SRK Electronics MFTX100, Руководство по эксплуатации и обслуживанию, Страница: 26 / 100

MFTX AM TRANSMITTERS
SRK Electronics 26
8.1 RF Functions
8.1.1 Set RF power
The RF output of the transmitter is controllable in the range 10 to 100% of
maximum rated power. This is achieved by having a SET POWER, calibrated in
watts.
SET POWER is the nominal carrier rest power that the transmitter will generate
into a non-reflective load. Loads that are not exactly 50
± j0 Ω may result in
slightly different output powers.
SET POWER and FORWARD POWER are related in the following way; forward
power is the actual average forward power at any given moment. The nature of
AM means that the average output power increases with greater modulation
depth and is also highly dependent on the crest factor of the audio modulating
the transmitter. The set power parameter allows the user to set the output RF
power generated by the transmitter at 0% modulation (sometimes referred to as
“carrier rest”). Thus under program audio conditions the forward power will be
fluctuating but always equal to, or greater than, the set power.
Similarly, reflected power is dependent upon forward power and the
characteristics of the antenna system. Ideally, reflected power should read zero
watts.
8.1.2 Forward and reflected power metering
The forward and reflected power meters of the MFTX series are true average
reading. This is unlike normal transmitter power meters which are actually
average
voltage
reading.
Average power measurement gives a far more accurate indication of actual
power delivered to the antenna system. The following example shows why this is
the case.
Consider the two RF waveforms shown below. The one on the left is a carrier
with no modulation. The one on the right is the same carrier with 100%
sqaurewave modulation (time scales are exaggerated for the sake of clarity).