Re: Max S/N for RSSI readings?-FARMERIK


sdwillingham
 

The Si4734 chip limits the reported S/N to 25. This is simply
a limit on the reported number, not a limit on the actual
signal's quality. I think the reason that it is limited is
due to the algorithm by which it is determined. It becomes
inaccurate at higher levels. It is also inaccurate if the
carrier frequency is detuned, which is why it then reports
zero. Further, for the purposes of the radio, it is moot to
determine the exact SNR above 25. The reason it computes SNR
in the first place is to improve the auto-search algorithm.
If the SNR is past 25, a station is "good" and no further
information is needed.

For the RSSI indicator, the Si4734 does *not* limit at 63.
Some Tecsun radios, e.g. the PL-380, impose this extra limit.
I don't know the actual reason, but my suspicion is that
they might be doing some math on the chip's reported values
(such as averaging). The value of 63 is the max that can
be represented in 6 bits. Perhaps the additional 2 bits in
their 8-bit controller are needed for headroom to avoid
overflow in some calculation. Or, maybe it's even simpler:
a 7-bit value could exceed 99, overflowing the 2-digit LCD
display.

-Scott-

--- In ultralightdx@..., "farmerik" <farmerik@...> wrote:

The highest S/N I am getting seems to stop at 25. Is that the top of the range? [PL-360 with Hi-Q coils as antennas]. - FARMERIK

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