A preliminary study on noise canceler for MW DX

    For MW DXing, the problem of noise is annoying matter. It will be the worst experience if noise can be heard on all the MW band in the case of going to remote DX expedition place even though spending a lot of time to prepare for the expedition with much efforts.
    I often talk about problems of noise with my friends of MW DXer. Most of them worry about the issue.
    As far as I know, Figure 1 shows the relationships between frequency and medians of measure noise in 2005 and 2007 and the pink line represents the ITU-R P.372-8 noise figure estimated in the business category (Ref.1).  The frequency for the measured noise is on HF band, but unfortunately, we could say the noise level on MW band become much larger than the noise figure shown in the ITU-R P.372-8 that data was measured in the 1970s (Very old data !! ).  Even in the rural area, I believe the noise figure has the same tendency.  It is so pity, but it can't be helped at all.

Fig.1
Ref.1 Miki Iwama, "Estimation of Background Noise in HF-band",2008 APEMC

    To overcome this annoying problem of noise, I started a preliminary study of noise canceler. There are a lot of reviews on noise cancelers among ham DXers and MW and SW DXers. In Japan, the famous noise canceler circuit would be one that introduced by JA1DI, Mr.Yamaguchi.  His original circuit is shown here by JO7NLI, and many Japanese DXers made this circuit to eliminate noise. The author of "Saitama Radioholic Intl" reported the fight with a neighborhood air-conditioner noise with his modified circuit in his blog, and Mr Kageyama also introduced his modified circuit based on the JA1DI's circuit too, and  All of them have confirmed that the circuit works very well for certain kinds of noise.
    Figure 2 shows the circuit block diagram designed by JA1DI's noise canceler. Thanks to the detail description by JA1ACW, I could understand how this circuit works to eliminate noise.

Fig.2 Circuit block diagram of noise canceler designed by JA1DI

   It is interesting to know the all-pass filter works as a phaser. Changing phase means that the inputted signal is also delayed. But please note that the time delay of the signal depends on the frequency of the signal. And the group delay characteristic of the all-pass filter is NOT constant. It means that the delay time is not constant for all the frequencies. It is very important to understand that this noise canceler can eliminate the noise on a certain minimum frequency band at the same time and I think this canceler is not appropriate for a reception recording all MW band by SDR (Software Defined Radio).   One of my friend of MW DXers whom I respect suggested to me that the noise comes through multipass including skywave reflected by ionospheres and the phase of noise changes with time and the phase and gain adjustment on the weighted adder would be needed accordingly. I will show the group delay characteristic of this circuit on this blog later.