Terminator II

(Originally published in EDN Magazine, March 30, 2006)
 

In my last column, I presented a rather troubled system architecture with incredibly stringent signal-quality requirements. The signal source is a low-impedance ECL driver with a 400-psec rise time, which connects to the receiver by 3 ft of RG58 A/U coaxial cable.

Somebody else already built the signal source, so you are stuck with the low-impedance ECL output. By government contract, you must design the receiver using an off-the-shelf FPGA combined with an external 50Ω±1% terminating resistor.

So far, the termination strategy sounds good, but unfortunately, the FPGA has a 9-pF input capacitance (Figure 1). When it connects directly to the external 50Ω end-terminating resistor with R₂=0Ω, the input capacitance interferes with the operation of the termination, creating massive reflections. The reflections return every 8 nsec, commensurate with the round-trip delay of the coaxial cable.

Quantifying the impedances in this circuit, the 400-psec rise time corresponds to a frequency of roughly F= 0.35/(400 psec)=875 MHz. At that frequency, the 9-pF capacitance of the FPGA inputs presents an impedance magnitude (1/(2πFC _IN)) of just 20Ω. This low impedance loads the 50Ω termination, preventing the terminator from doing its job.

Now, insert resistor R₂with a value of 50Ω in series with the FPGA input. This circuit trick raises the effective input impedance of the FPGA to something at least 50Ω or greater. In the worst-case scenario, if the capacitor impedance were absolutely zero, the parallel combination of R₁ and R₂ would make a termination impedance of 25Ω, creating a reflection coefficient no larger than one-third.

Resistor R₂ acts as an isolation component, preventing the FPGA capacitance from directly loading the terminating resistor. A value of 50Ω for R₂ in this circuit yields a fourfold reduction in the reflected signal amplitude.

At the same time, resistor R₂ degrades the received-signal rise time. This situation occurs because the combination of R₂ and C_IN acts as an RC lowpass filter. Setting R₂ equal to 50Ω in this circuit doubles the signal rise time.

If you are worried about late reflections shifting the apparent times of arrival for subsequent edges, the induced jitter varies in proportion to the product of the reflected signal amplitude (as a fraction of the signal swing) times the signal rise time. If the reflections decrease fourfold but the rise time doubles, then you win a twofold reduction in jitter. That outcome is good, but I want to do better.

In my next column, I will show you how to force the termination impedance to equal exactly 50Ω with minimum degradation of the received-signal rise time.

All Publications by Dr. Howard Johnson except as noted.
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