When is impedance matching critical?

One of the questions I asked myself in my previous post was when does it become critical to have a impedance-matched traces? Today I came across an interesting presentation by Rick Hartley, then a Senior Principal Engineer at L-3 Avionics Systems, now Principal Engineer at RHartley Enterprises.

In his presentation, Rick Hartley states that impedance matching becomes critical when the trace is 1/16th the wavelength of the signal in the propagation material. Here's the relevant slide, giving the formulas.

Proof I'm not an RF engineer

Back in 2013 I did a project with a Nordic Semiconductor nRF24LE1 chip. This is an 8051 compatible microprocessor with a built-in 2.4 GHz wireless transceiver. Conscious of how poor my knowledge of RF design is I followed the sample layout of the RF section exactly for the antenna. And by "exactly", I mean I overlaid the sample Gerber file onto my PCB layout and used that to position the components and route tracks. Without the proper tools to tune an inverted-F antenna like on the original, I used a commercial 2.45 GHz chip antenna with a 50 ohm impedance.

Everything seemed to work exactly as designed except the RF section. I could not get the board to talk to a commercial board using the same nRF24LE1 chip. Clearly I'd botched something.

I called a friend who is an Extra-class amateur radio operator and asked his advice. He reviewed my choice of passive components in the RF section and had me change out a couple which he deemed inappropriate for use at 2.45 GHz. This made them work when talking to each other, but they still wouldn't talk to the commercial boards and had significantly reduced range. Without the test equipment necessary to look at a 2.45 GHz signal, the project got shelved.

Testing... testing?

In a comment on my last post, Klaus Scheffler asked: 

How do you plan to test the register board? I first populated only all the DRAM cells without column and row selection logic, connected an Arduino to all column and row lines, and tested the DRAM bit by bit.

That's an excellent question.

Scratchpad board assembled

I've finished assembling the Scratchpad Register board. Here it is, in all its glory:

Poor Prior Planning Problems

Having received my new Scratch Pad Register boards from JLCPCB, I collected the parts I needed to populate the board. Everything was going well until I opened the container of solder paste: it was completely hard and dried out. I really should have anticipated this, since the last time I used it was three years ago. *sigh*

Scratchpad Register board delivered

This afternoon a DHL courier dropped a package at my doorstep. JLCPCB shipped it with a signature release required, but I waived the signature in advance.

I'm old enough to remember when placing a domestic mail order came with an "Allow 6 to 8 weeks for processing and shipping" disclaimer. Yet I sent this order to China electronically only 10 days ago. A week ago this physical package was in China, and now I'm holding printed circuit boards.

Scratchpad Register board ordered

After starting the layout of the Scratchpad Register board in August 2012 — yes, 11 years ago — it has finally been sent out for fabrication by JLCPCB in China. Here's what they say it will look like:

 

If they keep to schedule, production should start Monday (China time) and take 3-4 days. This would have the boards and stencil ship Friday-ish. I opted for the default DHL Express (2-4 day) shipping so they should arrive sometime the following week.