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ST-7000 OPERATOR'S MANUAL
PAGE16
signals, care has been taken to assure that receive and transmit data
polarities are identical - you may also use the ST-7000 for 300 baud Baudot or
ASCII RTTY (or 100 baud AMTOR).
3.1.3 THRESHOLD Control:
This control sets. the signal/no-signal detection threshold for the Carrier
Detect circuit.
It
operates in a similar fashion to a squelch control on a VHF
radio.
Tune the receiver to an unoccupied frequency and rotate the control
clockwise (CW) until the CD LED comes on. Now, "back" the control CCW until
the CD light goes OFF. Tune to an active HF packet channel and confirm that
the CD LED flashes ON and OFF as bursts of packet signals are heard.
The
setting of THRESHOLD will change with receiver volume control setting unless
you have a constant audio output connection on your receiver.
Practice setting
this control.
3.1.4 Tuning Indica tor:
The ST-7000 tuning indicator is a unique device, unlike any other ever offered.
A total of 20 LED segments are arranged in two parallel bars. A fully lit LED
segment corresponds to approximately 50 Hz change in the audio frequency of the
received tone(s).
Picture the two parallel bars as the vertical "legs" of the
letter "U".
The lowest audio frequency (approximately 900 Hz) will light the
top left LED segment.
As you tune to increase the tone frequency, the
illuminated LED segment moves down the left bar and then back up the right bar
to the highest frequency (approximately 3000 Hz).
The "center" of the display
(l/2 way between the bottom dots of the two rows) corresponds to the 1900 Hz
center frequency of the ST-7000.
A packet radio signal will appear as a band
of dots, the width of which is proportional to the shift and data rate.
The
dots actually correspond to the frequency spectra of the received signal.
Correct tuning of the receiver is achieved when the signal is centered on the
ST-7000 input filters - centered on the tuning display.
Therefore, you need
only tune until an equal vertical deflection is obtained on the two parallel
bars.
When correctly tuned, a 200 Hz shift signal will illuminate the bottom three
bars in both vertical lines - approximately to the "200" marks on the front
panel.
A 600 Hz shift signal will have a balanced deflection near the "600"
marks on the front panel.
With a little practice, it becomes VERY easy to
precisely tune an HF packet signal on the display.
Use some patience when
tuning - a given packet burst may appear "biased" to one side or the other -
this is because of the bit-stuffing and NRZI format used for packet data. With
just a little practice, you can easily tune to within 20 Hz or closer.
Watch
some HF packet signals on a busy channel. You will be amazed at the frequency
error of some stations.
The ST-7000 itself is very forgiving of minor tuning
errors (50 Hz on 200 Hz shift, 100 Hz on 600 Hz shift), but strive to be as
accurate as possible when tuning.
The resolution of the tuning indicator is a
lot better than the 50 Hz bar spacing might lead one to believe since each
segment partially illuminates as its "center frequency" is approached.
Resolution of 20 Hz or better is quite easy to achieve.
If
absolute frequency
accuracy is required on a net frequency, a crystal in the transmitter/receiver
is still the best approach.
Examples of correct and incorrect tuning are shown
in Figure 3.2.
HAL880815
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