Measuring a converter's output noise using 3 instruments. - Page 1
Forum section: electronics / projects

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 16, 2022, 11:44:52 pm
content: Test Equipment1) GWInstek 1202B , a 2 channel, 200 MHz DSO2) Tek 466 Analog Storage O'scope 100 MHz3) HP141T Spectrum Analyser with 8553B RF, 110 MHZDUT is a home brew converter for 13.8V 20 Amp UPS , ham radio use.Test setup:An isolation transformer with grounded interwinding shield was used to power the oscilloscopes and DUT.It has previously been found that the GW1202B scope "probes shorted" noise pickup level is lower when powering the scope along with DUT on the secondary of the isolating transformer.A metal load plate contains the dummy load , (4) of 55 Watt H7 halogen lamps, and an attenuator consisting of a 10dB SMA Tee and a further 6dB resistor divider.The load also consists of the UPS battery, a #27 Marine Duty with 85 AH capacity. The 20 Amp battery leads are about 2 metre long. Maybe they should be shortened.The load plate was placed close to the converter's metal box and was connected to it with short grounding cables at each end. The DUT was connected to the isolating transformer by its 120 Vac 3 pin supply lead. The protective ground goes through the isolating transformer frame to the DUT and load plate.The isolating transformer secondary is floating.The base noise of the 3 instruments was checked.The scope probe was connected to the load plate with the probe and ground clip shorted to the plate.GW1202B with DUT off (as reported by "Measure")Probe X10, Bandwidth Full, noise = 36 mV p-p , 7.2 mV rmsProbe X10, Bandwidth 20MHz, noise = 38 mV p-p , 3.5 mV rmsProbe X1, Bandwidth Full, noise = 4.5 mV p-p , 0.48 mV rmsProbe X1, Bandwidth 20MHz, noise = 3.5 mV p-p , 0.35 mV rmsTek 466 with DUT off, probe same as aboveProbe X10, Bandwidth Full, noise = 25 mV p-p of 108 MHz FM station, and 30 mV p-p of about 100 kHzProbe X10, Bandwidth 20MHz, noise = 30 mV p-p of about 100 kHzProbe X1, Bandwidth Full, noise = 2 mV p-p mainly of about 10 MHz, a receiver was on 30 m band nearbyProbe X1, Bandwidth 20MHz, noise = 2 mV p-p mainly of about 10 MHz, a receiver was on 30 m band nearbyHP141T Spectrum Analyser. The level was adjusted by +16dB to compensate for the attenuator.The attenuator has a series DC blocking, hi pass 1kV capacitor, corner about 200 kHz.With DUT off, the floor of the HP141T was -90dBm, fairly flat from 2MHz to 100 MHz.Test was with DUT on, running 3.3 Amp load with battery charged.The test was run with the probes in X1 and Bandwith Full on both O'scopesThe appearance of the 13.8 V output was approximately same on both O'scopes,Sinusoidal firing frequency ripple : approx 10 mV p-pOn-transients consist of a burst containing mainly about 48 MHz (21 nS period)Off-transients were about half the level, with similar content.GW1202B posted level of 144 mV p-p for the worst On-transientsTek 466 showed about 120 mV p-p for the same.GW1202B was increased to record length of 100k and FFT was selected with Hanning, 10MHz/div, centred on 50 MHz, vertical dBVComparing peaks of the HP141T and the 1202B :HP 141T ( including the 16 dB attenuators) :9.6 MHz: -69 dBm , 80 uV rms19.2 MHz: no peak above the floor48 MHz: -56 dBm , 350 uVGW1202B9.6 MHz: -75.2 dBV slightly fluttering , peaking 173 uV rms19.2 MHz: -77 dBV fluttering , peaking 141 uV rms48 MHz: -70.4dBV , 301 uVScrots:https://app.box.com/s/im7t8nfui9qf1hf7b9qe8slf1g7y2cisGWInstek 1202B: Yellow is the DC output with the pwm phased back with battery chargedRed is the FFT as described above.https://app.box.com/s/br0e2vcjd4qx5r9z8cor62cv7l14eediA better scrot as above with the pwm phased up to about 50%, charging the batteryhttps://app.box.com/s/c852iawjw4i9mq4v9ps2nfy1a8o7jfnqTek 466 showing the ringing at 21 nsec periodhttps://app.box.com/s/lfdiu767psqeqdv4542a78smnc4gd4c3HP141 showing the 48 MHz peak with 50MHz centre and 10 MHz/div each sidehttps://app.box.com/s/bx4ziyn0apeb5ddhpub43mjvi2hs21isThe load plate containing the lamps and attenuator.https://app.box.com/s/heinaarximaxkc33rjqld6p82hazncm3The HP141T being displayed on the GW1202B showing the converter ringing at 9.6 MHz, 48 MHz and an FM radio station at 97 MHz

author: jonpaul
author_rank: Super Contributor
timestamp: November 17, 2022, 06:58:49 am
content: bonjourSee the many HP, TEK and other app notes on exactly the noise and PARD measurement of SMPS.Probing is critical, most 10x probes are useless, Due to high CM noise.Line conducted noise needs a LISN.send schematic, pix, diagrams of the test and probing setup.Jon