Re: Just got a 475, nothing showing on display


Brian Cockburn
 

Hi,

Jake's point about having an issue or uncertainty about measuring ripple with a multimeter reminded me about a similar question that's been in my mind for a while.

In the service manual of the 2245A (the 'scope I have), Table 4-1 calls for a 'DMM DC Volts range: 0 to 140 V. DC voltage accuracy: ±0.15%. 4½ digit display. e.g. Tektronix DM 501A'. It makes no mention of being able to measure ripple values with a DMM, suggesting instead that a 'scope is used. So is measuring ripple with a DMM valid? Is it useful given that maximum allowable ripple is given for two distinct bands: 60-150 Hz and 20-40 kHz (which I take to mean 20 kHz to 40 kHz, rather than 20 Hz to 40 kHz). Do I need a DMM that 'goes' to at least 40 kHz on ACV to make a useful measurement here?

For instance, my 5½ digit 220,000 count bench DMM claims to have a 3 dB bandwidth of 300 kHz. On the 200 mVAC range it clearly doesn't meet the required ±0.15% accuracy figure, but it's what I've got. (Accuracy is given as %rdg + counts: 20 Hz to 50 Hz => 1.00 + 100, 50 Hz to 10 kHz => 0.35 + 100, 10 kHz to 20 kHz => 0.6 + 200, 20 kHz to 50 kHz => 1.5 + 250, 50 kHz to 100 kHz => 5 + 400.) Do I just need to make the calculations about my DMM's worst case error in each (of its) bands and make sure that my measured value including the error is below the required (specified) level?

Thanks, Brian.

Join TekScopes@groups.io to automatically receive all group messages.