Author Topic: Keysight 3000T why dBV for FFT ? ?  (Read 215 times)

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Offline EE-digger

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Keysight 3000T why dBV for FFT ? ?
« on: August 10, 2019, 09:17:19 pm »
Why, oh why ?

I think every scope I've used expressed FFT amplitude units in dBm in a 50 ohm system.

On my 3000T, I had to look at some signals this morning and was surprised to find amplitude 13.01 dB down.  WTF?  Of course, they are in dBV, with no option to change.

As a sanity check, I fired up my 52486A and there they were ... dBm.

What were they thinking? 
 

Online DaJMasta

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Re: Keysight 3000T why dBV for FFT ? ?
« Reply #1 on: August 10, 2019, 09:21:22 pm »
No option to change seems silly, but since they let you use the FFT function on the 1M inputs, dBm wouldn't really be the right thing to use there.  Do they let you set a reference level and just use dBc?
 

Online JDubU

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Re: Keysight 3000T why dBV for FFT ? ?
« Reply #2 on: August 10, 2019, 09:30:48 pm »
Excerpt from the "Keysight InfiniiVision 3000T X-Series Oscilloscopes User's Guide" (page 97):

"...
FFT (Magnitude) Units

0 dBV is the amplitude of a 1 Vrms sinusoid.

When the FFT source is channel 1 or channel 2 (or channel 3 or 4 on 4-channel
models):

• FFT units will be displayed in dBV when channel units is set to Volts and
channel impedance is set to 1 MΩ or the probe attenuation setting is
something other than 1:1.

• FFT units will be displayed in dBm when channel units is set to Volts, channel
impedance is set to 50Ω, and the probe attenuation setting is 1:1.

For all other FFT sources or when a source channel's units has been set to Amps, FFT units will be displayed as dB.


FFT (Phase) Units
 
In this case, the vertical units are degrees or radians.
..."

Did you find that not to be true?

(Note: edited for 3000T series instead of 3000A series)
« Last Edit: August 10, 2019, 09:44:01 pm by JDubU »
 
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Offline EE-digger

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Re: Keysight 3000T why dBV for FFT ? ?
« Reply #3 on: August 10, 2019, 10:03:04 pm »
Excerpt from the "Keysight InfiniiVision 3000T X-Series Oscilloscopes User's Guide" (page 97):

"
• FFT units will be displayed in dBm when channel units is set to Volts, channel
impedance is set to 50Ω, and the probe attenuation setting is 1:1.

..."

Did you find that not to be true?

(Note: edited for 3000T series instead of 3000A series)

You win the prize.  The channel is 50 ohm but because I'm running an active probe with 10:1 attenuation, dBV took over.  I actually RTFM but missed that  |O.

Thank you!

DaJMasta - I didn't see any other options and the math function does not allow FFT as an argument, otherwise Mx + B would be handy.
 


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