Electronic – What are the limitations, if any, of free EDA tools with respect to high speed design

Length matching is about timing so if you want to know how tightly you have to length match you have to understand the timing budget for your interface. Signals will leave your source, and arrive at your destination with some timing relationship. Your receiver requires a certain timing relationship between clock and data in order to be guaranteed to function correctly. This is usually defined as setup and hold time, or how long before the clock edge your data needs to be valid, and how long after it must stay valid.

There are several things in a system which eat into this budget, one of which will be your routing. Sometimes a manufacturer will tell you this information, other times you must derive it from the input and output timing data of your transmitter and receiver. Of course it's easy to just say well I must match exactly, because then you don't have to think about it :)

But let's think about it for a minute. You have a 170Mhz signal? That's a period of 5.882ns. What would happen if you routed all your data to within one inch of clock. What would be the worst case timing difference. Propagation time for a top layer trace, a microstrip is about 150ps/in. So a 1 inch difference will skew a data signal from clock +/- 150ps. That's really not bad at all considering your 5.882ns clock period. In fact 170Mhz is really not that fast.

If you understood the output skew from your transmitter, and your setup and hold times for your receiver you could come up with a number for the acceptable routing delay. Of course there are other factors, clock jitter, ISI, etc, but this should give you a good idea of what you can do.

There are techniques that can be used for prototyping. If you're prototyping a whole system you're probably not doing it right- but to test smallish bits of analogish circuitry, it's practical. It's good to go to a PCB layout early, but not necessarily as the first step.

Here's one hacked together circuit by a fellow I happen to know John Larkin- (he's since moved to gold-plated for the boards)

And another (the high-speed section is kept very small)

This is done with shears, dental burrs etc. The ground planes under everything mean that the circuits are fairly quiet. You can also stack breakout boards for SMT chips on the ground planes (you can buy them or have your own made).