Bessel-Thomson reference filter 1.25 Gb/s

The TA123 Bessel-Thomson Reference Filter 1.25 Gb/s is a specialised measurement accessory for optical communication and Gigabit Ethernet signal analysis.

It is intended for 1.250 Gb/s GBE signals and forms part of Pico Technology’s Bessel-Thomson filter family, where each filter is matched to a specific serial data rate.

The TA123 is used in the electrical path after optical-to-electrical conversion. A typical measurement chain is:

Optical transmitter → O/E converter → TA123 Bessel-Thomson filter → Sampling oscilloscope

Pico states that placing the correct BT filter between the O/E converter output and oscilloscope input helps the displayed waveform represent what an equalised optical receiver would see.

Designed for 1.25 Gb/s Gigabit Ethernet

The TA123 is specifically matched to:

• 1.250 Gb/s serial data
• Gigabit Ethernet
• Optical transmission testing
• Optical eye-diagram analysis
• Telecom and networking signal characterisation

Pico identifies TA123 as the dedicated 1.250 Gb/s GBE model in its Bessel-Thomson reference filter range.

Reference Receiver Response

The TA123 is not simply a general-purpose low-pass filter.

Its purpose is to apply the appropriate Bessel-Thomson reference response to the electrical signal before measurement.

This helps engineers evaluate parameters such as:

• Eye opening
• Rise time
• Fall time
• Pulse shape
• Overshoot
• Undershoot
• Timing behaviour
• Signal symmetry
• Receiver-oriented waveform quality

Using the correct reference response is important because measuring the same signal with excessive oscilloscope bandwidth can produce a waveform that does not accurately represent the receiver response used by the communication system.

Optical Eye-Diagram Analysis

The TA123 is well suited to optical eye-diagram measurements at 1.25 Gb/s.

Eye-diagram analysis can be used to evaluate:

• Eye height
• Eye width
• Crossing behaviour
• Rise and fall transitions
• Timing variation
• Pulse symmetry
• Overshoot and ringing
• Overall signal quality

The filter helps condition the waveform before it reaches the oscilloscope so the result is more representative of a reference optical receiver.

Optical-to-Electrical Measurement Chain

The TA123 works on the electrical signal, not directly on the optical fibre.

The correct sequence is:

1. Optical signal is generated or received.
2. An O/E converter converts the optical waveform into an electrical signal.
3. The TA123 applies the 1.25 Gb/s Bessel-Thomson response.
4. The oscilloscope captures and analyses the filtered electrical waveform.

This distinction should be clear on the RevineTech page because the TA123 is not an optical filter, transceiver or O/E converter.

Compatible PicoScope Systems

Pico Technology currently lists the TA123 filter family for use with:

• PicoScope 9221A
• PicoScope 9231A
• PicoScope 9321

These instruments can use the appropriate Bessel-Thomson filter to display measurements corresponding to the required optical receiver response.

Gigabit Ethernet Testing

The TA123 is suitable for applications involving optical Gigabit Ethernet systems and related 1.25 Gb/s serial communication links.

Typical uses include:

• Gigabit Ethernet transmitter testing
• Optical transceiver validation
• Fibre communication testing
• Network equipment development
• Optical module characterisation
• Telecom interface validation
• Research and educational laboratories
• Production signal verification

Passive Measurement Accessory

The TA123 is a fixed reference filter used directly in the signal path.

It does not require:

• USB control
• Software configuration
• Network connection
• PicoSDK
• Electronic switching
• User-selectable filter settings

The correct response is selected by choosing the appropriate filter model for the required data rate.

Bessel-Thomson Filter Family

Pico currently lists the following models:

Why Correct Filter Selection Matters

Each Pico BT filter is intended for a specific serial data rate.

Using the wrong filter can affect the apparent:

• Rise time
• Fall time
• Eye opening
• Pulse shape
• Overshoot
• Timing response
• High-frequency content

For 1.25 Gb/s Gigabit Ethernet measurements, TA123 is the correct Pico reference filter.

Applications

The TA123 is suitable for:

• 1.25 Gb/s optical signal testing
• Gigabit Ethernet signal characterisation
• Optical eye-diagram analysis
• Optical transceiver testing
• Fibre communication development
• Telecom equipment validation
• Network interface testing
• Optical module development
• Production optical testing
• Research and development
• Engineering education
• Communication laboratory training