Hi Guys,
I'm really confused by the AC capacitor in a serial link, who can detail its function? In our simulation,how to deal with it?
I want to use the S parameter model, which can be found from its supplier. But when the upper frequency of the model is only 6G Hz, i can't use it in my 6G Hz simulation again. I also found another option to deal with this problem, its main thought is model the ESL and ESR of a capacitor, not directly use it but divide ESL/ESR value to some small inductor/resistor, as below showing. But how does this model working?
(ESpice ".subckt Cap2 1 2
C1 1 0 0.15p
R1 1 3a 0.001
L1 3a 4a 0.2n
R1a 4a 3 0.001
L1a 3 4b 0.2n
R1b 4b 3ab 0.001
L1b 3ab 4ab 0.2n
R1ab 4ab 3b 0.001
L1ab 3b 4 0.2n
* CB 4 5 100n * uncomment to use 100nF series cap
R2 4 6a 0.001 * change node 4 to 5 when using cap
L2 6a 2a 0.1n
R2a 2a 6 0.001
L2a 6 4c 0.1n
R2c 4c 6ac 0.001
L2c 6ac 2ac 0.1n
R2c 2ac 6c 0.001
L2c 6c 4b 0.1n
R2b 4b 6ab 0.001
L2b 6ab 2ab 0.2n
R2ab 2ab 6b 0.001
L2ab 6b 2 0.2n
C2 2 0 0.15p
RS 1 2 100k
.ends Cap2
" )
Hi Bruce,
One of your quesitons was "I'm really confused by the AC capacitor in a serial link, who can detail its function?"
This capacitor serves to remove the DC component that results from an un-even number of 1's and 0's in the bit stream from the AC signal so it switches around 0V. Here's an article from Howard Johnson that you might find useful in explaining why you need them:
http://www.sigcon.com/Pubs/news/4_15.htm
The tricky part for simulation is that these capacitors have an RC time constant and they don't charge up instantly. When they're in the actual hardware, they charge up seemingly fast (a few miliseconds). The problem is, in simulation, you typically don't simulate out to a few miliseconds, you're simulating a few nano-seconds, or if you're really patient, maybe microseconds. So that means that your signal in simulation always looks bad unless you skip thousands of bits of simulation data. The alternative is to use a SPICE capacitor model and set an initial condition on the nodes of the capacitor so that it is effectively charged right from the beginning.
-Steve