# Decibel help

Discussion in 'Homework Help' started by c2morg, Feb 4, 2004.

1. ### c2morg Thread Starter New Member

Feb 4, 2004
2
0
hi everyone, im stuggling to keep my head above water in the course im doing, hope you can help !

the questions goes - oh and im not looking for an answer, that will get me no where, but if someone could walk through it, that would be amazing !!

"Calculate the voltage measured across a 600 ohms load for power level of 0dBm"

Im ok with working out directly each of ohms laws, and pretty ok with minor decibel stuff, but I just dont even know where to begin with this !

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Are you familiar with:

Votage Ratio in dB = 20*log(base 10)[V1/V2]

I am trying to remember whether the voltage ratio is taken as a reference to dBw or dBm, at which case you would have to convert from dBm in your example, to dBw.

For 600dBm:

dBw = 10*log(base 10)(0.6) = -2.218 dBw

It is along time since I did this so I'm a little rusty :blink:

3. ### c2morg Thread Starter New Member

Feb 4, 2004
2
0
cheers m8

been taking a stab at it, i remember the formula

right

remember that P= V2 / R (voltage squared) / resistance

swap it too v2 = p x r power X resistance

uve got the resistance ®600

times

power(p) 1mw-(reference) (0dBm is equal and a referance to 1mW remember!)

=0.6v

square routed=.775v

anyone verify im completly wrong and dave is right ?

4. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
I'll go with your answer c2morg, I think I'm venturing up the wrong tree with my response. And I can't see a problem with your calculation.

Good effort