help me with a scope/sg issue - Page 1
Forum section: electronics / testgear

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 23, 2013, 02:12:48 am
content: ok. have a Tek 2213 scope and a HP204C oscillator. i am somewhat new to scopes although I can build a 40w push pull tube amp from a schematic (so feel free to use correct terms). here is what I want to know.i set the hp204 for a 1k sine wave and adjust the output potentiometer so that reading across the terminals with a digital vom reads 1v.i throw the output to the scope and it reads much higher 3v. i don't think my scope is bad since it will properly register a 9v dc battery as 9vdc.i think my scope is properly calibrated. any thoughts? I noticed that the sig gen says 600 ohm over the two output terminals. is this an issue.my main reason for using this gear is to check the output of my tube amps and feed the amps a 1k 1v sine and use the crossover notch bias method of biasing the output section of the amps (usually a pair of 6L6 or EL34 tubes.)thankstim

author: Chalky
author_rank: Regular Contributor
timestamp: February 23, 2013, 02:30:10 am
content: Which scope reading are you referring to? If using scope's measurements, make sure your reading the same thing as the multimeter is - AC volts, and RMS?

author: c4757p
author_rank: Super Contributor
timestamp: February 23, 2013, 02:31:30 am
content: How are you reading the voltage from the scope? Are you aware of the different ways to measure AC voltage? 1V RMS (your meter measures RMS or "pseudo-RMS") is 2.8V peak-to-peak.What is your multimeter's bandwidth? If it's a really cheap one it may be meant only for very low frequency, like mains.

author: w2aew
author_rank: Super Contributor
timestamp: February 23, 2013, 02:44:16 am
content: The AC volts setting on the digital VOM is designed to measure the rms value of the sinewave. Inexpensive meters will only be accurate for sine waves, and only for lower frequencies. 1kHz should be low enough to read accurately, but that will depend on the meter.The rms value is the "equivalent" DC voltage that would produce the same average power as the AC signal.Better digital VOMs (DMMs) will read accurately for higher frequencies (but still rarely above audio frequencies), and will also read accurately for non-sinusoidally signals. In other words, they give True RMS readings.All that being said, the scope shows you the actual waveform. You're seeing the full peak-peak voltage of the sine wave. To convert between an RMS value to a peak to peak value, you would multiply the RMS value by 2.828. Thus, 1Vrms = 2.828Vpp. So, what you're seeing is normal.

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 23, 2013, 02:58:44 am
content: thanks guys. my vom says true rms but maybe that is not the case.will do some more checkingtim

author: w2aew
author_rank: Super Contributor
timestamp: February 23, 2013, 03:12:35 am
content: Quote from: gooseEL34 on February 23, 2013, 02:58:44 amIt sounds like your readings are pretty close. 1Vrms on your meter equals 2.828Vpp on the scope. That's pretty close to the 3v that you mentioned, so it doesn't look like there's any real problem.