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Switching from
Internal
to
External
and vice versa may result in a temporary loss of lock to
satellites.
The
External Frequency Source Parameters
controls allow the user to adjust the pa-
rameters related to external frequency mode:
•
With the
Ext. frequency value
edit box the user specifies nominal external fre-
quency value. Here one can enter the values between 2 and 40 MHz. The default value
is 10 MHz. The box is associated with the receiver parameter
/par/frq/ext
.
•
The field
Ext. frequency source status
shows whether the external frequency
is being used or not:
♦
off
– indicates that the receiver is using the internal crystal oscillator.
♦
wait
– indicates that the receiver is waiting for the external frequency lock.
More precisely, this value will be returned in the following three cases:
1. If the external frequency oscillator is disconnected;
2. If the amplitude of input signal is too low;
3. If the actual external frequency is different from that specified in the
Ext.
frequency value
edit box.
♦
locked
– indicates that the receiver is using the external frequency.
The field is associated with the receiver parameter
/par/frq/stat
.
•
With the
Ext. freq. signal amplitude
field the user can estimate the external
frequency signal amplitude:
♦
off
– indicates that the internal oscillator is actually being used;
♦
low
– indicates that the external frequency signal’s amplitude is lower than
needed;
♦
ok
– indicates that the external frequency signal’s amplitude meets the specs.
This field correspond to the receiver parameter
/par/frq/amp
.
The
Oscillator Offset Reduction
group box allows the user to govern the behavior of the
internal crystal oscillator.
•
The
Off
radio button means that the oscillator frequency offset reduction mode is
turned off.
•
The
Freq. Locked
radio button means that the receiver will adjust the internal oscil-
lator's frequency until the measured frequency offset is reduced to zero. By using the
incoming satellite signals, the receiver will force the internal oscillator to generate a
very stable 20 MHz frequency signal. This frequency output is available via the corre-
sponding receiver output pin.
