GB2113047A - Noise blanking in a radio receiver - Google Patents

Abstract

The receiver includes a controllable noise blanking attenuator 10 which is arranged in the I.F. signal path 3 of the receiver prior to a narrow band I.F. filter 5. A control unit 9 is provided which is responsive to received noise pulses and produces a blanking control pulse for the attenuator 10 whose duration depends upon the magnitude of the received noise pulse. Detectors 11 and 13 may be provided to detect the signal level at various points in the I.F. signal path and these detectors provide control signal inputs to the control unit 9 which control the threshold of operation of the control unit 9 together with the ultimate magnitude and/or duration of the noise blanking control pulse. The invention is particularly useful in mobile radio receivers. <IMAGE>

Description

SPECIFICATION Radio receiver TECHNICAL FIELD This invention relafes to a radio receiver and particularly to a receiver which includes a noise blanker, otherwise known as an extender.

Noise blankers are particularly useful in mobile radios fitted into vehicles, in order to control the effects of noise pulses due to the vehicle's ignition system.

The presence of a noise pulse is particularly troublesome in the narrow band l.F. filter of the receiver and causes this filter to ring. In a receiver fitted with a noise blanker the blanker may take the form of a controllable attenuator located in the l.F. signal path prior to the narrow band filter.

BACKGROUND ART In a known noise blanking receiver a detector is provided to detect the presence of a noise pulse in the received radio frequency (R.F.) signal and a control pulse is generated to control the controllable attenuator to attenuate the l.F. signal to blank out the detected noise pulse.

In the above known receiver the control pulse fed to the controllable attenuator is of fixed duration and amplitude and produces a hard attenuation of the l.F. signal. This hard attenuation can cause problems under certain reception conditions. For example in the presence of strong signals having neighbouring carrier frequencies to that of a desired channel, the action of the blanking attenuator modulates these carriers with a narrow pulse which in consequence has a wide bandwidth. Sidebands are therefore produced which extend across the narrow band l.F. of the receiver and produce interference, known as "splatter". This effect can be completely devastating when there are a large number of powerful nearby carriers and can make the receiver unusable, whilst a receiver without the noise blanking would be relatively unaffected.

This invention seeks to provide a noise blanking radio receiver in which the above described problem with known blanking receivers is at least mitigated.

BRIEF SUMMARY OF INVENTION According to this invention there is provided a radio receiver including a controllable noise blanking attentuator arranged in the I.F. signal path prior to narrow band I.F. filtering means and means for producing a blanking control pulse for the attenuator whose amplitude and/or duration is dependent upon at least one of the amplitude of the noise pulse and the amplitude of I.F. signals in the receiver.

The means for producing a blanking control pulse may include means for adjusting the amplitude and/or duration fo the blanking control pulse in response to a signal representative of the amplitude of the l.F. signal at a point in the l.F.

signal path prior to the controllable attentuator.

The means for producing a blanking control pulse may include means for adjusting the amplitude and/or duration of the blanking control pulse in response to a signal representative of the amplitude of the l.F. signal at a point in the I.F.

signal path after the narrow band I.F. filtering means.

In an embodiment of the invention the means for producing a blanking control pulse includes means for stretching a pulse derived from a received noise pulse.

The means for stretching may include capacitive mens arranged to be charged from a relatively low impedance source and to be discharged into a relatively high impedance.

A third pulse stretching stage may be provided, the effect of the third stage being dependent upon the stretched pulse duration provided by the first and second stages.

In a preferred embodiment the means for producing a blanking control pulse includes a pulse detector for detecting the presence of a noise pulse in the received R.F. signal.

Means may be provided for adjusting the threshold of action of the pulse detector.

The means for adjusting the threshold of the pulse detector may be operative to adjust the threshold in the presence of a steady R.F. carrier.

The means for adjusting the threshold of action of the pulse detector may include means operative to adjust the threshold in dependence upon'the rate of occurrence of noise pulses.

The means for adjusting the threshold of action of the pulse detector may include means for adjusting the threshold in dependence upon the amplitude of I.F. signals in the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS An exemplary embodiment of the invention will now be described with reference to the drawings in which; Figure 1 shows an embodiment of a radio receiver in accordance with the present invention.

Figure 2 shows an alternative embodiment of radio receiver in accordance with the invention and Figure 3 illustrates one form of control unit suitable for use in the embodiments of Figures 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION Referring now to Figure 1, R.F. radio signals received at an aerial I are fed to an R.F. front end i 2 which includes R.F. amplifier, local oscillator and mixer circuits of any suitable known design and which produces an intermediate frequency (I.F.) signal which is fed over an I.F. signal path 3 via a noise blanking attenuator 4 to a narrow band l.F.

amplifying and filtering stage 5. I.F. signals after being amplified and filtered by the l.F. stage 5 are finally fed to an audio signal recovery detector 6 which produces an audio output signal at an output terminal 7 for use by a suitable loadspeaker.

Radio frequency signals received at the aerial 1 are also fed via an R.F. amplifier and band limiting filter 8 to a control unit 9. The control unit 9 includes a pulse detector for detecting interference noise pulses and produces a blanking control pulse which is fed to a control input 10 of the noise blanking attenuator 4.

As explained above the effect of the amplitude modulation of the noise blanking attenuator 4 is to cause splatter interference in the I.F. stage 5 of the receiver. This problem is particularly severe in prior art noise blanking receivers in which the control pulse fed to the noise blanking attenuator 4 is of fixed amplitude and duration.

The blanking pulse which is required to effectively blank a large amplitude interference pulse is longer than that required by a week interference pulse. Also the modulation side bands created by the blanking pulse are directly related to pulse width. Consequently the signal to noise degradation caused by the unwanted side bands can be reduced if the blanking pulse is reduced in duration to an extent which is just sufficient to blank the interfering pulse. In accordance with an embodiment of the present invention the control unit 9 generates in response to a detected noise pulse a control pulse whose duration depends upon the amplitude of the detected noise pulse.

Consequently the control unit 9 generates longer blanking pulses when the received noise pulses are of large amplitude and generates a shorter control pulse when the noise pulses are of lower amplitude.

The blanking pulse required to blank effectively a large amplitude interference pulse, in addition to being longer than that required for a weak pulse also needs to be of higher amplitude and the control unit 9 therefore provides a blanking control pulse of amplitude dependant upon the amplitude of detected noise pulses.

The amount of blanking required is dependent upon the level of the received signal and if the received signal is very strong it may not be necessary to effect any blanking at all.

The adjustment of the threshold level at which blanking takes place is effected in the embodiment illustrated in Figure 2 to which reference will now be made.

In Figure 2 where like parts to those in Figure 1 bear like reference numerals the control unit 9 feeds a control pulse to the control input 10 of the noise blanking attenuator 4 whose amplitude and width are related to the amplitude of the interference noise pulse detected by the pulse detector forming part of the control unit 9.

An amplifier and detector 11 is coupled to the - l.F. signal path 3 prior to the noise blanking attenuator 4 and provides a detected output signal whose level is dependent upon the level of the relatively broad band I.F. signal fed to the l.F.

signal path from the R.F. front end 2. This signal is fed to a control input 12 of the control unit 9 and is effective to adjust the threshold level at which the pulse detector is able to detect interference noise pulses.

In simiiar manner a signal power detector 13 is coupled to the I.F. signal path at a point after.the narrow band stage 5 and provides an output signal whose level is dependent upon the power in the channel to which the receiver is tuned. The signal provided by the detector 13 is fed to a further control input 14 of the control unit 9 and this control signal is also effective to control the operating threshold of the pulse detector.

Consequently as the signal power in the receiver increases the threshold level at which blanking takes place is increased and also the amplitude of the blanking pulse is made dependent upon the amount by which the noise pulse amplitude rises above the set threshold level. Since the duration of the blanking control pulse depends upon the amplitude of the detected noise pulse this duration will also be automatically adjusted by a threshold adjustment. By providing control of the amplitude of the noise blanking control pulse in addition to its duration in dependence upon noise pulse amplitude and received signal level a more nearly optimum blanking control characteristic can be obtained.

The control unit 9 of the embodiment of Figure 2 will now be described in more detail with reference to Figure 3.

Referring now to Figure 3, interference pulses fed from the amplifier and filter 8 are received at a terminal 1 5 of the control unit and are fed to base 16 of an NPN transistor Ti. The base 16 of the transistor T1 is biased by means of resistors 17 and 18, which are connected to reference potentials 1 9 and 20 respectively, the potential 20 being in the present example earth potential. The transistor T1 has an emitter 21 which is coupled to the earth potential 20 via a biasing resistor 22 across which is connected a decoupling capacitor 23. The transistor Ti also has a collector 24 which is fed from the reference potential 1 9 via a resistor 25, a diode 26 and a further resistor 27 connected in series with one another.

The transistor T2 has a base electrode 28 coupled to the junction between the resistor 27 and the diode 26. The transistor T2 has an emitter electrode 29 coupled to the reference potential 1 9 and a collector electrode 30 which is coupled via series connected resistors 31 and 32 to one electrode 33 of a capacitor 34. The electrode 33 of the capacitor 34 is also connected to the emitter electrode of the transistor Ti, whilst the opposite electrode of the capacitor 34 is connected to the reference potential 20. A further capacitator 35 is connected between the junction of the resistors 31 and 32 and the earth potential 20.

The transistors T1 and T2 together act as the pulse detector of the control unit in a manner to be described later.

The collector electrode 24 of the transistor T1 is coupled to base electrode 36 of the transistor T3 whose collector electrode 37 is directly connected to earth reference potential 20 and whose emitter electrode 38 is coupled via a resistor 39 to the reference potential 1 9. A capacitor 40 is coupled between the base electrode 36 of the transistorT3 and the earth reference potential 20. The collector electrode 38 of the transistor T3 is connected to base electrode 41 of a transistor T4 and a capacitor 42 is coupled between this base electrode 41 and the reference potential 20. A further series combination of a resistor 43 and a capacitor 44 is also coupled between the base electrode 41 of the transistor T4 and the potential 20.The transistor T4 has a collector electrode 44 connected directly to the earth potential 20 and an emitter electrode 45 coupled via a resistor 46 to the reference potential 19 and via a resistor 47 to an output terminal 48.

Finally amplitude control of the noise blanking pulse in response to signal levels within the receiver is effected by means of a transistor T5 having a base electrode 49 coupled to an input terminal 50.

The transistor T5 also has an emitter electrode 51 coupled via a resistor 52 to the reference potential 1 9 and a collector electrode 53 which is directly connected to the emitter electrode 21 of the transistor TI . The operation of this amplitude control arrangement will be described in due course.

In operation the transistors T1 and T2 acting in combination provide a pulse detector, a full description of the operation of which has been given in U.K. patent application No 7914882.

Interference pulses fed via the amplifier and filter 8 arrive at the terminal 15 and appear on the base electrode 1 6 of the transistor Ti. This transistor T1 is biased so as to be only just conducting and it therefore has a rectifying action passing only interference pulses, of a single polarity which rectified pulses appear at the collector electrode 24.

The transistor T2 is effectively connected as a feedback control between the collector electrode 24 of the transistorT1 and the emitter electrode of that same transistor and this feedback determines and controls the biasing and hence the conduction point of the transistor T1. Under conditions in which a large steady carrier is fed to the terminal 15 the time constant of the capacitor 34 which is charged from the feedback biasing transistorT2 will ensure that the emitter biasing of the transistor T1 is changed so as to tend to turn off that transistor.

The detected pulse appearing at the collector electrode 24 of the transistor T1 charges the capacitor 40 effectively from a low impedance source but in view of the presence of the resistors 25 and 27 this capacitor can only discharge into a high impedance with the result that the pulse appearing at the base electrode 36 of the transistorT3 is stretched in duration. This stretched pulse is further stretched by the action of the transistorT3 which in a like manner charges the capacitor 42 from a low impedance source which capacitor can also only discharge into a relatively high impedance via the resistor 39.

A still further stretching is produced by a series combination of the resistor 43 and the capacitor 44 but the time constant of this combination is such that this further stretching is progressive in its action being more effective for longer pulses than for shorter ones. The final stretched pulse is fed via the transistor T4 which is connected as an emitter follower buffer amplifier to the output terminal 48 at which appears the final blanking control pulse for feeding to the control input 10 of the noise blanking attenuator 4.

The detected pulse appearing at the collector electrode 24 of the transistor T1 and hence that appearing at the output terminal 48 will of course be dependent upon the magnitude of the R.F.

interference pulse arriving at the input terminal 1 5 but of course this magnitude will be relative to the threshold level for detection of the transistor T1 set at the emitter electrode 21 of that transistor.

As mentioned previously control of the amplitude of the blanking control pulse in dependence upon signal levels through the receiver is effected by means of the transistor T5.

Signals representative of the broadband I.F. power and the narrow band signal power level provided by the detectors 11 and 1 3 respectively may be combined and fed to the input terminal 50 and hence to the base electrode 49 of the transistor T5. The collector electrode 53 of the transistor T5 is coupled to effect the biasing of the transistor T1 via the emitter electrode 21 of that transistor so that as the signal presented at the terminal 50 increases in amplitude, indicating greater signal strength in the receiver the transistor T1 has its operating conditions changed so that it requires an interference pulse of greater amplitude in order to exceed the turn on threshold of the transistor T1.

Therefore for relatively low amplitude interference pulses the transistor T1 remains turned off and no blanking takes place.

It is also undesirable to effect noise blanking in the presence of a large number of closely spaced interference pulses and therefore the amplitude of the noise blanking control pulse is also controlled in dependence upon the rate of arrival in the receiver of interference pulses. This will now be described with reference to Figure 4 where like parts to those in Figure 3 bear like reference numerals.

Referring now to Figure 4 the collector electrode 24 of the transistor T1 is coupled via two series connected resistors 54 and 55 to base electrode 56 of a transistorT6. Biasing of the transistor T6 is effected by means of two series connected diodes 57 coupled between the reference potential 1 9 and the junction of the resistors 54 and 55. A capacitor 58 is coupled between the base electrode 56 and the transistor T6 and the potential 19. The transistor T6 has an emitter electrode 59 coupled directly to the reference potential 1 9 and a collector electrode 60 which is coupled to the emitter electrode 21 of the transistor T1 in order to control the quiescent bias conditions of that transistor.

In operation detected pulses appearing at the collector electrode 24 of transistor T1 are integrated by the combination of the resistor 55 and the capacitor 58 and an integrated potential appears at the base electrode 56 of the transistor T6, the magnitude of this potential depending upon the rate at which pulses appear at the collector electrode 24. The potential appearing at the collector electrode 60 of the transistor T6 will depend upon that at its base 56 and this potential is operative to tend to turn off the transistor T1 when the rate of appearance of interference pulses is very high.

It must be understood that this invention has been described by way of example only and modifications may be made without departing from the scope of the present invention. For example although the control unit illustrated in Figure 3 is intended to include a control of the blanking pulse in dependance upon signal levels in the receiver, the transistor T5 together with the detectors ii and 1 3 can be omitted if desired and a similar circuit to that shown in Figure 3 can then be utilized as the control unit in the arrangement of Figure 1.Further although as described the invention provides blanking control pulses whose amplitude and duration are dependent upon the amplitude of the interference pulses and upon signal levels in the receiver, this is not essential and an improvement in the performance of the receiver under "splatter" conditions can be obtained by purely varying either amplitude or the duration of the noise blanking control pulses in dependence upon one or other of the amplitude of noise pulses and signal levels in the receiver.

In the described embodiment of Figure 3 the controls of amplitude and duration of the blanking control pulses are interdependent. Also control of the threshold levels affects both the amplitude and duration of the blanking control pulses. This is not essential and all the controls of the blanking pulses may be made independent of one another if so desired. Such independent control will of course be more complex than the embodiments descirbed herein.

Claims (12)

1. A radio receiver including a controllable noise blanking attenuator arranged in the I.F.

signal path prior to narrow band l.F. filtering means and means for producing a blanking control pulse for the attentuator whose amplitude and/or duration is dependent upon at least one of the amplitude of the noise pulse and the amplitude of l.F. signals in the receiver.

2. A radio receiver as claimed in Claim 1 wherein the means for producing a blanking control pulse includes means for adjusting the amplitude and/or duration of the blanking control pulse in response to a signal representative of the amplitude of the IF. signal art a point in the IF.

signal path prior to the controllable attentuator.

3. A radio receiver as claimed in Claim 1 or 2 wherein the means for producing a blanking control pulse includes means for adjusting the amplitude and/or duration of the blanking control pulse in response to a signal representative of the amplitude of the l.F. signal at a point in the l.F.

signal path after the narrow band l.F. filtering means.

4. A radio receiver as claimed in any preceding claim wherein the means for producing a blanking control pulse includes means for stretching a pulse derived from a received noise pulse.

5. A radio receiver as claimed in claim 4 wherein the means for stretching is arranged in at least two stages.

6. A radio receiver as claimed in Claim 4 or 5 wherein the means for stretching includes capacitive means arranged to be charged from a relatively low impedance source and to be discharged into a relatively high impedance.

7. A radio receiver as claimed in Claim 5 or 6 wherein a third pulse stretching is provided, the effect of the third stage being dependent upon the stretched pulse duration provided by the first and second stages.

8. A radio receiver as claimed in any preceding claim wherein the means for producing a blanking control pulse indicates a pulse detector for detecting the presence of a noise pulse in the received R.F. signal.

9. A radio receiver as claimed in Claim 8 wherein means is provided for adjusting the threshold of action of the pulse detector.

10. A radio receiver as claimed in Claim 9 wherein the means for adjusting the threshold of action of the pulse detector is operative to adjust the threshold in the presence of a steady R.F.

carrier.

ii. A radio receiver as claimed in Claim 9 or 10 wherein the means for adjusting the threshold of action of the pulse detector includes means operative to adjust the threshold in dependence upon the rate of occurrence of noise pulses.

12. A radio receiver as claimed in any one of Claims 9 to 11 wherein the means for adjusting the threshold of action of the pulse detector includes means for adjusting the threshold in dependence upon the amplitude of l.F. signals in the receiver.

1 3. A radio receiver substantially as herein described with reference to and as illustrated in the drawings.