Brymen dBm Readings - Page 1
Forum section: electronics / testgear

author: Electro Fan
author_rank: Super Contributor
timestamp: February 05, 2021, 08:13:09 pm
content: Just checking to see if any Brymen DMM users can confirm your dBm readings are as expected.I'm using a Brymen 789, feeding it with signals from a Keysight 33522B generator. FWIW, the Keysight will only allow amplitude to be set in dBm units when the output channel is set to 50 ohm (not 1M ohm). When using the Keysight 50 ohm output setting the Brymen only agrees with the Keysight when the Brymen is set for 200 ohms. At 200 ohms on the Brymen DMM and the Keysight generator agree with each other to within .01 dBm (pretty good, good enough). But when the Brymen is set for 50 ohms the Brymen reads 6 dBm too high. Maybe there is a voltage divider issue I'm missing?Thx

author: bdunham7
author_rank: Super Contributor
timestamp: February 05, 2021, 08:19:44 pm
content: Yes, you're missing the 50 ohms!Put a 50R resistor across the circuit somewhere.

author: Electro Fan
author_rank: Super Contributor
timestamp: February 05, 2021, 09:01:02 pm
content: Quote from: bdunham7 on February 05, 2021, 08:19:44 pmOk, makes sense.I guess I faked myself out with my initial test use case of the DMM.....I wanted to use a DMM that would measure dBm. So I ran the generator into a counter that displays dBm along with frequency with both the generator and counter set for 50 ohms. They agreed on dBm (and frequency), so next I ran the generator into the DMM - but there was no dBm agreement with the DMM set for 50 ohms. As you pointed out, I had failed to account for no 50 ohm load.So next when I added the 50 ohms per your suggestion (with a 50 ohm though terminator) everything snapped into agreement.Thx for getting me fixed up.

author: TimFox
author_rank: Super Contributor
timestamp: February 05, 2021, 10:19:55 pm
content: By the way, another common voltage unit is "dBu", where 0 dBu is the voltage across a 600 ohm load that would dissipate 1 mW. However, this unit is used to measure voltage at any arbitrary impedance, while "dBm" should be reserved for power in the actual reference impedance (e.g., 50 or 600 ohms).

author: Electro Fan
author_rank: Super Contributor
timestamp: February 05, 2021, 11:18:02 pm
content: Quote from: TimFox on February 05, 2021, 10:19:55 pmRoger that. I have other meters that are fixed at a 600 ohms. One of the nice features of the 789 is that the impedance has several settings from 4 - 1200 ohms.A little bit off topic: When you push the button to change the impedance it advances through a loop of available settings, and it stays on the last setting if you leave dBm and return to dBm (which is very good, of course). When you return to the impedance function it very briefly flashes the impedance setting you most recently selected but once you are in dBm if you hit the impedance selection button the first press will start advancing away from the set point and force you to cycle through the loop. A small enhancement to the user interface would be that when you go into dBm it should show you the current impedance setting and leave it displayed while you are in dBm. Also, (and this might be early user error, I just started using it) but I haven't figured out the relationship between frequency and dBm; it seems to be most accurate on dBm within it's preferred frequency range. Aside from dBm, on the frequency measurement spec it supposedly can measure 5 Hz to 1 MHz but the display seems most useful up to 99,999 Hz with it's 5 digit display (actually for some reason after 99,998 Hz it gives up the last digit and starts rounding to 100 kHz. On the other hand as a bonus it will display up to 5.000 MHz.) Overall when compared against other meters and simple standards it seems very accurate and it has a bunch of functions. It's very similar in size to the Fluke 179 and for about 2/3s the price it seems very competitive on accuracy and highly competitive on features.

author: Electro Fan
author_rank: Super Contributor
timestamp: February 13, 2021, 04:03:41 am
content: Another one a little off topic.Had a chance to compare the Brymen 789 to a Keysight 34465A. The Brymen is a keeper.