Noise blanking system for an AM radio receiver

To a standard AM radio receiver there is connected an impulse-noise suppression system comprising a preliminary blanking gate adapted for connecting to and for interrupting the AM-modulated signal path at the input of the IF section, and an audio blanking gate adapted for interrupting the audio circuit. Both blanking circuits detect impulse noise at the RF amplifier and with appropriate delays blank both points. Audio blanking masks the audio disturbance caused by the blanking in the AM-modulated-signal path. Audio blanking time is preferably from 2 to 3 times the duration of the blanking of the AM-modulated-signal path and is thus kept very short causing a minimum interruption of the wanted audio signal. Associated with the audio-signal-path blanking circuit is a sample and hold circuit for smoothing the blanked audio signal and virtually eliminating an audio disturbance or noise that is otherwise generated by the audio blanking circuit itself.

BACKGROUND OF THE INVENTION 
This invention relates to AM radio receivers having a noise blanking 
circuit and more particularly to such a receiver wherein the blanking 
circuit responds to a noise spike by blanking both the modulated-AM-signal 
path and the audio-signal path. 
This invention relates to all kinds of AM radio receivers and the term 
"modulated-AM-signal path" is thus meant to include the signal path in the 
RF section of a tuned radio frequency (TRF) receiver as well as to the 
signal path through the tandem combination in a superheterodyne receiver 
of the RF, mixer and IF sections. Also, the term "blanking" as applied to 
a signal path is used broadly herein to mean blocking the signal flow 
along the signal path, such as by interrupting the signal path, shorting 
the signal path or removing the electrical energizing source from a stage 
through which the signal path is routed. 
The suppression of impulse noise in AM receivers has been accomplished by 
clipping off any impulse at the antenna that is greater than the amplitude 
modulation at the moment. This and other clipping circuits represent the 
simplest and least expensive AM noise blankers. Such noise clipping 
circuits do not eliminate the noise but just reduce its amplitude. 
Another noise blanking system senses the noise in the RF or IF sections of 
a superheterodyne receiver and blanks the audio signal path. This system 
must employ a long blanking time that results in a thump sound from the 
receiver. This disadvantage is explained by the fact that nothing is done 
to protect the receiver RF and IF sections from overload and the pulse is 
stretched to a value equivalent to at least the period corresponding to 
the entire selectivity (bandwidth) of the receiver. As the noise impulse 
energy rises, the corresponding audio noise pulse is stretched more and 
more since the gain of the receiver will amplify the start and finish 
point of the initially stretched pulse in the IF filter farther and 
farther into IF amplifier saturation. This produces a long and variable 
length pulse. A low energy noise impulse is stretched to a minimum length, 
e.g. 150 microseconds, in the audio section determined by the IF passband, 
e.g. 452 KHz to 458 KHz. The audio noise pulse contains audio frequency 
components that are lower than the broadest audio frequency response that 
is determined by the IF passband, and thus at higher impulse energy 
levels, the audio noise pulse is even longer, e.g. 500 to 1000 
microseconds. Also for high impulse noise the receiver AGC may be 
activated to the point that the desired signal is heavily attenuated. 
A more complex but more effective and widely used noise blanking system in 
AM radios is one that senses the impulse noise in an early portion of the 
IF section of the receiver and blanks the signal path downstream at a 
point in the AM-modulated-signal path, e.g. in a later portion of the RF 
section of the radio. Such a system was first described by James J. Lamb 
in the paper entitled "A Noise Silencing I.F. Circuit for Superhet 
Receivers", QST, February 1936, pp. 11, 12, 13, 14, 38, 90, 92, 106, 108, 
110 and 112, and in his patent U.S. Pat. No. 2,101,549 issued Dec. 7, 
1937. Subsequently, a modified Lamb noise suppressor system senses the 
presence of a noise pulse in the RF section and blanks downstream in 
either the RF section of IF sections of the receiver. However, as is 
further explained below, this kind of noise suppressor system operative in 
the RF or IF sections of the receiver results in a bop sound at each 
incidence of a noise impulse. That sound is more pronounced in high 
fidelity AM radio receivers such as the recently introduced stereo AM 
receivers wherein such disturbances are even more objectionable. 
It is therefore an object of the present invention to provide a noise 
blanking system that upon being connected to an AM radio receiver more 
effectively suppresses impulse noise therein. 
SUMMARY OF THE INVENTION 
To a conventional AM radio receiver there is added a preliminary blanking 
means and an audio blanking means. In one particular aspect of the 
invention, the preliminary blanking means is connected to a detection 
point and to another point, the RF blanking point that is either 
coincident with or downstream of the detection point in the 
AM-modulated-signal path of the receiver. In another particular aspect of 
the invention, the preliminary blanking means includes a separate antenna 
for detecting the noise impulse. the preliminary blanking means is for 
generating a preliminary blanking pulse of predetermined fixed duration 
and for blanking the signal in the AM-modulated-signal path for said 
duration at the RF blanking point when a noise pulse is detected. 
The audio blanking means is connected to a sensing point in the 
AM-modulated-signal path, that may be the above-mentioned detection point, 
or to the separate antenna, and is connected in the audio-signal path of 
the receiver for interrupting the audio-signal path for a period of from 2 
to 5 times that of the preliminary blanking pulse duration. A sample and 
hold circuit means connected downstream of the audio blanking point of the 
audio-signal path is for sampling and holding the voltage constant there 
during the above-noted period of audio-signal-path interruption. This has 
the effect of smoothing the interrupted portion of the audio signal. 
In a conventional radio receiver the audio bandwidth is limited and 
determined by the bandwidth of the radio-frequency selective circuits in 
the preceeding AM-modulated-circuit path. And in that path the most narrow 
and controlling bandwidth is commonly found in the IF section. Thus the 
least width of a noise pulse that occurs in the audio section in response 
to an impulse of amplitude modulation at the input of the IF section is 
inversely related to the bandwidth of the IF section. 
However, this invention recognized that the width of a disturbance that 
occurs in the audio section in response to RF blanking and the resulting 
square pulse of amplitude modulation in the input of the IF section will 
always be twice or a little more than twice the width of the square pulse 
of amplitude modulation no matter what bandwidth the IF section may have. 
Therefore, the width of the audio blanking pulse is preferably made from 2 
to 3 times larger than the width of the RF blanking pulse. If that audio 
blanking pulse is given the appropriate amount of delay, e.g. 10 sec, to 
register it with the audio disturbance from the RF blanking pulse, optimum 
noise suppression is achieved. Alternatively, if in the interest of 
simplicity no such delay in the audio blanking pulse is provided, then it 
may be necessary to increase the audio blanking pulse width to from 3 to 5 
times the width of the RF blanking pulse. Thus the RF (or IF) blanking 
removes electromagnetic impulses whether from meteorological sources, from 
fluorescent lamps, light dimmers with silicon controlled rectifiers, motor 
commutation systems, or relays but produces a square hole in the RF or IF 
carrier (a time of zero modulation) that produces a disturbance itself in 
the audio section. The audio blanking, and filling by a sample and hold 
circuit, renders that disturbance essentially imperceptible by a radio 
listener. 
The noise suppression system of this invention is especially compatible 
with high performance AM radios, such as the recently introduced AM stereo 
radios, that are sold at premium prices. In this case each audio path will 
have an audio-signal path blanking gate. Other such applications include 
AM radios for navigation and military equipment where high quality 
performance also has a particularly high value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the radio receiver 10 includes an RF section 14 with 
an input connected to an antenna 12, a local oscillator mixer section 16 
and an IF section 18. Thus in the receiver 10, the main 
AM-modulated-signal path passes through the tandem connected RF, mixer and 
IF sections to the AM detector 20. The main audio signal path of the 
receiver 10 subsequently passes through the audio amplifier 22 to the 
speaker 24. 
A noise impulse 26p, that may be superimposed on an RF carrier is 
illustrated by waveform 26 in FIG. 2A. Impulse 26p appears at the input of 
the receiver 10, at point A in FIG. 1. In response, a "pulse" of 
substantial width as illustrated by the waveform 28 in FIG. 2B appears at 
point B at the output of the tuned RF section 14 having a band width of 
about 10 KHz. This "pulse" 28 at point B is a transient oscillation of the 
frequency to which the RF section 14 is tuned. In conventional AM 
broadcast receivers tunable over the band of 0.5 to 1.5 Khz, pulse 28 
typically lasts for about 50 microseconds, a quantity that is inversely 
related to the bandpass of the RF section 14. Also its beginning is 
slightly delayed from the impulse 26. The IF section 18 of the receiver 10 
has a piezoelectric filter 18' that resonates at the IF frequency and 
establishes an IF section bandpass of about 12 KHz. IF section 18 further 
includes an amplifier 18". 
In passing through the IF filter 18', the transient becomes even wider 
owing to the narrow IF bandpass. In fact, from point B to point C, the 
pulse id transformed in three respects. The RF pulse 26 is transformed to 
transient pulse 28 that oscillates at the IF frequency after being 
heterodyned by the mixer 16. The filter 18' delays the start of this 
oscillation by about 50 microseconds and lengthens it to about 200 
microseconds. The pulse at point C illustrated in FIG. 2C as waveform 30 
is a transient oscillation at the IF frequency, e.g. 455 KHz. All pulse 
widths are measured at a level of 10% of peak pulse amplitude. Because of 
the IF-bandpass-related exponential decay characteristic at the trailing 
edge of the pulse 30 at point C and because of the early saturation of the 
high gain amplifier 18", the pulse width of the amplified and clipped 
pulse 32 at point D at the output of the amplifier 18" is much greater 
yet. The AM detector 20 preserves only the envelope of pulse 32 to produce 
at point E an "audio" pulse 34 of the same width as is pulse 32. 
For noise pulses of increasing energy, the amplitudes of pulses 28 and 30 
increase proportionally while their pulse widths remain about constant. 
However, for noise pulses of increasing energy IF amplifier 18" soon 
saturates and the pulses 32 and 34 have no greater amplitudes but their 
pulse widths increase, e.g. to as much as 800 and 1000 microseconds for a 
typical AM broadcast band receiver. 
Referring to FIG. 3, a noise suppression circuit 38 of this invention is 
connected to the radio receiver 10. The noise suppression circuit 38 has 
an amplifier 40 followed by a one shot multivibrator 42 and an 
electrically activatable gate 44. These three elements constitute a 
preliminary noise suppression or blanking means that is operative in the 
AM-modulated signal path. The noise suppression circuit 38 additionally 
includes another one shot multivibrator 46, another electrically 
activatable gate 48 that momentarily blanks for a period determined by the 
blanking control pulse from block 46, and a sample and hold circuit 50. 
These later three circuits are the key components of an audio noise 
blanking means having noise-blanking efficacy only in the audio signal 
path of the receiver 10. 
When the receiver is tuned to an RF carrier signal 52 and a noise impulse 
54 is picked up by the antenna 12 as in FIG. 4F, an oscillating transient 
pulse 56 appears at the output of mixer section 16 in receiver 10 at point 
G. At the same time the amplifier 40 filters out the RF signal 52 and thus 
selectively amplifies the noise impluse 54. The amplified noise impulse 
triggers the one shot multivibrator 42 after a short delay (e.g. 10 
microseconds) to produce at the output of the multivibrator 42 a first 
blanking gate control pulse 58 about 70 microseconds wide that is 
coincident in time and a little larger in width than that of the pulse 56. 
Control pulse 58 causes the MOS gate 44 to open and thus momentarily blank 
the AM-modulated signal path for the period when the control pulse is on. 
By this blanking means the noise pulse is prevented from getting through. 
However, during the blanking of pulse 56, the carrier as well as the noise 
is prevented from getting through and at point J in the receiver the 
waveform 60 consists of the IF carrier with a hole in it, as in FIG. 4J. 
This hole represents a period of zero AM modulation and after transmission 
through the narrow bandpass (e.g. 6 KHz) of the IF section 18 appears at 
point K as the IF signal with waveform 62 as shown in FIG. 4K. The 
envelope of pulse 62 appears downstream of the detector 20 at point L in 
the signal path as the "audio" signal having a waveform 64 shown in FIG. 
4L. 
The one shot multivibrator 46 produces an output blanking pulse 66 that is 
delayed about 45 microseconds from the noise impulse 54 and that has a 
width of about 190 microseconds. The audio blanking pulse 64 turns off the 
gate 48 opening the audio signal path beyond point L and turns on the 
sample-and-hold circuit 50 to clamp the voltage at point N for the 
duration of pulse 66 at the level it had been at the time of initiation of 
pulse 66. As a result, the noise impulse causes essentially no disturbance 
in the audio signal at point N that is represented in FIG. 4N as a 
straight line waveform 68. 
Referring again to the foregoing discussion of a standard superheterodyne 
receiver 10 without noise suppression means as in FIG. 1, the width of a 
pulse appearing at the output of the bandwidth determining portion 18" of 
the IF section 18 in response to an impulse of infinitesimal width at the 
input of the IF section 18 will always be about equal to 1/2BW where BW is 
the IF bandwidth. For example, that IF output pulse at points C will be 
the reciprocal of the IF bandwidth halved, 170 microseconds, plus the 
amount of the input pulse width, 50 microseconds, totalling 220 
microseconds. Actually in this case it was closer to 200 microseconds but 
this rule of thumb is always useful and points up the fact that the width 
of pulses along the signal path in the receiver that stem from noise 
impulses are a known function of the bandwidths of the tandem connected 
receiver sections through which the AM modulated signals are processed, 
except when any section is allowed to saturate, e.g. as in FIG. 2D. That 
causes signal clipping which can expand the pulse width many times due to 
the amplification and exponential tailing off of the preliminary-blanking 
disturbance. 
The preliminary blanking system represented in FIG. 3 by circuit blocks 40, 
42 and 44 prevents saturation and produces a short 150 microseconds wide 
pulse of AM modulation at the output of the IF section, point K, as shown 
in FIG. 4K. The width of this IF output "noise pulse" is not dependent 
upon the bandwidth of the IF section. Its width is basically only 
dependent upon the duration of the AM-modulated-signal path blanking pulse 
(determined by one shot multivibrator 42), namely about twice that 
duration. This is the case because the IF output disturbance has a falling 
portion initiated at the delayed onset of the RF blanking pulse and 
symmetrically therewith a rising portion initiated at the delayed 
termination of the RF blanking pulse. 
A preferred MOS audio blanking gate circuit and a preferred MOS-bipolar 
sample-and-hold circuit are shown merged in FIG. 5. The MOS gate 
transistor 70 is a P-channel depletion device having a source connected to 
a biasing voltage divider of resistors 72 and 74 and to an input terminal 
76 corresponding to input terminal 76 in FIG. 3. The gate of transistor 70 
is connected to control input terminal 78 and a high (positive) signal 
here turns off transistor 70 whereas a low signal here turns on transistor 
70. Capacitors 82 and 84 each have a value of 0.1 picofarads and serve to 
"compensate" and thus cancel the rise and fall portions of the audio 
blanking pulse that tend to couple into the input of audio amplifier 22.. 
The drain of gate transistor 70 is connected to the network comprised of 
resistor 86 (e.g. 100K ohms) and capacitor 88 (e.g. 10 picofarads) that 
are connected at the gate of the N- channel transistor 90. Transistor 90 
serves as a high-input-impedance linear buffer amplifier with an output 
connected to the Darlington connected transistors 92 and 94. 
When the gate transistor 70 is conducting as is the case when there is no 
impulse noise, transistor 90 and transistors 92 and 94 pass the audio 
signal from terminal 76 to output terminal 96 that is in turn connected to 
the input of the audio amplifier 22. But as soon as a noise impulse causes 
a positive pulse at control terminal 78, transistor 70 opens and the gate 
voltage at transistor 90 is held at the level last appearing at terminal 
76. The audio signal at terminal 96 is "frozen" until the blanking pulse 
at control terminal 78 again connects the audio signal from the detector 
20 to amplifier 22. 
In the second preferred embodiment of FIG. 6, a stereo AM radio receiver 
100 has an antenna 102, a tuned RF section 104, a mixer 106, an IF section 
108, an AM stereo detector 110 with a "left" audio signal path 112 amd a 
"right" audio signal path 114, two audio amplifiers 116 and 118, and two 
speakers 122 and 124. 
A noise blanking system 126 has its own antenna 128 and its own RF section 
130. RF section 130 is a broadband RF amplifier. Large impulse noise 
triggers the one-shot multivibrator 132 that turns off the normally on MOS 
gate 134 effecting blanking of the very same noise impulse having been 
simultaneously picked up by the radio antenna 102. 
Another one shot multivibrator 136 is also triggered by the same noise 
impulse but has a built-in delay of about 40 microseconds, a similar 
feature to that of the multivibrator 46 described above. Multivibrator 136 
produces an audio gating pulse of about 190 microseconds. This turns off 
for 190 microseconds both the audio blanking gates 138 and 140, each of 
which includes a sample and hold circuit. In this way both left and right 
audio signals are blanked and smoothed during the audio signal disturbance 
created by the preliminary blanking of the impulse noise in the 
AM-modulated signal path. 
Of course, the antenna 102 may or may not be a part of the receiver 100 in 
FIG. 6. Likewise, the antenna 128 may or may not be part of the noise 
blanking system 126. Also, the receiver 100 and the noise blanking system 
126 may have their RF sections, 104 and 130 respectively, connected to the 
same antenna. Also, in principle the blanking MOS gate 134 may 
alternatively be connected between the antenna and the RF section 104 or 
anywhere else in the AM modulated signal path down to the IF filter. And 
even more generally, this blanking circuit may be employed in a T.R.F. 
receiver having no IF section (not shown).