Bessel-Thomson reference filter 155 Mb/s

The TA121 Bessel-Thomson Reference Filter is designed for engineers working with optical communication systems where accurate time-domain characterisation of 155 Mb/s OC3/STM1 signals is required.

In an optical test setup, the optical signal is first converted into an electrical waveform using an optical-to-electrical converter. The TA121 is then connected between the converter output and the oscilloscope input.

Pico Technology states that this arrangement helps the oscilloscope display a representation of the signal that an equalised optical receiver would see. This is particularly important for optical eye-diagram and waveform measurements where excessive receiver bandwidth can otherwise exaggerate high-frequency components, peaking or ringing.

The TA121 is part of Pico Technology's family of Bessel-Thomson reference receiver filters for standard serial data rates.

Designed for 155 Mb/s OC3/STM1

The TA121 is specifically matched to:

• 155 Mb/s nominal data rate
• OC3
• STM1
• SONET/SDH optical transmission testing

Pico's product family assigns a separate Bessel-Thomson filter to each supported communication rate. TA121 is the model intended for the 155 Mb/s OC3/STM1 application.

What a Bessel-Thomson Filter Does

A Bessel-Thomson reference receiver filter provides a controlled frequency response that approximates the response defined for optical communication receivers.

Its purpose is not simply to remove noise. It shapes the overall measurement bandwidth so the captured waveform better represents how a compliant receiver responds to the transmitted signal.

This helps reduce:

• Excessive waveform peaking
• High-frequency ringing
• Unrealistically sharp transitions
• Measurement differences caused by excessive oscilloscope bandwidth
• Eye-diagram distortion caused by an incorrect receiver response

Pico specifically states that its Bessel-Thomson filters are used to reduce peaking and ringing and are selected according to the bit rate being analysed.

Optical Signal Characterisation

The TA121 is intended for optical transmission testing after optical-to-electrical conversion.

A typical measurement chain is:

Optical transmitter → Optical fibre → O/E converter → TA121 Bessel-Thomson filter → Sampling oscilloscope

This configuration is useful for evaluating:

• Optical eye diagrams
• Rise and fall times
• Pulse shape
• Overshoot
• Undershoot
• Timing quality
• Signal amplitude
• Receiver-oriented waveform behaviour
• SONET/SDH transmitter performance

The filter provides the required reference receiver response before the electrical waveform reaches the oscilloscope.

OC3 and STM1 Applications

OC3 and STM1 represent closely related SONET and SDH transmission levels operating around 155 Mb/s.

The TA121 is therefore suitable for measurements involving:

• SONET OC3 transmitters
• SDH STM1 transmitters
• Fibre-optic communication modules
• Optical transceivers
• Telecom line cards
• Optical receiver development
• Fibre transmission test systems
• Optical component validation
• Research and educational telecom laboratories

Pico lists TA121 explicitly as the 155 Mb/s OC3/STM1 filter in its reference receiver filter table.

Compatible PicoScope Systems

Pico Technology currently identifies the Bessel-Thomson reference filter range for use with:

• PicoScope 9221A
• PicoScope 9231A
• PicoScope 9321

These oscilloscopes incorporate or support optical-to-electrical measurement functions suitable for optical communication characterisation.

Older PicoScope 9200A documentation also identifies TA121 for use with the optical-to-electrical converter on the PicoScope 9221A and 9231A.

Use with Optical-to-Electrical Converters

The TA121 is an electrical reference filter. It is not connected directly to an optical fibre.

The optical signal must first pass through an optical-to-electrical converter.

The converted electrical signal is then routed through the TA121 before entering the oscilloscope.

This distinction should be clear on the product page because the TA121 is:

• Not an optical filter
• Not a fibre-optic attenuator
• Not an optical receiver
• Not an O/E converter
• Not a standalone signal analyser

It is a reference receiver filter used as part of an optical measurement system.

Eye-Diagram Measurements

Eye diagrams are widely used to evaluate digital communication quality.

With the correct Bessel-Thomson response applied, engineers can more meaningfully evaluate:

• Eye height
• Eye width
• Crossing points
• Rise time
• Fall time
• Timing jitter
• Signal symmetry
• Overshoot and ringing
• Receiver margin

The TA121 helps prevent the measurement system from displaying a waveform response that is significantly different from the receiver response defined for the applicable transmission rate.

Helps Reduce Peaking and Ringing

Pico specifically identifies reduction of peaking and ringing as one of the purposes of its Bessel-Thomson reference receiver filters.

This is important because an excessively wide measurement bandwidth can display high-frequency waveform content that a real receiver would attenuate.

Applying the proper reference filter helps produce a more representative signal shape.

Reference Receiver Testing

The TA121 is useful when measurements need to approximate the behaviour of a standardised optical receiver.

This can support:

• Design verification
• Transmitter development
• Pre-compliance testing
• Production validation
• Research measurements
• Comparative device testing
• Telecom training laboratories

The filter does not itself determine whether a product passes a communication standard. Compliance still depends on the complete test method, oscilloscope, optical converter, masks, trigger configuration and applicable specification.

Passive Measurement Accessory

The TA121 is a passive RF measurement accessory.

It does not require:

• USB power
• External DC power
• Software control
• Firmware
• Electronic switching
• Calibration software

Its function is determined by its fixed analogue Bessel-Thomson response for the specified bit rate.

The RevineTech page should therefore avoid describing it as an active digital signal-processing accessory.

Bessel-Thomson Filter Family

Pico Technology currently lists the following reference filters.

This makes it important to select the filter according to the signal's communication bit rate rather than choosing one solely by connector type or oscilloscope model.

Why Correct Bit-Rate Selection Matters

Each reference receiver filter has a response optimised for a particular serial data rate.

Using a filter intended for another bit rate could change:

• Measured rise time
• Eye opening
• Overshoot
• Pulse shape
• Noise response
• Jitter appearance
• Mask-test results

For 155 Mb/s OC3 or STM1 testing, the correct Pico reference filter is TA121.

Applications

The TA121 is suitable for:

• OC3 optical transmitter testing
• STM1 optical system testing
• SONET signal analysis
• SDH signal analysis
• Optical eye-diagram testing
• Fibre-optic transmitter development
• Optical receiver validation
• Telecom equipment testing
• Optical module characterisation
• Pre-compliance testing
• Production-line optical testing
• Research laboratories
• Telecom training
• Engineering education
• Fibre communication system development

Why Buy from RevineTech?

RevineTech can supply the TA121 reference filter as part of a Pico Technology optical measurement setup.

Technical product-selection support can include:

• TA121 filter selection
• PicoScope compatibility
• Optical-to-electrical converter configuration
• Sampling oscilloscope selection
• Matching the filter to the required data rate
• Related RF adaptors and cables
• Eye-diagram measurement setup
• Telecom measurement accessories