The Flex 6300 Wideband Noise Blanker is a marvel for eliminating power line noise at your receiving location.
Recently, my local power line noise came back. It’s been around five years since the last time it was fixed by the local utility, Enmax. While waiting for a repair, have been experimenting with the noise blankers in my radios, particularly the Flex Radio 6300. Here is a demonstration of how well it works.
Power line noise has a unique signature. It is basically a spark, or impulse noise, with a fundamental frequency of 120 Hz. In other words, a spark is created twice each voltage cycle. Set your receiver to AM and watch the audio output on a spectrum analyzer. You will see a strong spike at 120 Hz and weaker spikes on its harmonics. If you watch the panadaptor on you SDR, you will see a rise in the entire noise floor. Power line noise can be very strong, depending on your distance from the source. Typically, it buries signals in noise.
Communications receivers have noise blankers. These normally work at IF frequencies. When it detects a strong pulse, a NB will literally turn off the receiver for a few milliseconds. Noise impulses can be detected because they have very rapid rise times, compared to normal signals.
The problem with traditional noise blankers is that the impulses have already made it most of the way through your receiver before they are attacked. Impulse noise has a very fast rise time, almost instantaneous. As the noise moves through your receiver, it get stretched out by filtering. Even with Low IF SDR noise blankers, this stretching out makes it difficult to differentiate impulses from signals.
Flex 6300 Wideband Noise Blanker is Really Different
Noise reduction should work best the earlier it tackles the problem. The Flex 6300 Wideband Noise Blanker works on the raw samples produced by front end analog to digital converter. In this radio, the ADC produces around 12 million samples per second after initial decimation. At this speed, impulse rise times of well under 1 μS can be detected. This is more than 50 times faster than with a Low IF noise blanker.
When I activate WNB on my Flex, I can make the power line noise almost disappear across an entire band, as shown above. When it detects a noise pulse, the Flex replaces the sample with an estimate of the desired signal. It actually might replace 100,000 samples – or about 1% of the total – every second to blank the noise.
The manufacturer has never published the algorithm for the Flex 6300 wideband noise blanker. So, the rest of my comments are just a guess. The samples will flow through some sort of delay line. Various techniques can be used to replace samples. These range from simply setting a noisy sample to zero, to more sophisticated techniques involving a prediction of what a noise-free sample would look like. A delay line is required so the software can look ahead and look back to make these estimates.
Two things I have discovered. First, WNB works best when the power line noise is stronger. With weaker noise, the threshold has to be set quite low and the algorithm gets confused. Second, when a really strong signal shows up, it can reduce the blanking effect slightly.