Yes, the P3 automatically bypasses its internal preamp whenever the
input signal level overloads the ADC. It uses a fairly complicated
algorithm to try to minimize the inconvenience. The first time the ADC
overloads, the preamp is switched out for a fairly short period of time
and then it is switched back again to see if the overload is still
present. If so, the preamp is turned off again, but this time for a
longer period. This continues until eventually the OFF period is quite
long. (I don't remember the exact time periods.)
It takes quite a strong signal to overload the ADC so most operators
will rarely see this.
The gain of the internal P3 preamp is about 19 dB. The software is
supposed to take that into account in the display so that signal levels
remain accurate when the preamp is switched out.
Alan N1AL
On 1/27/19 2:40 AM, Roger wrote:
Hello,
since I worked on 80 and 160 m the last couple of weeks I noticed that the
P3 panadapter starts clicking relays whenever a very strong local signal
(-10 dBm to 0 dBm) or a number of strong signals within the bandpass are
present. I could not find a relation yet when it happens because it takes
place in irregular intervals. I don´t notice this behavior on the upper
bands where signal levels and even the sum signals are not as strong as on
the lowbands. I also don´t notice it if I switch to a nonresonant antenna
and dropping the signal level or terminate the receiver to actually mute it.
So it has to do with the signal level which makes me think it is an
automatic adaption to keep the A/D converter in the dynamic range. It also
seems to have a time constant as it doesn´t switch around quickly but keeps
a certain state until it switches again. The fact that the noise floor on
the screen jumps up by 50 dB or and then drops down again hints to an
automatic adaption mode. Attenuator setting on the radio doesn´t have any
effect.
As I didn´t find any information about this issue in the manual I wonder
whether somebody with a P3 panadapter could confirm (or not) whether he/she
experiences the same thing. It was quite present this morning on 160 m (CQWW
CW 160 m) as the band was filled with 9+++ signals between 1810 and 1900
kHz.
I hear the sharp noise of the relays clicking thru the headphones and it
gives an uncomfortable feeling to hear the relays cycling all the time.
Maybe a future update of the firmware could make the switching process a bit
smoother
The firmware in use is the latest - ver. 1.60
73/Roger - DL5RBW
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