Automatic IF selectivity for radio receiver system

A microprocessor controlled radio receiver system for switching bandwidths of an intermediate frequency stage from a normally scanning mode using a relatively narrow bandwidth to a second mode using a relatively wide bandwidth automatically when there are no interfering signals from adjacent channels of a channel or station selected for receiving. Upon selection of a channel for listening the microprocessor controlled synthesizer causes the synthesizer to scan up one adjacent channel and down one adjacent channel to determine if there is an interfering signal therefrom and then returns to the selected channel. In the event of no interfering signal from an adjacent channel, switching is effected in the IF stage from a narrow bandwidth to a wide bandwidth to improve the quality of reception. If there is an adjacent channel signal detected of sufficient IF energy next to the selected channel, then the narrow bandwidth mode used during the scan will be maintained.

BACKGROUND OF THE INVENTION 
This invention relates to radio receivers having dual bandwith intermediate 
frequency (IF) stages and, more particularly, to a bandwidth switching 
system to provide improved IF bandpass selectivity. 
It is well known that there are radio broadcast receivers which offer 
switchable IF selectivity. In such prior art receivers, typically an 
interference-free radio signal condition is obtained with a narrow 
band-pass IF filter and higher fidelity is obtained with a wide bandpass 
IF filter. However, the existing systems have not been altogether 
satisfactory in providing better audio quality, especially when listening 
to a relatively weak station or channel which is located adjacent to a 
relatively strong station or channel. Furthermore, in many prior art IF 
selectivity arrangements, the choice of IF selectivity is usually 
dependent upon the signal characteristics of the selected channel and not 
upon adjacent channel characteristics or interference. 
SUMMARY OF THE INVENTION 
It is therefore the principal object of this invention to improve IF 
bandpass selectivity in radio receivers depending upon reception 
conditions and adjacent channel conditions. 
It is another object of the present invention to enable automatically 
switching the bandwidth of the intermediate frequency filter from a 
relatively narrow bandwidth to a relatively wider bandwidth. 
It is still another object of the present invention to provide for a 
microprocessor controlled synthesized radio receiver capable of operating 
in different modes of operation to effect different IF bandpass 
selectivities. 
It is still a further object of the present invention to improve upon the 
design of radio receivers and to simplify their construction while still 
enabling many functions and multiple modes of operation to thereby effect 
less distortion and better audio quality from the receivers. 
Briefly, the present invention is an improvement over the prior art systems 
in that it enables scanning of the broadcast band while using a relatively 
narrow bandwidth or higher selectivity and then automatically switches to 
a relatively wider bandwidth or lower selectivity upon stopping at a 
channel selected during scan after it has been determined that there is no 
interference from an adjacent channel signal. Basically, the bandwidth 
switching system for the radio receiver includes means for scanning 
predetermined broadcast channels, means for receiving signals from the 
scanning means and providing predetermined IF bandwidths in response 
thereto, the signal receiving means being further characterized by having 
a first mode of operation with a first bandwidth and a second mode of 
operation with a second bandwidth, detector means for receiving the 
predetermined IF bandwidth signals from the signal receiving means and 
converting the IF signals into audio signals, station detecting means 
coupled to an input of said detector means for detecting the presence of a 
signal of sufficient strength within the predetermined IF bandwidth and 
for supplying an output to said scanning means if such signal is present 
so as to enable said scanning means to operate to scan up and down at 
least one adjacent channel spacing, and controlling means for controlling 
the mode of operation of said signal receiving means in response to 
signals from said station detecting means whereby either said first mode 
of operation or said second mode of operation is selected depending upon 
whether an adjacent channel is detected. The scanning means preferably 
includes a phase locked loop frequency synthesizer coupled to a 
microprocessor. Typically, the first mode of operation uses a narrow 
filter and the second mode of operation uses a wide filter. A narrow 
filter is preferably used during scanning of the predetermined broadcast 
channels.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1, there is shown portions of a typical radio 
receiver including an antenna 10, and a radio frequency (RF) amplifier 
stage 12 which provides amplified input RF signals to a mixer 14. Also 
providing an input to mixer 14 is a synthesizer 16 which is controlled by 
a microprocessor controller 20 capable of operating in the manual scan up, 
manual scan down, auto scan up, and auto scan down modes of operation in 
response to user initiated inputs at input lines 22, 24, 26 and 28, 
respectively. The RF signal associated with the selected channel or 
station is converted to an IF signal by mixer 14, the output of which is 
then routed via a switch 36 to one of a pair of bandpass filters 32 or 34. 
In accordance with one of the aspects of the instant invention, the IF 
selectivity is switchable to provide either narrow or wide bandwidth. This 
is accomplished by using a wide bandwidth filter 32 and a narrow bandwidth 
filter 34 together with switch 36. Switch 36 is depicted as an 
electromagnetic type of switch, for example, a relay, for simplicity in 
illustrating the invention. It will be realized by those skilled in the 
art that other types of electronic switches may also be satisfactorily 
utilized. 
The output from filter 32 or filter 34 is supplied to an IF amplifier 40 
which provides an amplified IF signal at an input 42 of an audio detector 
44 and to a line 46 to a station detector 48. Audio detector 44 supplies 
the detected audio signal to an audio amplifier 52 which in turn supplies 
an amplified audio signal on a line 54 to a speaker (not shown). 
Station detector 48 senses sufficient energy within the IF band in any 
suitable manner known to those skilled in the art. When there is 
sufficient energy within the IF band, which is indicative of reception of 
a station on the selected channel or frequency, a signal is supplied by 
station detector 48 on a line 56 to the microprocessor controller 20. For 
example, if a station is detected, the output on line 56 may be in a high 
logic state and when no station is detected, the output on line 56 may be 
in a low logic state. 
Due to the relatively uncomplicated tasks which microprocessor controller 
20 is required to execute, virtually any current available microprocessor 
can be used. The amount of memory space required is minimal. Preferred 
microprocessor types are part number TMS1000 which is commercially 
available from Texas Instruments, part number MC141000 which is 
commercially available from Motorola, Inc., or part number MM57170 which 
is commercially available from National Semiconductor. 
Synthesizer 16 may be any one of a wide variety of commercially available 
integrated circuits. One preferred example is part number DS8907 for an 
AM/FM digital phase-locked loop frequency synthesizer available from 
National Semiconductor. A six page combined specification sheet and 
application note dated January 1980 is also available from National 
Semiconductor for this part number. For example, in the present invention, 
line 30 in FIG. 1 illustrates, collectively, separate lines for 50 Hz, 500 
KHz, clock data and enable signals between synthesizer 16 and 
microprocessor controller 20. Data to update synthesizer 16 when 
controller 20 wishes to change channels is serially encoded on the single 
data line. 
As previously mentioned, microprocessor controller 20 has a plurality of 
user initiated inputs for controlling channel or station selection. The 
manual scan up and manual scan down inputs at lines 22 and 24, 
respectively, are considered optional under the present invention. 
Basically, these inputs provide the capability of manually incrementing or 
decrementing to the next adjacent channel or station whether or not that 
channel or station is actually used. This permits the user to manually 
tune the radio receiver. Of course, these inputs could also be used to 
increment fractions of an adjacent channel step to provide the radio user 
with a fine tuning feature to further minimize distortion or interference 
from a strong adjacent station. 
In accordance with another aspect of the present invention, there are 
provided auto scan up and auto scan down inputs on lines 26 and 28, 
respectively, to the microprocessor controller 20. With reference to FIG. 
2, there is illustrated a flowchart for the microprocessor controller 20 
when either of the auto scan inputs at lines 26 or 28 are initiated. If 
the auto scan up input 26 is initiated, the microprocessor controller 20 
sets the IF bandwidth output on line 50 in a high logic state condition. 
This causes the electromagnetic winding of switch 36 to be energized 
thereby causing switch 36 to change from the state shown in FIG. 1 to 
provide IF signals from mixer 14 to the narrow filter 34. At the same 
time, microprocessor controller 20 sends information on line 30 to 
synthesizer 16 to cause synthesizer 16 to increment by one channel spacing 
by changing its output to mixer 14. As long as station detector 48 
continues to remain in a low state indicative of no station present, 
microprocessor controller 20 continues to increment channel by channel. As 
soon as a station is detected by station detector 48, the output on line 
56 goes high and the microprocessor controller increments an additional 
channel. At this point, an adjacent channel to the desired channel is 
being tested to determine whether it also contains a broadcasting station 
which results in an IF signal of predetermined magnitude. If so, 
microprocessor controller 20 decrements one channel and leaves switch 36 
in the energized position thereby utilizing the narrow filter IF bandwidth 
because of the presence of the adjacent station. If sampling of the 
adjacent channel indicates that there is no adjacent channel on the upper 
side of the desired channel because station detector output on line 56 
remains in a low logic state, then microprocessor controller 20 decrements 
two channels to determine if there is a station in the channel on the 
lower adjacent channel side of the desired station. If so, microprocessor 
controller 20 increments one channel to get back to the desired station 
and keeps switch 36 energized so as to utilize the narrow filter 
capability of the IF stage. If no station is detected on the lower channel 
side of the desired station, microprocessor controller 20 increments one 
channel to get back to the desired station and switches its IF bandwidth 
output on line 50 to a low logic state thereby de-energizing switch 36 so 
that switch 36 is now in the condition shown in FIG. 1 to use the wide 
filter IF bandwidth capability since no stations were detected in either 
of the upper or lower adjacent channels. 
The operation in the auto scan down mode is similar except that initially 
channels are decremented, rather than incremented, until a station is 
detected which results in the output of station detector 48 on line 56 
assuming a high logic level. At this point, microprocessor 20 begins the 
same procedure, as in the auto scan up mode, of checking the adjacent 
channels to determine if any stations are present therein. If there are 
any stations broadcasting in the adjacent channels, microprocessor 
controller 20 returns to the desired station and keeps switch 36 energized 
to use the narrow filter capability of the IF stage. If checking the 
adjacent channels indicates that no stations are present therein, 
microprocessor controller 20 de-energizes switch 36 by setting its IF 
bandwidth output on line 50 to a low logic state such that the wide filter 
capability of the IF stage is utilized. 
It will now be appreciated that, by virtue of the above-described 
invention, it is possible to automatically switch from one bandwidth to a 
second bandwidth, which is relatively wider than the first bandwidth, in 
the intermediate frequency stage of a radio receiver when no potentially 
interferring signals are present in adjacent channels, thereby providing 
better audio and listening quality to the user. 
While an embodiment of the invention has been shown and described, it will 
be obvious to those skilled in the art that changes and modifications may 
be made without departing from the invention in its broader aspects, and, 
therefore, the aim of the appended claims is to cover all such changes and 
modifications as fall within the true spirit and scope of the invention.