Low-Power Analog Front-end in 10Gbase-T
September 16, 2014, Custom Integrated Circuits Conference, San Jose, CA—Jan Westra from Broadcom described their efforts in making a multi-channel 10Gbase-T chip. The alternative of copper over short distances can easily supplement the optical fiber for the rest of the 10G Ethernet network in the datacenter.
One problem with copper is that the wire bundles generate cross talk and noise across the pairs. In a typical switch, the connector panel holds 48 connectors, configured as 12 Ethernet driver chips with 4 channels each. Each channel needs analog front end functions, a modulator and filters. The criteria for the chips are low power, small die area, low EMI, and robust. The chips must also manage cross-talk and echoes.
Low-density predictive codes are used for error detection in the encoder and decoder stages. The 10G signals are split into 4 x 2.5Gb/s lines to feed the 4 twisted pairs in the Ethernet cable. This type of cable has can cause losses of up to 20.7dB, so the modulation and error correction encode 3.5 bits per signal on each pair. Modulation is PAM 128.
One issue with cables for data at 10Gbps is inter-symbol interference, which closes the signal eye. Efforts to improve the signal integrity have included adding filters and feedback in linear equalizers which add high-frequency noise. Decision feedback equalization propagates errors and adds high delays. Pre-emphasis and forward error correction help, but over peaking forces a need for a high dynamic range receiver.
A workable alternative that overcomes many of the shortcomings is to add or subtract the full signal in Tomlinson-Harashima pre-coding to expand the constellation. The receiver needs to have a 10dB additional margin for this to work. In a similar manner, the echoes and crosstalk have to be compensated. Echoes come from the transmit signals getting into the receiver, but this effect can be overcome with linear filters.
Crosstalk from near end to far end can use the adjacent channel receiver as an input to cancel the crosstalk signal. This costs an extra 5dB in noise margin. Alien crosstalk, noise from other channels and lines is handled in the same way.
The specifications for the lines call for a BER of 10-12 and overcoming the Shannon limit of 20.7dB with uncoded 128 DSQ needs 31.5dB. You gain 8dB with LDPC. The full receiver error budget is 20dB for the Shannon limit, 3dB for the LDPC, 5dB for near and far end echoes, 5dB for alien crosstalk, 10dB for precoding, 5dB for equalization, and 12dB to account for peak to average ratios. The total is 60dB
This level of gain is integrated into the analog front end architecture with a 10 TFLOP DSP to handle the transmit, receive and hybrid functions, which separate the transit and receive signals with high linearity. The transmit linearity is included in the 802.3 spec, but the hybrid is not so linear due to the unknown loading from the cables. Previous efforts to address this error source use a mirror buffer with a 1:N ratio that is programmed to be the lumped equivalent of the actual cable. This mirror buffer relies on non-linear cancellation and delay echoes and can be fairly high power.
Alternatively, a mirror 1:N DAC replaces the buffers. The DAC is a standard current steering DAC plus a cascade scaled hybrid DAC with 12-bit accuracy in a 6+6 configuration. The DAC runs at 1.6G samples/second and exceeds the spec and design goals of 60dB. In a similar fashion, the receiver uses a tuned amplifier and sample and hold to feed the ADC. This version uses a 2.5 V supply with thin oxide devices in a 40nm process. Pre-charge and fast pull down circuitry reduce power requirements and a programmable gain amplifier with non-Miller frequency compensation provides the gain.
The chip measurements are BER of 10-15, and a signal-nose level of over 28.5dB. In a noise and crosstalk test with a cable surrounded by 6 other cables, alien crosstalk was 25.6dB at 100 Meters.
The design of a 10Gbase-T chip requires an overall gain of 60dB and echo cancellation is a big enabler. Improving transmit linearity enhances robustness, and over-sampled DACs reduce the effects of EMI. The thin oxide in the I/O sections help reduces power consumption.
