What are the different types of Oscilloscopes?
There’s mainly Real Time Oscilloscopes and
Equivalent Time Based Oscilloscopes also known as sampling scopes.
Real-time
Scope:
Real time Scope uses Horizontal scale to
trigger and capture the desired waveform.
Below shows the reconstructed waveform.
Fig.
1. Real time Scope Waveform acquisition (Keysight)
Real time scopes are very useful for:
●
Non repetitive signals
●
Able to display one-time
transient events
●
Large memory depth
●
No external trigger needed
Sampling
Scopes:
An equivalent time sampling oscilloscope,
sometimes simply called a “sampling scope,” measures only the instantaneous
amplitude of the waveform at the sampling instant. In contrast to the real-time
scope, the input signal is only sampled once per trigger. The next time the
scope is triggered, a small delay is added and another sample is taken. The
intended number of samples determines the resulting number of cycles needed to
reproduce the waveform.
Fig.
2. Equivalent time Scope Waveform acquisition (Keysight)
Equivalent time Sampling scopes are very
useful for:
●
Lower sampling rate allows
higher resolution ADC conversion
●
Wider bandwidth
●
Lower noise floor
●
Lower intrinsic jitter
●
Can include front end optical
modules
●
Can achieve solutions at a
reduced cost
Probes:
This section is mostly applicable for Real
time scopes although there are some solutions for ET Sampling scopes as well.
Probes can be broadly categorized into 2
types:
●
Passive Probes
●
Active probes.
Passive
Probes:
Passive probes are simple probes that use
only passive components like resistors, capacitors, and wires—no amplification
or active circuitry.
●
1:1,10:1 attenuation
●
These probes are adequate in
measuring signals <100MHZ
●
High Voltage signals.
●
General purpose probing etc.
Active
Probes:
Active probes include active electronic components (like FETs or op-amps) near the probe
tip to buffer and amplify the signal before it travels to the oscilloscope.
●
Used for Signal Integrity
Measurements
●
High Speed Signals( lower Rise
time and faster Switching frequency)
Examples
Rise time/ BW measurement:
In the example below I am measuring the
Maximum Bandwidth that my Sampling Scope can measure.
Signal in question is generated using
Leobodnar’s pulse Generator which provides both Trigger and actual signal which
is helpful when using a sampling scope.
Equipment:
Agilent DCA 86100B with HP83483A Dual Channel 20GHz Module
Mean - 37.8ps
Minimum-24ps.
 |
| Fig. 3. Rise-Time / Bandwidth measurement example on a Sampling-scope. |
NRZ
Signal at 26.5GHz using Pattern Lock
Equipment:
Agilent 83484A Dual Channel 50GHz Module
What
is an Eye diagram?
It's a time-domain
waveform of a digital signal, usually captured using an oscilloscope or a
BERT (Bit Error Rate Tester).
Multiple bits are superimposed on top of
each other by synchronizing to the clock, forming an image that resembles a human eye — hence the name.
It is critical that all transitions are
part of the pattern otherwise it won’t plot an Eye and mere look like a square
wave.
The Eye Diagram is used because it
provides a quick and intuitive way to
evaluate signal integrity. Specifically, it helps assess:
- Signal Quality: How clean and distinguishable the 1s and 0s are.
- Jitter: Variability
in timing (horizontal eye closure).
- Noise: Random voltage
variation (vertical eye closure).
- Rise/Fall Times: How
fast the signal transitions from low to high and vice versa.
- Inter-symbol Interference (ISI): When previous bits affect the current bit.
- Timing Margin:
Available margin before sampling errors occur.
- Amplitude Margin: Voltage range for correct detection.
That is typical Eye Diagram measurement in
serial Channel Characterization.
 |
| Fig. 4. Eye-Diagram on a sampling-scope. |
All these measurements are in the time
domain and can also look in the frequency domain using the built in FFT
function and look at the Harmonics of the signal.
In the example above the pattern is
generated using an FPGA and pattern is a PRBS (Pseudo Random Bit Stream) pattern.
Real-Time
Scope:
DSA7330D-
A Digital Serial analyzer is a scope that is
designed to analyze high speed serial link in sub GHz
As it can be seen the rise time measurement
of the signal is dependent not only on the front end analog BW of the
scope(33GHz) in our case but also the sampling rate (100G/s).
BW= 0.35/(trise) — Gaussian method.
In a real time scope it takes about 3-5
samples to recreate a signal and hence we can see that our scope can measure
~40ish ps on average and ~32ps as the minimum.
100G/s is 10ps resolution so that makes
sense.
Also another thing to consider is the in
herit Jitter in the signal which is present in our source and hence that can
also affect the measurement.
In future measurements will cover topics of
Jitter and its effects.
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