms conversion of the difference) is appropriate. And the output node > is quite high impedance so is sensitive to any loading at all. I don't have experience at 455 KHz but some at 32. I found it hard to measure using two standard probes in differential mode because you are influencing the phase, so even compensating for the amount you calculate the voltage drop you create you don't get a clear idea of the current since you can't trust that the phase has shifted on one side enough that your differential voltage is now different. And since the impedance is radically different on the two sides it is very frustrating. What I did to make it work was add a 90 megohm resistor on the tip of each scope probe and probe across the crystal. I used several small resistors to make the 90 megohms and the metal guts of a small connector to grip it onto the probe tip. With a x10 probe this makes it x100. Make sure your scope is set to AC coupling because even 100 megs of DC to ground will load some processors (mine was a Sanyo 4-bit that had start-up and run drivers for the crystal and run was very weak). You wll get some waveform which you can't completely trust but enough to satisfy you aren't totally wrong. > The various manufacturer app notes etc always use either a current > probe or a series resistance to derive crystal current but I'm not > having too much luck this time. Yeah, I eventually figured they either made their own test equipment or made it all up :( Cheerful regards, Bob --=20 http://www.fastmail.fm - A fast, anti-spam email service. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .