When designing area electronics, notably through the early prototyping stage or if qualification or flight {hardware} doesn’t operate as supposed, the standard oscilloscope is usually used to confirm the presence, timing and high quality of key alerts.
When debugging your area electronics utilizing an oscilloscope, many alternative measurement sorts are actually doable, e.g., voltages, currents, energy rails, digital logic, EMC, optical, TDR, and excessive voltage. For every of those purposes, the specification of the probe that makes contact along with your gadget beneath check (DUT) determines the standard of your check, e.g., its frequency response and bandwidth, the way it hundreds the DUT, whether or not it matches the enter impedance of your scope, the place and the way you connect the sign and floor contact suggestions. Typically the probe is ignored or taken as a right. Throughout a lot of my visits to clients, I’ve seen the flawed prognosis resulting from poor probing strategies or incorrect decision-making due to the specification of the oscilloscope and/or probe. Finally, this has impacted the power of shoppers to ship {hardware} and sub-systems to price and schedule.
A perfect probe wouldn’t load the DUT, would transmit the sign beneath investigation from its tip to the oscilloscope with good constancy, have zero attenuation, zero capacitance, zero inductance, infinite bandwidth, and linear-phase traits in any respect frequencies.
In actuality, a probe is a circuit with its personal electrical traits and when it makes contact along with your DUT, it immediately turns into half of a bigger system with its specification combining with that of the circuit of curiosity. To make a measurement, the probe should “steal” some power from the DUT and switch this to the scope’s inputs such that it doesn’t load the DUT whereas stopping degradation of the sign to be measured and won’t impair the performance of the DUT. Probes and oscilloscopes have an inherent capacitance making a low-pass filter that impacts larger frequencies, i.e., bandwidth, slowing rise instances. Probes have an intrinsic resistance, forming a voltage divider which reduces sign amplitude. Leads connected to probes add undesirable inductance leading to overshoot and ringing on the show. Leads may also act as antennae, picking-up electrical noise from the encircling setting. None of those results may very well be current within the sign you are attempting to measure, so it’s not a shock if assessments are deceptive and the prognosis flawed!
For general-purpose testing, the bottom line is to make use of probes which have a excessive enter resistance to attenuate the present taken from the DUT, usually 1 to 10 MΩ, in addition to low enter capacitance to make sure excessive impedance at larger frequencies, often 10 to 30 pF. A low impedance would adversely load the DUT impacting the measured sign degree.
As frequencies rise, to keep away from reflections resulting from capacitive and inductive reactances, the supply, load, and probe attribute impedance must be matched, often 50 Ω. As a information, an interconnect may be thought-about a transmission line probably inclined to reflections if its time delay, or its important size, is bigger than one third the sign rise time.
Determine 1 compares the sign integrity measured from a 10-MHz clock utilizing each 1 MΩ and 50 Ω enter impedances. The previous (gentle blue hint) accommodates reflections whereas the constancy of the latter (inexperienced hint) is superior!
Though I nonetheless have a number of, it’s uncommon for me to make use of the much-abused, 50-Ω, BNC oscilloscope coax cable with crocodile clips to precisely check and measure the most recent area electronics. Nevertheless, sometimes I do: the query is, when can you utilize this $10 “probe” somewhat than the costly $10k ones?
Final week I had an FPGA that wasn’t speaking with its JTAG programmer and wanted to examine the board was receiving the TCK, TMS, and TDI inputs, and outputting TDO. All of the costly, attractive probes have been getting used, nevertheless, because of the low frequency of the JTAG alerts, I knew the trusted 50-Ω BNC coax cable (Determine 2) may confirm these alerts with good integrity.
Determine 2 The ever present 50-Ω BNC oscilloscope coax cable present in most labs. Supply: RS PRO
At present’s satellite tv for pc and spacecraft electronics function at larger frequencies, with digital alerts having quicker edges and decrease voltages, near bigger switching currents and delicate analog alerts. Many small satellites squeeze all these capabilities onto one tiny PCB. To precisely measure alerts, observe occasions, and make the proper choices, the specs of the probe and the oscilloscope grow to be paramount; particularly bandwidth.
From Fourier evaluation, the bandwidth of a digital sign with a 50% responsibility cycle and a ten to 90% rise time may be approximated by:
You may not know the sting charges of the digital alerts you’ll have to measure sooner or later, however you probably have an appreciation of the best frequency of curiosity, e.g., a clock, an estimate of rise time and therefore bandwidth may be calculated. For instance, if one assumes rise time contains 7% of the entire interval, the sign bandwidth may be estimated as, 5*Fclk, or as much as the fifth harmonic!
Understanding the utmost sign frequency permits you choose the suitable oscilloscope and probe bandwidths for correct measurements. To reduce the in-band results of their respective 3-dB amplitude roll-offs, these must be 3 to five instances larger than the most important harmonic contained throughout the sign of curiosity.
For instance, for a sine wave with a basic frequency of 700 MHz, the oscilloscope and probe bandwidths ought to every be between 2.1 and three.5 GHz. Likewise for different analog alerts resembling modulated carriers, select bandwidths at the very least thrice bigger than its highest frequency part.
For a digital sign with an edge charge of 0.5 ns, its ensuing bandwidth may be approximated by 0.35/0.5 = 700 MHz. The measurement bandwidth of the scope and probe must be between 2.1 and three.5 GHz to precisely seize the constancy of the fifth harmonic. Equally, for rise and fall instances, if you wish to precisely see these transitions, the sting charges of your probe and oscilloscope must be three to 5 instances quicker the sign of curiosity. If you wish to validate the specs of your instrumentation, enter a pulse that has rise/fall instances three to 5 instances quicker!
Determine 3 and Determine 4 illustrate the affect of oscilloscope bandwidth on measurement constancy when verifying a 100 MHz clock with 500 ps edges: a 100 MHz scope solely passes by the basic frequency whereas a 500 MHz one captures as much as the fifth harmonic preserving the supposed waveform, however its personal rise time specification is limiting the measurement of the particular sign edge charge. A 1 GHz scope has 20% accuracy whereas a 2 GHz one affords 3%.
Determine 3 The affect of scope bandwidth on waveform constancy and rise-time. Supply: Keysight
A phrase of warning, there may be such a factor as an excessive amount of bandwidth as measurements can pick-up high-frequency noise as proven under, impacting the system’s efficient variety of bits (ENOB). The 20 MHz waveform on the left was captured utilizing a bandwidth of 6 GHz, whereas the one on the fitting had 100 MHz. Use the bottom bandwidth doable whereas nonetheless having sufficient to precisely seize the frequencies contained inside your sign of curiosity. If doable, restrict measurement bandwidth utilizing the oscilloscope’s built-in {hardware} or software program filters, and/or the specification of the probe.
Determine 4 The affect of an excessive amount of measurement bandwidth on waveform noise. Supply: Keysight
Going again to my drawback of debugging the uncommunicative FPGA utilizing a one meter, 50Ω BNC coax cable as a probe; how did I do know this could be high quality for verifying the JTAG alerts? The delay the sign experiences because it travels down the one metre cable is roughly 5 ns. For rise instances longer than 3 * 5 ns = 15 ns (important size), the ensuing bandwidth may be approximated by 0.35/15 ns = 23 MHz. The basic frequency of the JTAG alerts is round 1 MHz (properly under 23 MHz) with a enough variety of odd harmonics (bandwidth) captured to show the waveforms with good integrity and sharp edges. I additionally knew the BNC cable has a bandwidth of at the very least 1 GHz and the oscilloscope 8 GHz. Don’t trash these $10 cables simply but!
Many various kinds of probes can be utilized with fashionable digital oscilloscopes enabling a wide range of measurement sorts. Firstly, must you select a passive or an lively probe? The previous are sometimes shipped with oscilloscopes, are decrease price, rugged, and good for general-purpose testing usually as much as a number of hundred MHz. Internally, these solely include passive elements that reply to the sign being measured.
Passive probes have an attenuation issue that impacts DUT loading and the measurement bandwidth: a 1x or 1:1 probe doesn’t change the enter amplitude providing higher sensitivity for low-voltage alerts, whereas a 10x reduces the enter magnitude by an element of ten (Determine 5). These are used to guard the oscilloscope’s most rated voltage and supply higher SNR as any noise picked up by the probe can be attenuated, thus enhancing sign high quality. Using a ten:1 probe ends in a better inside resistance, usually 10 MΩ in comparison with 1 MΩ, which reduces circuit loading. The addition of capacitance within the tip cancels the scope’s enter capacitance, rising bandwidth and enhancing the measurement of upper frequencies and quicker edges.
Verify whether or not your oscilloscope auto-senses the probe attenuation or whether or not it’s important to manually swap between these. Passive probes additionally include compensation to match the probe and oscilloscope enter impedances. With out adjustment, the capacitive loading of the probe might filter out high-frequency elements and warp the sign beneath investigation.
At frequencies past 500 MHz, the output capacitance of most passive probes degrades the upper harmonics and edge charges. Moreover, they will severely load the DUT because the oscilloscope’s enter impedance shouldn’t be considerably larger than the circuit’s output impedance.
Lively probes don’t use sign energy from the DUT, however alternatively make the most of wideband amplifiers to allow high-frequency measurements. Lively probes have excessive enter resistance and low capacitance, usually lower than 1 pF, providing bandwidths as much as 20 GHz. The probe in Determine 6 is a single-ended probe, measuring a sign with respect to floor.
Determine 6 The everyday schematic of an lively probe. Supply: Rohde and Schwarz
Differential probes will measure the potential distinction between any two factors and are appropriate for verifying low-amplitude, high-frequency I/O resembling LVDS as utilized by many Earth-Remark imagers. Determine 7 reveals a typical schematic for a differential probe and Determine 8 reveals an off the shelf lively differential probe from Rohde and Schwarz (R&S). Differential probes supply excessive common-mode rejection over a broad vary of frequencies and use an inside differential amplifier to transform the distinction between two inputs right into a voltage that may be despatched to a typical single-ended scope enter.
Determine 7 A typical schematic of a differential probe. Supply: Rohde and Schwarz
Determine 8 An lively differential probe. Supply: Rohde and Schwarz
Energy-rail probes can help you measure AC ripple, high-frequency noise, and transients at excessive bandwidths on provide voltages with giant DC offsets, after which analyze the spectrum of this interference. EMC probes allow electrical and magnetic (E&H) near-field debugging of EMI points, whereas present probes present a non-invasive methodology for measuring present move by a conductor. A DC probe makes use of a hall-effect sensor to detect the magnetic area generated by a present because it passes by the probe’s ferrite core, whereas an AC probe makes use of a transformer to measure present move by its core.
Determine 9 reveals a latest present measurement from an Earth-Remark payload to confirm its in-rush habits at power-up. Thanks to my mates Giovanni and Nick from R&S UK for serving to me with this check.
One subject usually ignored is the parasitic impact of the ideas probing the DUT, generally known as the “connection bandwidth”. How and the place you probe is equally as vital because the specification of your check gear: lengthy connections degrade the measurement bandwidth, slowing edges, in addition to including undesirable inductance, leading to ringing and distortion when measuring high-frequency alerts. These might not really exist within the circuit beneath investigation! Parasitic elements to the left of the purpose labelled VAtn under decide the standard of the particular measurement. Determine 10 is a helpful picture from Keysight highlighting the affect of probe-tip size on measurement bandwidth.
Determine 10 The affect of probe-tip size on measurement bandwidth. Supply: Keysight
Right here’s my High Suggestions when contemplating oscilloscope probes to check your area electronics:
To conclude, the standard oscilloscope is usually used to confirm the presence, timing and integrity of key alerts through the early prototyping stage or, if qualification or flight {hardware} doesn’t operate as supposed. Many alternative assessments are actually doable utilizing your scope and selecting the proper probe is important. The selection would require an understanding on how its specification reconciles and interacts along with your DUT, its parasitic results, and the way and the place the probe is used, as this may all affect the standard of your measurements.
We may probe additional
Spacechips will probably be instructing its coaching course, Testing Satellite tv for pc Payloads, subsequent 12 months and you may contact, oc*******@sp***********.com, for extra info.
Dr. Rajan Bedi is the CEO and founding father of Spacechips, which designs and builds a spread of superior, AI-enabled, L to Okay-band, ultra-high-throughput transponders, Edge-based on-board processors, SDRs and MMUs for telecommunication, Earth-Remark, navigation, web, space-domain consciousness, space-debris retrieval and M2M/IoT satellites. The corporate additionally affords Area-Electronics Design-Consultancy, Technical-Advertising, Enterprise-Intelligence, Avionics Testing and Coaching Companies. You too can join with Rajan on LinkedIn.
Spacechips’ Design-Consultancy Companies develop bespoke satellite tv for pc and spacecraft {hardware} and software program options, in addition to advising clients methods to use and choose the fitting elements, methods to design, check, assemble and manufacture area electronics.
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