FR2492188A1 - FREQUENCY MODULATED SIGNAL DEMODULATION CIRCUIT - Google Patents

Abstract

FREQUENCY MODULE SIGNAL DEMODULATOR CIRCUIT INCLUDING A MULTIPLIER CIRCUIT 12 RECEIVING THE MODULE FREQUENCY SIGNAL AND A DELAY CIRCUIT TO SEND A SIGNAL WHICH HAS PASSED THROUGH LEDIT DELAY CIRCUIT TO MULTIPLIER CIRCUIT IN THE FORM OF ANOTHER INPUT SIGNAL DEMODULATOR HAVING A BANDPASS FILTER 13 WITH A FILTERING FREQUENCY BAND CHARACTERISTIC SHOWING SIDE BANDS AND A PHASE DIFFERENCE CHARACTERISTIC FROM THE FREQUENCY OR PHASE DIFFERENCE BETWEEN THE INPUT AND OUTPUT SIGNALS FOR A FREQUENCY SPECIFIC WITHIN THE FREQUENCY OF THE SAID SIGNAL OF ABOUT 90.

Description

The present invention relates generally to

  frequency modulated signal demodulator circuits (desi-

  below simply by FNI signals), and more particularly

  a kind of FM signal demodulator circuit of the capa delay line type to perform demodulation without introducing significant noise even when there are gaps in the FM signal reproduced from a medium

rotary recording.

  There are various types of conventional demodulator circuits.

  to demodulate an FM signal. A type of old-fashioned circuit

  reader having a fine linear characteristic in its demodulated output characteristic comprises a circuit for

  delay line demodulation. The demodulation circuit

  delay line type controller includes a delay circuit for delaying an input FM signal by a predetermined duration

  To, and a multiplier circuit consisting of a gate OR-

  exclusive to multiply the input FM signal with the signal

  FM above delayed by delay circuit.

  If a carrier frequency, when there is no modulation,

  is represented by fo, the frequency with respect to the character-

  demodulated output terodulator of the conventional delay line type demodulator circuit is a frequency whose

  demodulated output becomes maximum for a frequency of 2fo.

  When the frequency decreases from this frequency of

  2fo, the demodulated output decreases linearly. The character-

  output demodulated above is substantially linear within a range of frequencies

between zero and 2fo.

  In the case where an FM signal is reproduced from a recording medium such as a rotating disc or a magnetic tape, and when the signal fails due to a gap, the frequency becomes instantly zero in the part o there is a gap. As a result, when the signal

  FM is a signal having a carrier which is modulated in

  by a video signal such that the maximum width of the frequency deviation is within a range of 6.1 MHz to 7.9 MHz and the peak end of a synchronization signal or 6.1 MHz, the demodulated output in the part where the gap is found is introduced into the demodulator circuit of the delay line type above and exceeds the demodulated level of the terminal tip of the synchronization signal. In addition, the demodulated outlet projects widely towards the smaller side of the demodulated outlet. So, when there is a gap, it results in a significant noise and the inconvenience coming from the

  made a sharp deterioration in image quality.

  In addition, in the conventional type retara line demodulator circuit, the slope of the characteristic of the demodulated output is relatively small. It is therefore

  unable to gain sufficient gain, which is a

  comes. Consequently, a general object of the present invention is to create a new FM signal demodulator circuit and

  useful, of a kind of delay type, in which are

  mounted the disadvantages described above.

  Another more specific object of the present invention is to create an FM signal demodulator circuit comprising a bandpass filter instead of a delay circuit in the delay line type demodulator circuit. The FM signal demodulator circuit according to the present invention has a demodulated output characteristic in which there is

  a return in the opposite direction within a range of frequencies

  which is less than the carrier frequency of the FM signal. Thus, according to the circuit of the present invention, even when there are gaps in the reproduced FM signal, there is no introduction of projections inside the demodulated output of parts corresponding to the parts where the gaps are introduced. As a result, a strong reduction in noise due to the gaps is obtained. In addition, because the slope of the demodulated output characteristic of the circuit according to the present invention is large, the sensitivity to demodulation is high. In addition, because the bandpass filter is used as a delay circuit, we

  reduces the noise level outside the frequency range

  necessary, and you can get a demodulated output with a high signal to noise (S / N) ratio. The circuit according to

  the present invention is very stable because the characteristic

  linear ristic of the demodulated output characteristic is determined by the phase difference characteristic

of the bandpass filter.

  Other objects and characteristics of the invention appear

  ra: will be more clearly and not limited to the

  reading of the detailed description which follows with reference

to the accompanying drawings.

  FIG. 1 is a diagram which explains a characteristic

  that demodulated output of a demodulation circuit of

conventional FM signal.

  FIG. 2 is a block diagram representing an embodiment of an FM signal demodulator circuit according to

the present invention.

  FIGS. 3 (A) to 3 (E) are diagrams respectively representing the waveforms of the signals in each part of the system represented by the block diagram of the

figure 2.

  Figure 4 is a diagram showing a filtering frequency band and a phase difference characteristic with respect to the frequency of a bandpass filter

shown in figure 2.

  FIG. 5 is a diagram which explains the characteris-

  demodulated output tick of the demodulator circuit shown

in Figure 2.

  FIG. 6 is a diagram of a circuit representing an embodiment of a bandpass filter shown in FIG.

figure 2.

  An FM signal demodulator circuit of the conventional delay line type consisting of a delay circuit and a multiplier circuit has a frequency with respect to the demodulated output characteristic which is indicated by a line I in FIG. 1. If l 'we denote by fo the carrier frequency when there is no modulation, the characteristic I above curves in the shape of a mountain for a frequency of 2fo. A signal FH F1 of a frequency modulated video signal so that the frequency deviation is in the range of 6.1 MHz to 7.9 MHz is demodulated as indicated in Dl using the characteristic I. When it exists gaps in the FM signal reproduced Fi, the frequency is strongly deviated towards zero in the part where the gap exists, and there is introduction of a gap as indicated in Pl. The part Pl above corresponding to the gap is introduced in the form of a gap noise inside the demodulated output as indicated in Ql, according to characteristic I. The level Lb at the peak end of the noise Ql coming from the gap is strongly separated from a level La located at the peak end of a synchronization signal, and noise Ql is largely

  protruding from the tip end of the syn-

  chronization. Consequently, in the FM signal demodulator circuit of the conventional delay line type, there is introduction of a significant noise Ql when there is a gap, and this is a

  disadvantage because you cannot get a good quality image

  you. In addition, since the slope of characteristic I above is relatively small, the gain in demodulation

  is low, which is also a drawback.

  The present invention has eliminated the drawbacks described above of the conventional and general delay line type FM signal demodulator circuit. We will now make a

  description of an embodiment of a circuit according to the

  present invention with reference to Figure 2.

  In FIG. 2, an FM signal shown in FIG. 3 (A) and reproduced from a recording medium (such as a disc) is sent to a limiting amplifier il via a terminal d input 10, and converted into a signal having the waveform shown in Figure 3 (B). This converted signal is sent to a multiplier circuit 12 and to a bandpass filter 13. Because the bandpass filter 13 has a delay characteristic, the signal which has passed through the bandpass filter 13 is delayed by a duration x relative to the signal from the limiting amplifier 11, as shown in Figure 3 (C). The above signal which is

  thus delayed by time t is sent to the multiplicating circuit

  tor 12. This multiplier circuit 12 is constituted by an exclusive OR gate and produces an output which is indicated in FIG. 3 (D) in relation to the two inputs above. The output of the multiplier circuit 12 passes through a low-pass filter 14 and is obtained in the form of a demodulated output indicated in FIG. 3 (E) by means of an orifice 15. Thus, because the filter passes -band 13 has a delay characteristic, the circuit shown in Figure 2 operates as an FM signal demodulator circuit of the delay line type

  even when the bandpass filter 13 is used as a representation

felt in Figure 2.

  The bandpass filter 13 above has a filtering characteristic II indicated in FIG. 4. FIG. 4 shows the

  filtering characteristic and the difference characteristic

  phase, and a vertical axis on the right side indicates

  level, while a vertical axis on the left side indicates

  that the phase difference between the input signals and

  exit. A horizontal axis in this figure 4 indicates the frequency

quence.

  In the case where the bandpass filter 13 has a characteristic

  filtering tick of a frequency band as shown

  felt in Figure 4 between the lower limit frequency (2 MHz for example) of the lower sideband of the FM signal and the upper limit frequency (12 MHz for example) of its upper sideband, the phase difference characteristic of the filter passes band 13 reaches the state indicated by the dashed line IIE in FIG. 4 when the bandpass filter 13 is formed from a bandpass filter

  ordinary. In other words, in this case, the characteristic

  that the phase difference of the bandpass filter 13 reaches a state in which the phase acquires a delay for frequencies greater than the frequency f for which the pnase difference is zero, and the phase advances when it

  these are frequencies lower than the frequency f above.

  Consequently and in this case, the demodulated output characteristic of the assembly demodulator circuit becomes that indicated by a dotted line V in FIG. 5. This means that in characteristic V above, the demodulated output becomes minimal at frequency f when the phase difference of the phase difference characteristic becomes zero, and the demodulated output becomes maximum for frequencies where the phase difference becomes 1800

  or -180o. Therefore, characteristic V below presents

  te angles as shown in Figure 5. As a result, the above demodulator circuit is not practical in the

form where it occurs.

  However, according to the present invention, the characteristic

  phase difference tick of the bandpass filter 13 is chosen so that it is that represented by a solid line IV in FIG. 4. In other words, the characteristic IV of the phase difference is chosen so that the phase difference of the carrier frequency fo when there is no modulation of the FM signal is 900, that the phase difference for a specific frequency f1 between zero and the frequency fo is zero, and that one obtains a fine linear characteristic between the values

  of the phase difference between zero and 1800.

  An embodiment of a concrete circuit diagram of the bandpass filter 13 is shown in Figure 6. The bandpass filter 13 shown in Figure 6 consists of capacitors Ci to C5, and inductors L1 to L3 connected respectively as shown in the figure. The phase IV difference characteristic indicated in

  figure 4 can be obtained by choosing a constant value

  te of each of the elements of the circuit of FIG. 6 as follows:

CAPACIîANCE INDUCTANCE

  CI = 270 pF Ll = 37.8ByH C2 = 270 pF L2 = 7) 1H C3 = 30 pF L3 = 7y1H C4 = 56 pF C5 = 30 pF By giving the bandpass filter 13 the phase IV difference characteristic, the demodulated output characteristic of the demodulator circuit shown in FIG. 2 becomes that indicated by an angular continuous line VI in FIG. 5. As can be clearly seen in this FIG. 5, the demodulated output characteristic VI is a characteristic into which are introduced angles at frequencies f1 and f2 where the phase difference becomes zero

  and 1800, the demodulated output becoming minimal for the

  quence f1 and the demodulated output becoming maximum for the frequency f2. When the frequency band is less than the frequency fl above, the characteristic VI therefore includes a characteristic part VIa where the demodulated output becomes

  more important as the frequency decreases.

  When an FM signal F2 is demodulated by a demodulated circuit

  reader having the demodulated output characteristic VI, a demodulated output indicated by D2 in FIG. 5 can be obtained. The demodulation of the FM signal F2 is carried out using the demodulated output characteristic between the frequencies

fl and f2.

  In the case where a gap P2 exists in the FM signal F2, and the fact that a gap causes the failure of a signal, the tip end of the part where the gap P2 exists deviates towards a frequency close to zero. However, since the demodulated output characteristic VI has the reverse characteristic part VIa between the frequency f1 and the lower frequency part, the gap

  P2 above is introduced inside the demo output

  signaled in the form of a noise indicated in Q2. An Lc level at

  the peak end of the noise Q2 is of an approximate order

  vement equal to the level Ld of the peak end of the synchronization signal of the demodulated output D2. Consequently, the peak end of the noise Q2 does not protrude from the peak end of the above synchronization signal. According to the demodulator circuit of the present invention, no significant noise is introduced even when there is a gap, as can be clearly seen by comparing Figures 1 and 5. In addition, the slope of the demodulated output characteristic VI is greater by comparison with that of the classic example, and the demodulation gain (demodule output) is significant. In addition, the noise level outside the necessary frequency band is lowered due to the characteristic of the bandpass filter 13, and a fine S / N ratio can be obtained with respect to the demodulated output. In addition, due to the linear characteristic of the demodulated output characteristic determined by the phase characteristic of the bandpass filter, the operating stability of the demodulator circuit is

important.

  It will further be specified that the present invention is not limited to these embodiments and that numerous variants and modifications can be made to it without

  deviate from its scope.

Claims (5)

  1. Frequency modulated signal demodulator circuit, comprising a multiplier circuit (12) to which a frequency modulated signal is sent as an input signal, and a delay circuit to which the input frequency modulated signal is sent, for sending a signal having passed through said delay circuit to said multiplier circuit in the form of another input signal, characterized in that said   circuit is a bandpass filter (13) having a characteristic   filtering frequency band tick (II) covering a frequency band of the frequency modulated signal comprising side bands, and a phase difference characteristic (IV) with respect to the frequency o the difference of   phase between input and output signals for a frequency   specific frequency (fo) within the frequency band of said frequency modulated signal is approximately 90, and the sign of said phase difference is reversed for frequencies below another specific frequency lower than said specific frequency, said demodulator circuit having a frequency characteristic with respect to the demodulated output (VI) having an angle   for said other specific frequency (fL).   2. demodulator circuit according to claim 1, charac-   characterized in that said bandpass filter has a characteristic   frequency tick with respect to the phase difference (IV) which varies substantially linearly within a range of phase differences between zero and 1800.   3..Demodulator circuit according to claim 1, charac-   terized in that said specific frequency is a frequency   this carrier (fo) said frequency modulated signal when it there is no modulation.   4. demodulator circuit according to claim 1, charac-   terized in that said input frequency modulated signal is a frequency modulated signal reproduced from a   recording medium, sometimes containing gaps.   5. demodulator circuit according to claim 1,   characterized in that said demo output characteristic   dule is such that the demodulated output is produced so that a demodulated output level (Lc) with respect to a frequency approximately equal to zero is within a range of the demodulated output level (Ld) of said signal frequency modulated.