Method and apparatus for the efficient test of the center frequency of bandpass filters

A method and apparatus for determining the center frequency of a bandpass filter. The center frequency of a bandpass filter is determined by applying a bias voltage to a bandpass filter circuit receiving a pair of differential input voltages and producing a pair of differential output voltages. The differential output voltages are modified to exhibit a notchband magnitude characteristic. A minimum value of the differential output voltages exhibiting the notchband characteristic over a predetermined range of frequencies is determined and corresponds to the center frequency of the bandpass filter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the field of bandpass filters, and more 
particularly, to bandpass filters used to capture a wobble signal utilized 
in the read head and write head circuits of optical disk apparatus. 
2. Description of the Related Art 
Bandpass filters are commonly used in Digital Versatile Disk (DVD-RAM) and 
Compact Disk (CD-RW) apparatus. Specifically, bandpass filters are used in 
the read channel and write circuits of these devices. In operation, the 
bandpass filter serves to capture the wobble signal used in the positional 
control of the read and write heads associated with the optical disk 
apparatus. The wobble signal is typically embedded in a group of 
sinusoidal signals transmitted to the heads of an optical disk apparatus, 
with the frequency of the signal typically being in the range of 80-200 
Khz. 
Capture, which is the selective passing of the wobble signal through the 
bandpass filter, is a function of the center frequency of the bandpass 
filter. The more accurate the determination of the center frequency of the 
bandpass filter, the greater the accuracy of the filter in passing only 
the frequency associated with the wobble signal without passing additional 
or extraneous signal frequencies unassociated with the wobble signal. 
Accordingly, it is desirable to have an efficient and accurate means to 
determine the center frequency of a bandpass filter in order to optimize 
the capture of a wobble signal. 
Conventionally, measurement of the center frequency of a bandpass filter is 
accomplished by measuring the maximum of the magnitude characteristic of 
the filter in question. However, bandpass filters generally have 
relatively flat magnitude characteristics about the center frequency, 
which makes difficult accurate measurement or test of the center frequency 
within, for example, an accuracy of 1%, since there is such a small 
differential in the magnitude of the characteristic of a bandpass filter 
about the center frequency. 
Conventional devices have been proposed such as those disclosed in Whitten 
(U.S. Pat. No. 3,643,173) and Cabot (U.S. Pat. No. 5,136,267). These 
conventional devices incorporate tuneable microelectronic bandpass filters 
to achieve an effective inductance--and, hence, tuning of the center 
frequency--over a band of frequencies. To the inventor's knowledge 
however, none of the prior art devices presents a method or apparatus for 
measuring the center frequency to the degree of accuracy afforded by the 
present invention. 
SUMMARY OF THE INVENTION 
The present invention is directed to a circuit for the measurement of the 
center frequency of a bandpass filter that overcomes the limitations 
associated with conventional methods of measurement of the center 
frequency of a bandpass filter. 
Accordingly, it is an object of the present invention to provide for the 
accurate measurement of the center frequency of a bandpass filter of 
better than 1%. 
Another object of the invention is to provide for the accurate search for 
the center frequency of a bandpass filter by effectively increasing the 
rate of change of the magnitude values around the center frequency of the 
bandpass filter, thereby allowing faster identification of the center 
frequency of the filter. 
By way of summary, the present invention pertains to a test circuit coupled 
to a bandpass filter circuit in which the test circuit, in a first 
operating mode, does not affect the magnitude characteristic of the 
bandpass filter circuit. In a second operating mode, the test circuit 
causes the bandpass filter magnitude characteristic to change to that of a 
notch filter. In this manner, and because the magnitude characteristic of 
a notch filter is more sharply defined than that of a bandpass filter, the 
center frequency of the filter (which is the same for both bandpass and 
notch filter operations) can more easily be detected. 
It is to be understood that both the foregoing general description and the 
following detailed description are merely exemplary and explanatory in 
nature and are intended to provide further explanation of the present 
invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawings. 
The magnitude characteristic of a typical bandpass filter for a range of 
frequencies is shown in FIG. 1. If the bandwidth of the filter is not very 
narrow, the rate of change of the magnitude characteristic is low. As a 
result, finding the maximum magnitude presents some difficulty. This 
condition is depicted by FIG. 2, which shows in greater detail the apex of 
the gain versus frequency characteristic of a filter and how moderate its 
rate of change may be across its center frequency. As shown therein, the 
magnitude changes are on the order of approximately 0.8% around the center 
frequency, which requires the capability to detect a variance of 8 mV in 
1V in order to achieve the desired degree of accuracy of measurement. As a 
practical matter, these changes are difficult to detect under normal 
conditions. 
As shown in FIG. 3, a test circuit according to the present invention 
comprises a bandpass filter circuit that includes at least two 
sub-circuits: a bandpass filter circuit 22; and a bandpass filter test 
circuit 23. In bandpass filter circuit 22, input signals V.sub.inp, 
V.sub.inn are connected to terminals 1 and 2, respectively, and are fully 
differential input voltages. V.sub.op, V.sub.on are fully differentiable 
output voltages produced by a bandpass filter 24, as amplified by 
subsequent circuit elements and outputted at terminals 3 and 4. 
Specifically, the differential output voltages produced by bandpass filter 
24 are coupled to a pair of transistors 5 and 6 (designated M.sub.BP and 
M.sub.BN, respectively). Transistors 5 and 6 are cross coupled to 
transistors 9 and 10 (designated M.sub.FP and M.sub.FN, respectively) via 
circuit nodes 7 and 8. The differential output voltages provided by 
transistors 5 and 6 are coupled to operational amplifier 20, which has a 
gain of approximately 1000 in a presently preferred embodiment. 
Test circuit 23 comprises transistors 11 and 12 (designated M.sub.TP and 
M.sub.TN respectively), and switches 14 and 15 with an inverter 21. 
Transistors 11 and 12 are connected to input voltages V.sub.inp, V.sub.inn 
at nodes 16 and 17, respectively, and are cross coupled to transistors 10 
and 9, respectively. Switches 14 and 15, which can be implemented as 
N-type MOSFET transistors, are connected to inverter 21 to ensure that 
switch 14 is deactivated when switch 15 is activated and vice versa. A 
TEST signal is applied to test circuit 23 at terminal 18. The TEST signal 
is responsible for selectively activating and deactivating test circuit 23 
in a manner explained in further detail below. Switches 14 and 15 are 
connected in series between voltage levels V.sub.G (indicated by reference 
character 13) and V.sub.POS (indicated by numeral 19), both of which are 
constantly applied during operation. Voltage V.sub.G which is constantly 
applied at terminal 13, has a value less than the difference between an 
output common mode voltage (V.sub.o) and the absolute value of the 
threshold voltage of the transistors of bandpass filter circuit 22. 
V.sub.POS has a value of 5 volts in a presently preferred embodiment. 
The TEST signal is applied at terminal 18 in order to switch the circuit 
between a non-TEST mode and a TEST mode. The test signal, as discussed 
below, can be of either a HIGH or LOW value in order to achieve this 
change in operation. In non-TEST mode, the present invention operates as a 
bandpass filter in the usual fashion, and exhibits the magnitude 
characteristic shown in FIG. 2 with respect to the outputs at terminals 3 
and 4. This results from the cross-coupling of transistors 5 and 6 
(M.sub.BP and M.sub.BN) with transistors 9 and 10 (M.sub.FP and M.sub.FN) 
in non-TEST mode. In this mode, a TEST signal (e.g., a LOW signal) is 
applied at terminal 18 such that switches 14 and 15 are respectively, in 
open (nonconductive) and closed (conductive) states. These states cause 
the gates of transistors 11 and 12 to be connected to the highest possible 
supply voltage in the circuit, namely, V.sub.POS. 
To switch to TEST mode, an instantaneous bias voltage or TEST signal at a 
HIGH level--5 volts in a presently preferred embodiment--is applied at 
terminal 18. This signal changes switches 14 and 15, to their closed 
(conductive) and open (nonconductive) states, respectively, so that 
constantly applied voltage (V.sub.G) at terminal 13 is connected to the 
gates of transistors 11 and 12. 
In TEST mode, when the HIGH level TEST signal is applied, transistors 11 
and 12 (M.sub.TP and M.sub.TN) operate transistors 5 and 6 (M.sub.FP and 
M.sub.FN) to convert the magnitude characteristic of the bandpass filter 
output to a notchband magnitude characteristic by subtracting the bandpass 
filter output signal from its input signal. The effect of this conversion 
is shown in FIG. 4, which illustrates a notchband magnitude characteristic 
measured at the output of operational amplifier 20. 
The presently preferred embodiment involves a bandpass filter with a gain 
of unity at its center frequency. The desired gain is achieved by choosing 
the ratio of the sizes of transistors 11/12 (M.sub.TP /M.sub.TN) which 
correspond to transistor channel width/length ratio (W/L).sub.TP,N, and 
the sizes of transistors 5/6 (M.sub.BP /M.sub.BN) which correspond to a 
ratio (W/L).sub.BP,N according to the relationship: (W/L).sub.TP,N 
=A.sub.BP .times.(W/L).sub.BP,N where A.sub.BP is the desired bandpass 
filter gain at the center frequency. 
The input voltage/output voltage subtraction function of the TEST mode is 
verified by Kirchoff's current law equation for the currents coming into 
and leaving the input nodes of operational amplifier 20 in FIG. 3 as 
follows: 
##EQU1## 
where V.sub.op, V.sub.on and V.sub.ip, V.sub.in, are the differential 
output voltage and input voltage pairs; s is the complex frequency 
variable; and G.sub.B, G.sub.F and G.sub.T are the conductances of the 
P-type MOSFET transistors 11, 12, 5, 6, 9 and 10 (i.e., the pairs of 
transistors M.sub.BP, M.sub.BN, and M.sub.FP, M.sub.FN, and M.sub.TP, 
M.sub.FN, respectively). a.sub.T describes the switching in the test mode. 
That is, a.sub.T is 1 if in TEST mode and 0 in non-TEST mode, i.e., when 
the system is in a bandpass filter mode. 
As can be seen from FIG. 4, the notchband magnitude characteristic has a 
very sharp minimum at the center frequency of the bandpass filter under 
test. This minimum is easier to identify than, for example, the maximum 
shown in FIG. 2 corresponding to the center frequency of the bandpass 
filter. In FIG. 2, a 1% precision corresponds to the detection of an 8 mV 
variance around a 1V maximum, whereas as shown in FIG. 4, the same 
precision corresponds to a detection of 60 mV around a minimum that is 
ideally 0V. 
For optimal, fully differential operation when cross coupled, the 
transistor pairs 11/12 (M.sub.TP /M.sub.TN), and 5/6 (M.sub.BP /M.sub.BN) 
shown in FIG. 3 should be matched. Transistor pairs 11/12 (M.sub.TP 
/M.sub.TN) and 9/10 (M.sub.FN /M.sub.FP) should also be matched to yield a 
better minimum magnitude, corresponding to the center frequency of the 
bandpass filter under test. 
The method of testing a bandpass filter according to the present invention 
is depicted FIG. 5 The method allows the center frequency of a bandpass 
filter circuit 32 (such as that corresponding to filter 24 in FIG. 3) 
which receives a diffrerential input voltage and produces a differential 
output voltage 35, to be tested by coupling a test circuit 30 (such as 
that corresponding to test circuit 23 of FIG. 3) to bandpass filter 
circuit 32. The differential output voltage 35 is amplified by an 
operational amplifier 34 (such as that corresponding to operational 
amplifier 20 in FIG. 3). This operation generates a differential output 
voltage 36 that exhibits a notchband magnitude characteristic with a 
minimum magnitude corresponding to the center frequency of the bandpass 
filter. Thus, as has been shown, the method of the present invention, as 
shown in FIG. 5, exceeds the limitations of conventional methods and 
allows efficient and accurate determination of the center frequency of a 
bandpass filter. 
As shown in FIG. 6, the present invention (denoted by elements 33 and 25 
can be integrated into an optical storage or playback system such as a 
CD-RW or DVD-RAM device, comprising a wobble signal generator 31, read 
circuit 27 and write circuit 29. Various optical systems having read and 
write circuits 27 and 29, as well as wobble signal generator 31 and 
bandpass filter circuit 33, are already known and need not be explained in 
further detail for purposes of the present invention. An improvement in 
such an arrangement, particularly as to the bandpass filter being used 
therein, is also shown in my co-pending application identified above. The 
present invention further provides a circuit 25 to function in combination 
with these other structures so that accurate testing of bandpass filter 
characteristics--and, hence, optimal operation of the overall device--can 
be obtained. Measurement of the center frequency of the bandpass filter 
could be used, for example, to develop a signal that would achieve 
(whether manually or, through other firmware, automatically) adjustment of 
the bandpass filter characteristics so as to optimize detection of the 
wobble signal and overall performance of the device. 
Bandpass filters are also used in apparatus other than optical disk 
devices. The present invention tests for the center frequency of a 
bandpass filter and can accordingly be extended to many other applications 
that use bandpass filters in their operation. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the bandpass filter test circuit of the 
present invention without departing from the spirit or the scope of the 
invention. Thus it is intended that that the present invention cover the 
modifications and variations of this invention provided they come within 
the scope of the appended claims and their equivalents.