WD2XES Low Frequency Exciter
What it Does:
Many Part 15 operators have the entire transmitter located at the feed point
of the antenna. While this simplifies the transmitter design, it does make it
more difficult to maintain and accurately change the frequency, or to switch to
different modes. For the higher power Part 5 Experimental service, it makes much more
sense to have the exciter and transmitter in "the shack," with a conventional feedline
to an outside antenna tuning unit. My exciter is a rack-mount unit mounted in a rack
above the power amplifier. It provides a linear 10 milliwatt output into 50 ohms, and
runs through a bandpass filter into the power amplifier.
The exciter uses an Analog Devices AD9850 Direct Digital Synthesis (DDS) chip.
The chip is clocked by another AD9850 chip! The idea is to take an external 10 MHz
signal from a GPS-disciplined oscillator, and generate 4294.967295 kHz. This is
a "magic number" for the 32-bit DDS chip, and allows precise frequency entry down
to 1 milliHertz without any rounding-off errors. The process is not exact, as the
real "magic number" is actually 4294.967296 kHz. Unfortunately, the 32-bit math does
round off that last digit. The error in the LF range is on the order of 34 microHertz,
which is quite acceptable!
The final DDS chip is controlled by an Atmel 89S8252 microcontroller. Programming of
the controller is done in assembly language, and the printout is getting pretty
thick. A 12-button keypad and 2-line LCD screen are mounted on the front panel of
the exciter. They provide a menu system which allows selection of frequency, mode,
keying speed and ID. No computer is necessary to run the exciter/transmitter in
CW, QRSS, DFCW or graphical text modes. The frequency may be entered with the
keypad down to 10 millihertz. The transmitter can be keyed manually through a jack
on the rear panel, if computer control is necessary.
The basic exciter presently allows CW at 3, 6 and 12 wpm, QRSS at 3, 10, 30, 60, 90
and 120 second dots, DFCW at those QRSS speeds, and some experimental FSK graphical
modes. As the DDS chip easily allows 0, 90, 180, and 270 degree phase shifts, many
other possibilities exist if the timing requirements can be met.
The rackmount case also includes a phasing-type SSB exciter. Audio from a rear
panel jack is fed through an excellent 90 degree active phase shift network to
two SBL-3 doubly balanced modulators. In this mode, RF from the DDS exciter is
routed through a 90 degree phase splitter and sent to the DBM's. The exciter is
wired for upper-sideband operation, and the carrier and lower sideband are suppressed
more than 40 dB. In normal operation, audio centered on 800 Hz from a computer
sound card is applied to the audio jack. The DDS frequency is then set 800 Hz
lower than the desired output frequency. This allows transmission of WOLF, PSKAM10,
Jason and a variety of other modes. Since the present WD2XES power amplifier is linear,
all of these modes can be transmitted in a reasonable bandwidth. Use of a switching amplifier
would require some extra attention to the shaping of the keying.
The DDS output is very low-level. Following a 200 kHz low pass filter, a common-base
stage drives an LM6321, providing a -6 dBm 50 ohm output for the phasing exciter.
The output stage is an MJE181/171 pair that provides +10 dBm. An external tunable bandpass filter
with a 3 dB BW of 1.6 kHz is used between the exciter and the power amplifier. This helps to remove
some of the synthsizer artifacts in the LF range.
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