WEBVTT NOTE Test Equipment and Its Limits 1 00:00:00.000 --> 00:00:02.062 The spectrum analyser. 2 00:00:02.062 --> 00:00:11.062 Question. Which of the following parameters does a spectrum analyzer display on the vertical and horizontal axes? 3 00:00:11.962 --> 00:00:15.828 Answer. Signal amplitude and frequency. 4 00:00:16.428 --> 00:00:31.539 Which of the following test instruments is used to display spurious signals and /or intermodulation distortion products from a transmitter? Spectrum analyzer. The frequency-domain instrument. 5 00:00:31.539 --> 00:00:44.988 A scope shows you a waveform; only a spectrum analyser shows you what else your transmitter is producing, and at what level. Digital oscilloscopes and their failure mode. 6 00:00:44.988 --> 00:00:54.007 Question. Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope? 7 00:00:54.907 --> 00:01:00.210 Answer. Sampling rate of the analog-to-digital converter. 8 00:01:00.810 --> 00:01:08.370 Nyquist again, in a different instrument. And the failure when you exceed it is insidious: 9 00:01:08.370 --> 00:01:15.938 Question. What is the effect of aliasing on a digital oscilloscope when displaying a waveform? 10 00:01:16.838 --> 00:01:23.376 Answer. A false, jittery low-frequency version of the waveform is displayed. 11 00:01:23.976 --> 00:01:42.462 Not a blank screen, not a warning, a plausible-looking waveform at the wrong frequency. If a digital scope shows you something slow and unstable that you did not expect, suspect aliasing before you suspect the circuit. 12 00:01:42.462 --> 00:01:43.056 Probes. 13 00:01:43.056 --> 00:01:48.507 Question. How is compensation of an oscilloscope probe performed? 14 00:01:49.407 --> 00:02:00.847 Answer. A square wave is displayed, and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible. 15 00:02:01.447 --> 00:02:13.356 A 10 times probe forms a divider with the scope's input capacitance, and the divider is only flat with frequency when a trimmer in the probe matches it. 16 00:02:13.356 --> 00:02:28.007 Uncompensated, square-wave tops slope up or down and every amplitude reading is wrong. Every scope has a calibration output for this. It takes twenty seconds and it is skipped constantly. 17 00:02:28.007 --> 00:02:34.119 Question. Which of the following is good practice when using an oscilloscope probe? 18 00:02:35.019 --> 00:02:40.099 Answer. Minimize the length of the probe's ground connection. 19 00:02:40.699 --> 00:02:45.784 The ground lead is an inductor in series with your measurement. 20 00:02:45.784 --> 00:03:03.459 The standard long crocodile lead adds enough inductance to make a fast edge ring convincingly, you measure ringing that exists only because of how you measured. For fast signals, use the short ground spring. Triggering. 21 00:03:03.459 --> 00:03:13.459 Question. Which trigger mode is most effective when using an oscilloscope to measure a linear power supply's output ripple? 22 00:03:14.359 --> 00:03:16.890 Answer. Line. 23 00:03:17.490 --> 00:03:35.010 Ripple is synchronous with the mains, so triggering on the mains itself gives a stable display. Triggering on the ripple waveform gives an unstable one, because the ripple is small and noisy. Frequency counters. 24 00:03:35.010 --> 00:03:41.370 Question. What is the purpose of using a prescaler with a frequency counter? 25 00:03:42.270 --> 00:03:48.550 Answer. Reduce the signal frequency to within the counter's operating range. 26 00:03:49.150 --> 00:04:04.750 A divider ahead of the counter. A counter good to 100 MHz with a divide-by-10 prescaler reads to 1 GHz, at a tenth the resolution. Antenna analysers. 27 00:04:04.750 --> 00:04:13.918 Question. Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge? 28 00:04:14.818 --> 00:04:21.578 Answer. Antenna analyzers compute SWR and impedance automatically. 29 00:04:22.178 --> 00:04:27.858 Two advantages, and the second is the significant one: no transmission. 30 00:04:27.858 --> 00:04:41.618 You can sweep an antenna without putting a signal on the air, which means no interference, no identification requirement, and no risk to a transmitter into an unknown load. 31 00:04:41.618 --> 00:04:47.378 Question. Which of the following can be measured with an antenna analyzer? 32 00:04:48.278 --> 00:04:52.160 Answer. Impedance of coaxial cable. 33 00:04:52.760 --> 00:05:12.000 Question. Which of the following is used to measure SWR? A. Directional wattmeter. B. Vector network analyzer. C. Antenna analyzer. D. All these choices are correct. 34 00:05:12.900 --> 00:05:16.420 Answer. All these choices are correct. 35 00:05:17.020 --> 00:05:32.962 Check yourself. Your digital scope shows a slow wandering waveform on a circuit you know oscillates at 50 MHz. What is happening? Square waves on your scope have sloping tops. What do you adjust? 36 00:05:32.962 --> 00:05:39.420 You need to check an antenna's resonance without transmitting. What instrument? 37 00:05:45.420 --> 00:06:01.742 Aliasing, the sample rate is below twice the signal frequency, so you are seeing a false low-frequency version. Probe compensation, the trimmer in the probe, against the scope's calibration square wave. 38 00:06:01.742 --> 00:06:07.802 An antenna analyser, which computes SWR and impedance without transmitting.