How to measure harmonics on DC-DC converter output? - Page 1
Forum section: electronics / beginners

author: dl2sba
author_rank: Newbie
timestamp: May 01, 2017, 09:09:46 am
content: Hi everybodyI had some issues running an WSPR transmitter powered by one of these cheap LM2596 based DC-DC step-down converters.The DC-DC converter switching frequency was mixed on the carrier signal, which gives a nice wide-band signal :-/I can use an oscilloscope to measure the output ripple on the converter. Mainly there I can see the switching frequency. My idea is now to hook up a spectrum analyzer to the output of the DC-DC converter, so that also the harmonics created by the converter can be seen.On the input socket of my spectrum analyzer is written: Max +30dBm/50VDC.Now my questions are:(1) Can I directly hook up the converter output (5VDC) to the analyzer?I assume yes, because 5VDC < 50VDC.(2) Should I use a 20dB attenuator to protect the analyzer's input?The 50Ohm attenuator will draw 100mA, so 0,5W is dissipated in the attenuator. So measuring the output ripple without load is not possible.(3) Should I use such a DC blocker?The capacitor in the DC blocker will form a high-pass filter. If the DC blocker is specified from 300kHz to 8GHz, I assume, that I will miss the first harmonics of the DC-DC converter - right?If anyone can give me some hints, how to perform harmonics measurements on the DC-DC converter, this would be great.Thanks in advanceDietmar

author: ebclr
author_rank: Super Contributor
timestamp: May 01, 2017, 09:16:01 am
content: Use these probes on your spectrum analyzerhttp://www.aaronia.com/products/antennas/Cheaper oneshttps://www.amazon.com/Tekbox-TBPS01-EMC-Near-Field-Probe/dp/B00OD57CRW/ref=sr_1_1?ie=UTF8&qid=1493630227&sr=8-1&keywords=near+field+probes

author: woodchips
author_rank: Frequent Contributor
timestamp: May 01, 2017, 09:23:47 am
content: If your analogue scope has a signal out, then use a stanard 10:1 scope prope to pick the signal up and connect the scope sigal out to the SA, might need an attenuator. Has the advantage of using the robust scope input to protect the SA. Won't work with a DSO.

author: dl2sba
author_rank: Newbie
timestamp: May 01, 2017, 01:07:49 pm
content: @woodchips@ebclrThanks for your answers.Is this really the correct way to measure the emissions on the output terminals of the DC-DC converter?When using such antennas, I assume to catch all emissions from the converter, not only those relevant to the load?!//Dietmar

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2017, 02:33:28 pm
content: You want a bias tee. DC goes one way, AC goes the other way. You want the crossover frequency in the 1-100kHz range. This is easy enough to construct with some perfboard and spare components. This way, you can adjust the load on the converter, while monitoring its noise level.Bandwidth only needs to be up to a few hundred MHz, so construction isn't terribly critical.Don't forget that the DC-DC converter input wires are probably quite noisy. Create a ground plane, or a single point ground reference, to filter all three wires against (IN+, IN-, OUT+, and let OUT- == GND). Use LC filter networks to keep switching noise inside, and add R+C damping to the capacitors as needed to reduce peaking.Tim