Digital oscilloscopes

01

Set the bandwidth from clock rate or edge speed

Keysight defines oscilloscope bandwidth on a low-pass response: "most scopes with bandwidth specifications of 1 GHz and below typically have what is called a Gaussian response", while those above 1 GHz "typically have a maximally–flat frequency response". As a rule of thumb, bandwidth should be at least five times the fastest digital clock, capturing up to the fifth harmonic. For accurate edge measurements it gives fknee = 0.5 / rise time (10–90%) and, for a Gaussian-response scope, bandwidth of 1.0 × fknee for 20% timing accuracy, 1.3 × for 10% and 1.9 × for 3% (1.0, 1.2 and 1.4 × for maximally-flat). [1]

Keysight puts a Gaussian-response scope's own 10–90% rise time at about 0.35 / bandwidth and a maximally-flat scope's at about 0.4 / bandwidth, noting that this is not the fastest edge the scope can accurately measure. Its worked example: a 500 ps edge has fknee = 0.5 / 500 ps = 1 GHz, so 1 GHz bandwidth gives about 20% timing error and roughly 2 GHz is the better choice for 3%. [1]

Editorial calculation: a 200 MHz Gaussian-response scope has a rise time of about 0.35 ÷ 200 MHz = 1.75 ns; by the five-times rule it suits clocks up to about 40 MHz. [1]

Give the supplier the fastest clock rates and edge times, required timing accuracy, signal amplitudes and offsets, number of analog and digital channels, the longest time span to capture at full resolution, trigger and decode needs, probes required, and bench or portable use.

02

Set sample rate, memory depth and update rate

Keysight recommends a maximum sample rate at least four to five times the specified real-time bandwidth for Gaussian-response scopes, so the sin(x)/x reconstruction filter can reproduce fast waveforms; maximally-flat scopes can theoretically specify bandwidth at fS/2.5. Because scopes reduce sample rate at slow timebases, it gives Acquisition memory = Time span × Required sample rate: an entry-level scope with 1 GSa/s and 10 k points drops to 100 MSa/s when capturing 100 µs. It also stresses waveform update rate as a key specification for catching infrequent glitches. [2]

Editorial calculation: to keep 1 GSa/s across a 1 ms capture, a scope needs 1 × 10⁻³ s × 1 × 10⁹ Sa/s = 1 million points of memory; with 10 k points it would sample at only 10 MSa/s. [2]

Among suppliers linked to this page, Semight lists wide-bandwidth sampling oscilloscopes among its instruments for optical transceiver and component testing, and FNIRSI lists handheld and benchtop units such as a 2-channel 100 MHz benchtop model and a 50 MHz, 250 MS/s handheld combining oscilloscope, multimeter and signal generator. [3] [4]

03

Compare what suppliers offer

The suppliers linked to this page list oscilloscopes; their cards link to the pages reviewed.

Ask any supplier for bandwidth and frequency-response type, rise time, sample rate per channel when all channels are on, memory depth, vertical resolution, input impedance and maximum input voltage, waveform update rate, triggers and decodes, probes supplied, calibration certificate and safety category.

04

Trace the instrument blocks

This is a functional reference built from the sources cited; it is not one maker's specification.

05

Fill the specification fields with values for your signals

An oscilloscope quotation needs values for your signals. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the signal and capture data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Signals: [fastest clock; fastest edge; amplitude]
Accuracy: [timing accuracy needed]
Capture: [longest span at full rate]
Channels: [analog; digital; probes]
Features: [triggers; decodes; update rate]
Use: [bench or portable; calibration]

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