dB vs. dBm

[ssylee]

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

Thanks in advance!

 

[John Fields]

On Fri, 24 Oct 2008 12:38:02 -0700 (PDT), ssylee <stanigator@gmail.com>
wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

---
You just add them and keep the dBm unit.

For example, if your spectrum analyzer shows a 0dBm signal coming out of
the attenuator, then the signal at the input of the attenuator will be
+20dBm, or 100 milliwatts

JF

 

[terryS]

On Oct 24, 5:46 pm, John Fields <jfie...@austininstruments.com> wrote:

On Fri, 24 Oct 2008 12:38:02 -0700 (PDT), ssylee <staniga...@gmail.com
wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

---
You just add them and keep the dBm unit.

For example, if your spectrum analyzer shows a 0dBm signal coming out of
the attenuator, then the signal at the input of the attenuator will be
+20dBm, or 100 milliwatts

JF

I agree. dBm is a measurement related to Zero dBm being one milliwatt.
Decibels are a logarithmic ratio.

So 100 milliwatts compared to 1 milliwatt.
The logarithm of 100 = 2.0; so 100 milliwatts would be 2 Bels larger/
louder/stronger etc.

Since we are talking 'decibels', 2 bels = 20 decibels (tenths of a
Bel).

Btw a quick way to think about dBs is as follows.
Every time a power ratio doubles. For example from one m.watt to two
m.watts, or from 50 to 100 m.watts it is aratio of approx. 3 decibels.
Derived from logarithm of two = 0.3xxx (approx).

So power ratio:
1 to 2 = 3 dB.
2 to 4 = 6 dB.
4 to 8 = 9 dB.
8 to 16 12 dB.
16 to 32 15 dB.
64 18 dB.(Getting there!)
20 dB.*This 20 dB is pwr. ratio 100 times more/less.
128 21dB.
256 24 dB.
512 27 dB.
& 1024 = 30 dB.

 

[Eeyore]

ssylee wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

You also need to know the impedance of the circuit. Typically 50, 75 or
600 ohms.

dB is a RATIO (either power or voltage or SPL)

dBm is a measure of POWER

I think wikipedia covers it quite well.
http://en.wikipedia.org/wiki/Decibel

Graham

 

[Eeyore]

ssylee wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal

One thing puzzles me here. Attenuators REDUCE signal level NOT add to it.
http://en.wikipedia.org/wiki/Attenuator_(electronics)

Can you clarify what's going on here please ?

Graham

 

[John Fields]

On Fri, 24 Oct 2008 23:17:25 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:

ssylee wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

You also need to know the impedance of the circuit. Typically 50, 75 or
600 ohms.

---
Nope.

As long as the impedances are the same, throughout, it doesn't matter
unless you're concerned about the voltages and/or currents, which the OP
clearly wasn't.
JF

 

[SUREN]

On Oct 25, 3:17 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

ssylee wrote:
Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

You also need to know the impedance of the circuit. Typically 50, 75 or
600 ohms.

dB is a RATIO (either power or voltage or SPL)

dBm is a measure of POWER

I think wikipedia covers it quite well.http://en.wikipedia.org/wiki/Decibel

Graham

same as the friends aa toldd
a c

 

[Eeyore]

John Fields wrote:

Eeyore wrote:
ssylee wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal
measured on a spectrum analyzer, which has the units of dBm. From
reading some search results on google, I'm aware the dB is a measure
of relative power where dBm is a measure of power relative to 1 mW. In
order to convert between the two units, do I have to know about one of
the power involved in the dB unit, which is the attenuator?

You also need to know the impedance of the circuit. Typically 50, 75 or
600 ohms.

---
Nope.

As long as the impedances are the same, throughout, it doesn't matter
unless you're concerned about the voltages and/or currents, which the OP
clearly wasn't.

Let's say it's advisable to know that the impedances have to be the same.

Graham

 

[Jasen Betts]

On 2008-10-25, Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:

ssylee wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal

One thing puzzles me here. Attenuators REDUCE signal level NOT add to it.
http://en.wikipedia.org/wiki/Attenuator_(electronics)

Can you clarify what's going on here please ?

me guessing:
OP wants to know how much to add to the readings his device gives to
get the unattentuated value.

Bye.
Jasen

 

[Eeyore]

Jasen Betts wrote:

Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:
ssylee wrote:

Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal

One thing puzzles me here. Attenuators REDUCE signal level NOT add to it.
http://en.wikipedia.org/wiki/Attenuator_(electronics)

Can you clarify what's going on here please ?

me guessing:
OP wants to know how much to add to the readings his device gives to
get the unattentuated value.

Could be. It wasn't clear to me.

Graham

 

[langwadt@fonz.dk]

On 25 Okt., 02:18, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

ssylee wrote:
Not sure if this is an appropriate group to ask this question.
Basically, I'm trying to add 20 dB (from an attenuator) to a signal

One thing puzzles me here. Attenuators REDUCE signal level NOT add to it.http://en.wikipedia.org/wiki/Attenuator_(electronics)

Can you clarify what's going on here please ?

Graham

He's measuring something on a spectrum analyser through a 20dB
attenuator and now has
to add the 20dB to the values seen on the analyser to get the true
values

-Lasse