[Elecraft] XG1

[Jim Brown]

On Thu, 29 Jan 2004 19:53:32 -0600, Bob-AG5Q wrote:

>dBm means the power is measured relative to one milliwatt.  A one milliwatt
>signal is zero dBm and a 1 watt signal would be +30 dBm.

Yes, but:

Remember that dB is simply 10 log of the ratio of two powers (or 20 log
the ratio of two voltages). When you stick the "m" at the end, you
define one of those powers  (the denominator) as 1 mW.  

There are many impedances to which the 1 mW can be referenced. To
define the voltage, you must also define the impedance. For example,
the most commonly used reference in pro audio used to be 600 ohms, and,
in the old days, all audio circuits were terminated, mostly in 600 ohms
(although mic circuits commonly used 50, 150, and 250 ohms). Nowadays,
audio circuits are NEVER terminated, and pro audio uses dBu, which is
20 log of the voltage referenced to 0.775 volts (which happens to be
the voltage that corresponds to 1 mW in 600 ohms). 

By unterminated, I mean that modern audio equipment has a very low
output impedance and drives some impedance that is at least 10X greater
so that the source can be seen as a constant voltage source, and the
system can be best analyzed on the basis of voltage gain.  Example --
the output impedance of the speaker amplifier in the K2 is probably on
the order of a tenth of an ohm, and it drives a nominal 4 ohm load.  A
low impedance pro microphone typically looks like a 150 ohm source, and
it drives a preamp with an input impedance of 750 - 1,500 ohms. Most
pro "line level" inputs look like a 10K - 50Kohm load; a consumer
product typically looks like 50K - 500K ohms. 

There are many references for dB units, each defined by a specific
application or industry, and with a reference that makes sense for that
industry. You can, for example, use 1 microvolt as a reference, and not
specify the impedance. The telephone companies have several common
references. RF folks have others. 

There are also many acoustic dB references that define loudness. dBSPL
is one of them.  

Jim Brown  K9YC