WO1996012351A1

WO1996012351A1 - Filter diplexer

Info

Publication number: WO1996012351A1
Application number: PCT/US1995/012517
Authority: WO
Inventors: Brian J. Green
Original assignee: Scientific-Atlanta, Inc.
Priority date: 1994-10-17
Filing date: 1995-10-12
Publication date: 1996-04-25
Also published as: AU3945495A

Abstract

A filter diplexer (133) is provided for matching a filter (132) to a mixer (134) upconverting an intermediate frequency signal to an output signal by mixing the intermediate frequency signal with a local oscillator (152). The filter (132) filters the intermediate frequency before applying it to the mixer (134). A diplexer (133) is provided between the filter (132) and the mixer (134) for matching the impedance of the filter (132) to that of the mixer (134). The diplexer (133) includes an inductor (302) connecting the filter (132) to the mixer (134) and a capacitor (306) and a resistor (308) in series in shunt configuration with the inductor (302).

Description

FILTER DIPLEXER

BACKGROUND OF THE INVENTION

The invention relates to the field of television signal transmission, and more particularly, to a filter diplexer which appropriately matches the impedance of a

filter to a mixer and transmission line. A baseband television signal is frequently modulated on an intermediate frequency carrier to form an intermediate frequency signal. The intermediate frequency signal is then mixed with a local oscillator signal to convert the intermediate frequency signal to a signal of a desired output frequency for transmission. The transmission path may be over the air, via coaxial cable, optical fiber or other transmission media.

In order to convert the intermediate frequency signal to an output frequency signal, it has been proposed to mix in a mixer the intermediate frequency signal with a local oscillator signal, thereby forming a signal having the output frequency. It has further been proposed to provide at least one filter in these devices in order to deliver to the mixer an intermediate frequency signal of suitable purity. Prior art filters used in such devices do not provide a broadband impedance match to the mixer. Therefore, in the past, it has been necessary to provide both a signal filter and its topological dual in order to provide the proper impedance match. This is because the signal filter provides a good match in the filter's passband, but is a reflective source to the mixer at frequencies significantly outside the filter's passband.

Referring to Figure 1, a prior art filter 10 is shown. It consists of a signal filter 20 and its dual 30. Signal filter 20 consists of inductors 22,24 and capacitor 26. The dual 30 consists of capacitors 32,34 and inductor 36. In addition, a resistance equal to the characteristic impedance of the system (ZQ) is provided at the end of the dual 30 opposite the end connected to the output of signal filter 20.

More recently, signals filters have become more complicated as the required center frequency for such filters has increased due to increased channel capacity for television systems. For instance, multi-order microstrip filters have been developed. As more complicated filters are used in these transmission systems, the provision of topological duals becomes more complicated and expensive. Thus, there remains a need for a filter component to provide proper impedance matching to a mixer without the provision of a topological dual.

SUMMARY OF THE INVENTION

In accordance with the invention, an output converter is provided for accepting an intermediate frequency signal and converting that signal to an output frequency signal for transmission. The output converter is a three-stage device including an intermediate frequency conversion module, a local oscillator module and an output converter module. The output converter module includes a signal filter having a multiple resonator structure employing variable capacitors. The filter is implemented with microstrip technology. A diplexer structure is provided for impedance matching to the mixer. Therefore, no complicated topological dual

is required. The diplexer structure is simple, inexpensive, takes up very little space, requires no tuning and requires little maintenance. Accordingly, its use is

a significant improvement over the topological dual scheme of impedance

matching. The filter diplexer can be used in any mixer conversion scheme that

uses any combination of input, output, or local oscillator frequencies where a

microstrip combline filter may be used, or for any circuit application where a

broadband match is required for a microstrip combline filter. The description of the preferred embodiment below is given as an example of an application of the

filter diplexer as was designed for a particular television modulator; however, the present invention should not be considered so limited and the present filter diplexer

may be used in any application where a broadband match to a filter is required.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a prior art signal filter arrangement with impedance

matching circuitry.

Figure 2 shows a block diagram of an output converter in accordance with

the instant invention.

Figure 3 shows a signal filter in accordance with the instant invention.

Figure 4 shows a transmission line representation of a signal filter in accordance with the instant invention.

Figure 5 shows a diplexer in accordance with the instant invention. Figure 6 is a detailed schematic of a diplexer in accordance with the instant invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 2 shows a block diagram of the output converter of the instant invention. The output converter performs a dual frequency conversion. The first conversion converts a first intermediate frequency signal to a second intermediate frequency signal centered near 172 MHz. Of course, 172 MHz is given by example only and other frequencies may be used as the second intermediate frequency signal. The second conversion converts the second intermediate frequency signal to an output signal, for example, of between 550 and 1000 MHz. Such a frequency range is given by way of example of typical optical fiber or coaxial cable television distribution plant. Of course, other, wider frequency ranges may be used for high-speed laser systems and the like. IF converter module 110 receives a first intermediate frequency television signal at IF Input 112. The first intermediate frequency signal is passed to an attenuator 114 where the system gain is set. The signal then passes to mixer 116. In mixer 116, the first intermediate frequency signal is mixed with a first local oscillator signal from local oscillator 118 to generate a second intermediate frequency signal. The local oscillator is a synthesized source that is phase-locked to a crystal oscillator.

Below applicants describe embodiments for two well known television formats, NTSC and PAL. Of course, the invention is applicable to other formats as well, including the advent of high definition television formats.

In an NTSC embodiment, the first intermediate frequency signal is at 45.75 MHz and is converted to a second intermediate frequency signal at 170.25 MHz. The first intermediate frequency sound subcarrier is at 41.25 MHz. This subcarrier is converted to a signal at 174.75 MHz. The limits of the second

intermediate frequency signal are 169-175 MHz. The frequency of the local oscillator 118 in this embodiment is 216 MHz.

In a PAL embodiment, the first intermediate frequency signal is at 38.9 MHz and the second intermediate frequency signal is at 169J MHz. The first intermediate frequency sound subcarrier is at 32.9 MHz and is converted to a

signal at 175.1 MHz. The limits of the second intermediate frequency signal are 167.85-175.85 MHz. The frequency of the local oscillator 118 in this embodiment is 208 MHz.

The output signal of mixer 116 is then passed to amplifier 120 where it is amplified. The amplified signal is then output as the second intermediate frequency signal.

The local oscillator module 150 includes a tunable local oscillator 152. In a preferred embodiment, the local oscillator can generate frequencies from 380 to 830 MHz in order to generate output frequency signals of 550 to 1000 MHz for transmission when the local oscillator signal is mixed with the second intermediate frequency signal. Local oscillator module 150 may also include buffering and amplifying circuitry.

The output converter module 130 receives the second intermediate frequency signal from the second intermediate frequency converter module 110 and receives a local oscillator signal from the local oscillator module 150. The output converter module converts the second intermediate frequency signal to a signal of between 550 and 1000 MHz for transmission by mixing the second intermediate frequency signal with a local oscillator signal from local oscillator module 150.

The output converter accepts the second intermediate frequency signal from the second intermediate frequency module and passes the signal to a bandpass filter 132 centered at 172 MHz. The purpose of this filter is to attenuate unwanted mixing products resulting from the mixing occurring in mixer 116 of the second intermediate frequency module 110. This filter also removes most of the leakage from local oscillator 118 and attenuates most of the broadband noise in the signal. Diplexer 133 is shown placed between the bandpass filter 132 and mixer 134 to provide a broadband match for the mixer. This is by way of example only. The diplexer could be placed on either side of filter 132 for proper impedance matching. Similarly, a diplexer could be placed on either side of filter 140 to provide impedance matching for that filter.

The second intermediate frequency signal then passes to a mixer 134, where it is mixed with the signal from local oscillator 152 after the signal is amplified by amplifier 136. Mixer 134 upconverts the second intermediate frequency signal to an output frequency signal of between 550 MHz and 1000 MHz depending on the frequency to which the local oscillator 152 is tuned.

The upconverted signal is further amplified by amplifier 138. Amplifier 138 in this case consists of a two-stage cascaded transistor amplifier. The amplified signal is then passed to a tunable 550 to 1000 MHz filter 140 to remove unwanted mixing products and any local oscillator leakage. In a preferred embodiment, filter 140 is a pair of bandpass filters, each of which is a two-pole coupled resonator design as will be discussed later. The signal is then amplified to its final level by amplifier 142. In a preferred embodiment a directional coupler 144 is employed and splits the amplified signal into two signals. The first signal

is supplied as the output signal from output port 146. The second signal is supplied to test port 148 for monitoring the operation of the output converter.

The structure of bandpass filter 140 will now be described in regard to Figure 3. The filter 400 is composed of two filters 410,420, each of which is a two-pole design. Resonators 422,424,426,428 each have a variable capacitor 430,432,434,436 associated with them for tuning purposes. The resonators are implemented through microstrip technology.

The structure of bandpass filter 132 will now be described in more detail with reference to Figure 4. The second intermediate frequency is received on input strip 210. Multiple resonators 212,214,216,218 are formed from microstrip. At the end of each resonator is supplied a variable capacitor 220,222,224,226. The combined length of the segments A and B comprising a resonator in this case was chosen to be 17 degrees long at the 172 MHz center of the passband, where 17 degrees refers to degrees of wavelength of the center frequency passband. The ratio of lengths of the A and B segments determines the output impedances to which the filter is matched. In this case, the filter is matched to 50 Ω. Similarly, there exists segments C and D that determine the input impedance to the filter. An output strip line 228 is provided to carry the filtered signal to a diplexer. Alternatively, both fixed and variable capacitors can be associated with each resonator. In that case, fine tuning of the resonators is accomplished with the variable capacitors. In any event, by the use of the variable capacitors, each of the resonators is tuned to a center frequency of 172 MHz. The impedance of either side of the filter has a high return loss in the passband but makes a rapid transition to pure reflection outside the passband. In

fact, the out of band impedance is nearly a short circuit as the frequency is increased above the passband. For instance, at high frequencies, the resonator segments having lengths A and B and C and D are nearly short circuited and the impedance seen at either the input or output of the filter at frequencies significantly above the passband are nearly the parallel combination of microstrip inductor segments A and B and C and D, respectively. The diplexer makes use of the short circuit, transitioning to inductance, at frequencies above the passband by placing an inductor in series with the main output followed by an R-C series network in shunt position. The diplexer of the filter is shown in Figure 5.

Figure 5 shows a diplexer for impedance matching the filter. Resonator 312 and variable capacitor 310 correspond to resonator 218 and capacitor 226 in Figure 4. An inductor 302 is placed between the resonator 312 and output strip transmission line 304. A capacitor 306 and resistor 308 are also tied to the output strip transmission line 304. The value of the resistance 308 is chosen to be approximately equal to the characteristic impedance of the system (ZQ). Here, the characteristic impedance of the system means the circuit components on each end of the filter. That impedance may or may not be the overall impedance of the television distribution system. The value of the inductor 302 is chosen such that the series impedance at the filter passband frequency does not cause significant insertion loss but increases in impedance with frequency above the passband. The capacitor 306 is chosen so that at some crossover frequency significantly above the filter passband the capacitive reactance is equal to but opposite to that of the inductor 302.

The choice of the inductive, capacitive and resistive values, as described above, causes a coordinated transition in port impedances above the filter passband

such that the combined source impedance is nearly ZQ, the characteristic impedance of the system. The impedance transition occurs because the inductor presents an increasing impedance to the filter short circuit as the capacitor presents a decreasing impedance to the resistor as frequency increases. The result is a transition from a near short circuit just above the filter passband to a resistive load of ZQ well above filter passband. This network provides good return loss to the mixer connected to the filter output in the important regions of the frequency spectrum (i.e., those within the filter passband of 168 to 176 MHz, the local oscillator frequency of 380 to 830 MHz and the output frequency of 550 to 1000 MHz).

A simple block diagram of a portion of the output converter module is shown in Figure 6. This diagram includes values for capacitance, resistance and inductance components according to the preferred embodiment. The resonator filter 310 corresponds to the structure in Figure 4. The characteristic impedance of the system is 500, as is nearly resistor 316. Capacitor 312 is provided to be 12 pF, while inductor 314 is provided to be 30 nH. As discussed in detail above, the signal from the filter structure is fed to mixer 320, which mixes the filtered signal with a local oscillator signal of between 380 and 830 MHz supplied by local oscillator 322. The resultant signal is a channel signal, for example, a television channel, of between 550 and 1000 MHz. The improved match to the mixer at the local oscillator and output frequencies helps reduce unwanted n x m mixer spurious products and two-tone third order products. The diplexer structure can be implemented for filters of order other than the four order system shown in Figure 4. For example, a diplexer for a fifth order filter can be implemented with similar results. In addition to higher order filters, higher order diplexers may be used to obtain different transition points and the like as will be appreciated by persons skilled in the art. Such diplexers would include multiple capacitors and inductors. Furthermore, even better band match of the filter can be achieved by adjusting the tap point on the output resonator of the filter after the filter is connected. The diplexer does cause a slight increase in passband filter loss. But the loss is small and is not a concern in a typical system.

The diplexer scheme can also be used with other components to obtain an improved return loss below the passband. The diplexer scheme is applicable to filters other than bandpass filters. The diplexer is also applicable to bandpass filters where the input and output connections are made via separate additional coupling elements parallel to the filter resonator. Also, for sequential double conversion applications, the filter could have a diplexer circuit provided on both sides of the filter. The implementation of the inductor and/or capacitor can be through either lumped or distributed components. The diplexer will work with filter constructions other than those employing microstrip construction, even though the preferred embodiment uses a microstrip combline filter.

Similarly, the diplexer may be located anywhere an impedance match to a filter is needed. Referring again to Figure 2, a diplexer could be located between the output of amplifier 120 and filter 132. Similarly, a diplexer could be located on either side of filter 140 for better impedance matching to that filter.

The invention has been described in detail with reference to the appended drawings. However, many other variations of the invention as possible as will be

appreciated by persons skilled in the art. Therefore, the invention should be construed as limited only by the claims.

Claims

1. A frequency converter for converting an intermediate frequency signal to a signal of a different frequency, comprising: a filter for filtering the intermediate frequency signal; a local oscillator for producing a local oscillator signal; a mixer for mixing the filtered intermediate frequency signal and the local oscillator signal and producing an output signal having a frequency different from either the intermediate frequency signal or the local oscillator signal; and a diplexer for impedance matching the filter to the mixer,

wherein said diplexer comprises an inductor coupled between the output of the filter and the mixer and a capacitor and resistor in series placed in a shunt configuration with said inductor.

2. A frequency converter as recited in Claim 1, wherein said resistor is chosen to be approximately the characteristic resistance of the system.

3. A frequency converter as recited in Claim 1 , wherein said filter is a microstrip filter.

4. A frequency converter as recited in Claim 3, wherein said microstrip filter includes a plurality of resonators and plurality of variable capacitors, one variable capacitor associated with each resonator.

5. A frequency converter as recited in Claim 1 , wherein said local oscillator is a tunable local oscillator.

6. A frequency converter as recited in Claim 5, wherein said tunable local oscillator produces local oscillator signals having frequencies of between 380

and 830 MHz.

7. A frequency converter as recited in Claim 6, wherein said output signal has a frequency of between 550 and 1000 MHz.

8. A frequency converter as recited in Claim 4, wherein the output of the microstrip filter is provided to an ouφut microstrip line, said output microstrip line connecting to one of said resonators dividing said resonator into two segments, a first segment and a second segment, the lengths of those segments determining the output impedance of the filter.

9. A frequency converter as recited in Claim 4, wherein the input of the microstrip filter is provided by an input microstrip line, said input microstrip line connecting to one of said resonators dividing said resonator into two segments, a first segment and a second segment, the lengths of those segments determining the input impedance of the filter.

10. A diplexer circuit for impedance matching a filter to another circuit component, comprising: an inductor coupled between said other circuit component and said filter; and a capacitor and resistor in series placed in a shunt configuration with said inductor.

11. A diplexer circuit as recited in Claim 10, wherein said resistor is chosen to have a resistance equal to the approximate characteristic impedance of

the system.

12. A diplexer circuit as recited in Claim 10, wherein said inductor,

resistor and capacitor are formed of lumped components.

13. A diplexer circuit as recited in Claim 10, wherein said inductor,

resistor and capacitor are formed of distributed components.

14. A diplexer circuit as recited in Claim 10, wherein said inductor,

resistor and capacitor are formed of any combination of either lumped or

distributed components.