Noise Blanker, causing noise

I was chatting to my mate on UK muppets the other night when some annoying static came up on the channel making working conditions difficult.   Tried changing frequency, my mate put a bit more power on but it made little difference.   I could still hear him though,   so just put up with it for a while.   Just out of curiosity, I hit the NB button (turned it off) and suddenly, crystal clear mod from my mate, all static gone!   I find the NB / ANL very useful on SSB /AM, espectially in the mobile.   But never experienced it actually exacerbating the noise!

Hi Chris,

I'm not surprised by the results of utilising the Noise Blanker / Automatic Noise Limiter on your radio but when you first discover it for yourself it can be a bit of a shock.

As you rightly point out it is very useful for AM/SSB modes and works best to remove ignition noise from a vehicle mobile installation. Hardly surprising when you consider how old such circuitry is, (decades old designs still in use today!), and those noise scenarios were exactly what they were designed for.

I won't bore you with the history and technical details of such circuitry, (oh believe me, I really could! ), but suffice to say you have found a perfect example of where not to use them, namely Frequency Modulation and a noise source other than the repetitive pulses from an ignition system.

In those instances due to the nature of such circuitry you actually degrade the signal path so of course when you turned it off your FM reception actually improved. Seems counter intuitive at first but that's pretty much it in a nutshell.

You'll find other examples of when such circuits improve or degrade your reception such as signal to noise ratios, (SNR). Sometimes turning the RF gain down with the NB/ANL on suddenly improves things because you have brought everything into line where such circuitry performs at it's best. (Another counter intuitive example....turning DOWN the RF gain to improve the reception?!?! Can't be right....yet certainly is. )


It's always nice to make such discoveries to help improve your radio work so take it as a learned bonus.


You can always tell an 'old hand' watching them use a radio, a signal comes in, buttons are quickly switched, the odd twiddle of a knob here or there and the signal is as clear as they can get it. Watching an old timer on an equally old radio is an art performance in itself, they can twiddle those array of knobs and flick switches like a mad professor bringing life forth and sure enough turn that ragged signal into a BBC broadcast reception quality. 

It's when they receive someone where they don't have to twiddle a single thing that it's known as an 'armchair copy'.


Anyway, enough of my waffle. 

But suffice to say I've lost count of how many times I've heard, "NB switch? Nah, don't do nuffin mate!", and let out a little sigh.


All the best,
Victor

Hi Victor

I've always just left the noise blanker on and not given a it a thought.  I always turn it off on FM now.   I have experienced good results from turning down RF gain.   One night, the person I was talking to was almost being wiped out by noise.   WIth the RF gain backed off, it cut out the noise and brought him down to S0 from S7, but I had him on radio 5.

The noise blanker will make the selectivity of a receiver worse meaning you're more susceptible to interfence and bleedover from stations on nearby frequencies.

Turning down RF gain as you state is the best way to do this as it improves the dynamic range of the reciever, attenuating unwanted noise more than wanted signal. ideally you'd wind the RF gain down so the S meter was reading S0 on an empty channel. On AM/SSB just increase the volume knob to compensate for the quieter audio.