ES2237294A1 - Tunable filter for filtering radio frequencies and microwaves in e.g. TV, has filtrate network incorporating syntony elements, and additional network electronically reducing intermodulation introduced by syntony elements - Google Patents

Abstract

The filter has a filtrate network for filtering radio frequency frequencies and microwaves. The filtrate network consists of printed line segments provided with multiple connections. Frequency of signals is determined by frequency of resonance of lines. The filtrate network incorporates syntony elements. An additional network electronically reduces intermodulation introduced by the syntony elements.

Description

Tunable filter for radio frequencies and microwave.

Object of the invention

The present invention relates to a step filter band that is feasible to be tuned electronically and that characterized by its high selectivity.

The scope of this invention is extends from the radio frequency (VHF and UHF bands, with frequencies between 100 MHz and 1 GHz, typically), up to frequencies microwave (frequencies between 1 GHz and 30 GHz) and millimeter band  (frequencies above 30 GHz), and, depending on the bandwidth of the application, from narrow bandwidth systems to systems broadband

Although it is especially indicated to be used in applications in IV-V band of television (400-900 MHz), radio frequency bands for mobile applications, radio link applications with different types of carrier modulations both analog and digital, the invention can also be applied to any other frequency band such as microwave frequencies more high (greater than 1 GHz).

Background of the invention

Currently, the growing demand for services from communications demands the best use of the spectrum of Available frequencies set for communications applications. This demand is being met on the one hand using techniques that  take better advantage of the spectrum by getting more of information, as well as with a reduction in the bands of protection around the frequencies used to transmit information. This requires greater frequency selectivity and, therefore, greater selectivity in the filters, to avoid interference with other applications at nearby frequencies. He frequency segment to be selected by the filter should can be easily selectable allowing to use the same filter for several frequencies without repeating it. This implies a tuning capacity of the filters.

On the other hand, in our days the systems of analog communications are being replaced by systems digital Digital systems are capable of transmitting greater amount of information in the same frequency range as the analog systems, as well as in general improve their quality. In the digital systems is crucial to reduce group delay and make it similar for all transmitted frequencies. It is known that filters are one of the elements with the greatest group delay and greatest  dispersion in it produced in the systems of communications

Other types of band pass filters are known tunable, consisting of cylindrical or prismatic pieces that are fixed to the walls of a metal cavity, while in the other wall changes the capacity depending on the penetration of screws, which once set are fixed with nuts, or Well another type of mechanical variable capacitors.

This type of filter presents several problems, on the one hand, the difficulty of achieving optimal coupling between input and output, distance or separation being critical between the conductors described above and on the other hand the tuning difficulty, as this must be done manually, fixing the fixing nuts being difficult during the adjustment process. Another problem stems from his selectivity limited by the number of conductors used, Increasing losses as this number increases, while increasing the number of adjustments by means of screws or tuning capacities to be performed.

The filters closest to the invention are filters made with printed line technology (circuits printed), which have the inconvenience of cancellation selectivity. Although there are topologies that allow to increase the selectivity, these require filters with a greater number of resonators, hindering its design and limiting its application to electronically tunable filters due to the high number of tuning elements (in general as many as resonators) necessary to shift the filter response in frequency.

Explanation of the invention. a) Introduction

This invention consists of a filtering network step band signals, made by metallic printed lines type "microstrip" (also called printed lines "micro tape"), that is, printed circuit lines taxed by photolithography on a dielectric substrate with mass plane lower. The specificity of this invention with respect to others microstrip filtering networks consists of the combined use of The following effects:

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He resonance effect, whereby the device has a strong variation depending on the frequency that is applied, allowing the step at the output of frequencies near the frequency of resonance. There are different resonator implementations, the used in the present invention is by using printed microstrip lines.

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Electrical and magnetic coupling between Two next printed lines.

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Effect overlapping the radio frequency signal, which runs two trajectories: a direct path and a feedback path that causes the appearance of frequencies at which there is no transmission (transmission zeros) causing an increase in selectivity.

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The characteristic impedance of the resonators. Through his modification, the electronic variation of the resonance frequency

The main ones are presented below features, improvements and advantages of the network proposal.

The main features are:

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Low step attenuation

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Low group delay and high flatness in the band of He passed

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High frequency selectivity adjacent

The invention offers the following improvements:

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Tuning capacity

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Higher mechanical simplicity of the topology

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Dimension Reduction physical

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Techniques for performing filters with low intermodulation

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Interconnection techniques of filters at different frequencies

The previous improvements introduce the following advantages:

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Higher repeatability in its manufacture compared to other structures, in particularly those that incorporate mechanical adjustments in the drivers

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Cost reduction of manufacturing

In order to better understand the operation of the present invention, is set forth in detail below. the description of it, attaching figures and drawings that they facilitate explanations, although these are presented by title merely illustrative and do not mean any limitation of the present invention

The description that is made of the network of filtered object of this report refers to its realization using microstrip transmission line technology for being this the most interesting technology from the point of view of manufacturing; notwithstanding the effects, and topologies described can be used with other types of line technologies transmission.

b) Detailed explanation of the invention

In order to facilitate the explanation, it accompanying the present specification some drawings in which some cases of realization have been represented, which are cited in title as an example The statement of each figure is included below. of this subsection "Detailed explanation of the invention".

To get a filter with the characteristics described above and in order to detail its operation, reference is made to figure 1, where the structure is shown basic of the filter object of the invention. Entry and exit are perform by point 1 and 2 of figure 1, respectively, that are the ends of two connected microstrip transmission lines using the capacitor 5. There are two possible paths along the which passes the signal, a direct path (marked as 3 in the figure), and a feedback path (marked as 4) through two coupled microstrip resonators and two other microstrip resonators coupled to the microstrip input and output lines, respectively. This last trajectory is the cause of the appearance of transmission zeros, when combined at the exit in contract the two signal contributions from both roads. There is only signal transfer from the input to the output in the frequency band around the frequency of resonance resonators. These resonance frequencies are can be adjusted using capacitors 6 and 7 of figure 1, the which modify the impedance and therefore the frequency of resonance. The pass band depends on the signal coupling between the resonators, and adjusts with the separation distance ("S" in the figure) between both resonators. The separation of the transmission zeros is adjusted with the capacitor 5 of the Figure 1.

The capacitors used for tuning of the filter (elements 5, 6 and 7 of figure 1) can be fixed, variable capacitors, varactors, capacitors micromachining (MEMS), as well as sections of transmission lines or other adjustment elements, giving rise to different embodiments of the network object of the invention. Figure 2 shows a possible implementation scheme using varactors (elements 3, 4 and 5 in figure 2) as tuning elements. By varying the reverse polarization voltage of the varactors is modified their capacity varying the resonance frequency of the resonators that make up the filter, as well as the separation between zeros of transmission. Shock resistance and inductances have been added (which have a high impedance at frequencies of radio frequency but allow the passage of the continuous component) to prevent leakage of the radio frequency signal to the polarization circuits (power and ground).

In general, if such tuning elements do not they are totally passive, as is the case with tuning elements based on varactor diodes, can cause intermodulation between carriers or signals that pass through the filter. Therefore, in the Figure 3 shows a new configuration, object of the invention, where networks are used as indicated (box 4) to reduce intermodulation. In these networks the varactors 4 and 5 of figure 2 for a new element consisting of two varactors connected in series to which, in turn, it is connected in parallel a fixed capacitor (element 6), and another capacitor fixed (item 5) also connected in series. They are also added Continuous polarization networks of the varactors, which in turn block the radio frequency signal, formed by resistors and shock inductances (8), as well as return inductances of continuous polarization (element 7). The connection of Varactor set with parallel capacitor and serial capacity, in series reduces the level of harmonic frequency generation and of odd intermodulation. This degree of reduction can be modified. with the capacitor parallel to the varactor and the series capacitor the same. The control of transmission zeros (through the variable capacitor 3) can also be done with the new network or using variable capacitors or other elements of adjustment mentioned above. The tuning of the filter can be electronically control by generating tensions control on nodes 9 and 10, using a digital converter to analog (block D / A in the figure) controlled by a microprocessor, or microcontroller (block M in the figure).

Figure 4 shows parameter measurements of dispersion (parameters S) of an embodiment of the filter according to the scheme of figure 1. This figure shows the losses of return expressed in decibels (parameter S11) and losses of insertion in decibels (parameters S21). The band is observed 685 MHz centered intern, with two transmission zeros pronounced around 665 MHz and 775 MHz, for which the two Road contributions show a 180 ° offset, being eliminated when combined at the filter outlet, presenting in around these frequencies a high attenuation.

Figure 5 shows the delay measure of filter group of figure 4, where the main feature is the flatness of the delay in the pass band. This flatness is of great importance when filtering modulated signals digitally, since group delay differences in frequencies of the pass band of the filters introduce a increase in the error rate in the demodulation of the signal in Current communication systems.

Figure 6 shows parameter measurements of dispersion (parameters S) for an embodiment of the filter in millimeter band centered at 41 GHz. This embodiment shows that the filter of the structure object of the invention can be used in any band, both radio frequency (frequencies up to 1 GHz) and microwave (frequencies higher than 1 GHz).

Figures 7 and 8 show the measurements of dispersion parameters (insertion losses or S21 and loss of return or S11, respectively), for an embodiment based on the filter of figure 2, using varactor elements such as electronic tuning elements. In these figures you represents the variation depending on the voltage applied to the resonator varactors (elements 4 and 5 in figure 2). Be observe how the frequencies of the pass bands of the filter network can be modified or tuned continuously by varying the voltage of polarization of the varactors, without practically modifying the network bandwidth This is because with the tension reverse polarization of the varactors, the capacity that present these decreases, thus increasing the frequency of resonance resonators. There is a certain increase in insertion losses for low polarization voltages due to the quality factor of the varactor, which is lower for low polarizations Such behavior can be reduced using tuning elements with higher quality factor, such as MEMS varactors or variable capacitors with manual adjustment.

Figure 9 shows the power of the output signal for the fundamental frequency (comprised within the pass band of the filter) and third order intermodulation products, for an embodiment of the filter of Figure 2 at the center frequency of 621 MHz , when the input consists of two frequency tones (f1 and f2 respectively) separated 1 MHz (f2-f1 = 1 MHz) and with the same input power, Pin, expressed in dBm in the figure. Figure 10 shows the relationship between the output power (at f1 and f2) and the power associated with the third order intermodulation product (corresponding to frequencies 2f1-f2, and 2f2-f1, said power difference is known as intermodulation distortion, or IMD ), depending on the input power for the same embodiment of Figures 2 and 9. This measure known as the two-tone test studies the generation of spurious components of the filter to signals that constant of several electrical components (for example several carriers).

Figures 11 and 12 show the results of the tests of the same two-tone experiment corresponding to the measures of figures 9 and 10, but using the network to improve intermodulation of figure 3. It shows an improvement of 35 dB in intermodulation (IMD) for an input power of 0 dBm, which goes from approximately 35 dB to 70 dB. For the network of intermodulation enhancement the same varactors have been used in Both realizations. It is concluded that the new network produces a remarkable reduction of the signal generation effect of intermodulation at the exit. This reduction is of great interest practical for the realization of amplifiers in systems digital communications as well as analog television systems or digital Thus, in these an important application is presented (Figure 13), where two selective filters are used according to one of the versions of figures 1, 2 or 3, with a broadband amplifier (block 3 in figure 13) connected between the output of the first filter and the input of the second filter (nodes 2 and 4, respectively, in Figure 13). This way you get an amplifier with high output power, and of programmable tuning according to the central frequency of the filters, with rejection of frequencies corresponding to channels or unwanted interference, taking advantage of the high selectivity of the filters presented, as well as the reduced intermodulation using the network of figure 3. This type of amplifiers tunable has a great industrial application, since Currently, adjustments are usually made manually. Using the previous application, amplifiers could be developed single-channel for tunable analog and digital television electronically

Figure 14 shows an application for interconnection of this type of single-channel amplifiers to different frequencies, in combination with a technique commonly used in Cable Television amplifiers, the so-called Z technique. The application is as follows. The filters in Figure 13 are given add some clues that define two new accesses (nodes 6 and 7 in the figure), at the input and output (nodes 2 and 4) of the amplifier (item 3), respectively. The signal input would be node 1, and the channel corresponding to the pass band of the filter tunable would be amplified while the other channels they would be rejected and the signal would pass through 6 to the entrance of Another single channel amplifier. Similarly, the outputs (points 5 and 7) would be combined with the outputs of other amplifiers single-channel

Statement of the figures

In order to facilitate the explanation, it accompanying the present specification some drawings in which some cases of realization have been represented, which are cited in title as an example

Figure 1:

It represents a basic circuit of the basic filter where the entry, exit and elements that make up the filter.

Figure 2:

Represents an embodiment of variable filter in VHF / UHF band with tuning elements for tuning electronic frequency band filter through.

Figure 3:

Represents an embodiment of variable filter in VHF / UHF band with several varactor elements for tuning electronic frequency band through filter reducing the appearance of intermodulation products.

Figure 4:

Represents insertion loss measures (parameter S21) and return losses (parameter S11) depending of the frequency of one embodiment of the filter according to the circuit described in figure 1 in UHF band (680 MHz center frequency for this particular embodiment).

Figure 5:

Represents the group delay measured for a filter embodiment according to figure 1.

Figure 6:

Represents insertion loss measures (parameters S21) and return losses (parameter S11), in frequency function of an embodiment of the filter according to the circuit described in figure 1 in frequency band millimeter (center frequency 41 GHz for this embodiment particular)

Figure 7:

Represents insertion loss measures (parameter S21), depending on the frequency of an embodiment of the filter of figure 2 for various tuning voltage values which applies to frequency control varactors ..

Figure 8:

Represents return loss measures (parameter S11), depending on the frequency of an embodiment of the filter of figure 2 for various tuning voltage values It applies to frequency control varactors.

Figure 9:

Represents the output power at the frequency fundamental (-) and the power corresponding to the component of third order intermodulation (-o-) when the the two tones at the entrance, both of the same power (Pin) and 1 MHz apart, for one embodiment of the filter of Figure 2.

Figure 10:

Represents third order intermodulation in function of the input power of two separate 1 MHz tones, for an embodiment of the filter of figure 2.

Figure 11:

Represents the output power at the frequency fundamental (-) and the power corresponding to the component of third order intermodulation (-o-) when the the two tones at the entrance, both of the same power (Pin) and 1 MHz apart, for one embodiment of the filter of Figure 3.

Figure 12:

Represents third order intermodulation in function of the input power of two separate 1 MHz tones, for an embodiment of the filter of figure 3.

Figure 13:

Represents an application of filters tunable for the realization of tuning amplifiers electronically programmable

Figure 14:

Represents an application of filters tunable for interconnecting amplifiers type single channel at different frequencies, using the technique of Z combination, both at the entrance and at the exit.

Claims (6)

1. Tunable band-pass filtering network, essentially characterized by being constituted by two coupled resonators realized by means of printed transmission lines type microstrip (figure 1), whose resonance frequency is adjustable with an alternative path for the signal (feedback) that includes other two microstrip resonators coupled, and that causes the appearance of transmission zeros (figures 4, 5, 6). 2. Tunable band-pass filtering network according to claim 1, characterized by the use of adjustment elements for the resonance frequency and separation of the transmission zeros incorporating one or several fixed or variable capacitors, varactors, or variable capacitors made by microstructures (MEMS), connected between the ends of the feedback resonators of claim 1 and ground. 3. Band pass filtering network according to claim 1, characterized by the use of a structure for intermodulation reduction by means of a network formed by several variable capacitors (varactors) together with fixed capacitors, according to figures 3, 9, 10, 11 and 12, which is connected between the ends of the feedback resonators of claim 1 and mass. 4. Band pass filtering network according to claims 1, 2, and 3, characterized in that it is tunable by means of the voltages applied to the varactors of claims 2 and 3, controlled by a digital device, giving rise to a signal filter device digitally programmable (figures 7, 8). 5. Application of the bandpass filtering network to single-channel amplifiers, characterized by two filter networks such as those described in claims 1, 2, 3 and 4, between which an amplifying device is inserted (Figure 13), giving rise to a fixed or tunable frequency selective amplifier, if adjustment elements are used as described in claims 1, 2, 3 and 4. 6. Application of the bandpass filtering network to single-channel amplifiers according to claim 5, characterized by the combination of the input and output signals, respectively, of several mono-channel amplifiers tuned to different frequencies, by using the combination technique Z described in figure 14.