RF Design - Microstrip vs. Lumped Components

Question

I currently working on a RF design, with frequencies up to about 2GHz.

What is the "rule of thumb" for when to stop using lumped components (Capacitors, Inductors, Resistors), and start using microstrips ect. ?

P.S. I can see that lumped components are available up to about 10GHz, but they are pretty expensive.

Edit: What about decoupling capacitors, on supply lines to pll's ect., should they always be associated with RF Chokes to work probably (at 2GHz) ?

Answer 1

The trouble with discrete parts is that they come with parasitics; e.g. an inductor will have a parasitic resistance, parasitic capacitance, and parasitic capacitance to the board. Generally speaking, as you approach 1GHz, parasitics start affecting your frequency response, even on small SMD components. Even if you buy "10 GHz" components, they'll have parasitics. The hard part of RF design is modeling the parasitics accurately (and knowing where they can be ignored). Also, there's a trade-off between precision and cost.

Microstrip elements are more abstract, so you generally have more control over what's going on. As others have mentioned, though, they are bigger.

Modeling components is time-consuming. Keep in mind, though, for a one-off project, you might find that tweaking the circuit is enough to get it to work. Repeatability over hundreds or thousands of units is a different matter. For this spectrum analyzer project, you don't need to worry about the 10.7 MHz IF blocks, but you will have to be careful with the 1-2GHz stuff. The bandpass filter looks like the most difficult part.

Answer 2

I'm not an expert in RF, but my guess is that the following tradeoffs come into play:

lumped components:

• you don't have to design them
• as long as they're physically much smaller than the wavelength, they should work well
• except for high-power or high-voltage, should be available in 0402 or even 0201
• if you need to change frequencies, you can do so by changing the BOM + can leave the circuit layout unchanged

microstrip elements:

• tolerance is controlled by geometry, so can be much better than lumped components (esp. relative tolerances) with higher-Q/sharper rolloff
• you can do all sorts of things you can't do with lumped components (e.g. resonators, quarter-wavelength transformers, directional couplers, etc)

Wavelength is definitely the key -- 1GHz has a wavelength of about 30cm in air (less in a high-K material like FR4). I've heard various rules of thumb like lambda/2pi, lambda/10, lambda/8, but if your board traces have dimensions of >3cm you probably need to deal w/ transmission line effects for signals > 1GHz.

Answer 3

There really isn't a hard and fast rule.

The Advantage of lumped components is that they are smaller at these frequencies, so your board area will be smaller. If you go to microstrip elements for filters etc, you should also use a controlled impedance board, otherwise you're in for a bunch of hand tuning.

As far as 50Ohm traces, that's good practice to use those to interconnect circuit blocks, but unless the trace length is greater than 10% of the wavelength, you probably don't need it.