Icom IC-251A Instruction Manual page 25

SSB OPERATION
1.
RECEIVING
After connecting an antenna, microphone, etc., set knobs and switches as follows.
POWER SWITCH OFF (button out position)
T/R SWITCH RECEIVE (UP)
VOX SWITCH OFF (UP)
AGC SWITCH SLOW (UP)
NB (NOISE BLANKER) SWITCH OFF (UP)
MODE SELECT SWITCH USB or LSB
VFO/MEMORY SWITCH "A"
AF GAIN CONTROL Fully Counterclockwise
RF GAIN CONTROL Fully Clockwise
SQL CONTROL Fully Counterclockwise
DIAL LOCK SWITCH OFF (UP)
RIT CONTROL Center (12 o'clock) position
The other knobs are unrelated to receiving and need not be set at this time.
In SSB operation there is both a USB (upper side band) and an LSB (lower side band).
USB is usually used on the band, while LSB is usually used on the downlink of the satellite
communications. Select the mode according to your use.
Now turn ON the power switch. The meter lamp will be illuminated and ''145.000.0" will be
shown on the frequency display. Slowly turn the AF gain control clockwise to a comfortable
level. Rotate the Tuning Knob until a signal is received. The multi-function meter needle will
move according to the signal strength, so tune for the highest possible meter reading and the
clearest audio. If you cannot get a clear signal, you may be receiving in the opposite sideband.
If so, change the mode switch to the proper sideband.
The VFO's in the IC-251A/E change in 100Hz steps. Therefore the maximum possible dif-
ference in your.receiving frequency and the received signal frequency is 50Hz. However, for
normal voice signal reception, a difference of 50Hz is negligible and indeed may be hard to
distinguish. Since the frequency is changed in 100Hz steps with the IC-251A/E, as opposed to
continuous variable frequency systems, tuning is simplified and more accurate.
If squelch operation is required to cut out noise when no signal is received, turn the SQUELCH
Control clockwise until the noise from the speaker stops and leave it just below this threshold.
NB (NOISE BLANKER)
Turn the NB switch ON (down) when there is pulse type noise, such as ignition noise from
automobile motors. Noise will be suppressed and even weak signals will be received com-
fortably. The noise blanker may not work as effectively when there is a strong signal on a
nearby frequency or when there is continuous (not pulse type) noise.
AGC (AUTOMATIC GAIN CONTROL)
The IC-251A/E has a fast attack/slow release AGC system which holds the peak voltage of
rectified IF signals from the IF amp circuit for a certain period. Therefore, during the pauses
in normal speech of the received signal, uncomfortable noise will not be heard. The multi-fun-
ction meter indicates the peak value for a certain period, facilitating reading of the meter 'S"
function.
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