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Signal-Noise Ratio – The Essence of Radio

signal-noise ratio

Discerning signals from noise is what radio reception is all about. We always seek to improve the signal-noise ratio. 

Whispering works in a quiet pasture. Not so much at a party. Radio users really care about the SNR or signal-noise ratio. Generally, for spoken word, you need a signal that has three times more amplitude than the background noise. That is 10 dB signal-noise ratio. Ideally, you want 30 to 50 dB SNR for good speech communications.

The concept of SNR is the same for speech, radio or video. The key factor is the bandwidth of the receiver. With narrow bandwidth signals such as Morse Code (a single tone) you can get by with an SNR of 3 to 8 dB. For WIFI networks, you need at least 4 dB for 1 Mbps data, and 20 dB for higher speeds. Your cable television needs > 15 dB for digital video.

Now, all of these signal-noise ratios assume white noise, where the noise components are independent and typically evenly distributed. The moment your noise has some pattern – in effect becoming an interfering signal – all bets are off.

When manufacturers make sensitivity claims for their receivers, they are referencing the radio’s internally generated noise. Since you have to also put up with atmospheric, galactic and man-made noise, you receiver’s sensitivity specification for a 10 dB signal-noise ratio is not that meaningful.

Finally, many hams use digital modes that claim a negative signal-noise ratio. You might see SNR descriptions of -10 or -20 dB for modes like CW, teletype or PSK. Don’t be fooled. The standard for measurement is a 2500 Hz bandwidth. As you reduce the bandwidth down to 100 or 10 Hz, noise reduces but signal strength remains. There is no such thing as a workable negative SNR for reception.

Signal-Noise Ratio Improvements You Can Achieve

Adding a low noise preamplifier ahead of your receiver can improve SNR at frequencies where you are not limited by atmospheric noise. Surprisingly, this can really help for Medium Wave as well as VHF. These low noise preamplifier reduces the overall noise figure of your receiver.

Learn to adjust the Automatic Gain Control of your receiver. Modern gear lets you adjust AGC threshold, slope, hang and decay times. You can use these controls to help the desired signal dominate noise as it works its way through your receiver.

A Noise Blanker will remove impulse noise, such as from a car ignition. Your Noise Reduction will significantly reduce white noise typically increasing your signal-noise ratio by 10 dB. It can also remove correlated noise such as tones and birdies.

And consider digital modes which use much smaller bandwidths. Some of these even include forms of error correction which is particularly useful if signals and noise are fluctuating up and down.

Discerning signals from noise is the essence of radio. You have lots of tools and techniques available, even if you live in a noisy neighborhood. For this situation, use an external noise canceler – they really work.

 

2 comments

    • John VE6EY says:

      Thanks for your note. Yes, more on the way. I am about to start a series on how to use a noise canceler (ANC4) and setting up noise antennas. Should compliment your post nicely. Cheers from Canada. 73 John

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