| Products > Test Equipment |
| Impedance Measurements? |
| (1/1) |
| Lowkus:
I'm a beginner at electronics and have a goal of eventually selling in volume what I design, so I want my designs to be professional quality. I've recently been planning a project that is going to involve a high-speed ARM chip and I want to match the impedance correctly as it communicates with other devices on the board... 1) Is there an inexpensive way to verify that impedance is matching between devices? Or do engineers favor using hardware measurement tools? 2) Should I buy a vector network analyzer to determine impedance? Or some other type of test equipment? 3) If I should buy a VNA, I will probably want one that can do 3GHz to 6GHz speeds. Is there a brand/model of VNA you recommend that might fit (or exceed ) that specification? 4) I was reading that VNA's are easy to damage. Is that true, and if so how would I use it correctly and safely? 5) Does anyone have URL's to the YouTube videos Dave has made related to impedance and how to measure it? |
| noisyee:
For high-speed digital interconnection, like DDR, PCIe, we usually assume devices themself are impedance matched correctly and we designers only care about PCB trace (sometimes plus connectors). We generally have little to no method to verify those devices because 1. they are active termination (not a passive 50ohm resistor) 2. some have variable termination value depending on function or setting 3. some are bi-direction IOs and 4. some have advanced feature to compensate impedance mismatch. Even PCB trace, verify it's characteristic impedance is not cheap. A trusty VNA is already very expensive, fixtures and probes to test trace impedance can easily cost more than an entry-level VNA. Not to mention differential trace require 4-port VNA. Although some DIY probes, like an open coaxial, can perform the test, they are not that accurate and sometimes cause false negative result. If you are serious about your design but can't afford those expensive instruments, I recommend: 1. Find a reliable PCB manufacture, their technical team will help you do proper controlled impedance design, some even provide verification report. 2. Design and order a small test board if necessary, with only controlled impedance trace (which later will be used on your project) and coaxial connector, then it can be tested and characterized without costly fixture or probe. 3. Carefully follow IC manufacturers' design guide and protocol specification. If you really want an instrument, an entry-level VNA would do the job, presume you don't need that high level of measurement accurate or need to provide compliance test report, your main goal is to study the concept and do some experiments. VNA and other RF instruments are sensitive to port overload/over voltage and ESD damage. Follow your instrument user guide and it should last for decade. |
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