DE2536453A1 - Occupied communications channels detector - frequency demodulates IF signal and filters result and uses frequency discriminator - Google Patents

Abstract

The detector, for automatically recognising occupied channels, ensures quick recognition in the shortest time possible by avoiding having to determine absolute noise level. Instead the signal to noise ratio is determined from the phase spectrum of the IF signal -- ie by evaluating the time derivative of phase, frequency. The IF signal (from the channel being examined) is frequency demodulated and evaluated by filtering in an HP or BP filter. The detector consists of a chain of the following in the order given: an amplitude limiter (1), a frequency discriminator, a BP filter, a rectifier, an integrator and a comparator (6).

Description

"Automatic detection of channel occupancy" The invention applies a method for the automatic detection of channel occupancy and an arrangement to carry out the procedure.

With automatic search receivers (channel sampling), the level exceeding a certain predetermined threshold by the IF level as Criterion for a channel occupancy, d. H. for the presence of a signal in the channel, used. The threshold must be chosen so that a reliable differentiation de useful signal from the interference level that is always present - in particular ash - is possible. Of the Noise level changes over time and as a function of the adjusted reception frequency. Should signals also be recognized that are only slightly above the interference level, the threshold value must be corrected from time to time become, which causes difficulties in an automatically operating system The invention is based on the object of a method and an arrangement of the initially described called type to create, bypassing the difficult noise level determination for the level threshold a reliable automatic detection of channel occupancy in enable as short a time as possible.

According to the invention, this object is achieved in that for assessment of the signal-to-noise ratio, the phase spectrum in the channel to be tested by limiting and frequency demodulation of the Z signal with subsequent filtering in a high or bandpass filter is evaluated.

In an advantageous arrangement for carrying out the method it is provided that an amplitude limiter is sequentially equipped with a frequency discriminator, a high-pass or band-pass filter, a rectifier, an integrator and a comparator with an input for a reference voltage Uref are connected downstream.

The invention is based on the decomposition that it is cheaper instead of the absolute level to evaluate a relative variable, namely the signal-to-noise ratio. A statement about the signal-to-noise ratio can be made with most of the known types of nodulation win over the phase spectrum of the IF signal. It is easier instead of the Phase angle setLne time derivative, the frequency, to be evaluated.

An embodiment of the invention is shown in the drawing and is described in more detail below.

Fig. 1 shows an arrangement for performing the method according to the invention. The IF signal is limited in an amplitude limiter 1 and a Frequency discriminator 2 supplied. The output signal of the frequency discriminator is applied to a high-pass or band-pass filter 3, which has a frequency or phase modulation cut off the useful signal. The spectrum then mainly contains noise components, can therefore be used approximately as a measure of the noise power in the IF channel will. Since the total output is determined by the limitation, one thus obtains at the same time a statement about the signal-to-noise ratio. The limited, frequency demodulated and high- or band-pass filtered IF signal is via a rectifier 4 and a Integrating stage 5 is fed to a comparator 6. By comparing of A level with a threshold value Uref fed to the comparator is thus obtained Criterion for the presence of a signal in the channel (maximum evaluation level with pure noise).

The applicability of the procedure and the arrangement is not affected by it affects that the spectrum width of the useful signal is smaller than the Z bandwidth is. When limiting a signal-to-noise mixture, the spectrum broadens, so that noise components can be separated for evaluation in each case. the Spectrum broadening generally occurs when limiting signals that have more than one spectral line and for which the resultant of each Vibrations do not have a constant amplitude (in contrast to e.g.

for frequency modulation). It is an intermodulation, which is particularly clear as phase modulation of the strongest oscillation by the weaker can be interpreted (Fig. 2 ?. When several useful signals are present produces an effect similar to that caused by the noise.

However, an eçanal occupancy display for the strongest signal is possible, if its level is approx. 6 dB above that of the others.

Claims (2)

P a t e n t a n s p r ü c h e 1. Method for the automatic detection of a channel occupancy, thereby characterized in that for assessing the signal-to-noise ratio, the phase spectrum in the channel to be tested by limiting and frequency demodulating the ZF signal cit subsequent filtering in a high-pass or band-pass filter is evaluated. 2. Arrangement for performing the method according to claim 1, characterized characterized in that an amplitude limiter (1) is sequentially a frequency discriminator (2), a high or Bandpass filter (3), a rectifier (4), an integrator (5) and a comparator (6) are connected downstream with an input for a reference voltage Uref (Fig. 1). L e r s e i t e