Test & measurement TRANSMITTER
TX FFE (feed-forward equalizer) CTLE (continuous time linear equalizer)
RECEIVER
FFE (feed-forward equalizer)
DFE (decision feedback equal- izer)
Amplifies high-frequency energy at symbol boundaries
Inverts the low-pass characteristic of the transmission channel
+ Compensates for simple low- pass behavior
+ Compensates for simple low- pass behavior - Amplifies noise
-Delayed and weighted signal copies -Operates at the symbol rate -More taps possible than TX FFE
+ Reduces impedance discontinuities - Can amplify noise
-Non-linear, with feedback loop -Operates at the symbol rate and requires clock data recovery -Often combined with CTLE/FFE
+ Best compensation for discontinuities in the frequency response + Can suppress noise
case, signal distortion leads to errors in the receiver, which can no longer distinguish data symbols correctly. Figure 1 shows an example of this signal distortion. The eye pattern at the transmitter output is wide open but completely closed at the receiver.
EQUALIZATION FILTERS FOR IMPROVED SIGNALS
Equalization filters on both the transmitter and receiver sides can reduce signal loss with fast data interfaces and even recover signal symbol information because they compensate for frequency-dependent losses in the transmission channel.
The USB 3.2 Gen 2 Standard defines a 3-tap FIR filter for pre-distortion at the transmitter and a 1st-order CTLE filter with a 1-tap DFE filter at the receiver. Depending on channel quality, the filter parameters and interaction of the filters can be optimised even further. The table below outlines the design and function of common equalization filters.
TEST REQUIREMENTS
To measure the compliance of fast data interfaces, an adapter (test fixture) is necessary for the contact between the measuring device and the transmitter. However, signal loss caused by the test fixture should not affect test results. De-embedding makes it possible to compensate for losses on the signal path from the transmitter
Instrumentation Monthly April 2025
to the outputs of the test fixture using a filter. However, interface standards also define other test points within a transmitter-receiver transmission link. As such, specified transmission paths can be added using filters to emulate signal losses for the measured transmitter signal. This method is called embedding. Last but not least, the test specification also defines how to emulate the previously mentioned equalization filters. For the actual signal analysis automatic amplitude and time measurements as well as data eye pattern and jitter/noise analyses are available.
TEST SOLUTION WITH THE R&S®RTP OSCILLOSCOPE
The R&S RTP-K121 de-embedding and R&S RTP-K126 embedding and equalization options for the R&S RTP oscilloscope allow you to define relevant signal paths. Test fixtures and additional transmission paths can be added or removed based on S-parameters that define relevant transmission losses. Common equalization filters such as CTLE, FFE and DFE can be configured according to interface standards and individual requirements. Figure 3 shows an analysis example with a USB 3.2 Gen 1 data signal (5 Gbit/s). In this example, signal integrity is measured at three different points and compared using an eye pattern: (1) directly at the transmitter, (2) after embedding a channel designed in accordance with the specifications of the USB Implementers
Fig. 3: Comparison of waveforms and eye patterns at different test points for a USB 3.2 Gen 1 signal (5 Gbit/s): - Diff1 / Eye1: Original measurements at transmitter output - Lane1 / Eye2: Additional transmission channel embedded - Lane2 / Eye3: Additional CTLE equalization filter applied
Forum (3-meter cable and 5-inch PCB) and (3) after emulation with CTLE filter.
SUMMARY
The R&S RTP high-end oscilloscope offers a range of tools for analysing signal integrity. By emulating additional transmission paths and equalization filters, users can easily include other system conditions in their analyses.
Rohde & Schwarz
www.rohde-schwarz.com 39
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