This firmware will give the possibility to
enter all frequencies, the limits are determined by the VCO. Frequencies can be stored in memory channels,
there are 7 memory groups with each 100 memory channels available. Every memory channel can contain a name of
23 characters. There is no PC needed.
The available channel spacings are 5, 6.25, 10, 12.5, 15, 20 or 25kHz. The software gives
the possibility to use CTCSS and sel-call, but the hardware of the SE550 must be upgraded with
the CTCSS board and the FX003 to use these options. They are (or were) avaible from Ascom.
The VCO voltage coming from the PLL can only be 8V, that's not enough for the
whole frequency range, therefore the range is divided over 64 parts.
The software adjusts a 6 bit D/A converter when a frequency is selected, this DAC gives a DC
voltage to select the range roughly and the PLL does the finetuning. This DAC voltage is also used to tune
the front-end. When lock errors occur, it is usually due to these values. In software you can
optimize the sensitivity with the settings "DAC=0" and "DAC=63" in the submenu Hardware, but, when the
values are messed up the PLL is not able to lock.
The default DAC settings are optimised for the version which is most used in the Netherlands. DAC=0 is set to the lowest frequency
where the SE550 should work with, this corresponds with the lowest VCO range and lowest voltage on the
varicaps in the front-end. With DAC=63 you have to determine the maximum frequency, the highest voltage
for the VCO and front-end.
First you should set both DAC settings to a very high frequency (eg. 490MHz for UHF and 200MHz for VHF),
in this case the DAC will always be set to 0 by the software on the working frequencies, now you can
find the frequency where the PLL is able to lock for the lowest DAC value. DAC=0 should be set to
this frequeny in the menu afterwards. Do the same to find the DAC=63 (set both values to 310MHz for
UHF and 30MHz for VHF). These are roughly the values you have to choose, afterwards you can optimise
the values for the best sensitivity. Be aware of the fact that false DAC values can cause unlocking
on some frequencies, even in the middle of the frequency range, you should check the frequencies where
the SE550 is about to switch to another DAC value. These are visible in the status menu. The last
step you can optimise the front-end as well.
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