Questions about DMM Impedance and dBm Measurements - Page 1
Forum section: electronics / beginners

author: Electro Fan
author_rank: Super Contributor
timestamp: November 12, 2019, 11:59:33 pm
content: Looking for a discussion/tutorial on the typical impedance settings of DMMs (handheld and bench), and more to the point - to the extent a DMM can display dBm - what are the typical abilities of decent DMMs to set DMM impedance in manner that facilitates dBm readings for various DUTs?Along these lines, any discussion that explains why the standard reference impedance of 600 Ohms is used with dBm measurements would be helpful. I think I get the explanation in the addendum below but maybe there is more to it. Maybe it has to do with the difference between analog meters and digital meters? Just guessing... For example, how does the 600 Ohm "standard" relate to typical bench DMMs that might offer dBm measurements but seemingly only offer settings of 50 ohms and High Z...? For example, on an Agilent 32500B series DMM the DMM can be set to either 50 Ohms or High Z but in voltage readings the Agilent DMM can display dBm if it is set to 50 Ohms but not High Z (maybe when amplitude units are set to dBm and the DMM is set for 50 Ohms the DMM is really overriding the 50 Ohm setting and setting itself to 600 Ohms? But if so, why couldn't it override the High Z setting if the user selects dBm amplitude units?)On a practical product level, what are some of the most popular entry level handheld DMMs that provide dBm readings and how flexible are they with respect to accommodating various impedances? Or is it more likely that a bench DMM should be used to accommodate dBm measurements?FWIW, what started all this was that I installed a FA-2 counter and it happens to measure and display dBm, and the dBm reading (after the FA-2 was calibrated) lined up nicely with my Agilent signal generator’s dBm reading. So, next I figured I’d plug the output of the signal generator into a Radio Shack DMM that supports dBm, but the RS DMM dBm reading is about half of what the sig gen says – so I figured it’s an impedance mismatch issue but I can’t quite figure out what’s what.Addendum: I saw the article in this link which seemed to make sense.https://www.globalspec.com/learnmore/labware_test_measurement/multimeters_electrical_test_meters/analog_dbm_meters(but I think it has some typos that I tried to fix in the second version below).Analog dBm meters are instruments that measure signal power. They display values on a dial, usually with a needle or moving pointer. Signal power, which can be expressed in decibels (dB) when the unit of reference is known, is measured in dBm to represent the decibels referenced to a power level of one milliwatt (mW). Expressed as a logarithm, dBm = 10 log (P/P Ref), where P is the measured power and P Ref is the reference power of 1 mW. Analog dBm meters that convert dBm into voltage (V) measure impedance in ohms (W) so that any voltage across any resistance (R) that results in 1 mW equals 0 dBm. Expressed as an equation, 0 dBm = V^2/R = 1 mW, where V is the voltage and R is the circuit resistance. Consequently, 0.775 V across 600Wis 0 dBm, 1 V across 1,000 isW0 dBm, and both voltages result in 1 mW.Maybe instead of referring to impedance in ohms as “W” it should have just said Ohms in which case it would read as follows:Analog dBm meters are instruments that measure signal power. They display values on a dial, usually with a needle or moving pointer. Signal power, which can be expressed in decibels (dB) when the unit of reference is known, is measured in dBm to represent the decibels referenced to a power level of one milliwatt (mW). Expressed as a logarithm, dBm = 10 log (P/P Ref), where P is the measured power and P Ref is the reference power of 1 mW. Analog dBm meters that convert dBm into voltage (V) measure impedance in ohms (Ohms) so that any voltage across any resistance (R) that results in 1 mW equals 0 dBm. Expressed as an equation, 0 dBm = V^2/R = 1 mW, where V is the voltage and R is the circuit resistance. Consequently, 0.775 V across 600 Ohms is 0 dBm, 1 V across 1,000 is Ohms 0 dBm, and both voltages result in 1 mW.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 13, 2019, 01:12:49 am
content: Further investigation and findings:When the Agilent sig gen is set for Hi Z, 200 Hz, and 1.414 Vrms the Radio Shack DMM reads 200 Hz, 1.412 V, and 5.2 dBm.(The Agilent won't display dBm when set to Hi Z - not sure why.) So maybe this means the Radio Shack is set to 600 Ohms when reading dBm?- If this happens to be correct, maybe what I am looking for is a DMM (handheld or bench) that would enable the dBm setting to be tied to a particular impedance (such as 50 Ohms or 1 MOhm) - something user selectable.(When the Agilent sig gen is set for 50 Ohms, 200 Hz, and 1.414 Vrms it will show 16.02 dBm; the Radio Shack DMM reads 200 Hz, 2.836 V, and 11.3 dBm. Again, this seems to indicate that the Radio Shack has an impedance of 600 Ohms when measuring dBm.)What I think I'm looking for is a low cost but reasonably accurate way of measuring dBm that has the ability to set an impedance matching various DUTs.... (I know I could just measure Volts and cacluate but I'd kind of like a measuring device that displays dBm.)Thx in advance for any educational info

author: ve7xen
author_rank: Super Contributor
timestamp: November 13, 2019, 01:53:01 am
content: This feature is typically designed to be used in 600Ω audio systems, and doesn't usually provide a 600Ω load impedance, just calculates dBm assuming a 600Ω load. If the reference impedance isn't configurable, it's probably safe to assume it's using 600Ω.Some meters do let you set the reference impedance though, for one the Keithley 2000/2015. I'm not aware of any that actually present a variable impedance load, though, you'd probably need something like an electronic load to get that functionality.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 13, 2019, 02:22:12 am
content: I'm not familiar with the Keithley 2000/2015 but I saw in the Keithley 6500 manual that the dBm reference value can be set from 1-9999 Ohms. Just curious why 9999 is the upper limit rather than say 10 MOhms (or higher)?Edit: In the Keysight 34465A manual regarding dBm scaling it says "the reference resistance value (Ref R) may be 50, 75, 93, 110, 124, 125, 135, 150, 250, 300, 500, 600 (default), 800, 900, 1000, 1200,or 8000Ω." Not quite as nice as user selectable through the range like on the DMM6500 but a similar max value (8000 Ohms), so there must be a practical reason for hitting the threshold around 10k. Just not sure what the reason is...

author: vk6zgo
author_rank: Super Contributor
timestamp: November 13, 2019, 03:06:39 am
content: Quote from: Electro Fan on November 12, 2019, 11:59:33 pmYes---but!Most such instruments actually measure the voltage across a known impedance, corresponding to a given power level.QuotedBm is only valid when referring to power levels which are measured at the same impedanceYes, you can have many dBs of power gain between points at different impedances, but you can't quote those power levels in dBmQuoteThis is gobbldegook!I think they are trying to say:-"Analog dBm meters actually read the voltage across a given impedance (Z) in Ohms.For convenience they normally use a resistance (R) to provide this value (if, & only if, there are no reactive components in an impedance, we can simplify to Z= R)Commonly provided values of R are 50 & 600 Ohms."QuoteI don't know why they picked 1000 Ohms, apart from easy maths.There may be some one 1000 Ohm systems around, but I've never seen one-----there are 1200 Ohm ones, though.A more usual one would be"0.224 (approx) volts across 50 Ohms to result in 1mW."As for the use of "W"This is a common and annoying failure of attempts by many websites to reproduce the "" symbol.The (false) logic involved seems to go like this:-"An 'Ohms' symbol is a upper case 'omega' symbol, so we can use a lower case 'w' which looks a bit like a lower case "omega" instead.On second thoughts, better make it an upper case 'W'."Fairly obviously, whoever thought up this sequence was not aware of the use of lower case "omega" & upper & lower case "W" for other things in Electronics!A pretty dumb mistake in something that purports to be an Electronics text!

author: Electro Fan
author_rank: Super Contributor
timestamp: November 13, 2019, 09:07:01 pm
content: vk6zgo - Thanks for all the info. I think the key point is:"dBm is only valid when referring to power levels which are measured at the same impedance"So, just 2 more questions.1. Is it correct that two power measurements could be taken (in two different circuits), each using a different reference impedance unique to it's circuit, and thereby establish a value in dBm for each circuit, at which point the two measurements could be compared with one another in dB?- I think this is what the Siglent manual is saying on page 84-85....?http://siglentna.com/wp-content/uploads/dlm_uploads/2017/10/SDM3055x_UserManual_UM06035-E02A.pdf2. Still trying to understand why DMMs seem to max out the user selectable impedance reference value at around 8,000 - 10k Ohms.... rather than going to 10M Ohms or higher?Thx

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2019, 09:52:42 pm
content: With higher impedance one does not generally use impedance matching. The 10 M are more used hoping that this is much higher than the source resistance. With only slightly higher frequency one can not ignore the capacitance - so the AC impedance is rarely at 10 M.The 8 to 10 K upper limit could be just there to avoid using kOhms. Even 10 K does not really makes much sense for the reference impedance. The important ones are 50 Ohms, 75 Ohms and 600 Ohms.

author: ve7xen
author_rank: Super Contributor
timestamp: November 13, 2019, 10:16:09 pm
content: All a multimeter in dBm mode is doing is measuring the voltage andassumingthat the load impedance, is the reference one you specify. It does notprovidethe load impedance, so assumes that you're either taking a measurement across the load, or that the system has feedback and will maintain that voltage with a load applied.It's just measuring voltage and doing:\$\log_{10} \frac{(V_{in}^2)/Z_{ref}}{1mW}\$You can do this with a calculator, or probably with many multimeters that have math functionality but no explicit dBm mode.I assume larger impedances aren't supported because the numbers get very very tiny, and pretty much meaningless.QuoteI suppose you could, but I don't know why you'd want to. The measurement should indeed meaningfully reflect the power ratio between the two different loads though.

author: TimFox
author_rank: Super Contributor
timestamp: November 14, 2019, 02:22:16 pm
content: In audio systems, it is common to express voltages measured across arbitrary impedances in "dBu", where the reference level is the voltage that makes 1 mW in a 600 ohm load. Therefore, dBu = 20 log (V/[0.775 V])

author: Electro Fan
author_rank: Super Contributor
timestamp: January 15, 2021, 10:14:46 pm
content: Old thread... I nowIn continuing the search for a reliable and convenient meter to measure dBm for various impedances including 50 ohms, it looks like a Brymen 789 should do the trick? Yes?https://brymenmeter.com/brymen-bm789-multimeter.html

author: srb1954
author_rank: Super Contributor
timestamp: January 16, 2021, 06:15:03 am
content: Quote from: vk6zgo on November 13, 2019, 03:06:39 amI believe the 600impedance figure is historical and it came from the early days of telephony where the open wire telephone lines had a nominal characteristic impedance around about this figure.Configuring the open wire lines for an impedance of 1000for simplification of the maths would not be feasible as it would require much thinner conductors for the open wire lines which would result in excessive line loss.

author: radiolistener
author_rank: Super Contributor
timestamp: January 16, 2021, 10:20:42 am
content: Quote from: Electro Fan on November 13, 2019, 01:12:49 amYes, you can select impedance from the list with RANGE button on the Brymen DMM when you measure dBm value. Note it shows power for a loaded source. There is no load in DMM, so you needs to attach some dummy load to get correct values.

author: CaptDon
author_rank: Super Contributor
timestamp: January 17, 2021, 02:49:11 am
content: DBM meters are assumed to be 'bridging' which means they are in fact very high input impedance meters, basically 10meg like any other DMM normally has. So when you select an impedance you haven't changed anything about the input impedance of the meter, you have only changed the way it scales and converts the voltage it sees. Normally most DBM comparisons are at the same impedance. For instance you may have an R.F. amplifier with a 30DBm input and a 50DBm output and it has a gain of 20db (in this case it is simply a 20db gain NOT called a 20DBm gain because it is 20db of gain no matter what the input level (within reason of course). As I recall 30DBm is 1 watt 40=10 watts 50=100 watts and 60DBm is a kilowatt. When I was working on radars we measured in DBK decibels referenced to a kilowatt. 6DBk peak (4kw pulse) and 12DBk (12kw pulse) were very common for small aircraft weather radar and pleasurecraft boat search radars.