MFJ MFJ-1270C, Системное руководство

Страница: 108 / 264

MFJ-1270C/1274C MANUAL ADVANCED OPERATION
Page 99
to determine the actual operating frequency for band edge or netting purposes.
Remember to consider that you will be occupying a few hundred Hz on either
side of F(emission).
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 n
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 a narrow (approximately 500 Hz) filter is to be used in the radio for HF
packet and RTTY operation (and this is STRONGLY recommended), it may
be necessary to use a tone pair centered on the radio's filter rather than the
"standard" 2125/2295 or 1600/1800 Hz pair. 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 which provide for direct FSK RTTY operation use the radio's
narrow CW filter if one is installed. 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 required modem center
frequency for the particular radio's FSK mode has been determined, the
modem alignment procedure presented in the trouble shooting / calibration
section of the owners manual may (should) be used to align the modem for
this mode. 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.