Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
2000-04-04
2002-09-10
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S184000
Reexamination Certificate
active
06448873
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to printed filters and more particularly relates to LC-type printed circuit filters realized using a parallel resonant combination of suspended printed inductor and suspended interdigital capacitor that may be formed over any dielectric substrate.
BACKGROUND OF THE INVENTION
Wireless radio frequency (RF) communications are becoming more and more prevalent in the world today. Products touting wireless RF communication links are becoming increasingly popular among consumers. A multitude of new products including redesigned existing ones is being built with wireless RF links today. Most RF communication circuits employ some form of resonant circuitry in their transceivers. Due to the explosive consumer demand for products sporting wireless communication links there is a need for low cost, high accuracy and high reliability filters that are suitable for mass manufacture using conventional techniques.
RF filters are necessary circuits in transmitters and receivers that operate in a both wireless and non-wireless, i.e., cable, environments. In a transmitter, the amount of suppression that an RF filter must provide is determined by regulatory requirements or by the amount of interference that the transmitter might cause as a result of unwanted spectral components. RF filters are even more essential in receivers of communications systems especially when the communications system is wireless and is likely to suffer from reception of interfering signals in addition to the natural (thermal) reception noise. In a receiver, the quality of the filtering dramatically effects the reception performance, especially when considering certain types of interference. A particular receiver may deliver an output of degraded quality (i.e., higher error rate in a digital receiver, or severe signal distortions in an analog receiver), if the frequency response of its filters is compromised.
When defining the filtering requirements in a receiver, the following factors should be considered: (1) the frequency band in which the receiver is to operate, (2) the frequency conversions and IF to be used, (3) the spectrum of the modulated signal to be demodulated by the receiver and (4) the nature of any interfering signals to be encountered and the associated rejection requirements.
The filter preceding the demodulator would normally be the narrowest and should allow the minimum amount of additive noise and interference to enter the demodulator. Its bandwidth is normally close to that of the modulated signal for which the receiver was designed. The selectivity of the receiver, i.e., the rejection of adjacent frequencies that may cause jamming or performance degradation, is determined by the steepness of the frequency response curve of the filter.
In a single frequency receiver, such as in the case of simple pagers, the narrow filter may precede most of the electronic circuitry of the receiver, thus reducing the possibility of intermodulation produced within the receiver. In receivers intended for an entire band of frequencies, from which one frequency among many is in use at any one time, the filtering at the input of the receiver usually has a bandwidth at least as wide as that of the entire band used. These filters cannot provide rejection for in band interference and usually do not have significant attenuation even for out of band frequencies that are close to the edges of the band. In such receivers, the IF filtering provides effective rejection of such interference as long as an inband intermodulation product was not generated before the signal reaches the narrow IF filtering.
The wide filters located at the input to the receiver are intended to provide image rejection and out of band interference rejection, which is effective for signals sufficiently far from the edges of the frequency band. A typical filter, e.g., surface acoustic wave (SAW) filter, for use in the 900 MHz ISM band, for example, costs over $1.00 and has a frequency response with limited out of band attenuation.
Most RF filters and circuits for communication applications make use of one or more inductors in their design. Previously, these were lumped inductors or printed inductors that have been formed on printed circuit boards using a variety of techniques such as stripline, microstrip, slotline, etc. Inductors formed using any of these techniques are typically constructed in the form of a planar spiral with the spiral being circular or square in shape. A disadvantage, however, of forming inductors, such as microstrip inductors, on printed circuit boards is that they are very sensitive to the characteristics of the printed circuit board material. The characteristics of the printed circuit board material directly affect the characteristics and performance of the inductors formed thereon. Parameters of the PCB material such as thickness and dielectric constant affect the characteristics of the inductor. The sensitivity of microstrip inductors to the dielectric constant of the printed circuit board material results in variations in the resultant self-resonant frequency. In addition, variations in the thickness of the PCB material causes variations in the value of the inductance that results in frequency response errors of any filter constructed therefrom.
Another disadvantage of constructing printed inductors on printed circuit boards using traditional techniques is that the repeatability of the value of the inductance is too low for mass production. As described above, the characteristics of the inductor are very sensitive to the parameters of the printed circuit board material. In addition, most printed circuit inductors constructed utilizing conventional techniques have limited values of the quality factor Q of the inductor. This is due to the nature of the conventional inductor that is constructed having a ground plane. Further, since the ground plane is separated from the printed inductor traces by the printed circuit board material there is typically significant parasitic capacitance between the inductor and the underlying ground plane. In some applications, this parasitic capacitance can be problematic because it causes a reduction in the self-resonant frequency of any LC combination formed using the inductor.
An alternative to using printed circuit inductors, such as microstrip and stripline inductors, is to use discrete inductor elements. A disadvantage, however, of using discrete elements is the high cost typically associated with high Q lumped coils.
A limitation, however, of the use of filters that utilize printed inductors is the sensitivity of the center frequency and other filter characteristics to variations in the PCB etching process. One means of compensation is to design wider filters to accommodate the PCB etching tolerances. This, however, is not always a viable solution, as the requirements of the particular application may demand sharp narrow filters.
Another prior art solution to realizing filters with invariable center frequencies is to use precise PCB etching techniques. High precision PCB manufacturing, however, entails higher manufacturing costs.
The effect of underetching of the traces results in a lower inductance value due to less magnetic flux linkage since there is less space for the magnetic field in between the traces. Conversely, overetching results in a higher inductance for the opposite reason. For example, a variation in trace width (e.g., 8 mil rather than the desired 10 mil) of 2 mil may result in a center frequency deviation of 2% (or a 20 MHz error in the desired center frequency of a 900 MHz band filter). In practice, variations in PCB etching tolerances may result in a filter having a variation in center frequency of 3%. Furthermore, the bandwidth of such a filter may be extended from 28 MHz to 45 MHz. In the ISM bands allowed by the FCC for unlicensed operation, for example, such a filter is too wide to sufficiently filter out unwanted signals from outside the desired band.
SUMMARY OF THE INVENTION
The present invention is a suspended p
Bettendorf Justin P.
Brady III Wade James
Neerings Ronald O.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
LandOfFree
LC filter with suspended printed inductor and compensating... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with LC filter with suspended printed inductor and compensating..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and LC filter with suspended printed inductor and compensating... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2833814