LF and the QRM-problem.
At my QTH there sometimes exists a serious QRM problem. In general the noise level is already rather high, probably due to the continuous burning of over 100 fluorescent lamps on the inner corridors of the flat building. I have to live with that. But other noise sources like light dimmers, speed regulators of hand tools and the like can be partially cured, provided it is an impulse noise related to the mains frequency. To find this out, connect an oscilloscope to your receiver's audio output and trigger it to the mains (50 Hz) frequency. If the result is a still picture of the noise you then can find out if it has either a 50 Hz or 100 Hz relationship.
I have made a noise blanker system based on the mains frequency. The 50 Hz or 100 Hz source can be derived from the receiver's mains transformer or otherwise. After clipping it into sharp pulses or a square wave it triggers two time-variable monostable MV's in sequence in order to obtain the correct phase relationship with the noise. Then a third monostable is triggered to provide an adjustable gating signal for the series switch in the receiver's signal path. The monostables I used were CD4528 CMOS IC's.
The switching function can then be done by either a junction- or a MOSFET. It is my experience that the series switch has to be inserted in the path where the signal level is already rather high. Otherwise the switching action produces loud bangs making the cure worse than the evil. If you insert the switch in the audio path to speaker or headphone, do it prior to a small bandwidth audio filter, if you use one.
The procedure is as follows: Switch the noise blanker on and set the blanker pulse width to average. Carefully adjust the timing of one or both delay MV's, until the noise is at minimum. Then adjust blanker pulse width for an optimum. A slight readjustment of the time delay then may be needed. In this way a loud S9 plus noise can be reduced to some S4 or less.
Anybody interested for more details please write to me. There is no print lay out available as I never make PCB's. Use VEROBOARD or the like.
Harry Grimbergen, PA0LQ.
2341 HE Oegstgeest