EP2782187A1

EP2782187A1 - Structure of LOW-PASS filter in particular for DVBT-DVBH

Info

Publication number: EP2782187A1
Application number: EP13160398.7A
Authority: EP
Inventors: Enrico Carlo Maria Boracchi
Original assignee: 3B Elettronica di Boracchi Pietro & C SNC
Current assignee: 3B Elettronica di Boracchi Pietro & C SNC
Priority date: 2013-03-21
Filing date: 2013-03-21
Publication date: 2014-09-24

Abstract

A structure of a LOW-PASS filter, in particular for DVBT-DVBH, comprining a first STOP BAND filter with narrow band with insertion loss of <1.5dB across the transmitted band, an insertion loss at 790MHZ or 702MHz of <8dB, and an insertion loss at 739MHz or at 705MHz of >30dB followed by a second filter, in cascade, that can increase the insertion loss from 790 to 793MHz.

Description

BACKGROUND OF THE INVENTION

This device is a structure of Low-pass filter in particular for DVBT-DVBH.
The continuous progress of the mobile telephony has created new broadband access systems - LTE - with objective to achieve wireless high speed connections.
The frequency allocation of the LTE (Long Term Evolution) has reduced the number of available TV channels in the UHF frequency band.
In fact, the original 49 TV channels will be reduced to 40 in order to free the frequencies from 794 MHz to 858 MHz (TV channels 61 to 69).
So the LTE frequency band and TV frequency band will be adjacent with important development.
The LTE signal transmitted by the BTS will be received by the Mast Head amplifiers or the TV sets with built-in digital tuner covering the full UHF frequency band (including the 9 channels assigned to the LTE).
The total signal received at the input of the Mast Head amplifier (TV & LTE signal), in some cases, can exceed the maximum power input threshold of the amplifier, overloading the same and, because the amplifiers are not designed for excessive overloading signals, degradation/cancellation of the TV channels can occur.
This is particularly true for the TV sets receiving the adjacent signals to the DVBT band, mainly ch.60 that, exceeding some pre-set protection levels, will produce some degradation between the last DVBT channels and the first LTE frequency blocks.
The degradation of the TV signal due to the overloading of the Mast Head amplifier requires an adjustment of the domestic TV installation as per CEI regulations.
If this adjustment is not implemented, all the domestic Mast Head amplifiers shall be switched off.

SUMMARY OF THE INVENTION

In order to solve this problem, a filter shall be developed that will overcome this problem complying with CEI regulations about the LTE filters.
This development, together with other objectives, that will be clearer at later stage, is achieved with a LOW-PASS filter, in particular for DVBT-DVBH.
The filter structure is designed with a first STOP BAND filter with narrow band with insertion loss of <1.5dB across the transmitted band, an insertion loss at 790MHZ or 702MHz of <8dB, and an insertion loss at 739MHz or at 705MHz of >30dB followed by a second filter, in cascade, that can increase the insertion loss from 790 to 793MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of this device will be clearer during the analysis of the description of the preferred but not exclusive design of this device, illustrated in the attached drawings - this illustration is indicative and not limitative:

Fig. 1 shows the equivalent circuit of the first filter;
Fig. 2 shows the frequency response of the first filter;
Fig. 3 shows the equivalent circuit of the second filter;
Fig. 4 shows the equivalent circuit of every single resonator with distributed parameters;
Fig. 5 shows the equivalent circuit of the next filter;
Figs. 6 and 7 show two graphs with LTE curve response obtained with traditional filters and the response obtained with LOW PASS filter that is part of this patent application.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described in the introduction, it will be almost impossible to design a filter to comply with new norms and regulations.
A new LOW PASS FILTER shall be design with very tight specifications:

1^st SPECIFICATION
1. 1. Insertion loss in the transmitted band <1.5dB
2. 2. Insertion Loss at 790Mhz <8dB
3. 3. Insertion loss at 793Mhz >30dB
4. 4. Insertion loss from 830Mhz >30dB
2^nd SPECIFICATION
- 1. Insertion loss in the transmitted band <1.5dB
- 2. Insertion loss at 702Mhz <8db
- 3. Insertion loss at 705Mhz >30dB
- 4. Insertion loss from 830Mhz >30dB

A single Low Pass Filter cannot be designed due to the high number of cavities required even if with an elliptical type of filter. In fact the filter shall work across the 470-950 MHz band.
According to this invention, the solution is to utilize 2 STOP BAND FILTERS in cascade: the first filter with very narrow band to satisfy the requirements 1 to 3, the second, in cascade, must provide a very sharp side to improve the insertion loss from 790 MHz to 793 MHz.
To satisfy point 4 of the specifications the first filter shall have an equivalent circuit as per Fig. 1.
Fig. 2 shows the frequency response of the filter.
This filter has only 5 cavities with these results:

Insertion loss in band <1dB;
Insertion loss at 790 MHz = -5.5dB;
Insertion loss at 793MHz = -26dB;
Bandwidth @ 3dB = 40MHz.

To improve the sharpness, an additional cavity was added to the original design.
The second filter is a Stop Band filter with a bandwidth greater than 150MHz and cannot be designed as the first one.
However the best results, to achieve the objectives for point 1 to 3, will require reducing the bandwidth of the filter to 10MHz.
With similar performances, the losses must be very low, Q factors of the resonators very high and coupling factors very weak.
So the filter had to be designed with distributed parameters.
Every parallel resonator has been replaced by an inverter (750hm line impedance with a 90° phasor at the centre of the band of the filter) followed by a cascade resonator with high Q made by a λ/4 coax line with centre frequency 796MHz terminated to earth and to a capacitor.
In order to keep a high Q factor for each resonator, the matching is made with matched coax.
Fig. 3 shows this solution.
Every INV block is made by a 75Ω line whose length is calculated for the correct operation of the filter.
Every single resonator is made by distributed parameters. Here below is the typical description of the circuits.
Every resonator is matched to the next one with a 75Ω line whose length creates a bipolar inverter.
Practically after the inverter a cascade resonant circuit appears as parallel and vice versa. This solution allows using the same type of resonator for all the cells.
This filter cannot satisfy point 4 of the specifications so a second Stop Band Filter, with same characteristics, shall be connected in cascade. The equivalent circuit is shown in Fig. 5.
The very important matching with a capacitor towards the resonators greatly reduces the Q factor and increases the rejected band beyond 150MHz.
This solution satisfies the required objectives of the project.
In fact the structure of the filter is made with 2 filters in cascade satisfying points 1 to 4 of the specifications.
Any materials, components and dimensions can be used according to the requirements.

Claims

A structure of a LOW-PASS filter, in particular for DVBT-DVBH is designed with a first STOP BAND filter with narrow band with insertion loss of <1.5dB across the transmitted band, an insertion loss at 790MHZ or 702MHz of <8dB, and an insertion loss at 739MHz or at 705MHz of >30dB followed by a second filter, in cascade, that can increase the insertion loss from 790 to 793MHz.
A LOW-PASS filter structure, according to claim 1, characterized in that it is realized with distributed parameters.
A LOW-PASS filter structure, according to claim 1, characterized in that it includes several inverters each one followed by a resonator in cascade.
A LOW-PASS filter structure, according to claim 1, characterized in that every resonator in cascade is made by a high Q resonator made with a λ/4 coax line with centre frequency 796MHz terminated to earth and to a capacitor.
A LOW-PASS filter structure, according to any of the previous claims, characterized in that each resonator coupling is distributed with a matched coax.
A LOW-PASS filter structure, according to any of the previous claims, characterized in that it includes several INV blocks, each one being made by a 75Ω line whose length is calculated for the correct operation of the filter.
A LOW-PASS filter structure, according to any of the previous claims, characterized in that each resonator is realized with distributed parameters.
A LOW-PASS filter structure, according to any of the previous claims, characterized in that each resonator is matched to the next with a 75Ω line with appropriate length to realize a bipolar inverter.
A LOW-PASS filter structure, according to any of the previous claims, characterized in that it comprises the same type of resonator for all the cells.
A LOW-PASS filter structure, according to any of the previous claims, characterized in that the second stop band filter is realized with distributed constants coupled to the resonators via a capacitor.