Page 79
FT
DX
5000 O
PERATING
M
ANUAL
E
NHANCING
T
RANSMIT
S
IGNAL
Q
UALITY
A
DVICE
During Class-A operation, 10-Amps of Bias current will be flowing, regardless of the modulation level that leads to
actual power output. Therefore, if the ambient temperature in your operation location is high, the transceiver tempera-
ture may rise as well, due to the high bias level (which must be dissipated as heat). Depending on the temperature, you
may wish to reduce the BIAS level using menu item “
169 TGEN BIAS
”, to reduce the amount of heat being generated.
When the optional
DMU-2000
Data Management Unit and a video monitor are connected, you can monitor the heat
sink temperature on the video monitor; thus, you can always be aware of a rise in temperature during Class-A operation.
Normally, the temperature is below 80 °C. If the temperature rises to near or above this value, however, we recommend
you adjust the BIAS level toward Class-AB via menu item “
169 TGEN BIAS
” (decrease the numerical value to reduce
the heat being dissipated).
An innovative aspect of the “Class-A” mode is that the actual power output is always limited to 75 Watts. So even
though you might adjust the BIAS in the direction of Class-AB operation, the power output will not rise; this eliminates
the need to re-tune your linear amplifier, if used.
Q
UICK
P
OINT
Class-A operation provides a significant improvement in transmitter distortion suppression. During Class-A operation,
the 3rd-order IMD products are typically suppressed 45 dB. The 5th- and higher-order IMD products that can cause
“splatter” and interfere with others, will typically be suppressed 70 dB or more.
If you are using a linear amplifier such as the
VL-1000
, the low distortion produced by the
FT
DX
5000
’s transmitter
means these intermodulation distortion products will not exist to be amplified by your linear.
L
OW
- D
ISTORTION
CLASS-A O
PERATION
(
SSB M
ODE
)
The High-Power 200-Watt Final Amplifier Stage of the
FT
DX
5000
utilizes a pair of ST Micro Electronics Corp. SD2931
MOSFET devices operating at 50 Volts. The push-pull configuration provides low distortion along with high power
output. The 92 mm thermostatically-controlled cooling fan directs forced air across the heat sink, when triggered by a
rise in heat sink temperature.
Class-AB 200W PEP IMD
Class-A 75W PEP IMD
