I am glad you find the discussion interesting. Sometimes I
worry about alienating the non-tech group members with too
much technical stuff vs dxing strategies, techniques, and
"Does the radio somehow know the center of the signal I
am shooting for?"
Actually, the problem is that it doesn't know the center you
are shooting for when you tune to a different one!
Your question is a very good one and the answer is non-obvious
to anyone without inside information on the chip's design.
Essentially, the chip needs to know the desired carrier frequency
in order to do its 'smart' SNR signal processing and calculation.
When you off-tune, the chip no longer 'sees' the carrier where
you told it to 'look'. Under this condition, the processor
considers the SNR calculation to be invalid and returns a 0 dB
By the way, this is part of the issue dxer's have with soft-mute.
The soft-mute calculation is based on the measured SNR. So when
you off-tune, the SNR goes to zero and soft-mute (if enabled)
--- In ultralightdx@..., Tony Germanotta <germanotta.tony@...> wrote:
Thanks Jim and Scott for the info. This is fascinating stuff for a non-techie like me. I have a question, though. When I offtune my PL-310 to one side or another of a frequency, not only does the signal strength drop a few notches on the volume knob, I always get a 0 SNR reading, no matter how well the station is coming through, no matter how tight the bandwidth. Does the radio somehow know the center of the signal I am shooting for? Of course, it really doesn't matter, the important thing is intelligibility. But it has me wondering just how smart this chip really is. Is it reading my mind, or just my WRTV Handbook?
On Jan 26, 2010, at 11:17 PM, sdwillingham wrote:
Jim gives an excellent explanation of the RSSI and SNR indicators.
Here are some further comments for more technical readers.
1) The abbreviation RSSI stands for "Received Signal Strength
Indicator", not "Relative . . ." Jim correctly emphasizes
that this strength refers to the LNA input voltage, not
electromagnetic field strength.
2) The signal-to-noise ratio Jim describes refers to signal strength
compared to the noise generated by the radio circuitry. The SNR
metric from the Si4734 is more sophisticated than that. If one
substitutes a short-circuit across the AM antenna terminals, he
will see the RSSI drop to 2-5 dBuV. With an antenna connected
and "between stations", the RSSI can be 30 or more dBuV. Yet the
Si4734 does not report the SNR as 25 dB, i.e. the signal strength
above the receiver noise floor. Instead, the chip reports a
metric measuring the signal strength relative to atmospheric
noise and interference. Of course, if the atmospheric noise is
low enough (or antenna gain is inadaquate), the SNR calculation
will reflect the receiver circuit's noise floor. I wish I
could give more detail about the calculation, but I believe that
is proprietary information.