Western Electric 407A-4, Инструкция по эксплуатации

Страница: 72 / 136

the Transmitter
With the final stage adjusted to the pro per operating
conditions at the unmodulate d carrier output, it is necess ary
next to ob serve the operati on with modu lati on applie d.
For ideal distortionles s operati on with no carrier shift
the antenna current should rise 2 2 . 5 per cent at 1 00 per cent
modulation. In transmitters not employing negative fee dback,
a fai lure of the antenna current to ris e to this extent ordi­
narily indi cates distortion in the modu lated wave . With the
dis torti on removed by feedback, the only re sult of such "car­
rier shift" is a slight decrease in transmitted s ignal ; for
example , if the antenna cu rrent rises only 19 per cent, the
loss in transmitted s ignal with a dis tortionless wave is only
0. 25 db. The tendency for carrier shift to occur depends on
local conditions , such as instantaneous line regulation, as
well as on the adjustment of the transmitter .
This equipment is des igned to permit 100 pe r cent modu­
lation with no carrier shift, provided the line-voltage regu­
lation is such that the plate voltage on the power ampl ifier
does not change more than 2 pe r cent between the modu lated
and unmodulated condition. It is left to the judge!ll3nt of
the operator, however, whether the circuits should b e ad­
justed for z ero carrier shift or whether a small amount of
carrie r shi� ( downward ) should be tolerated for the purpose
of increasing tube life through a reduction in filament
voltage .
The adjustment for the des ired antenna curr ent rise in­
volves adj us tment of filament voltages on water-co oled tub es
and proper s etting of the coupling circuit between the modu­
lating amplifier and power amplifier, which determines the
load impedance for the former . A lower load impedance ( i . e.,
a higher plate current ) on the modulating amplifie r in­
creas es the antenna current ris e, provided the filament
6 5