FM threshold

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The FM-threshold ( English FM threshold effect ) is at angle modulation methods such as frequency modulation (FM), a limit value of the signal power, at its underflow leading to important disturbances of the signal to be transmitted.

background

Every signal transmission takes up a minimum bandwidth that must be allowed through by the filters of the receiver in order to avoid distortion. On the transmission path or in the receiver itself, disturbing noise is always added, which should be less than the useful signal. In the case of angle modulations, one can observe that the signal-to-noise ratio (SNR) after the demodulator deviates significantly from the carrier-to-noise ratio (CNR) of the modulated high frequency before the demodulator:

  • For signal powers below the FM threshold , the noise is significantly increased.
  • For high signal power, demodulation reduces the noise.

The transition between the two areas is known as the FM threshold . The specific value of the FM threshold depends, among other things, on the modulation index used and the type of FM demodulator. Usual values ​​are in the range of 10 to 20 dB for a signal-to-noise ratio of the modulated carrier  .

With amplitude modulation (AM), which does not count towards angle modulation, the SNR continuously degrades. That is one reason why AM is used in aeronautical radio .

description

Area of ​​the FM thresholds with a red background when the modulation index varies. The comparison line applies to AM with synchronous demodulator.

The signal-to-noise ratio upstream of the demodulator is referred to below as CNR , and the SNR of the demodulated useful signal in the baseband as SNR . In the sketch opposite, the SNR is plotted as a function of CNR to clarify the FM threshold. The relationship between the two signal-to-noise ratios is given above the FM threshold using the following linear equation:

The summand stands for the so-called modulation gain, which among other things depends on the modulation index . Depending on the modulation index , this gain is in the range of a few dB up to values ​​around 20 dB. Doubling the modulation index results in an improvement in the SNR by a factor of 2 or 6 dB (and not by a factor of 8, as is often the case with a faulty source).

With a larger modulation index, the Carson bandwidth of the modulated signal increases; at the same time, the modulation gain increases the SNR and the FM threshold as a kink in the curve, shown in the diagram as an area with a red background, is shifted to the right in the diagram. This means that with a higher modulation index a larger CNR is required in order to stay above the FM threshold (only applies to old demodulation methods , such as the push-pull edge discriminator, PLL demodulators are not affected). As a primary function of the modulation index, the group of points for the FM thresholds is in the range from 10 dB to just over 20 dB CNR.

If the CNR in front of an FM demodulator remains above the FM threshold, the SNR achieved after the demodulator can alternatively be achieved with a higher bandwidth and lower power in the HF range. Above the FM threshold, power and bandwidth can be exchanged for one another. The achievable modulation gain is z. B. used in satellite communication to save power and thus weight.

Effects

Below the FM threshold there is massive interference in the baseband signal, which is shown in the diagram by an almost vertical drop below the FM threshold to values ​​around 0 dB SNR and a negative modulation gain. In the case of voice transmission, this means a drastic increase in the noise level if the field strength only falls below a certain minimum, which is detected in two-way radio devices by a squelch and leads to the loudspeaker being switched off.

Especially with FM reception by car radios, the field strength fluctuations can be so great that a distant transmitter briefly generates higher field strength. Then the threshold effect leads to the fact that the stronger incident transmission signal is demodulated and the weaker one is almost completely suppressed, similar to noise.

See also

Channel capacity

literature

  • Rudolf Mäusel, Jürgen Göbel: Analog and digital modulation methods . Hüthig, 2002, ISBN 3-7785-2886-6 , pp. 101 .
  • Hans Dieter Lüke: Signal transmission . 6th edition. Springer, 1995, ISBN 3-540-58753-5 , pp. 237-241 .
  • John G. Proakis, Masoud Salehi: Communication Systems Engineering . 2nd Edition. Prentice Hall, 2002, ISBN 0-13-095007-6 , pp. 244-247 .

swell

  1. Der, Lawrence, Ph.D., Frequency Modulation (FM) Tutorial , http://www.silabs.com/Marcom%20Documents/Resources/FMTutorial.pdf , Silicon Laboratories, Inc., accessed Feb. 24, 2013
  2. Modulation and Noise