本帖最后由 Head4psi 于 2014-10-18 22:29 编辑
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DC Blocking Capacitor Value
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Detail please refer to Dr. Howard Johnson's webside :http://www.sigcon.com/Pubs/news/7_09.htm* N+ o% h0 i9 m p4 _3 g
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I shall begin with a quote from my newsletter vol 4, #15, "When to use AC coupling": [size=11.8181819915771px]To estimate the degree of DC wander possible when passing a particular code through a certain high pass filter HPF(f), first set up a complimentary filter LPF(f), defined thus: [size=11.8181819915771px]LPF(f) = 1 - HPF(f) [size=11.8181819915771px]Then pass the data code through the filter LPF(f) and look for the worst-case output. The magnitude of the output of LPF(f) equals the magnitude of the worst-case DC-wander error you will experience when passing your signal through HPF(f).
If that article is not familiar to you, take a moment now to look it over, as the remainder of this text builds on that theme: www.sigcon.com/Pubs/news/4_15.htm Filter Theory (Review)The remainder of this article requires that you know the relaxation time constant associated with a high-pass R-C filter. If your transmission setup is terminated at both ends with impedance Z0, as is customary with very high-speed links, then the total resistance in series with your DC blocking capacitor equals twice the line impedance, or 2Z0. A DC-blocking capacitor C placed in series with your serial link creates a simple high-pass filter [HPF] with a time constant: That statement assumes your line is terminated at both ends. If, on the other hand, your source happens to be a low-impedance driver and the line terminated only at its far end with impedance Z0 then the time constant becomes a different value: In either case the related complementary filter [1-HPF(f)] has the same time constant as HPF(f). I shall assume in the following analysis that your transmission line is terminated at both ends. If that is not the case, you must modify all the equations below. ApproachI propose that we develop an expression for the maximal size of the output from the complementary low-pass filter LPF(f). That expression relates the maximum amount DC wander to the time constant,τ and thus to the value of capacitance. If you know how much wander your system can tolerate, as determined from analysis of your eye margin budget, you can then calculate the capacitance required to achieve that goal. To read along with the following analysis you need to know how a one-pole LPF reacts to one individual bit. Figure 1 —Response of low-pass filter to a single bit with unit amplitude. A single bit of duration T when presented to the input to the LPF causes the LPF output to rise in a linear fashion during the bit interval, falling slowly with time constant τ, thereafter back to zero. This approximation assumes that τ vastly exceeds T, a condition consistent with the idea of a DC-blocking application. ' Q, ^* p+ T& o3 C; i+ n
If you transmit N similar bits in a row, it is a good bet that the LPF filter output will pump up to a value of NT/τ. How many bits in a row might you ever see? That is a very important question to ask about your data code; answers vary widely depending on who designed your code and whether they considered DC balance. I am going to define a term now, called running-disparity, or RD, that will help you understand how data codes are built. Every sequence of code bits x[n] implies a corresponding sequence RD[n], where: RD[n] equals the sum of all bits up to and including bit x[n] It is helpful in constructing these arguments if you think of a binary data sequence as having values +1 and -1 (or, more generally, +A and -A). For a DC-balanced sequence, the RD never strays far from zero. In fact, one excellent way to specify the degree of DC balance in a data code is to call out the maximum excursion of RD. Those of you well versed in calculus may be thinking that RD looks like an integrating operation. Precisely. Here is a basic theorem about RD. IF your data code guarantees |RD|<n
( k$ f7 P+ @8 M) \* w& KTHEN the DC wander signal z(t) is bounded by:
4 B1 u4 G2 p$ J y- `( q3 vWhere: - n is the bound on RD, in numbers of baud intervals,
- A is the binary signal amplitude (+/-A),
- T is the coded bit interval, and
- <Greek-tau> is the HPF filter time constant.
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This RD theorem assumes that the filter response is a single-pole filter with a monotonic step response (no zeros). With the RD theorem and the relation τ=2Z0C in hand, a specification M for the maximum permissible amplitude of DC wander then determines the required (minimum) value of capacitance C: Where: - n is the bound on RD, in numbers of baud intervals,
- A is the binary signal amplitude, in volts, assuming the signal swings from -A to +A,
- T is the coded bit interval,
- M is the maximum permissible amplitude of DC wander, in volts, and
- Z0 is the transmission line impedance (assumes both-ends termination).
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Any value of capacitance larger than this amount will work. The following sections review four popular data codes showing the values of n appropriate for each. " l& b/ R$ N" N9 b @( P# Y
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发表于 2014-10-17 15:50
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发表于 2014-10-17 16:49
