More experiments and analysis on the multipath filter 25-AUG-2019

## Experiments 25-AUG-2019

* Changing the height of the antenna (raised 1m from the previous height
  at the balcony) DID change the remaining noise level a little
  of no-sound output for NHK-FM Osaka 88.1MHz (-73.4 -> -73.8dB).

* The multipath filter did not work much on reducing QMM levels
  for the CATV rebroadcast/frequency-translated signals
  from J:COM Toyonaka-Ikeda (79.6MHz for NHK-FM Osaka).
  Possible reason: the output is amplitude-controlled before the translation.

* Adding a booster (DX Antenna UF35M, NF ~5dB, gain: 15 - 35dB, set to 20dB)
  did not change/increase the C/N and S/N levels, but decreased a bit
  (-73.4 -> -73.3dB)

## Thoughts

Hayashi-san personally suggested me to measure three possibility of noises as:

* 1) Noise inside the receiver: this shows up/down (invert-)proportionally
     to the input level, e.g., attenuating the input signal for 3dB
     results in the decrease of output S/N level for 3dB.

* 2) Artificial noise coming into the antenna: changing the antenna settings,
     e.g., antenna direction/height may change the S/N level.

* 3) Phase noise of the receiver: this shows up as the upper limit of the
     measured S/N by the receiver, due to the limitation caused by
     the jitter/phase noise of the local oscillators.

In our case:

* 1) Boosting/un-boosting the signal did not change the receiving signal
     quality much. Adding a booster, or an attenuator (-20dB) didn't change
     the listening signal quality measured by QMM, or the remaining noise
     of no-sound output.

* 2) Raising the antenna changed the S/N level up to 0.4dB this time.
     In a past experiment on February 2019, 3dB decrease of no-sound
     noise level was measured.

* 3) Using a GPSDO for 100MHz single-carrier, the measured noise was -96.6dB.
     This value can explain the practical limitation of the output noise level
     due to the internal phase noise of Airspy HF+ and airspy-fmradion.

In our case (addendum):

* 4) Using a GPSDO for 100MHz single-carrier with force-enabled stereo decoder,
     the measured noise was -75.45dB.  
     This value and the noise pattern corresponds very well with the
     no-sound noise with the stereo output.
