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Getting probably the most out of the accuracy of your oscilloscope will not be arduous nevertheless it takes some consideration to element. This text seems to be into numerous methods to enhance your measurements.
Use a number of show grids to take care of dynamic vary
Contemplate a standard observe that throws away one-quarter of the oscilloscope’s dynamic vary, attenuating alerts to make them match on a single frequent display as in Determine 1.
Determine 1 In case you attenuate acquired alerts to make them match on a single show grid you’re throwing away dynamic vary.
Digital oscilloscopes are set as much as map the complete vary of their analog to digital converter’s (ADC) to the complete or nearly full show grid. In case you attenuate the alerts from every channel to make them slot in one-quarter of the grid you may have thrown away two digital bits of dynamic vary. Moreover, offset must be added to show the hint inside the chosen two divisions. There’s an offset accuracy specification which provides a further error supply to the readings.
What it’s best to do is use a a number of grid show and place every channel in its personal grid. No attenuation or voltage offset is required, and the alerts are displayed at full dynamic vary as proven in Determine 2 the place a quadrature show exhibits 4 grids every at full dynamic vary.
Examine the highest traces in every determine; discover the decrease noise on every sign in Determine 2. Attenuating the enter sign by buying it at a better vertical scale studying reduces the alerts vertical displacement nevertheless it doesn’t cut back the interior noise within the oscilloscope channel.
Determine 2 A 4 grid show with a single channel in every grid shows the waveforms at full dynamic vary.
The result’s a decrease sign to noise ratio. Take a look at an overlaid comparability of the sign on channel 1 proven in Determine 3.
Determine 3 Overlaying a sign acquired at 50 mV/division (inexperienced hint) with one acquired at 200mV/division (yellow hint). The yellow hint is broader, much less nicely outlined, and noisier.
The hint which was attenuated by an element of 4 was acquired at 200 mV/division. When it’s displayed at 50 MV/division it seems with extra noise and fewer decision. The unattenuated hint, in inexperienced, exhibits a small perturbation driving on the sine wave that isn’t seen within the attenuated sign that is because of improved dynamic vary.
Not solely does the show exhibit this lack of amplitude decision however the loss impacts different measurements as nicely. The height-to-peak measurement may be very delicate to noise. Word that the peak-to-peak studying of the attenuated sign is studying that larger noise stage and is 44 mV larger than the sign acquired a 50 mV/division. The rms ranges are very shut. It’s because the rms course of integrates the sign decreasing the measured noise stage.
The upper noise stage of the attenuated sign additionally impacts the measurement of its frequency. Word that the measured uncertainty, as expressed by the usual deviation of the measurement, is twice as nice for the attenuated sign.
Simply say no to attenuating alerts to suit them on a single grid—use a number of grids show and present every sign at full dynamic vary.
Extra correct cursor studying
There are three measurement instruments accessible in an oscilloscope: the display graticule, cursors, and measurement parameters. Cursors are markers that may be moved over a displayed waveform and file the cursor’s location in time and the amplitude of the waveform on the intersection with the cursor. The accuracy of cursor measurement is dependent upon the person’s capability to position the cursors precisely on the specified level of the waveform.
You may enhance the accuracy of cursor placement with a number of easy tips. First is the cease the acquisition whereas inserting the cursors. The waveform varies from acquisition to acquisition, and you’ll find it simpler to position the cursors if the waveform isn’t altering on a regular basis. The second, and extra essential trace is to activate a zoom hint or traces. Cursors observe within the zoom area and the bigger show makes it simpler to position the cursors as proven in Determine 4.
Determine 4 Cursors observe within the zoom traces, use zoom traces, which present an expanded view of the waveform, for extra correct cursor placement.
Not solely does the expanded show within the zoom make it simpler to see the positive particulars on the waveform however the charge of change of cursor motion is diminished when the cursor enters the zoomed space. The slower charge of change gives higher management in inserting the cursors.
Within the instance proven the cursors are to be positioned on the zero crossings of the sine wave. Two zoom views are displayed, one for every crossing. Cursors are positioned manually whereas monitoring the cursor amplitude within the channel annotation field till the cursor amplitude reads 0 V.
Word that the measurement parameter P1 measures the imply interval of the sine wave as 99.9999 ns in contrast with the cursor worth DX = 100.04 ns. The measurement parameter has a a lot larger decision as a result of it applies twin interpolation operations to the willpower of the interval. Basically, the measurement parameters present probably the most correct measurement outcomes. Cursors, nevertheless, supply a extra basic measurement functionality, there’s not a measurement parameter for each measurement occasion. The place a measurement parameter exists, it is going to produce a extra correct outcome than cursors.
Selective measurements for parameters
Determine 5 exhibits an instance of a waveform that’s arduous for the standard measurement parameter to make with out a little assist. The waveform is the gated clock for an I2C serial interface. The 2 waveforms are an identical, and the frequency parameter can be used to measure the clock frequency utilizing completely different measurement setups.
Parameter P1 is measuring the frequency of the highest hint. It sees 162 cycles and measures frequencies from 68.518 kHz to 100.298 kHz. This isn’t stunning as a result of the waveform’s timing will not be uniform. The worth studying of P1 is the frequency of the of the final acquired cycle within the waveform and exhibits a frequency of 73.281 kHz. Wanting on the final cycle within the M2 waveform (inexperienced hint) you possibly can see that it’s longer than a lot of the different cycles which accounts for the decrease frequency.
Determine 5 Using measurement gates to selectively measure the frequency of the gated clock waveform.
To unravel this challenge there are a number of methods accessible within the parameter setup. The primary is gating, because the identify implies it permits measurements to be made solely between person positioned gates. Some oscilloscopes use the measurement cursors to gate the parameters measurements. This Teledyne LeCroy oscilloscope makes use of a separate set of gate markers proven as dashed traces on the decrease (yellow) hint. The gates are set in regards to the first clock burst and measure the frequency over the eight full cycles contained. The frequency parameter, on this case, reads from 99.914 to 100.109 kHz. Using measurement gates has efficiently restricted the vary of measurements to only these of the clock and ignored the gaps. Gating makes the measurement parameters a bit extra versatile.
The second measurement instrument is acceptance standards. This instrument permits the parameter to measure all of the values however solely shows these inside a person entered vary as proven in Determine 6:
Determine 6 Utilizing parameter acceptance standards to submit solely frequency values which are between 99 and 101 kHz.
Acceptance standards is about as much as show solely measured frequency values between 99 and 101 kHz in parameter P2. The variety of measurements inside the vary is 144 in comparison with the 162 listed in P1 which exhibits all of the measured values. The frequency parameter measures beginning with the primary constructive going edge so there are eighteen gaps within the clock waveform which are included within the measurement which is the same as the distinction between the full measured values and the accepted values. It’s legitimate to ask how you recognize the vary of values to make use of because the acceptance standards, the subsequent part exhibits how you are able to do that.
Observe and histograms
A observe is a mathematical operate, accessible in some oscilloscopes, which plots parameter values versus time. On this instance it’s helpful to see what elements of the waveform are related to the completely different waveform occasions. Determine 7 exhibits a observe based mostly on the frequency parameter.
Determine 7 Examples of the observe (F2) and histogram (F1) capabilities based mostly on the measured frequency parameters.
The observe of the frequency parameter seems in hint F2 (pink). The vertical scale of the observe is in hertz, models of frequency versus time. This hint is time synchronous with the supply waveform the I2C clock. It exhibits a pulse waveform with prime values of 100.3 kHz and base values of 68.52 kHz. The highest values are uniform in values however the base exhibits barely higher values at every finish of the observe equivalent to the start and finish of the supply waveform. The observe exhibits the place the variation in frequency happens. Word that the width of the 100 kHz segments is wider than the width equivalent to the 70 kHz and underneath sections. There are extra clock pulses within the 100 kHz group.
The hint F1 is a histogram of the frequency parameter. Histograms are graphical plots displaying the variety of occurrences inside a small vary of measured knowledge values plotted towards the info worth. It’s an estimate of the chance of a measured worth occurring. The information values used within the histogram could be acquired pattern amplitudes, timing values, or measured parameters. The histogram of the P1 frequency measurements from the earlier part is proven as hint F1 (yellow) in Determine 7.
The histogram is within the backside grid. The horizontal axis is the measured worth, on this case the frequency. The vertical axis is the variety of measured values inside a small bin. The bin measurement is customers selectable. On this instance the horizontal axis is damaged into 1000 bins. So, for the roughly 100 kHz vary the bin measurement is about 100 Hz. The histogram exhibits two apparent peaks and two a lot smaller peaks. The most important peak is at 100 kHz representing the clock the height depend within the 100 kHz bin is 34 with smaller counts within the adjoining bins, all collectively the depend can be 144. The one furthest to the left is at 68.6 kHz, that is the frequency of the cycles which embody the gaps. Two smaller peaks are additionally hole frequencies with values of about 72-73 kHz, that are related to the parameter measurements at both finish of the clock sign.
The construction of the histogram and observe, with the majority of the readings about 100 kHz, gives the data essential to selected and set the acceptance limits within the parameter measurement. Values beneath 100 kHz are related to the gaps in clock bursts and ought to be excluded from the measurement of the clock frequency.
The observe and histogram capabilities supply great perception into your measurements displaying the place the measured values unique and the way the values are distributed by worth.
Conclusion
This text has proven a number of methods to assist enhance measurements made in your oscilloscope together with maximizing show decision, cursor placement, and measurement parameters. For individuals who have an interest, the oscilloscope used was a Teledyne LeCroy LabMaster. Different oscilloscopes have related capabilities, however you’ll have to seek the advice of your person’s handbook to search out the corresponding options. Observe and histogram are most frequently related to jitter measurement instruments.
Arthur Pini is a technical assist specialist and electrical engineer with over 50 years expertise in electronics check and measurement.
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