"Glenn Booth" wrote in message
...
Hi,

"Don Pearce" wrote in message
...
On Sat, 18 Mar 2006 19:56:17 -0000, "Serge Auckland"
wrote:


"Don Pearce" wrote in message
. ..
On Sat, 18 Mar 2006 17:54:15 GMT, "don" wrote:

dbm is as stated a reference of two like power values to a 1mW
reference
however the impedence does not need to be 600 ohms, it can be any
value
of
ohms as long as both power values are based on the same impedence


Don,

Do you have a reference for this statement? In 34 years in Pro-Audio I
have
never heard it expressed in this way, always referred to 1mW into 600
ohms.


No, it is just one milliwatt - no ohms needed. You have only come
across it in relation to 600 ohms because you have been worked in
audio, and that is all you have been exposed to. If you ever worked in
RF, you would have found exactly the same power, referred to in
exactly the same way in 50, 62.5 and 74 ohms. The power is the same in
all of these - and 600 ohms too.

Agreed. We had the same conversation on uk.r.a back in 2003. I've
quoted a bit of it here as Serge might be interested:

I said:
I don't disagree that the reference must always be given, but for
measurements of power, such as those that reference dBm (dB referenced
to 1mW) surely the impedance is totally redundant? (Unless what one is
really trying to describe is voltage, but using a power ratio to do
so). The 50R says nothing that I can see about the power, it only
allows one to relate the voltage that will be dropped across that
particular impedance/resistance with that dB worth of power being
dissipated.

To which a certain Mr. Lesurf said this:

However, bear in mind two points:

1) That in most cases (in RF at least) the quoted systems will be based
upon assuming the system is impedance matched and then give the power that
will be delivered to the source. Hence quoting the impedance tells the
user
that this is the required matched impedance for optimum power transfer.


2) That in most cases the receiver will tend to be designed to work over a
given voltage range due to finite voltage rails, etc. Hence the impedance
is useful for establishing the voltage levels that must be expected.


It is therefore useful to confirm the assumed impedances. In RF/microwave
we have the annoyance that 50 Ohm is common for system and lab work, but
other impedances like 75 Ohm, etc, crop up for specific purposes/areas.


In principle, though: yes, once you've quoted the signal power in dBm
you've established the power available. You could then use a transformer
to
alter the impedance (and hence signal voltage) if so desired.


At the time, I hadn't considered case (1) that Jim pointed out, as I know

less than nothing about RF.

Who says Usenet doesn't go around in circles?

Regards,

Glenn.

Thanks to Don and Glenn. I've learnt something. Looking through my old
college texts books, I can see you're right. What comes of a narrow
upbringing.

S.