Getting the most out of your Measurements Workshop. Mike Schnecker

Getting the most out of your Measurements Workshop Mike Schnecker

Agenda Oscilloscope Basics Using a RTE1000 Series Oscilloscope. Probing Basics Passive probe compensation Ground lead effects Vertical System Overview Channel input coupling Effective use of vertical scale Ways to get more vertical resolution Horizontal Systems Sampling Methods Acquisition Rate Relationship of memory depth and sample rate Trigger System Trigger specifications Advanced triggers But the majority of what we ll discuss is scope agnostic it could be done with any other R&S scope or any digital scope for that matter. 10/1/2015 Oscilloscope Fundamentals 2

Basic things I assume you know ı Oscilloscopes measure Voltage (y-axis) vs. Time (x-axis) ı so that means oscilloscopes work in the time domain ı Oscilloscopes have been around a long time (1930s!) Started out with analog implementations Now we have digital storage oscilloscopes (DSOs) The General Radio Oscilloscope (1931), with sweep circuit (right). 3

Attenuation Bandwidth Definition ı Bandwidth is THE single-most crucial parameter used for the oscilloscope selection: 0dB -3dB Ensure the scope has enough bandwidth for the application! ı Oscilloscope bandwidth is specified at -3dB (-29.3%) f BW Frequency 0 db 6 div at 50 khz - 3 db 4.2 div at bandwidth The maximum bandwidth of an oscilloscope: The frequency at which a sinusoidal input signal amplitude is attenuated by -3dB.

Amplitude Bandwidth Requirements of the Test Signal ı Required scope bandwidth depends on test signals frequency components Digital square waveform is composed of odd sine wave harmonics Rule of thumb: BW Scope = 3-5x f clk of Test Signal (1.5 2.5x of the bit rate) f Fundamental f 3rd harm. f 5th harm. Frequency 5

Bandwidth Application Mapping l Data rates of typical I/O interfaces Interface Data Rate 101010 Frequency Oscilloscope Bandwidth Requirement 3rd harmonic 5th harmonic Oscilloscope Classes I2C 3.4 Mbps 1.7 MHz 5.1 MHz 8.5 MHz Value LAN 1G 125 Mbps 62.5 MHz 187.5 MHz 312.5 MHz Lower mid-range USB 2.0 480 Mbps 240 MHz 720 MHz 1200 MHz DDR II 800 Mbps 400 MHz 1.2 GHz 2.0 GHz Mid-range SATA I 1.5 Gbps 750 MHz 2.25 GHz 3.75 GHz Upper Mid-range PCIe 1.0 2.5 Gbps 1.25 GHz 3.75 GHz 6.25 GHz High-end entry PCIe 2.0 5.0 Gbps 2.5 GHz 7.5 GHz 12.5 GHz High-end 8

Oscilloscope Basics viewing the signal ı Viewing the signal on the oscilloscope ı ı Best done using the basic 3 knobs on the instrument: 1. Vertical scale and position get the signal on screen 2. Horizontal scale see the lowest frequency signal variation 3. Trigger level get a stable signal Possibly change trigger type to get a better picture Once the signal is on the screen: Measure sung observation Measure using markers Use automated measurements 10

Basic Controls 11

Workshop: Basic Controls ı Connect the passive probe to the 10MHz_CLK signal on the demo board ı Preset the oscilloscope ı Use vertical horizontal trigger controls to get a stable signal on the screen ı Connect the passive probe to the I2C_SCLK signal ı Preset the oscilloscope ı Use vertical horizontal trigger controls to get a stable signal on the screen 01.10.2015 Footer: >Insert >Header & Footer 12

Probe Basics: ı These three factors Encompass most of what goes into proper selection of a probe Physical attachment Minimum circuit loading what the circuit sees Adequate signal fidelity what the scope sees 13

Probe Basics: ı This situation is frequently encountered: Signal is not easy to reach Source impedance can vary widely Setup is sensitive to noise and will be frequency dependent For more than one signal to be measured, there will be slight propagation differences between probe tip and instrument input (skew) 14

Probe Basics: The ideal probe ı The ideal probe: Does not influence the source Displays the signal without distortion 15

Probe Basics: The real probe ı The real probe: Does influence the source Displays a distorted signal 16

Probe Basics: Passive Probes ı Passive Probes Least Expensive No active components, essentially wires with an RC network Input impedance decreases as the frequency of the applied signal increases 17

Probe Basics: Active Probes ı Active Probes ı Low loading, Adjustable DC offset, Auto recognition by instrument ı Incorporate field effect transistors that provide very high input impedance over a wide frequency range. ı In short, Active probes are recommended for signals with frequency components above 100MHz. 18

Passive Probe Input Impedance 19

Probing Best Practices ı Use appropriate probe tip adaptors whenever possible: Even an inch or two of wire can cause significant impedance changes resulting in distorted wave forms at high frequencies ı Keep ground leads as short as possible: Added inductance of an extended ground lead can cause ringing to appear on a fast transition wave-form ı Compensate the probe: An uncompensated probe can lead to various measurement errors, especially in measuring pulse rise or fall times ı Add test points when possible to original design 20

Probe Options 21

Probe Summary Best Practices ı Circuit changes when the probe is attached ı Probe effect minimized by having: High resistance 1MOhm vs 10MOhm probes, specified on data sheet Low capacitance Probe tip geometry (passive versus active, specified on data sheet) Generally active probes have lower capacitance Low inductance Lead length shorter has lower inductance Not specified in a data sheet, multiple accessories 22

Workshop: Probe Compensation ı Matches the probe cable capacitance to the scope input capacitance. ı Assures good amplitude accuracy from DC to upper bandwidth limit frequencies ı A poorly compensated probe can introduce measurement errors resulting in inaccurate readings and distorted waveforms ı Connect Probe to compensation output on RTO ı Use small screw driver to adjust POT in probe body to adjust wave-form ı Zoom in on wave-form for better resolution ı Press Measure/Acquisition/select averaging in wfm arithmetic ı Compensate probe Affects amplitude, rise time, etc 01.10.2015 Footer: >Insert >Header & Footer 23

Workshop: Probes Ground Loop Effects ı Study the effects of extended ground wires on wave-forms Use passive probe on probe compensation output Measure overshoot with long ground lead Zoom into edge and study positive overshoot Take a reference acquisition to save the wave-form to the screen Replace long ground lead with short spring lead Do a single shot to stop acquisition and compare the two waveforms Take a measurement of the positive and negative overshoot Affects overshoot, rise time, etc 01.10.2015 Footer: >Insert >Header & Footer 24

Workshop: Passive Probe vs Active Probe Rise Time ı Connect passive probe to Ch1 and Active probe to Ch2 ı Probe 10_MHZ_CLK on Demo Board ı Zoom in and measure rise time ı Rise time of passive probe wave-form is slower ı Discussion ı This capacitive loading affects the bandwidth and rise time characteristics of the measurement system by reducing bandwidth and increasing rise time. ı Passive probe = 9.5 pf ı Active has.8 pf Affects rise time, overshoot, etc 01.10.2015 Footer: >Insert >Header & Footer 25

The Function Blocks of a Digital Oscilloscope The Vertical System Vertical System Memory Att. Amp ADC Acquisition Processing Post- Processing Display Amp Trigger System Horizontal System 27

Vertical System Overview ı The controls and parameters of the Vertical System are used to scale and position the waveform vertically ı The vertical system detects the analog voltage and conditions the signal by the attenuator and signal amplifier for the analog-to-digital converter (ADC) Input Coupling Scale Position Offset Bandwidth

Workshop: Channel Input Coupling ı Defines how the signal spans the path between its capture by the probe through the cable and into the instrument. ı Broadest BW is achieved with 50 Ohm input coupling ı Passive probe is typically 1 M Ohm coupled limiting the bandwidth to 500 Mhz under all conditions ı Benefits to 1 M Ohm coupling is protection from high voltages ı Study the effects of scope termination on signaling Connect coaxial cable to SMA connector labeled RF OUT Select 50 Ohm coupling and measure signal amplitude Select 1 M Ohm coupling and measure signal amplitude Do you know what s happening? Affects impedance considerations 29

The Function Blocks of a Digital Oscilloscope The Vertical System Analog-to-Digital Converter Vertical System Memory Att. Amp ADC Acquisition Processing Post- Processing Display Amp Trigger System Horizontal System 30

{ Analog-to-Digital Converter (ADC) Sampling Taking samples of an input signal at specific points in time. Samples Interpolated Waveform Hold Time Needed for Digitizing Sample Interval T I ı ı ı Samples are equally spaced in time Sample Rate measured in Samples/Second (Sa/s, ksa/s, MSa/s, GSa/s) Sample rate: Clock rate of ADC typically 5 times higher than oscilloscope bandwidth

Maximizing the ADC input range ı Input range and position directly affects the resolution of the waveform amplitude ı The 10 vertical scales correspond to the full ADC input range Scale/div = 50 mv/div Scale/div = 100 mv/div Signal amplitude: 0.5 V Best ADC resolution 8 bit => 2 mv / bit reduced ADC resolution 8 bit => 4 mv / bit

Workshop: Vertical Scale ı Examine quantization errors introduced by using only half the ADC Connect passive probe to 10_MHZ_CLK signal on demo board Use half grid display and compare to full grid Save as reference Compare saved ½ grid and see levels Measure positive overshoot and show effects Affects Most Measurements 35

Vertical Summary Best Practices ı Signal should occupy 80% of the screen (not off screen causing amplitude overload) ı Avoid having any part of signal off the screen ı Avoid making the signal too small (<30%), reduces resolution 36

Improving Vertical Resolution When might it be helpful ı Typically low bandwidth signals where you want to see a small signal in the presence of a larger one ı Common Applications Power Analysis Measurement of small voltage variation in presence of large voltage, e.g. conduction Loss Small current analysis on component sleep state Accuracy in Ripple Voltage measurements Medical Weak cardio or neural signal Wireless communication High resolution suitable for NFC, Wireless Power Charging & design using small Amplitude Shift Keying in data transmission. Typically in lower BW. Embedded circuit designs Low power circuit with weak signals Sub threshold leakage measurements

Improving Vertical Resolution ı There are three ways to increasing vertical resolution in an oscilloscope Averaging High resolution (Enhanced res) BW Filtering ı Each of these increases signal to noise ratio thereby giving more signal resolution HighRes Decimation BW Filtering

Improving Vertical Resolution ı Benefits of improving vertical resolution Increases resolution up to 16-bits 8-bit = 256 levels 10V full screen = 39mV 16-bit = 65,536 levels 10V full screen = 152uV ı There are potential drawbacks Averaging: Requires a repeatable waveform High resolution: Sample rate reduction Unknown BW Higher resolution signal is not seen by the trigger

Improving Vertical Resolution ı BW Filtering Doesn t have drawbacks of averaging or high-res No sample rate reduction Known BW Higher resolution signal is seen by the trigger Quantization steps clearly visible. Hidden low level signal becomes visible. Signal characteristics can be measured.

Improving Vertical Resolution Lab ı Small Demo Board ı Channel 1 to I2C_SCL ı Preset ı Autoset ı Turn sample rate to 10GS/s ı Note small glitch to left of trigger ı Go to HD Mode Set to 3MHz and 16bits ı Draw a zoom box that covers just the glitch and the current edge we are triggering on ı Go to trigger mode and change hysteresis to zero ı Adjust to trigger on rising edge of glitch

Sampling Methods & Acquisition Modes Memory Vertical System Att. Amp ADC Acquisition Processing Post- Processing Display Amp Trigger System Horizontal System 42

Aliasing (Sampling too slow) ı Nyquist Rule is violated: Sampling rate is smaller than 2x highest signal frequency Signal is not sampled fast enough -> aliasing False reconstructed (alias) waveform is displayed!!! input signal alias Example -input: 1 GHz sine wave -sample rate: 750 MSa/s -alias: 250 MHz 43

Sampling Methods - Effects of Different Sample Rates A 10 khz Sine Wave Signal Input Signal: 10kHz Sine Wave Nyquist/ Shannon The sampling rate must be: f S Sampling Rate: 200kHz Sampling Rate: 50kHz Sampling Rate: 25kHz f 2 S f i f S : sampling frequency f : frequency of the i input signal Sampling Rate: 12,5kHz

Workshop: Affects of Aliasing ı Connect to the RF OUT signal (50 ohm coupling) Add a frequency measurement to verify 825MHz sine wave Set the horizontal scale to 1us/div Lower sample rate to 5GSa/sec, then 2GSa/sec and observe the frequency measurement 45

Acquisition Summary Best Practices ı Beware of Autoset sample rate may be too low ı Adjust record length first, sample rate second Sample rate maintains signal fidelity. As you adjust the sample rate lower, you lose signal detail (especially bad with transitions/edges) ı Faster update rate increases chance of catching signal details or glitches ı Screen refresh rate is much slower than the update rate 50

Agenda ı System Bandwidth Definition ı Probing Basics ı RTO Tour Workshop Passive probe compensation Ground lead effects De-skewing probes ı Vertical System Overview Workshop Channel input coupling Effective use of vertical scale ı Sampling & Acquisition Workshop Acquisition Rate ı Horizontal Systems Workshop Horizontal measurements ı Trigger System Workshop Edge Trigger Runt Trigger 51

Horizontal System l The horizontal system's sample clock determines how often the ADC takes a sample; the rate at which the clock "ticks" is called the sample rate and is measured in samples per second l The sample points from the ADC are stored in memory as waveform points; these waveform points make up one waveform record Memory Vertical System Att. Amp ADC Acquisition Processing Post- Processing Display Amp Trigger System Horizontal System 52

Horizontal System Buzz Words Resolution time between 2 samples Sampling Rate Record Length # of samples Time Scale time / div s Acquisition time 10 * time / div s Acquisition time Sample Rate 1 / Resolution Time Scale # of Div s x x = Record Length e.g. 10 GS/s x 100 ns/div x 10 Div s = 10K samples 10 GS/s x 100 s/div x 10 Div s = 10M samples 53

Horizontal System Memory ı Purpose of Memory Every sample has to be stored in acquisition memory Deeper memory of course stores more samples Longer periods of time captured means more samples to store if sample rate wants to be maintained (better signal reproduction & zoom) Sampling (Sample & Hold) Digitizing 1 0 1 1 1 0 0 1 (Convert to Number) 1 0 1 1 1 0 0 1 Memory Storage 1 0 1 1 1 0 0 1 1 1 1 1 0 1 0 1 (Sequence Store) Scope Screen 54

Workshop: Horizontal Measurements ı Understand the sample rate of the scope to ensure the measurement is accurate Use passive probe on probe compensation and take long acquisition Measure rise time Zoom in and see how many points are on the edge. Increase sample rate and check the rise time again (should be around 1.5ns) Ch1/Acquisition/Resolution Tab. Adjust Record Length Limit to 20Msa and Sample Rate to 2Gsa/s Affects Most Measurements 56

Horizontal Summary Best Practices ı More record length and faster sample rate together expand the length of time scales you can view Higher memory results in longer time capture Higher sample rate results in shorter time scales ı Autoset does not give you the optimum sample rate Minimizes record length for a given time scale ı Higher sample rate and deep memory advantages: Increased signal fidelity (more accurate signal reproduction) Better resolution between sample points Higher chance of capturing glitches or anomalies Observe high frequency noise in low frequency signal Capturing of longer time periods while maintaining resolution (fast sample rate)

Trigger System Memory Vertical System Channel Input Att. Amp ADC Acquisition Processing Post- Processing Display Amp Trigger System Horizontal System 59

Trigger System ı Motivation Get stable display of repetitive waveforms In 1946 the triggered oscilloscope was invented, allowing engineers to display a repeating waveform in a coherent, stationary manner on the phosphor screen Isolate events & capture signal before and after event Define dedicated condition for acquisition start 60

Trigger Accuracy ı Key accuracy parameter: Minimum detectable glitch (a small signal spike): what is the smallest pulse that can be triggered on typically [ps] Sensitivity: minimum voltage amplitude required for valid trigger typically [mv or div] Jitter: timing uncertainty of trigger, determines smallest measurable signal jitter typically [ps rms] 61

Trigger Types (I) ı Edge Trigger is the original, most basic and most common trigger type triggering is executed once a signal crosses a certain threshold rising edge falling edge rising and falling edge 62

Workshop: Trigger Basics ı Runt Pulse Example Set the demo board to mode 9. Runt 100/s Use passive probe on RARE_SIG Press Trigger/Type=Window/Vertical condition=stay within//time condition=longer/width=50ns Adjust upper and lower trigger limit until runt is visible in wave-form Change board mode to 0. Runt 1/s What is happening? Change trigger mode between Auto and Normal observe what happens Advanced While Auto triggering, use digital filter and observe change in signal. Set to 1MHz and apply to trigger. Observe what happens. Increase filter BW and observe changes 65

Trigger Summary Best Practices ı Advanced triggers can more accurately represent the signal on the scope Protocol triggers can capture specific addresses, values ı Trigger sensitivity is difference from the acquisition sensitivity May be able to see something but not trigger on the event Digital triggering eliminate this problem ı Auto trigger mode acquires waveforms even if a trigger event is not present. Normal mode will wait until a trigger event happens Normal mode is very useful for advanced triggers 67

High Definition Mode Summary Best Practices ı Uses oversampling and low pass filtering to enhance ADC resolution Superior to other methods since it does not require a repetitive signal and does not reduce sample rate ı Minimizes waveform distortion ı Combined with digital triggering, can trigger on very small signal changes ı Great measurement technique for: Small current levels in low power devices Measuring dynamic on resistance of transistors Low speed serial buses prevents 68

THANK YOU 69