Noise Cancelling Circuit

One of the problems of living in a built up area is local noise, this is particularly troublesome at higher frequencies where signals can be heard at a much lower level, where the noise should be much lower.

In my area of London I suffer noise from switched mode power supplies which cause a hash type noise of 2 to 3 S points on 20 through to 10 Metres. At the lower frequencies this noise is lost below the general band noise level and not usually so much of a problem.

On searching through various publications I came across a circuit in the RadCom handbook of a Noise Cancelling circuit using a phasing technique to eliminate local point source noise.

I decided to try this circuit, but on firing it up was very disappointed with the results. The circuit generated more noise than I was trying to get rid of in the first place! This was due to the poor front end design introducing cross modulation around the FET transistor.

I replaced the original design with an RF amplifier which I have used for number of years with great success in other projects.

This improved matters infinitely, the cross mod. vanished. Although this improved the strong signal handling capabilities, the circuit still did not seem to perform as well as I would have hoped.

The noise level certainly decreased, it was only by a few db and the tuning seemed fairly broad. My attention turned to the tuning arrangements prior to the FET. The original circuit used RF chokes to tune the various bands. Although this worked it was lossy and had low Q. The Q of the tuned circuit should not be too high as the circuit requires to “see” a fairly broad spectrum of noise to work correctly.

I experimented with different tuning arrangements and came up with the present circuit which seems to be broad enough to accept a reasonably wide band, but sharp enough to reject some of the broadcast bands that were causing inter mod problems.

In use, the circuit is able to reduce local noise on the 20 Metre band from S 8 to 9 down to zero with very little effect on received signals, apart from them leaping out of the noise. On the higher bands e.g. 17 and 10 Metres the noise reduced from about S 6 to 7 down to zero again.

On the lower frequency bands it will also work on local noise, for example, at the bottom end of 80 Metres in my area there is a strong wide band pulse type buzz which is received at about S 8 to 9. This little circuit is able to eliminate this to below the band noise.

This circuit will not eliminate signals because of it’s broadband characteristic. Also, it is not very effective with noise that is at a greater distance and is received via skip, as this tends to be multi-path, these end up as more than one signal from more than one direction.

I have not been able to try this circuit on power line noise as there are a lack of them in this area, therefore, I would be interested in feedback from anyone that tries this circuit on any such noise.

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