[Lowfer] Light Dimmer - Beyond Superposition...

[Ed Phillips]

Scott Tilley wrote:

>To expand the topic one can also consider a line-synchronous noise 
>blanker for LF use.  This is different the the asynchronous types in 
>your HF rig...
>
>These guys only work on noise coming from mains powered dimmers and such 
>but can be very effective on multiple sources of dimmer noise such as 
>can be found in an urban QTH.
>
>Have a look here for a great description of a very useful design:
>http://www.ka7oei.com/syn_blank.html
>
>I haven't seen much other literature on this method.  Anyone else got 
>anything?
>
>73 Scott
>
I built a synchronous blanker back in the late 1970's which is very 
effective for light dimmer and equivalent line synchronous pulsed noise. 
Uses 120 Hz triggers generated from power supply [resistor in CT of 
power supply transformer] to trigger four CMOS pulse generators with 
variable delay and width to handle up to four different interference 
signals. Output of the generator is a series of pulses applied to the 
gate of a VN10KM mosfet connected directly across the receiver antenna 
terminals to get around problems with "stretching" in the narrow 
bandwidth of the receiver. It will completely kill interference from a 
light dimmer in the next room. Doesn't help with garbage like "power 
leaks" which varies in phase or is nonsynchronous. There is an 
auxilliary sawtooth output for the horizontal sweep of the monitor scope 
whose vertical input is the receiver audio output. Helps in adjusting 
the delay and width of the blanking pulses to the minimum necessary to 
get rid of the interference and leaves the maximum amount of signal. 
It's surprising how a weak signal can be copied even though the receiver 
is blanked a good portion of the complete power line cycle.....

The general principle of the "two antenna" noise blankers goes back to 
the early days of VLF transatlantic communications, which were often 
interrupted by static from the tropics. The output of an auxilliary 
receiving antenna was combined with the main receiving antenna with 
suitable amplitude and phase and was pretty successful in supressing 
noise from any particular storm. In effect the scheme forms a null in 
the receive pattern in the direction of the interference. Same principle 
as the cancellers discussed here and can be quite effective in at least 
some interference situations. We use the same idea in radar systems to 
do "jammer cancelling" - one or more auxilliary antennas are placed 
around the edges of the main antenna and their outputs are combined with 
the main output via phase shifters and attenuators. With N antennas it's 
possible in principle to cancel [N-1] interference sources by forming 
nulls in their directions. When the sources are within the main beam the 
idea still works but of course there is a loss in the received signal as 
well. Same scheme widely used in communication systems and 
jammer-nulling antennas such as for GPS.

Ed