Hal Communications ST-6000, Инструкция по эксплуатации

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4-5
When the signal is incorrectly tuned, the amplitude of the major axis will be reduced and the two
traces may be rotated and/or no longer perpendicular. After using the tuning oscilloscope, the user
will discover that, in many cases, the trace positions and amplitudes can be used to determine
which direction the receiver dial should be adjusted for correct tuning.
Proper tuning adjustment of the receiver is much more critical when the autostart circuits of the
ST-6000 are active than when they are not. This is because the autostart circuitry senses the
"plus-plus" voltage which is quite sensitive to the centering of the signals in the discriminator fil-
ters. However, if the autostart is not active, the relatively broad bandwidths of the discriminator fil-
ters themselves will allow good reception of even poorly tuned RTTY signals. A good operating pro-
cedure, then, is to turn the autostart OFF (with the front panel switch) while tuning a signal. When
the signal is correctly tuned, the autostart can be activated, if desired. This technique is highly rec-
ommended for operators who are unfamiliar with the ST-6 and ST-6000 autostart circuits although,
with practice, you may prefer to tune with the autostart circuit on to avoid the garbled print while
tuning. Tuning when the autostart is active requires experience and faith in the tuning indicators
since there is a delay of 1.5 to 3.5 seconds after correct tuning is achieved before the printer is ac-
tivated.
The above tuning procedures in general apply only to the reception of FSK RTTY signals. Re-
ceiver tuning of AFSK-AM or FM signals will obviously not affect the frequency of the tones. There-
fore, tuning of the receiver is not at all critical for AFSK signals. The tuning meter or oscilloscope
will indicate the match between the transmitted audio tones and the ST-6000 discriminator filter
rather than correct receiver tuning. Obviously, the frequencies of the transmitted tones must
match the discriminator filters fairly well for satisfactory autostart operation. Most problems with
AFSK autostart systems can be traced to either off-frequency transmitter tones or misalignment of
the demodulator. The ST-6000 should NOT have these problems since the frequencies of the
transmitted tones are crystal controlled and the discriminator filters are aligned to these same
crystal-controlled tones.
4.4 Use of the ATC Circuit
The ATC (Automatic Threshold Control) circuit is designed to provide a degree of correction for
bias distortion which may be caused by the propagation of the signal. It is most useful when using
the "AM" mode of the demodulator (LIMITER switch OFF), but can also be used in an "FM" mode
of demodulator operation (LIMITER switch ON). The ATC circuit should generally be used only
when receiving a signal that is transmitted at close to the maximum data rate for the given speed
(transmitted at close to the "tape rate"). The ATC can cause distortion to signals that are hand-typ-
ed and it should be switched off if problems are encountered with printing hand-typed signals. The
distortion is, however, minimal in most cases and the ATC circuit can remain ON for most applica-
tions (as it is in the ST-6 demodulator).
4.5 Use of the DTH Circuit
The DTH (Decision Threshold Hysteresis) circuit will provide correction for distortion caused by
multiple-path propagation of RTTY signals. When the signal between a transmitter and receiver is
ionospherically propagated over more than one path, the lengths of these paths are usually quite
different. Therefore, the propagation time for each path is different. When these various signals
are summed in the receiving antenna, the antenna voltage can vary over a wide range (phase ad-
dition and cancellation) or the transition time from mark to space (or vice-versa) can be distorted.
The voltage variations (or fading) is neutralized in the high-gain limiter and detector circuits of the
ST-6000. The indeterminate transition time, however, can cause distortion and misprinting of the
RTTY signal. The DTH circuit provides enough hysteresis in the slicer stage to require a definite