Low cost digital automatic alignment method and apparatus

A technique for the automatic alignment and tuning of the front end of a vehicle radio. A tuning voltage is applied to a multiplying digital-to-analog converter before being applied to a front end tuning circuitry of the radio. A tuning voltage antenna number is applied to the tuning voltage at the converter to scale the tuning voltage, and adjust the center frequency tuning of the front end. During alignment, the tuning voltage antenna number is used to modulate the tuning voltage around a mean by reprogramming the tuning voltage antenna number a few values higher and lower in sequence. This modulation of the antenna tuning voltage creates an AC modulation component on the signal strength indicator signal passing through the front end. The signal strength indicator signal is measured and compared to its value before the tuning voltage antenna number was changed. If the signal strength indicator signal is higher for a higher tuning voltage antenna number, the front end is misaligned to the low side of the desired alignment point. Likewise, if the signal strength indicator signal is lower for an increasing tuning voltage antenna number, the front end is misaligned to the high side. If there is no change in the signal strength indicator signal for both increases and decreases in the tuning voltage antenna number, the front end is optimally aligned.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a technique for performing automatic 
alignment and tuning of a radio receiver and, more particularly, to a 
technique using a software algorithm in connection with a test/alignment 
station on a production line of vehicle radios to digitally and 
automatically align and tune the various tuning circuits and components in 
the radios. 
2. Discussion of the Related Art 
As is well understood in the art, radio receivers are responsive to radio 
frequency (RF) signals broadcast from a transmission antenna to convert 
the signals into a desirable format, such as speech or music. An antenna 
associated with the receiver captures the electromagnetic energy of the RF 
signals from the surroundings, and converts this energy into electrical 
currents that are subsequently processed. Typically, a radio receiver will 
be separated into an amplitude modulation (AM) portion and a frequency 
modulation (FM) portion. In the United States, the AM portion is tunable 
to RF signals in the frequency band from 530 to 1710 kHz, and the FM 
portion is tunable to RF signals in the frequency band from 88 to 108 MHz. 
In order to tune the received RF signals to a desired station for 
broadcast, the receiver will include a variety of tuned circuits. FIG. 1 
shows a schematic block diagram of a known FM receiver section 10 of a 
radio receiver that is responsive to RF signals, and can be tuned in the 
FM radio frequency bandwidths. The FM receiver section 10 is intended to 
represent various types of electronically tuned, superheterodyne FM 
receivers known in the art, and is especially intended to represent such a 
receiver for use in a vehicle. Electromagnetic energy in the form of RF 
signals is received by a radio frequency antenna 12, and is applied to a 
tunable bandpass filter circuit, particularly a tunable tank circuit 14, 
that is tuned to establish a bandwidth having a particular center 
frequency. The tank circuit 14 will be tuned to a particular center 
frequency for a radio station, and its bandwidth will cover several 
adjacent stations. When the tank circuit 14 is tuned to a particular 
frequency in the FM frequency bandwidth, the RF signals received from the 
antenna 12 and applied to the tank circuit 14 at the tuned frequency are 
passed by the tank circuit 14, and frequencies outside of this bandwidth 
are rejected. The tank circuit 14 provides RF selectivity, and is tunable 
to about a 2 MHz bandwidth, and thus limits the bandwidth so that 
frequencies entering the FM receiving section 10 are not outside the FM 
spectrum. Typically there will be one to three of these types of RF 
selective filters to provide this function. 
The tank circuit 14 includes a pair of back-to-back varactor diodes 16 and 
a coil 18 making up an LC circuit that establishes the resonant frequency 
of the circuit 14. Other capacitive components would also be included in 
the tank circuit 14 to establish the resonant frequency. A tuning voltage 
potential applied between the varactor diodes 16 acts to adjust the center 
frequency output of the tank circuit 14, and thus acts to tune the 
receiver section 10 to a particular FM center frequency usually the 
frequency of a station being tuned. The coil 18 would generally include a 
ferrite core that is selectively positionable within the coil 18 to set or 
tune the frequency of the tank circuit 14. 
The output of the tank circuit 14 is the RF signal received by the antenna 
12 that is bandpass filtered by the tank circuit 14. This selected RF 
signal is applied to a low noise RF amplifier 20 to be amplified. The 
amplifier 20 is a preamplifier that gives gain sensitivity and a higher 
signal-to-noise ratio. The amplified output from the amplifier 20 is then 
applied to a mixer 22 that mixes the selected and amplified RF signal with 
an RF signal from a local oscillator 24, also a tuned tank circuit, to 
establish an intermediate frequency, for example 10.7 MHz, that is 
suitable for subsequent processing by the components of the receiver 
section 10. The combination of the antenna 12, the tank circuit 14, the 
amplifier 20 and the mixer 22 is typically referred to as the "front end" 
of the receiver section 10. The input to the mixer 22 from the local 
oscillator 24 is selected so that no matter what frequency the tank 
circuit 14 is tuned to, the intermediate frequency will always be the 
same. By establishing a common intermediate frequency in this manner, 
inexpensive, highly repeatable components can be used in the receiver 
section 10 having suitable tolerances without the need for providing 
expensive, high tolerance components, as is well understood in the art. 
The receiver section 10 is tuned to a particular FM station by controls on 
a front panel (not shown) of the radio. The controls on the front panel 
are connected as inputs to a microprocessor 26. The microprocessor 26 
determines the desired FM channel frequency and sends a serial data string 
to a frequency synthesizer 28. The frequency synthesizer 28 includes a 
phase locked loop (PLL) (not shown) and other suitable components to 
generate a tuning voltage output, in a manner that is well understood in 
the art. The tuning voltage output of the frequency synthesizer 28 is used 
to apply a voltage to the varactor diode 16 in the tank circuit 14 and the 
varactor diode in the local oscillator 24 so as to adjust the tuning 
frequency of the circuits. 
The intermediate frequency signal from the mixer 22 is applied to an 
intermediate frequency filter circuit 32. The filter circuit 32 provides 
the selectivity which isolates the desired station within the selected FM 
frequency spectrum from the tank circuit 14. The filtered intermediate 
frequency from the filter circuit 32 is applied to an intermediate 
frequency amplifier 34 to amplify the intermediate frequency for 
subsequent processing. The filter circuit 32 selects the desired FM 
station by employing narrow band filtering. 
The amplifier 34 amplifies the intermediate frequency signal to a level 
high enough to drive an FM detector 36. The FM detector 36 extracts the 
transmitted information on the RF signal through a frequency-to-voltage 
conversion. The FM detector 36 can be any suitable detector for the 
purposes described herein, such as a quadrature circuit. The detected RF 
signal from the detector 36 is applied to a stereo decoder 38 that 
separates the signal into left-right and left -Eright channels. The 
separated signals from the stereo decoder 38 are applied to a matrix and 
audio processor 40 that converts the left+right and left-right signals 
into left and right audio signals. The left and right FM audio signals 
from the processor 40 are then applied to subsequent processing circuitry 
(not shown), and then to the system speakers (not shown). 
The tank circuit 14 is tuned to the frequency of a desired station and the 
local oscillator 24 is tuned relative to the frequency of the desired 
station so that the output of the mixer 22 is set at the intermediate 
frequency, here 10.7 MHz. In operation, the mixer 22 passes the difference 
between the selected frequency from the tank circuit 14 and the tuned 
frequency of the local oscillator 24. However, the operation of the mixer 
22 also passes the addition of the selected frequency from the tank 
circuit 14 and the tuned frequency of the local oscillator 24. In order to 
prevent this image frequency from being passed by the mixer 22, the range 
of the tank circuit 14 is limited to a somewhat narrow bandwidth. The 
selectivity of the tank circuit 14 is usually limited to be narrower than 
20 MHz to provide significant attenuation more than 20 MHz away from the 
desired frequency. Thus, the primary purpose of the tuned tank circuit 14 
is to provide selectivity in the front end of the receiver section 10 
before the mixer 22 to attenuate the image frequency so that it doesn't 
pass through the filter circuit 32. Even though the tank circuit 14 is 
tuned to a particular station, there is a number of other stations, for 
example about five stations, along the FM bandwidth selected by the tuned 
tank circuit 14. However, the center frequency of the tank circuit 14 has 
to be tuned to the desired station as the receiver section 10 is tuned to 
different radio stations. 
Because modern radios are electronically tuned, using the varactor diodes, 
a problem exists in that there is not perfect tracking between the 
adjustments of the center frequency of the tank circuit 14 and the local 
oscillator 24 from receiver to receiver. As discussed above, the RF 
selectivity of the tank circuit 14 is centered at the desired FM 
frequency, and the local oscillator 24 is tuned to the value of the 
intermediate frequency (10.7 MHz) above the center frequency of the tank 
circuit 14. Because the LC components in the tank circuit 14 and the local 
oscillator 24 are not identical, even if the same tuning voltage is given 
to both circuits, the same center frequency would not be achieved. The 
center frequencies of the tank circuit 14 and the local oscillator 24 
would not change the same amount for both filters for the same change in 
tuning voltage, and therefore tuning sensitivity would be lost because the 
RF selectivity of the tank circuit 14 would be attenuating the desired 
signal as it would not be centered relative to the local oscillator center 
frequency. Stated another way, by plotting out the center frequency of the 
local oscillator 24 on the X axis and the center frequency of the tank 
circuit 14 on the Y axis, it would be desirable to see a 45.sub.-- line 
shifted up by 10.7 MHz, or shifted down the direction being tuned. 
Therefore, for each radio receiver, the center frequencies of the local 
oscillator 24 and the tank circuit 14 must be adjusted relative to each 
other to get proper tuning. 
In the prior art, in order to properly adjust the cores of the inductors, a 
signal generator is connected to the receiver section 10 at the antenna 
12. The receiver section 10 and the signal generator are programmed to 
give a frequency at a particular location in the FM frequency band. The 
inductors in the tank circuit 14 and the local oscillator 24 are then 
adjusted by mechanically moving their ferrite cores so that the magnitude 
of the intermediate frequency signal is maximized, as provided by an 
automatic gain control plot. By adjusting the position of the cores of the 
inductors in the tank circuits 14 and 24, the center frequencies of the 
antenna circuit 14 and the local oscillator 24 will equal the output 
frequency of the frequency synthesizer 28 minus the intermediate 
frequency. This frequency would be the desired FM station frequency. 
The mechanical adjustment of the components in the filter circuits, as 
discussed above, is a relatively time consuming and labor intensive 
process. Therefore, new techniques have been developed in the art to 
provide computer assisted mechanical alignment. One of those improvements 
is an automatic alignment technique as disclosed in U.S. Pat. No. 
5,428,829 issued Jun. 27, 1995, titled METHOD AND APATUS FOR TUNING AND 
ALIGNING AN FM RECEIVER, assigned to the assignee of this application, and 
herein incorporated by reference. 
In the FM receiver section 10, there is a DC voltage, typically referred to 
as the signal strength indicator (SSI), that can be taken from the 
amplifier 34, or the circuit components, and is proportional to the signal 
strength of the RF signal received by the antenna 12, or applied to the 
antenna 12 by a generator during alignment. In order to get an indication 
of whether the center frequency of the tank circuit 14 is aligned to the 
desired tuning alignment point, the voltage of the SSI signal can be 
monitored. In order to accomplish this, a known RF signal is applied to 
the antenna 12 by a generator, and the DC value of the SSI signal is 
measured. By modulating the tuning voltage applied to the varactor 16, the 
center frequency of the tank circuit 14 will be modulated back and forth. 
This modulation gets propagated all the way through the receiver section 
10, and shows up as a square wave on the SSI voltage signal. Thus, a 
square wave is superimposed on the SSI voltage signal and is either 
in-phase or out-of-phase with the original modulating signal. Therefore, 
by looking at the SSI voltage signal as the tuning voltage is modulated, 
it is possible to tell if the tank circuit 14 is tuned to the desired 
alignment point. 
Referring back to FIG. 1, for the technique disclosed in U.S. Pat. No. 
5,428,829, the local oscillator tuning voltage from the frequency 
synthesizer 28 is applied to a tuning voltage (TV) scaling and modulation 
circuit 42 prior to being applied to the varactor diode 16 of the tank 
circuit 14. The circuit 42 scales the local oscillator tuning voltage to 
get a derivative of the tuning voltage that is applied to the varactor 
diode 16 in the tank circuit 14. The scaling is provided by a digital word 
tuning voltage (TV) antenna number that is used to set the center 
frequency of the tank circuit 14 for different stations during the 
operation of the receiver 10. 
FIG. 2 shows a schematic block diagram of the components of the TV scaling 
and modulation circuit 42. The tuning voltage from the frequency 
synthesizer 28 is applied to the plus terminal of an amplifier 44 that 
acts as a buffer. The output of the amplifier 44 is connected to resistors 
R.sub.1 and R.sub.2 which act as a voltage divider in series with a diode 
46. The divided voltage between the resistors R.sub.1 and R.sub.2 is 
applied to a positive terminal of an amplifier 48 which applies gain to 
the output of the amplifier 44, and also acts as a buffer. The output of 
the amplifier 48 is applied to a multiplying digital-to-analog converter 
(MDAC) 50. 
The MDAC 50 acts as a programmable voltage divider controlled by the 
microprocessor 26 which provides the TV antenna number represented as a 
digital word. What the MDAC 50 does is takes an analog input, and 
multiplies it by a digital word to scale the analog input. In other words, 
the analog input from the amplifier 48 is divided by the digital TV 
antenna number to provide a divided analog output from the MDAC 50. For 
example, if an 8-bit digital word representing a gain of 1 is applied as 
the TV antenna number, the analog voltage signal applied from the 
amplifier 48 passes straight through the MDAC 50. If an 8-bit digital word 
representing a gain of 0.5 is applied as a TV antenna number to the MDAC 
50, then the analog output of the MDAC 50 would be one-half the analog 
input from the amplifier 48 at the input to the MDAC 50. The analog output 
of the MDAC 50 is applied to the non-inverting terminal of an amplifier 
52. The amplifier 52 is DC referenced to the voltage drop across the diode 
46. 
A modulation source and synchronous detector 54 is provided that includes a 
synchronous detector to determine whether the square wave superimposed on 
the SSI voltage signal is in-phase or out-of-phase with the modulating 
signal, or whether there is a square wave at all. This information is 
applied to the microprocessor 26. The detector 54 also generates the 
modulating signal that is applied to a summing junction 56 within the TV 
scaling modulation circuit 42. In one embodiment, the rate of modulation 
signal is 1 kHz. The modulating signal modulates the antenna voltage from 
the amplifier 52. 
FIGS. 3(A)-3(C) show three graphs with voltage on the vertical axis and 
frequency on the horizontal axis. Each of the graphs gives a desired 
alignment point of the alignment between the center frequency of the tank 
circuit 14 and the local oscillator 24. The solid graph line represents 
the frequency bandwidth of the tank circuit 14 as tuned with the current 
tuning voltage antenna number. The dotted graph line represents the center 
frequency bandwidth of the tank circuit 14 for a subsequent higher tuning 
voltage antenna number as established by the microprocessor 26. As shown 
in FIG. 3A, an increase in the tuning voltage antenna number causes the 
center frequency of the tank circuit 14 to move away from the desired 
alignment point, and thus causes the SSI voltage signal to decrease 
because the designed alignment point is on the positive slope portion of 
the bandwidth curve of the tank circuit 14. This is a misalignment to the 
high side of the desired alignment point. FIG. 3B shows the situation 
where an increase in the tuning voltage antenna number causes the center 
frequency of the tank circuit 14 to move closer to the desired alignment 
point, and thus provides an increase in the SSI voltage signal because the 
desired alignment point is on the negative slope portion of the bandwidth 
curve of the tank circuit 14. FIG. 3C is the case where the tank circuit 
14 is basically tuned to the desired alignment point with the current TV 
antenna number, where an increase in the tuning voltage antenna number 
does not cause the SSI voltage signal to significantly change. This is 
because the alignment point is at the top, flat portion of the tuning 
curve. 
When the receiver section 10 is tuned by an operator, the microprocessor 26 
programs the frequency synthesizer 28 to the proper frequency in the 
conventional manner. For each channel frequency, the microprocessor 26 
provides the TV antenna number applied to the MDAC 50. This generates the 
proper tuning voltage to set the center frequency of the tank circuit 14 
to the correct channel. To align the tank circuit 14, generally three 
alignment points are determined, as discussed above, and the 
microprocessor 26 interpolates between the alignment point to tune the 
receiver section 10 to intermediate channels. 
The above described tuning procedure as disclosed in U.S. Pat. No. 
5,428,829 requires the circuitry as described to generate a modulating 
signal, a tuning voltage modulator, and a synchronous detector to detect 
when the SSI voltage is "ain" or "out" of phase with the tuning voltage 
modulating signal. This circuitry is used once initially to tune the tank 
circuit 14 during production of the radio, and is not used again after the 
radio is tuned. Therefore, improvements can be made for tuning an 
electronically tunable radio that eliminates these circuits, and their 
associated cost. It is therefore an object of the present invention to 
provide an automatic alignment and tuning technique for a radio that 
includes these advantages. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the present invention, a low cost 
technique is disclosed for digital automatic alignment and tuning of a 
radio. A tuning voltage is applied to a multiplying digital-to-analog 
converter before being applied to front end tuning circuitry of the radio. 
A tuning voltage antenna number is applied to the tuning voltage at the 
converter to scale the tuning voltage, and adjust the center frequency 
tuning of the front end. During alignment, the tuning voltage antenna 
number is used to vary the tuning voltage around a mean by reprogramming 
the tuning voltage antenna number a few values higher and lower in 
sequence. This modulation of the antenna tuning voltage creates an AC 
modulation component on the signal strength indicator signal. The signal 
strength indicator signal is measured and compared to its value before the 
tuning voltage antenna number was changed. If the signal strength 
indicator signal is higher for a higher tuning voltage antenna number, the 
front end is misaligned to the low side of the desired alignment point. 
Likewise, if the signal strength indicator signal is lower for an 
increasing tuning voltage antenna number, the front end is misaligned to 
the high side of the desired alignment point. If there is no change in the 
signal strength indicator signal for both increases and decreases in the 
tuning voltage antenna number, the front end is optimally aligned. 
In a specific application of the invention, a tester circuit is provided at 
the manufacturing level of the radio that applies a trigger signal to be 
compared with the signal strength indicator signal as the tuning voltage 
antenna number modulates the tuning voltage. The tester circuit determines 
whether the trigger signal is in-phase or out-of-phase with the signal 
strength indicator signal, and whether an AC signal is present on the 
signal strength indicator signal. These values are used to determine 
changes in the signal strength indicator signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following discussion of preferred embodiments directed to an automatic 
digital tuning and alignment technique for an FM radio is merely exemplary 
in nature and is in no way intended to limit the invention or its 
applications or uses. 
A description of how the SSI voltage signal is monitored to align and tune 
the FM front end of a FM radio receiver is discussed above and in U.S. 
Pat. No. 5,428,829. The present invention proposes using the TV antenna 
number and associated software to modulate the antenna tuning voltage, so 
as to eliminate certain circuitry previously necessary to align the radio 
receiver 10. Since the tuning voltage passes through the MDAC 50 before 
being applied to the varactor diode 16 in the FM front end circuitry, it 
is possible to vary the tuning voltage around its mean by simply 
reprogramming the TV antenna number a few values higher and lower in 
sequence. After each change in the TV antenna number, the SSI voltage 
signal changes. According to the technique of the present invention, this 
change in voltage is used to measure the phase of the SSI voltage signal 
relative to the direction of change in the TV antenna number. If the SSI 
voltage signal is higher for a higher TV antenna number, then the SSI 
voltage signal is in-phase, indicating the tank circuit 14 is misaligned 
to the low side. Likewise, if the SSI voltage signal is lower for 
increasing TV antenna numbers, then the SSI voltage signal is 
out-of-phase, indicating the tank circuit 14 is misaligned to the high 
side. If there is no SSI voltage signal change for both increases and 
decreases in the TV antenna number, the radio receiver is optimally 
aligned. This proposed technique would completely eliminate all of the 
circuitry associated with the modulation source and synchronous detector 
54 and the summing junction 56 from the receiver 10 shown in FIGS. 1 and 
2. 
To perform the technique of the invention as described above, a custom 
tester circuit 62 is provided, according to one embodiment of the present 
invention, as shown in FIG. 4. The tester circuit 62 would be located at a 
position suitable for providing alignment during the production of the 
radios, and thus would not need to be incorporated in each of the radios 
individually. The purpose of the tester circuit 62 is to easily and 
effectively determine two conditions of the radio receiver. First, whether 
there is AC information present on the SSI voltage signal as a result of 
modulating the TV antenna number. Second, if there is AC information 
present on the SSI voltage signal, whether the AC information is "in" 
phase or "out" of phase with the modulation of the antenna tuning voltage. 
This is an indication of the receiver being misaligned to the low and high 
side, respectively, therefore telling the tester circuit 62 which way to 
adjust the center frequency of the tank circuit 14. 
The tester circuit 62 includes a test computer 64 that stores and runs the 
alignment algorithm. The algorithm instructs the test computer 64 to 
generate a square wave trigger signal that is synchronous with the 
modulation of the TV antenna number, and thus in sequence with the 
modulation of the antenna tuning voltage. The square wave trigger signal 
is applied to each of the data input of a first digital latch 66, a rising 
edge detector circuit 68 that detects the rising edge of the square wave 
trigger signal, and a falling edge detector circuit 70 that detects the 
falling edge of the square wave trigger signal. An output of the rising 
edge detector circuit 68 goes high after a one-quarter cycle delay from 
the rising edge of the trigger signal, and this high output signal is 
applied to the latch input of the digital latch 66 and the latch input of 
a second digital latch 72. Additionally, the output of the falling edge 
detection circuit 70 goes high after a one-quarter cycle delay from the 
falling edge of the square wave trigger signal, and this high output 
signal is applied to the latch input of a third digital latch 74. 
The SSI voltage signal is also applied to the tester circuit 62 as a DC 
voltage signal and may include an AC ripple component superimposed on the 
DC signal depending on how the tank circuit 14 is aligned to the desired 
alignment point. The SSI voltage signal is AC coupled to the circuit 62 by 
an AC filter capacitor 76 to filter off the DC component. The AC 
component, if it is present, is then amplified by a gain amplifier 78, and 
is limited by a limiter 80 to produce a suitable square wave signal for 
the circuit 62. The square wave or limited SSI signal is applied to the 
data inputs of both the latches 72 and 74. The outputs of the latch 66 and 
the latch 72 are applied to the inputs of an AND logic gate 84, and the 
outputs of the latch 72 and the latch 74 are applied to the inputs of an 
exclusive--OR (XOR) logic gate 86. The output of the latch 66 is latched 
high after a one-quarter cycle delay of the rising edge of the trigger 
signal. The output of the latch 72 is latched high after a one-quarter 
cycle delay of the rising edge of the trigger signal if the limited SSI 
voltage signal includes the AC component, and is in-phase with the trigger 
signal. Further, an output of the latch 74 is latched high after a 
one-quarter cycle delay from the falling edge of the trigger signal if the 
limited SSI signal includes the AC component, and is out-of-phase with the 
trigger signal. An output of the AND gate 84 will be high if both the 
outputs of the latches 66 and 72 are high. An output of the XOR gate 86 
will be high if one of the inputs is high when the other input is low. The 
output of the AND gate 84 then gives an indication of whether the SSI 
voltage signal is in-phase or out-of-phase with the trigger signal, and 
the output of the XOR gate 86 gives an indication of whether the AC 
component is present on the SSI voltage signal. 
FIG. 5 shows timing diagrams for the trigger signal and for the three 
possible cases of the relation between the limited SSI voltage signal and 
the trigger signal based on the alignment of the tank circuit 14. In case 
1, the AC component on the limited SSI voltage signal is present and 
in-phase with the trigger signal, and indicates that the tank circuit 14 
is misaligned to the low side of the desired alignment point, as indicated 
in FIG. 3B. Case 2 shows that the AC component is not present on the 
limited SSI voltage signal, and indicates that the tank circuit 14 is 
correctly aligned with the desired alignment point shown, as in FIG. 3C. 
Case 2 can also occur if there is a malfunction in the tester hardware or 
the radio receiver 10. And, case 3 indicates that the AC component is 
present on the limited SSI voltage signal and is out-of-phase with the 
trigger signal, indicating that the tank circuit 14 is misaligned to the 
high side of the desired alignment point, as in FIG. 3A. Additionally, two 
vertical lines are shown, and labelled state 1 and state 2. State 1 is the 
output of the latch 72 when the limited SSI signal is latched to the 
rising edge of the trigger signal, and state 2 is the output of the latch 
74 when the limited SSI signal is latched to the falling edge of the 
trigger signal. 
The digital latch 66 is used to maintain the trigger signal high after the 
rising edge of the trigger signal. If the outputs of both of the latches 
66 and 72 are high, then the output of the AND gate 84 will also be high, 
indicating that the AC component on the limited SSI voltage signal is 
in-phase with the trigger signal. This is case 1, thus instructing the 
tester circuit 62 that the tank circuit 14 is misaligned to the low side 
of the desired alignment point. If the output of the latch 72 is not high, 
then the limited SSI signal is not in-phase with the trigger signal, and 
the output of the AND gate 84 will be zero, indicating that either case 2 
or case 3 is present. Therefore, it is necessary to ascertain additional 
information to determine the situation. The XOR gate 86 provides AC 
present information by XORing the outputs of the digital latches 72 and 
74. As mentioned above, the output of the digital latch 72 is the limited 
SSI signal at state 1, and the output of the latch 74 is the limited SSI 
signal at state 2. If the two signals are the same, the output of the XOR 
gate 86 will be low, indicting that no AC is present, or case 2. If these 
two signals are different, the output of the XOR gate 86 will be high, 
indicating that AC is present, and thus the limited SSI signal must be 
out-of-phase with the trigger signal, or case 3. Therefore, by monitoring 
the outputs of the logic gates 84 and 86, it is possible to tell if the 
tank circuit 14 is aligned, and if it is not aligned, how the antenna 
tuning voltage needs to be changed to cause the circuit 14 to be aligned. 
A truth table is provided below showing all of the states, where output 1 
is the output of the latch 66, output 2 is the output of the latch 72, and 
output 3 is the output of the latch 74. 
______________________________________ 
TRUTH TABLE 
OUT 1 OUT 2 OUT 3 CONDITIONS 
______________________________________ 
Low Low Low Error, OUT 1 must be High 
Low Low High Error, OUT 1 must be High 
Low High Low Error, OUT 1 must be High 
Low High High Error, OUT 1 must be High 
High Low Low Out-of-phase, No AC Present 
High Low High Out-of-phase, AC Present 
High High Low In-phase, AC Present 
High High High In-phase, No AC 
______________________________________ 
FIG. 6 shows a flow chart diagram 90 of the operation of the algorithm, in 
one embodiment, that the test computer 64 uses to operate the tester 
circuit 62, as discussed above, to determine whether the tank circuit 14 
is aligned properly. The test computer 64 initiates the algorithm by first 
setting an initial TV antenna number to a value that will be close to the 
desired alignment point for the tank circuit 14 based on previous 
experiences, as indicated by box 92, but will cause the tank circuit 14 to 
be misaligned to the low side. To perform the modulation of the antenna 
tuning voltage, the TV antenna number is adjusted to some value higher 
than this initial value, for example 5 points higher, and the trigger 
signal is set high, as indicated by box 94. Next, the TV antenna number is 
adjusted to the same value lower than the initial value, and the trigger 
output is toggled low, as indicated by box 96. This provides one period of 
the trigger signal for states 1 and 2, as indicated in the signal flow 
diagram of FIG. 5. 
After this cycle, the test computer 64 reads and stores the outputs of the 
AND gate 84 and the XOR gate 86, as indicated by box 98. Thus, the 
algorithm will know how the tank circuit 14 is tuned relative to the 
desired alignment point, as discussed above. The algorithm performs the 
steps of boxes 94, 96 and 98 a number of times to allow for settling of 
the hardware and to establish a steady state condition. Five times is 
chosen as a minimum number in that a larger number of times could be 
equally applicable. A decision diamond 100 determines whether a counter 
has reached five times indicating that the current outputs of the AND gate 
84 and the XOR gate 86 should be accurate. The algorithm then calculates 
the average of all the previously stored values of the in-phase and AC 
present signals from the gates 84 and 86, respectively, as indicated by 
box 102. If three of the five iterations provides a certain value for each 
of the outputs of the AND gate 84 and the XOR gate 86, these values are 
determined as the true values for these gates. 
Once the algorithm has calculated and stored the average AC present and the 
in-phase signals, the algorithm then determines whether the AC present 
gate 86 has been determined to be high, as indicated by diamond 104. If AC 
is not present, then the algorithm increments the TV antenna number in box 
106. Since AC was not present, no determination of phase relationship is 
possible. If AC is present, then the phase relationship is checked in 
diamond 108, and if in-phase sets the variable "low-side number" to the 
current TV number in box 110 and increments the TV antenna number in box 
106. The algorithm then returns to modulating the TV antenna number in 
accordance with boxes 94, 96 and 98 to determine whether the new frequency 
of the tank circuit 14 is tuned to the desired alignment point. In one 
embodiment, the amount of modulation of the TV antenna number is +/- 5 
counts around the current value, although other values are applicable. 
This process continues until the outcome of the diamond 108 shows that the 
limited SSI signal is out-of-phase with the trigger signal. Then in box 
112, the variable "high-side number" is set equal to the current value of 
TV antenna number. The algorithm then checks that the "low-side number" 
and "high-side number" have been set, indicating that the tank circuit 14 
has passed through the alignment point. It then calculates the midpoint of 
these numbers as the optimal alignment point in box 114. This is done this 
way because phase measurements can not be made due to a lack of AC on the 
SSI signal (case 2 of FIG. 5). The embodiment shows that there are several 
values of TV antenna number near the desired alignment point that produce 
case 2. So the desired TV antenna number is the midpoint of this set of 
numbers. This midpoint value is stored in the radio receiver 10 so that 
the microprocessor 26 can output the appropriate antenna tuning voltage to 
the tank circuit 14 so that the tank circuit 14 is tuned with the local 
oscillator 24 to provide the desired circuit alignment. In this manner, 
the receiver 10 is aligned at the manufacturing level of the vehicle in 
accordance with the technique of automatic alignment discussed above, 
without the need for the circuitry of the modulation source and 
synchronous detector 54 of the prior art. 
It is stressed that the particular algorithm discussed above in connection 
with the flow chart diagram 90 of FIG. 6 is by way of a non-limiting 
example. Other algorithms may be equally applicable in accordance with the 
teachings of the present invention to determine the alignment of the tank 
circuit 14 by modulating the antenna tuning voltage using the TV antenna 
number. 
The foregoing discussion discloses and describes merely exemplary 
embodiments of the present invention. One skilled in the art will readily 
recognize from such discussion, and from the accompanying drawings and 
claims, that various changes, modifications and variations can be made 
therein without departing from the spirit and scope of the invention as 
defined in the following claims.