DE1176214B - Receiver for frequency or phase modulated electrical oscillations with automatic bandwidth control - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Class: H 03 d

Number:
File number: Filing date:
Display day:

German class: 21 a4-29 / 01

C 17839 IXd / 21 a4
November 12, 1958
20th August 1964

The invention relates to a receiver for frequency or phase modulated electrical oscillations an intermediate frequency band filter with adjustable bandwidth for the purpose of noise suppression, which has a base bandwidth that is substantially greater than the frequency magnitude of twice the maximum Is the modulation frequency, and at its output an amplitude limiter and a downstream one Demodulator are connected and fed with an unlimited intermediate frequency signal Amplitude detector, the output of which is connected to the control input of the band filter via a low-pass filter.

Such known receivers have the advantage that with favorable reception conditions, i. H. greater Amplitude of the receiver input signal, the bandwidth is large, so that no undesired distortion whatsoever of the modulation frequency emitted at the output, while only when the Receiver input signal, the bandwidth is automatically smaller and thus the risk of distortion greater will. The invention has the task of improving the known bandwidth control, with The basic idea of the invention is through the special design of the control characteristic, d. H. of Relationship between the size of the input signal and the frequency of the bandwidth that To raise the limit of sensitivity of the recipient and at the same time, only insofar as it is absolutely necessary, the To affect the demodulation product emitted from the receiver output by distortion.

The one contained in receivers for the reception of frequency or phase modulated vibrations Limiter has the special peculiarity that it detects the vibrations received with the greatest peak amplitude strongly emphasizes or "captures" vibrations with a smaller peak amplitude. If a limiter should therefore emphasize a continuous sinusoidal oscillation against the noise, so the peak amplitude of this oscillation must be at least as large as the Gaussian noise be. Because the root mean square value of the energy contained in the noise is much smaller than that of a Is a sinusoidal oscillation, this results in a critical value for the signal-to-noise ratio at the limiter by lOdb. If this ratio of signal to noise at the limiter input is not reached, so the aforementioned "trap effect" for the oscillation is lost, as is the ratio of the signal to the noise at the limiter output falls quickly to zero, so that the oscillation can no longer be demodulated.

Based on these, at a later point

Receiver for frequency or phase modulated electrical oscillations with
automatic bandwidth control

Applicant:

Collins Radio Company Cedar Rapids, Ia.

(V. St. A.)

Representative:

Dipl.-Ing. K. A. Brose, patent attorney,

Pullach (Isar Valley), Wiener Str. 2

Named as inventor:

Howard D. Hern, Cedar Rapids, Ia. (V. St. A.)

Claimed priority:

V. St. ν. America November 14, 1957

(696 473)

explained in more detail, the invention is characterized in that between the output of the amplitude detector and the control input of the band filter is a non-linear transmission element is switched on, which is designed so that only when falling below the peak value of the noise Signal amplitude uses the bandwidth control.

The bandwidth, which is designed to be much larger than the frequency value of twice the modulation frequency of the receiver is therefore only restricted if intermodulation distortion is accepted will.

In a special embodiment of the invention, the non-linear transmission element contains a number of Diodes connected in series with resistors, each diode coming from a DC voltage source different voltage values is biased and each series connection between the output of the Low pass filter and earth is switched on. This way, depending on which of the diodes is through the control voltage output by the amplitude detector become conductive, the control voltage from this transmission element output control output voltage can be influenced non-linearly.

The non-linear transmission element is expediently designed so that when the input voltage falls the intermediate frequency band filter at the receiver

5u bandwidth at most up to the frequency amount of twice the maximum modulation frequency.

409 657/313

Preferably, according to a further embodiment of the invention, the non-linear transmission element in its transmission properties are adapted to the relevant receiver in such a way that with decreasing input voltage at the receiver the bandwidth is only reduced by an amount that is just enough to at the amplitude limiter input a ratio between the supplied signal energy and the noise energy of lOdb to obtain. .

The invention is explained in more detail with reference to the drawings, for example, in which shows

F i g. 1 is a block diagram of a receiver in which the non-linear transmission element according to the invention is used,

F i g. 2 a controllable device that can be used together with the non-linear transmission element according to the invention Band filter,

F i g. 3 details of the non-linear term,

F i g. 4 to 8 are diagrams used to explain the invention and

F i g. 9 is a circuit diagram of another type of non-linear element.

For a better explanation of the mode of operation and the design of the non-linear transmission element should first the computational relationships and then the entire receiver circuit in which this member Use is explained.

The total ratio of useful signal to interfering signal

at the receiver output | ] from can be viewed as having two components, namely:

1. the component

the end,,

which is based only on the noise generated in the receiver, and 2. the component ( ^s I from _{;) i} which is only based on the through

the bandwidth reduction generated distortion (hereinafter referred to as intermodulation distortion) is based.

The component ( ^Λ ) from _/; can be written as follows:

Ο

off ,, (in decibels) = 10 log H - 10 log 5 - 10 go . " Η! ? ■ ' B, ι

35

whereby

H is the critical signal amplitude at the limiter,

which makes 10 log H equal to 10 decibels, S is that part of the total receiver input signal that exceeds the critical height of the receiver input level,
Bif the frequency or phase modulated signal

bandwidth presented by the receiver, Ba the largest bandwidth of the demodulatable message transmitted by the received signal and

D is the ratio of the maximum carrier frequency change to the maximum modulation frequency.

In addition, the part S exceeding the critical level of the entire receiver input signal X can be expressed as follows:

Substituting equation (4) into equation (1) gives:

^S ) from ,, (in decibels) = 10 log -. £ "?" . (5) η j I Kl · j ,, ' Ba

Note that the receiver bandwidth is no longer included in equation (5). The component the ratio of useful signal to interfering signal

at the receiver output f ^s ) off ,, therefore changes

directly with the amplitude of the signal at the receiver input and is not affected by a change in the receiver bandwidth Bif.

Equation (3) can also be written as follows:

45

X ₁ = (KT- F ₁₁ ■ H) ■ Btf = C ₁

10 log 5 = 10 log

(2) X ₁ ,

where X _{1 is} the critical level of the receiver input signal.
It can also be demonstrated that

10log X ₁ = 10 log (KT · Bif) + 10 log (F ,,) -t- 10 log H

(3) is, where

K is Bolzmann's constant, T is the absolute temperature,

Bif is the IF bandwidth (which is a good approximation of the receiver's noise bandwidth which actually this factor is) and F _{n is} the receiver's noise figure.

Substituting equation (3) into equation (2) gives:

10 log 5 = 10 log

_HKT . _Bif . _Fii .

(4) where the constant C _{1 is} equal to KT -F _n -H . Accordingly, according to equation (7), the critical one changes

Bandwidth Bif linear with the value X ₁ , where Bif does not necessarily have to be the actual bandwidth of the receiver, but the bandwidth that the receiver must have if it is to receive a signal which exceeds the critical signal level X _1. In order for a signal to be received, it is important that the peak amplitude of the input signal X exceed the value X _{1 given} by the receiver's own input noise, pre-limiting RF gain and bandwidth. In conventional receivers with a fixed band filter, this value X _{1 is} a fixed value X ₀ . As already indicated, receivers are also known whose bandwidth changes automatically with the level of the received signal and which can therefore still receive signals with an amplitude that is already drowned in the noise in receivers with a fixed bandwidth. Based on this known state of the art,

5 6

a receiver is described below which has three tubes F ₁ , F ₂ , V ₃ , preferably designed as control tubes with variable tubes F 1, F 2, V 3, which are configured as a width control curve and which are particularly favorable receiving cathode followers. The anodes of these properties in the case of very small input signals, tubes are connected to a DC voltage source ß +, the dimensioning of the band 5 and connected by capacitors 54, 55, 56 with earth width control circuit in particular connected, while the cathodes, respectively about with the graphs in FIG. 4 to 8 resonant circuits 51, 52, 53 are connected to earth, is explained by means of. Capacitors 62, 63, 64 to the grids of the next

The in F i g. 1 shown in the block diagram of the following tube or with the IF output verEmpfänger has in a known manner one of an io bound, the grid of the tube V ₁ via antenna 10 with a frequency or phase modulation a capacitor 61 gelled the IF input signal Signal fed superimposing and mixing leads. The part 11 coming from the non-linear element, the bandwidth of which is sufficiently large to avoid DC control voltage via resistors 66, 67, 68 to avoid intermodulation distortion. applied to the grid of the tubes and changes the

From the output 12 of the part 11, this becomes the tube's steepness and thus its output amplitude the RF resistance fed by the antenna, which affects the IF center frequency output signal proportional IF signal to a bandwidth of different strengths in its tuned resonant circuits adjustable IF band filter 13 is supplied, which attenuates and thereby changed its bandwidth at one for the trouble-free operation of the receiver.

A sufficiently large RF input signal with a maximum of 20 In one embodiment, the limiter 28 takes over

Bandwidth is operated. At the same time, the function of the one discussed above comes from the output 14

the IF signal to an amplitude limiter 21 and amplitude detector.

to a conventional ZM or PM detector 22, e.g. B. The in F i g. 9 included limiter amplifier 88 σ,

a frequency discriminator, the output 88Z>, 88c, which are known per se, contain each

terminal 23 feeds the message content. 25 because a double triode V ₁₀ , V ₁₁ , F ₁₂ , the first

The amplitude of the incoming RF signal system as a cathode follower and its second system is determined in front of or in the limiter 21 by an amplitude as a grid base amplifier with a tank circuit HOa, tuddetektor 16, the IF band 110b, 110c being connected in the anode circuit, The filter does not have a disruptive effect, since in the coupling state, essentially all larger components of the signal circuit having cathode amplitudes are effected by a series connection of a lowest bandwidth. Diodes D ₁ , D ₂ , D ₃ are each transmitted across the spectrum. The alternating voltage setting contained in the output signal of the capacitors 113, 114, which enable a free potential amplitude detector 16, is switched in parallel with the cathode circuit by a low-pass filter 17. Parallel to each diode there is also a screened out, whose cutoff frequency is at least as high as 35 capacitor C ₁ , C ₂ or C ₃ and a resistor R ₁ , as the low-pass filter 89, which is usually contained by propagation conditions R ₂ or R ₅ Arrived- suitable choice of polarity of the diodes and the value of the RF signal. If the receiver is part of the resistors, the desired course of the multiple (diversity) receiver system, which works with tropospheric scattered radiation, is taken from the series-connected low-pass filters, then the 40 and the band filter 17, which can be varied in its bandwidth, are useful for a cut-off frequency of filter 13 supplied control direct current depending on 30Hz, since the highest rate of change received from the change in the RF input signal speed of the fading is usually below 30Hz. will.

The cut-off frequency should preferably be lower than that in FIG. 3 nonlinear super-lowest shown Be news frequency. 45 bearing link 18 contains the series to the counter

The signal emitted by the filter 17 is an equals R ₁ , R ₂ , R ₃ connected diodes D ₁ , D ₂ current signal, which has a relatively long, through the and D ₃ , which with their anodes with different time constants of the filter defined time span points of an average amplitude value of the RF input signal voltage divider 103 connected to a battery 104, which is proportional. In the case of a positive pole with respect to the noise 50 is grounded, so that the diodes are biased to the same or greater peak amplitude because of different potentials. The effect of the same amplitude of the useful signal and Rau diodes that the resistors R ₁ to R ₃ in see higher mean energy of the useful signal this non-linear way influence the direct current signal coming from the low-pass filter significantly more from the useful signal current. At very low signal span influenced than by noise. The direct current signal 55 voltages block all diodes, and their resistors of the filter 17 are not able to derive any current via the inventive non-current, the thus completely linear transmission element 18, which is explained in more detail later by one of the resistors 97 and 98, as a control signal 19 goes to the voltage divider in its standing position. A direct current bandwidth controllable IF bandpass filter 13 is supplied, bias voltage source 99 is connected in series via a resistor whose bandwidth is thus non-linear to the amplitude 60 stand 110 with the resistor 98. Above the RF input signal changes. One characteristic of an isolation resistor 102 from the connection of member 18 is shown in FIG. 4 and 5 and the control will be explained at a later point at point 100 of the voltage divider. voltage decreased. Since the battery 99 has a Die F i g. 2 shows a current known per se and, in the positive, greater current than the current delivered by the non-linear element in connection with the present invention, through the reversible band filter arrangement, the bandwidth of which is 98, the direction of flow can be reversed by a direct current control signal. the filter output. This switching arrangement essentially consists of the current at the filter output becoming a positive

control voltage, the level of which can be adjusted by changing the value of the resistor 110. On the other hand, if the negative voltage on capacitor 96, i. h at the filter output, large, then all diodes D ₁ , D ₂ and D _{3 place} their resistors in the circuit, whereby part of the current coming from the filter is diverted to earth and does not reach resistor 98. If the voltage output by the filter falls below the bias voltage which the voltage divider 103 causes on the diode D ₃ , the resistor R ₃ loses its dissipation effect. The remaining leakage current is determined by the value of the resistors R ₁ and R ₂ . If the signal amplitude continues to decrease, only D _{1 is found.} The mode of operation of a receiver equipped with bandwidth control is illustrated in FIG. 5 to 8 explained. According to equation (5) given earlier, the component of the ratio of signal to which is dependent on the receiver input noise changes (without bandwidth control)

Disturbances at output I " ) from" linear with height

of the receiver input signal X. This relationship is shown in FIG. 5 is represented by the line 31, the sizes being plotted in decibels. It can also be seen that this relationship only applies up to the critical value X _{0 of} the receiver input signal, which the vertical line 32 illustrates.

above the reverse voltage, and only the resistor R ₁ 15 light. With values smaller than X ₀ , the ampitude is effective, while with an even lower signal, the signal at the limiter is too small, so that the

that the whole _{M] tnis} / "

amplitude block all diodes so that the entire filter current flows through resistor 98.

The broken curve 82 in FIG. 4 shows the effect of the circuit according to FIG. 3. The last section 83 of curve 82 results when there is a large filter output that will cause all of the diodes to conduct so that resistors R ₁ , R ₂ and R ₃ can divert the current. The section 84 from «quickly falls to zero, like the unstable,

dashed line 33 shows.

The F i g. 6 shows how the bandwidth control increases the sensitivity of the receiver below the value X ₀ up to the value X _m m . If the input signal is larger than X ₀ , the normal bandwidth βmax is used. However, it sinks

if the input signal is below X ₀ at a lower signal amplitude, the diodes D ₁ and D _{2 will} conduct at the same time. The curve section 85 bandwidth is reduced. Point 34 on line 31

in Fig. 5. In

results when only the last diode D _{1 is} conductive and the last section 86 when no diode is conductive and no current is diverted through R ₁ , R ₂ or R _3.

When receiving small input signals, in addition to the automatic bandwidth control according to the invention, no automatic gain control is desired, since the determination of the amplitude as a guide value for the change in bandwidth requires a fixed, defined gain between the receiver input and the amplitude detector. However, a delayed automatic gain control is applicable to the subject matter of the invention, provided that the gain control begins only at an amplitude at which the value X ₀ mentioned earlier is exceeded. Such a delayed automatic gain control relieves the amplitude limiter in the case of very large input signals. If an automatic gain control is used at the same time as the automatic bandwidth control according to the invention, its additional effect must be taken into account when dimensioning the non-linear element 13.

in Fig. 6 corresponds to point 39 F i g. 8 shows the change in the critical bandwidth when the input signal X is less than the critical value X ₀ . Finally, if the input signal to the value X _m is fallen, the smallest value of the bandwidth B i _{n m} is achieved for the particular receiver, so that it lies in a further reduction of the input signal below the threshold of the receiver. It can be seen that the signal-to-noise ratio (in decibels) falls proportionally with the signal that is still above the reception threshold, although the signal can still be demodulated.

The theoretical smallest possible bandwidth of the amplifier 13 is known to be twice the bandwidth of the modulation signal, since the entire spectrum is not used with a smaller bandwidth.

In special cases, the smallest possible bandwidth is limited by other considerations, for example due to the increasing intermodulation distortion when the bandwidth is narrowed, their permissible level again depends on the area of application of the recipient, e.g. B. for multi-channel

The amplitude detector 16 in FIG. 1 from 50 receivers through the audible crosstalk,
giving signal level is the RF amplitude directly The F i g. 7 explains how, on the one hand, between the

proportional, if a linear detector is used, degradation of the output signal is caused. Hence, the one of this satisfies the control of the ... j 1 * ■ ί ^s \ jj

Band filter 13 detached detector for control ^by ^ modulation distortion (-) from ,, and other-

tensions do not meet the requirements of curve 81 in FIG. 4. Therefore, in FIG. 1 the invention non-linear member 18 is provided to provide the appropriate non-linearity between the control voltage amplitude and the amplitude of the signal emitted by the detector.

A typical characteristic of the non-linear element 18 is represented by the curve 82 in FIG. 4 reproduced, which consists of lines 83 and 84 as well as 85 and 86. Note that this curve 82 is somewhat lies above the curve 81, so that the DC control voltage supplied by the non-linear element 18 at any time becomes somewhat greater than the optimum DC voltage indicated by curve 81.

on the other hand, the deterioration of the output signal caused by the receiver noise feasible middle ground can be found. For many recipients, this is caused by both causes Impairment of the signal of concern. It depends essentially on the smaller of the both signal-to-noise (or interference) ratios, as this is the overall ratio at Significantly influenced receiver output. In Fig. 7th

Again, the line 31 represents the component I-out «, while the curve 37 represents the component

illustrated, which only applies to the intermodulation

10

Distortion relates. The line 32 represents the level of the input signal, from which with falling Input signal the bandwidth decreases. The curve branch 37 remains constant because of the constant Bandwidth does not change the intermodulation distortion. With increasing bandwidth reduction however, the intermodulation distortion increases (see left branch 37), mainly

because of the increasing denominator in (-) fromj ,. When the input signal falls from point 34 to point 38, the overall signal-to-noise (interference) ratio is deteriorated less by the component due to the intermodulation distortion than by the component due to the noise. When the point 38 is reached, both components exert an equally large influence, while with a smaller input signal than corresponds to the point 38, the influences are reversed. In many cases, therefore, the minimum bandwidth is determined by point 38. The minimum bandwidth can be a higher value of the input signal if above all audible crosstalk (ie intermodulation distortion) is to be avoided. However, if it is important to receive the message above all, even with a small amount of restricted crosstalk, a small minimum bandwidth can be provided, so that with input signals smaller than the point 38, reception of the deteriorated, but nonetheless possible still understandable message is possible.

Claims (4)

Patent claims: 1. Receiver for frequency or phase modulated electrical oscillations with one in its Bandwidth for the purpose of noise suppression adjustable intermediate frequency band filter, which is a Has base bandwidth which is substantially greater than the frequency amount of twice the maximum Is the modulation frequency and at its output an amplitude limiter and a downstream one Demodulator is connected, and with one of the unlimited intermediate frequency signal powered amplitude detector, the output of which is connected to the control input via a low-pass filter of the band filter is connected, characterized in that between the output of the amplitude detector and the control input of the band filter, a non-linear transmission element is switched on, which is designed in this way is that the bandwidth control is only activated when the signal amplitude drops below the peak value of the noise begins. 2. Receiver according to claim 1, characterized in that the non-linear transmission element contains a number of diodes (D ₁ , Z) ₂ , D _{3 ) connected in series with resistors (Ji 1} , R ₂ , R _s ) , each diode off a DC voltage source (103,104) is biased to different voltage values and each series connection between the output of the low-pass filter and ground is switched on. 3. Receiver according to claim 1 or 2, characterized in that the non-linear transmission element is designed so that when the input voltage at the receiver drops, the intermediate frequency band filter its bandwidth at most up to the frequency amount of twice the maximum modulation frequency decreased. 4. Receiver according to claim 1, 2 or 3, characterized in that the non-linear transmission element is adapted in its transmission properties to the recipient concerned, in such a way, that with decreasing input voltage at the receiver the bandwidth only increases by one amount is reduced, which is just sufficient to at the amplitude limiter input a ratio between the input signal energy and the noise energy of 10 db. Considered publications: Swiss Patent No. 261 287; Austrian patents No. 152 116, 153 942, 177; British Patent Nos. 491 183, 620 668; P i t s c h, "Textbook of radio reception technology", Leipzig, 1948, § 392 and p. 732; D a h 1 k e — N o w a c k, among other things, »Die Rohr im FM receiver ”, Part III, 1953, pp. 59 to 110; "Technisches Zentralblatt", 1958 / Elektro, p. 1284, 1285: Report from "Electronic Rundschau", 12 to 44, February 1958. For this purpose 2 sheets of drawings 409 657/313 8.64 © Bundesdruckerei Berlin