Homepage - RF Cafe
Webmaster: Kirt Blattenberger | KB3UON | Sitemap | ©1996-2014     Visit RF Cafe on
      Menu below is just a small sample of what is here!          Visit RF Cafe on Facebook Visit RF Cafe on Twitter Visit RF Cafe on LinkedIn
Custom Search
More than 10,000 searchable pages indexed.
•−•  ••−•    −•−•  •−  ••−•  •
RF Cafe Morse Code >Hear It<

dBm to Vrms - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


 Post subject: dBm to Vrms
Posted: Sat Oct 28, 2006 12:38 pm 
 
Captain
User avatar

Joined: Sat Mar 19, 2005 3:20 pm
Posts: 5
Dear all,
I generated a 50 MHz, -10 dBm sine-wave using a Rohde & Schwarz RF signal generator. I checked this using an Agilent spectrum analyser and, as you'd expect, I get my tone at 50 MHz with peak amplitude of -10 dBm. However, when I input my RF signal to a TDS 210 Tecktronix Digital Oscilloscope, the peak-to-peak voltage is 336 mV and the Cyc RMS voltage is 117 mV. Now, on the TDS210, Cyc RMS calculates the RMS voltage over one complete cycle of the waveform.

When I tried to calculate this I got a different answer:

P = -10 dBm, which is equivalent to 100 uW
Now, P = V^2 / R
I've taken R = 50 ohms
Therefore Vrms = sqrt(P x R)

That is: Vrms = sqrt(100 uW x 50 ohms)

So, Vrms = 70.7 mV

This is different to the 117 mV measured on the oscilloscope??


 
   
 
 Post subject:
Posted: Mon Oct 30, 2006 4:48 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
3 possibilities come to mind
1) error in calibration of the scope, particularly at the measurement frequency. If the Y video amp was 3 dB down at the measuring frequency, there is a 3dB error immediately
2) the impedance was higher than 50 ohms.
3) presence of harmonics in the generated waveform.






Posted  11/12/2012
A Disruptive Web Presence

Custom Search
Over 10,000 pages indexed! (none duped or pirated)

Read About RF Cafe
Webmaster: Kirt Blattenberger
    KB3UON

RF Cafe Software

RF Cascade Workbook
RF Cascade Workbook is a very extensive system cascaded component Excel workbook that includes the standard Gain, NF, IP2, IP3, Psat calculations, input & output VSWR, noise BW, min/max tolerance, DC power cauculations, graphing of all RF parameters, and has a graphical block diagram tool. An extensive User's Guide is also included. - Only $35.
RF system analysis including
frequency conversion & filters

RF & EE Symbols Word
RF Stencils for Visio

Product & Service Directory
Personally Selected Manufacturers
RF Cafe T-Shirts & Mugs

RF Cafe Software

Calculator Workbook
RF Workbench
Smith Chart™ for Visio
Smith Chart™ for Excel
Your RF Cafe Progenitor & Webmaster
Click here to read about RF CafeKirt Blattenberger... single-handedly redefining what an
                                 engineering website should be.

View the YouTube RF Cafe Intro Video Carpe Diem! (Seize the Day!)

5CCG (5th MOB): My USAF radar shop

Airplanes and Rockets: My personal hobby website

Equine Kingdom: My daughter Sally's horse riding website