Power Units
.
W (watts) – Basic unit of power; assumes 8 Ω (or user-settable)
load
.
dBm – Decibels relative to 1 milliwatt into 600 Ω (or
user-settable) load
The ATS-1 Access cannot directly measure power. However, power
can be calculated from amplitude and impedance (or resistance), so if
the value of the impedance is provided, a power reading can be
obtained from a voltmeter reading. Both of the power units use this
technique.
Since the old dBm-measuring instruments measured voltage, not
power, they only correctly read dBm when the circuit impedance is
600 Ω. However, since they measure voltage, they correctly read dBu
no matter what the circuit impedance. Those old meters calibrated in
dBm are actually dBu meters.
Unfortunately, the dBm unit is still mistakenly used when the circuit
impedance is well over 600 Ω. Modern equipment manufacturers may
ask that a +4 dBm signal be applied to a device for calibration when
the circuit impedance is 10 kΩ, which would result in a signal
amplitude of 5 volts. This may be so large as to overload the input
circuitry, causing it to clip. What is really desired is a +4 dBu signal,
which is 1.22 volts. This is a much more reasonable level for modern
audio equipment.
So how can you tell which specifications are truly dBm and which are
dBu? As a general rule, you should read ‘dBm’ as ‘dBu’ unless you
know you are working with equipment terminated in 600 Ω. If you are
using 600 Ω equipment, you can use dBm units. You can easily
measure the input impedance of a device using the ATS-1 Access’s
Gen Load function.
If you are testing equipment terminated in high impedances, or if you
are unsure of the input impedance of the device, it is best to select the
minimum impedance for the ATS-1 Access generator and use the dBu
units. In most cases this will be correct, and even in the worst case this
will only be off by a fraction of a dB.
& For information
about the Gen Load
function, see page
4-38.
3 Operation
Operational Overview Units
ATS-1 Access User's Manual 3-21
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