AC dB readings with Siglent SDM3045 and SDM3055, from Rigol DG1022Z - Page 1
Forum section: electronics / testgear

author: mtolesen
author_rank: Contributor
timestamp: March 23, 2017, 08:39:08 am
content: Hello,Please excuse me, but i have a question, proberly very basic, but it confuses me.I have purchased a Rigol DG1022Z waveform generator, and a Siglent SDM3045X multimeter.I want to measure AC dB readings from the Rigol DG1022Z on the Siglent SDM3045X multimeter.I use 1 kHz frequency from the Rigol, to be sure that i am way within the specs of the Siglent SDM3045X multimeter.Now, i select dB output setting on the Rigol, so that i can step 1 dB at a time on the rigol, and connect it to the Siglent Multimeter, and choose dB readings on the Siglent.Here is the problem.....i do not reas the same dB value on the rigol and the siglent, when the Rigor ea shows -2 dB for it's output, i real a different dB value on the Siglent meter.As i understand, 0 dB output on the Rigol generator, shall correspond to 0,774 Volt, and on the Rigol that is also right meaning, when i adjust the rigol to 0 dB output, and swich to voltage output reading, the Rigol shows 0,774 volt output voltage, BUT the Siglent multimeter shows 0,85 something....?I have adjusted the output impeadence in the Rigol generator to 600 ohm and adjysted the Siglent input impeadence to 600 ohm for match but i do not think i helps.I can actually compensate for the error on the Siglent multimeter by tweeking on the REL setting in the meter, but honestly, i do not relaay understand what is happening, and what i do wrong.So, why do i not read the same dB on the Rigol and the Siglent, and what shall i do to make corresponding dB readings on the Rigol and the Siglent...?, can somebody help me with this, maybe, simple problemThank you very much.

author: trevwhite
author_rank: Frequent Contributor
timestamp: March 23, 2017, 08:43:39 am
content: Does your sig gen think it is being connected to a 50R load? It adjusts what it thinks the output voltage would beBased on that assumption. Can you set the scope input to 50R

author: mtolesen
author_rank: Contributor
timestamp: March 23, 2017, 09:32:21 am
content: Well, i adjust the sig gen into 600 ohm output impedence, and then i also adjust the siglent voltmeter input impedence into 600 ohm, that should work, right?

author: 2N3055
author_rank: Super Contributor
timestamp: March 23, 2017, 01:00:45 pm
content: Quote from: mtolesen on March 23, 2017, 09:32:21 amNot if you don't load it with 600 ohms...

author: mtolesen
author_rank: Contributor
timestamp: March 23, 2017, 03:25:48 pm
content: Well, the load is in the volt meter, and i adjust this into 600 ohm load.

author: w2aew
author_rank: Super Contributor
timestamp: March 23, 2017, 09:36:27 pm
content: Quote from: mtolesen on March 23, 2017, 03:25:48 pmLooking through the user manual of the DMM, I don't see a way to change the input impedance to 600 ohms. It only shows that you can change the DC input impedance from 10M to 10Gohm and this only applies to DC Volts measurement. So, I don't see how you're changing the load presented by the meter into 600ohms.Also, a measure of dB is not an absolute value. It is a relative value. It expresses the ratio of two absolute measurements, it doesn't represent an absolute value on it's own. A common "exception" to this is in the audio world, some people equate a 0VU (volume unit) on a VU meter to 0dB - not exactly correct, but it is common enough. This corresponds to 1.228Vrms into a 600ohm load.Again, the manual describes dB measurement, but only in the context of subtracting on dBm measurement from another dBm measurement - which gives a dB result which represents the ratio between the two measurements.

author: crazyguy
author_rank: Regular Contributor
timestamp: March 24, 2017, 04:39:06 am
content: Quote from: mtolesen on March 23, 2017, 03:25:48 pmI don't think there is a adjustable resistance reference load built-in on the meter (50R ~ 8000R), it is just for let the processor know how to compute the dB value.Just like an DSO input attenuator 1:1, 10:1, 100:1, 1000:1 setting. If the setting do not matched with the probe, the reading will be wrong.

author: 2N3055
author_rank: Super Contributor
timestamp: March 24, 2017, 07:53:27 am
content: From your voltmeter datasheet:Input Resistance 600 mV 10 M? or 10 G? selectable6 V,60 V, 600 V and 1000 V Range 10 M? ± 2%What you set in a voltmeter is just a number for calculations, so voltmeter knows how to calculate power (dBm is power) from voltage it's measuring.It is expected that outside circuit is properly terminated. Voltmeter doesn't do termination, it is high impedance. You need to both terminate circuit (with a proper terminating impedance for that circuit) and then tell voltmeter what that impedance is. And then it will be ok.

author: mtolesen
author_rank: Contributor
timestamp: March 24, 2017, 03:16:22 pm
content: Hello All,Thank you so much for your answer to me, yes, i have misunderstood something about this multimeter input resistor, when i come home i will try your suggestions.

author: mtolesen
author_rank: Contributor
timestamp: March 24, 2017, 06:57:43 pm
content: OK, you guys were absolutely right, it was the external termination resistor i was missing, i was misunderstanding the volt meter setting and thought that the resistor setting was the actual input impedance resistor that i was choosing.