Automatic IF tangent lock control circuit

An improved radio receiver that maintains a stable predetermined IF frequency upon receipt of input carrier signals of differing frequencies. The IF signal (14) is processed through a signal demodulation unit (16) that includes a tangent processing unit (22) to yield a control signal that varies as a tangent function of both the demodulated IF signal and a reference signal from a reference oscillator (17). This control signal is used to control the frequency and phase of a reference signal (26) used by the IF stage (12) to center the IF signal with respect to the IF stage characteristics.

TECHNICAL FIELD 
This invention relates generally to radio receivers, and more particularly 
to IF processing and tangent locking principles. 
BACKGROUND ART 
Phase locked loops are well known in the prior art as a means of detecting 
a desired RF transmitted signal. Such loops detect a signal by comparison 
of a locally generated estimate of the signal with the received signal 
itself. The error in phase between the two signals is low-pass filtered 
(i.e., integrated), and then used to correct the local estimate to 
resemble the average frequency and phase of the received signal. 
In general, in most radio receivers, such phase locked loops are used to 
lock on to the IF signal, which comprises the originally received carrier 
signal as altered in frequency through the IF stage, as well understood in 
the art. At least one prior art reference, however, suggests that phase 
locking can be made a function of the locally generated estimate and the 
incoming carrier signal to the IF stage, thereby allowing the phase locked 
loop to lock with respect to the incoming signal itself. 
In the early 1960's, a tangent function phase locked loop was proposed. 
This loop phase compared an IF stage output signal with a locally 
generated estimate of the signal in a dual phase comparator that yielded 
both the sine and cosine of the phase difference between the IF signal and 
its estimate. These outputs could then be appropriately combined to yield 
a tangent function (sine divided by cosine), which resultant signal could 
be used as the correction signal for the loop. Since a tangent function 
varies its amplitude more significantly than a sine or cosine function, 
operation of a phase locked loop as a function of the tangent function led 
to increased phase comparator range and linearity, at least under some 
operating conditions. An example of a commercially available radio 
receiver part that makes use of tangent locking principles to lock on to 
the IF signal is the Motorola MC13020 AM Stereo Decoder integrated 
circuit. 
To date, no one has succeeded in establishing a satisfactory tangent based 
phase locked loop through the IF stage of a radio receiver. Given the 
stability, speed, range, and parts improvement that such an arrangement 
would provide, a need for such a configuration clearly exists. 
SUMMARY OF THE INVENTION 
These and other needs are met through provision of the device described 
herein. The device includes a frequency translation unit (FTU) and a 
reference signal unit (RSU) in the IF stage. The FTU responds to receipt 
of: (1) an input signal that comprises a carrier signal having an 
information signal modulated thereon; and (2) a reference signal from the 
RSU. In response, the FTU provides an output signal having a frequency and 
phase that relates to the carrier signal as translated by the reference 
signal, and this FTU signal comprises the IF signal of the receiver. The 
IF signal is demodulated in ordinary course, with at least part of the 
resulting demodulated signal being used to provide a control signal to the 
RSU, which control signal itself varies, at least in part, as a tangent 
function of at least a part of the demodulated signal. 
As a result, the receiver can lock to the original incoming carrier signal 
as a tangent function, while simultaneously providing a stable IF 
frequency regardless of the original input signal frequency. Therefore, 
the IF signal can be exactly centered to match the IF stage 
characteristics with significant associated improved performance.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, the device can be seen as generally depicted by 
the numeral 10. The device (10) operates with a radio receiver that 
receives an input signal at an input (11) (for purposes of this 
description, the input signal will be presumed to be an AM stereo 
modulated signal of the form (1+L+R)cos(wct+.phi.) (where .phi. has the 
form arc tan (L-R)/(1+L+R), L comprises left side information of a stereo 
signal, R comprises right side information and wct comprises the carrier 
signal). 
The input signal is received and processed by the FTU (12) as a function of 
the reference signal (26) from the RSU (13) to yield an IF signal (14) of 
the form (1+L+R)cos(wit+.phi.). A signal demodulation unit (SDU) (16) 
demodulates this IF signal (14), in part as a function of a demodulating 
signal (24) provided by a reference oscillator (17), to yield demodulated 
information signals (in this case, L+R (18) and L-R (19)). These 
demodulated signals can then be processed through a matrix (21) in 
accordance with well understood prior art technique to yield the L and R 
output signals as desired. 
In addition, a tangent processing unit (TPU) (22) processes at least a part 
of the demodulated signals (in this case, a signal related to L-R) and 
uses this signal to develop a control signal (23) for the RSU (13). This 
control signal will vary at least in part as an average of the tangent 
function of the phase as multiplied by the envelope of the IF signal (14) 
as compared to the demodulating signal (24). As a result, the phase 
difference between the input signal and the reference signal (26) will be 
substantially equal to the frequency and phase of the demodulating signal 
(24) from the reference oscillator (17), thereby providing a stable IF 
frequency regardless of input signal frequency, and while simultaneously 
obtaining the benefits of tangent locking. 
Referring now to FIG. 2, a more detailed description of the device (10) 
will be provided. 
A standard ganged capacitor tuning structure (31) functions to receive the 
input signal, which signal passes through the input (11) of the IF stage 
(32). The FTU (12) includes a Mixer/AGC (automatic gain control) (33), an 
IF filter (34) and an IF gain AGC section (36). Both AGC units (33 and 36) 
respond to an AGC drive (37) that itself responds to the L+R demodulated 
signal from the SDU (16) to control the gain of the AGCs (33 and 36) as a 
function of the L+R signal 
The IF stage (32) also includes the RSU (13), which may be a voltage 
controlled local oscillator (VCLO). The VCLO provides a reference signal 
(26) to the mixer (33) for use by the mixer (33) in translating the input 
signal and thereby creating a resultant signal having a frequency and 
phase related to the input signal as translated by the reference signal 
(26). 
The IF signal (14) output passes to the SDU (16), which includes a limiter 
(38) and an envelope detector/filter (39) to demodulate the IF signal (14) 
and yield a first demodulated signal (18) comprising the L+R component, 
all in accordance with well understood prior art technique. The IF signal 
(14) also passes to the TPU (22), which includes an analog divider (41) 
that divides the IF signal ((1+L+R)cos(wit+.phi.) where wit comprises the 
IF carrier) by cos.phi.. The resultant signal (41) passes to a Q 
detector/filter (42). The Q detector/filter (42) comprises a gated 
demodulator that flips phase 180 degrees at 90 degrees and 270 degrees 
with respect to a reference signal provided for that purpose, and that 
yields an output related to the vector quantity of the incoming signal 
that is in quadrature (at a given moment in time) with a reference signal 
provided for that purpose. In this device, the Q detector/filter (42) 
utilizes a signal from a quadrature generator (43) as such a reference 
signal (described below in more detail) to provide a signal of the form: 
##EQU1## 
which can otherwise be represented as: 
##EQU2## 
The above, of course, constitutes a tangent function (sine divided by 
cosine) that essentially reduces to L-R and comprises the second 
demodulated signal (19). A signal (44) at least related to this L-R 
component can be provided to a loop driver (46) which comprises a filter 
that at least partially integrates the incoming signal and which provides 
a current proportional to the L-R term. This current comprises the control 
signal (23) that controls the frequency and phase of the RSU (13). 
A reference oscillator (17) provides a demodulating signal (24) having a 
predetermined stable frequency (in this example, 3.6 MHz), which signal is 
provided to the quadrature generator (43). The quadrature generator (43) 
divides this signal (24) down to a desired frequency (in this example, 450 
KHz) and provides two outputs based thereon. The first output comprises 
the 450 KHz signal without any phase shifts (0 degrees) and the second 
output comprises the 450 KHz signal with a predetermined phase shift (90 
degrees). The Q detector/filter (42) described earlier uses the latter 
signal as set forth above to develop the L-R signal. 
In operation, the loop characteristics of the interconnections between the 
mixer (33), the analog divider (41), the Q detector (42), the loop driver 
(46), and the RSU (13) cause the loop to tangent lock to thereby cause the 
long term average of the L-R term from the loop driver (46) to be zero. In 
effect, the IF signal frequency becomes locked with respect to the 
reference oscillator (17) demodulating signal (24) frequency, such that 
the average phase difference between the input signal and the reference 
signal (26) is substantially equal to the frequency and phase of the 
demodulating signal (24). By appropriate selection of the IF stage (32) 
components, a stable IF frequency and phase of, for instance, 450 KHz and 
0 degrees, can be maintained. The VCLO will thereafter adjust its 
frequency and phase to cause this lock through the IF stage (32) with 
respect to the reference oscillator (17) to occur. 
Other components are depicted as well, including an I detector (47) and a 
pilot tone detector (48). The presence and function of these components is 
well understood, and additional information regarding such components can 
be found, for example, in U.S. Pat. Nos. 4,377,728 and 4,489,431. 
A number of advantages result through this configuration. It avoids the 
loss of stereo signal separation and distortion that can result when 
standard phase locked loops are used with single channel modulation that 
has an asymmetrical phase deviation with respect to the carrier that can 
arise in standard broadcasts. Further, by locking onto the incoming 
signal, as versus the IF signal, the entire system becomes locked in the 
center of the IF with resulting improvement in performance. 
Those skilled in the art will recognize that various modifications of this 
invention could be made without departing from the spirit and scope of the 
invention. The claims should therefore not be considered as limited to the 
precise embodiments set forth in the absence of express limitations 
directed to such embodiments.