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MFJ-1278B MULTI-MODE ADVANCED OPERATION
NOTE: Regardless of the type of modem, whether or not the modem has audio filtering built
in, 300 baud AFSK modem performance on a High Frequency linear mode (SSB as
opposed to NBFM) radio channel will NOT be optimum UNLESS a filter of
approximately 500 Hz bandwidth is used in the radio IF strip.
There are two reasons why this is the case. First, for the filtering to be fully effective, it has
to precede the first hard limiter in the system. This limiter is typically located in the first
stage of the demodulator. Second, no filter at audio can prevent an off channel interfering
signal from capturing the receiver AGC system and causing wide variation in the level of the
audio presented to the demodulator. All demodulators are affected by audio level variations.
If the user uses a narrow filter, say approximately 500 Hz. in the radio for HF Packet and
RTTY operations, then it may be necessary to use a tone pair centered on the radio's filter,
rather than the "standard" 2125/2295 or 1600/1800Hz. pairs. It is STRONGLY
RECOMMENDED that a narrow filter be used for HF Packet and RTTY operations. If the
radio lacks IF shift capability, this will almost certainly be necessary. If this is the case, it
will be necessary to determine the center frequency of the audio spectrum output from the
receiver when the narrow filter is selected and the radio is in the LSB mode.
Most radios that provide for direct FSK RTTY operation use the radio's narrow CW filter if
installed. The Kenwood TS-820 is one example. Packet operation using direct FSK is a
viable mode but extreme caution should be exercised to make sure TX and RX frequencies
are identical and that the transmitted frequency pair is centered in the radio's 500 Hz filter
passband. Once the user determines the center frequency for the particular radio's FSK
mode, the the user should use modem alignment procedure presented in the
troubleshooting/calibration section of the owners manual. Since there is no standardization
among the various radio manufacturers in this regard, a method for making the determination
of transmitter FSK output frequencies and their relation to the LSB carrier oscillator and
center of the 500 Hz filter is NOT presented here. Contact the manufacturer of the radio to
obtain this information.
Radios that have provision for an auxiliary "narrow SSB" filter can have the 500 Hz
bandwidth filter installed instead of the approximately 1.8 KHz wide "narrow" SSB filter.
This will allow direct selection of the narrow filter for AFSK work when in SSB mode. If the
filter is also desired for CW operation and you don't want to invest in 2 identical filters, it is
usually a relatively simple modification to cause the radio to also select this filter when in
CW mode. In the TS-430, for example, this requires moving one end of one diode on the IF
board. The 270 Hz filter can then be installed in the "normal" narrow CW filter position
making two bandwidths of narrow filter available for CW operation with all filters selectable
from the front panel.
Some of the newer HF radios (and many older ones) allow selection of filter independent of