# Test Instrumentation Noise MT

Question Answer True or false, voltage is the only quantity tested by a VOM— all others are determined by voltage. True What are some advantages of digital VOM (3) 1. More accurate, 2. More stable, 3. Easier to use *** What is the crest ratio for a sinewave? What does this equal in regard to RMS? 0.707… RMS/peak What is the issue with measuring AC signal? Positive and negative alternating half waves will cancel each other out What is the solution for this? Root Mean Square (RMS) What is the equation for RMS? How is RMS obtained? RMS= square root of (X1+X2)/n….. square root of each data point difficult by total # data points What does the RMS value give us in terms of the AC signal? Gives us the area under the curve The crest ratio only applies to what type of waves? Sine waves What is a less accurate way that RMS can be obtained? By measuring the peak of the wave and multiplying by crest ratio (0.707)…. 0.707=RMS/peak What are two ways that we can calibrate a VOM? Cannot do this clinically– need speical equipment What is a point check? When we connect two probes of VOM to measure the resistence… theoretically it should be 0 ohms…if off– accuracy issue.. smaller is better in terms of value What can we do to check accuracy of VOM? 1. Zero point check, 2. Battery check When measuring voltage with the VOM, does the meter need to be put into the circuit? No! How is restance measured using the VOM? At least one leg of the resister must be disconnected from the circuit…. R is measured by passing a *fixed current* via a resister and calculated from a voltage drop** Why do we need to remove one leg from circuit when measuring resistance? Because to measure resistance you you must allow current to pass through the resister and then measured the voltage drop off… we dont want to measure current going through another pathway– want to measure the current we KNOW!!! How do we measure current with a VOM? The meter must be put into the circuit What do we need when measuring current using a VOM to prevent short circuit Need certain amount of resistance What prevents short circuit in VOM? Fuse in VOM What are some other instruments used for testing? 1. Signal generator, 2. Frequency counters, 3. Oscilloscope