Yaesu FT-900 Operating Manual page 38

Operation
1700-Hz TAPA convention), and your TNC uses 2 1 t 5-
/2315-Hz tones. What frequency should your display
show?
Unlike RTTY and AMTOR. which imply the marl<
frequency when setting up skeds, packet frequencies
refer to the center of the two tones. With your modem,
the carrier ottset is in the middle between 23t 5 and
21t 5 Hz. or 2215Hz. So if you're using LSB mode, you
need to add this olfset to the specified QSO frequency
to got your displayed frequency: 14.1 0 t 30
+
0.0022t 5
(M H z) .. 14.103515, which disp lays as either
14.103.51 or 14.103.52. On the o ther hand, if you're
using USB mode. you subtract the ottset, and your
display shows 14.099.08 or 14.099.09.
Since tuning is very critical for F1 packet, selecting
2.5- or 5-Hz tuning steps will provide the most precise
tuning. Also, you should enable display of the IO·Hz
d•git. by holding the
• uP
button while switching the
transceiver on. Tune the transmitter and receiver
within 10 Hz of a signal to minimize repeats.
1200-Baud FM Packet
The equipment setup for 1200-baud FM pocket
(above 29 MHz) is the same as for 300-baud packet.
There is no squelch status output from the FT-900,
however, so performance will be better on noisy chan-
nels it your TNC has PLL-type DCO. Tuning is much
less critical in this rnode, requiring no special adjust·
ments.
To transmit FM packet, simply adjust the RF PWR
conuol for the desired power level.
Note: Computer-Generated RFI
When using a TNC connected to your transceiver,
or even having a PC located in the shack, the possibil-
ity exists that you may experience computer-gener·
ated RFI (Radio Frequency Interference).
The CPU in a personal comp uter operates wrth a
crystal-controlled oscillator (clock) and timing circuits.
Common clock frequencies include
a.
12. 16, 20 and
25 MHz. In addition. high-speed digital data switching
uses square waves, which produce odd·order har-
monic frequencies.
Computer-generated RFI may appear at seem-
ingly random frequencies (usually right where a rare
OX station is calling CO!) throughout the range o f your
transceiver, and may sound like constant ticking or
buzzing that may change as you type or work within a
program. Severe RFI may haveS-meter indications as
much greater than S-9, making copy of voice signals
difficult and data signals virtually impossible.
Computer-generated RFI is usually a result of in-
adequate shielding ol the PC's cabinet or
110
and
peripheral connections. Whilo computer equipment
may comply with RF emission approval standards, this
does not ensure that sensitive amateur radio receivers
will not experience RFI from tho device.
There are a few steps you can take to reduce or
eliminate computer-generated RFI. Tho first step is to
ensure that only shielded cables are used for TNC-to-
transceiver connections, carefully check RF ground
connections and re-orient your station equipment in
relation to the computer. Try mov•ng your PC and
Optional DVS-2 Digital Voice Recorder
The OVS-2 is a digital recorder that can greatly
enhance OXing, voice contesting and net opera·
tions. Tho FT-900 has a special jack for connecting
the OVS-2 on the rear panel. It offers two inde·
pendent functions: recording received signals for
playback later in the loudspeaker/headphone, and
recording signals by the microphone for playback
during transmiSSIOn. Operating details are provided
wrth the OVS-2, but here
is
an overview.
Receiver Recording
In this mode, the OVS-2 maintains a continuous
recording of the last 16 seconds of audio from the
receiver. This can
be
particularly h elpful in picking
out call signs during a pileup or net check in, as you
can replay the same recording with the push of a
button, as many times as you need. without having
to ask for re-transmissions.
page34
Transmission Recording
This mode allows the OVS-2 to record either two
8-second segments or four 4·second segments of
audio from the microphone. such as contest ex·
changes, station IDs or net calls. Each can then be
played back. either in a monitor mode (without
transmission), or directly over the air. The 8· and
4-second segments share the same memory, so
two 4-second segments can be combined •nto one
8-second segment. The digital memory used in this
mode is independent from that used for receiver
recording.