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Translation of the article by Arkadiusz Matysiak from Delta company
Because of questions on the filters we manufacture, I would like to clarify the terms that are associated with this field - to eliminate possible mistakes when making an order or implementing the filters in antenna systems. As an example I will analyze band-stop filter, but the principles should be observed for all kinds of the equipment.
We have taken the notation:
- FZK-(ch. number): channel-stop filter -(ch. number), e.g. FZK-52
- FZP-(ch. number, ch. number): band-stop filter -(ch. numbers), e.g. FZP-2934
An ideal band-stop filter has no attenuation in the pass-band, 90-degree steep slope, and infinitely high attenuation in the rejection band. The frequency characteristics (attenuation - T) would be like this:
In practice every filter is described by a set of parameters:
Tp - pass attenuation (the lower the better)
Tz - stop attenuation (the higher the better)
Fp1- Pass frequency 1 (attenuated at the level of Tp)
Fz1- Stop frequency 1, (attenuated at the level of Tz)
Fz2- Stop frequency 2, (attenuated at the level of Tz)
Fp1- Pass frequency 2 (attenuated at the level of Tp)
Tz - stop attenuation (the higher the better)
Fp1- Pass frequency 1 (attenuated at the level of Tp)
Fz1- Stop frequency 1, (attenuated at the level of Tz)
Fz2- Stop frequency 2, (attenuated at the level of Tz)
Fp1- Pass frequency 2 (attenuated at the level of Tp)
The frequencies (bands) are given as TV channel numbers.
The customer should specify all the parameters listed above.
Theoretically, the width of the rejected band is determined by the two points (to the left and to the right from the center frequency) where the attenuation level drops by 3dB. In practice, however, in TV antenna systems, we are not so interested in these points, but in the the rejected TV channels. When making an order, it is enough to specify these channels.
NOTICE 1.
Increase of the Tp /Tz ratio makes the filter more expensive.
The lower the difference between Fp1 and Fz1 (or between Fz2 and Fp2), the more expensive the filter.
Increase of the Tp /Tz ratio makes the filter more expensive.
The lower the difference between Fp1 and Fz1 (or between Fz2 and Fp2), the more expensive the filter.
NOTICE 2.
There are important only the four channels - the shape of the transitions ( between Fp1 and Fz1, and Fz2 and Fp2) is not essential - it depends on the kind of the filter and tuning method.
There are important only the four channels - the shape of the transitions ( between Fp1 and Fz1, and Fz2 and Fp2) is not essential - it depends on the kind of the filter and tuning method.
The shape of the transitions is not essential - all the filters fulfill their tasks:
The offered filters are not generally suitable for cable TV systems (possibility of interferences coming in between Fz2 and Fp2).
Below there are characteristics of two implementations of FZK-52 filter.
The filter on the right is not suitable for the application in the region, although it can be acceptable in another country (different Fp1). When ordering band-stop filters (FZK) it is important to specify not only stop channels, but also the active ones, especially in the case of close channels.
Tp value depends only on the structure of the filter (0.5-2 dB).
Tz value: -18 dB or -25 dB (in the case of pass-band filters -30 to -50 dB).
Tz value: -18 dB or -25 dB (in the case of pass-band filters -30 to -50 dB).
Example characteristics of pass-band filters.
Due to different structure, they are classified as F-5 or F-6. Below - characteristic of F-5 filter (ch. 50). The vertical lines indicate video and audio carriers.
In the case of close channels (the active and unwanted - one or two channel gap) there should be used F-6 filters. These filters are additionally equipped with notch filter for required channel. In general, the filters work effectively for at least one channel gap.
Label of band-pass filter for ch.50, with notch filter for ch.52: Filter F-6 k50 p52.
It is impossible to make LC filters that would pass one TV channel and stop the adjacent one - there is only several hundred kHz for the slope.
The F-6 filters ensure typical attenuation of carriers at 25 dB level (one channel gap, UHF - band V), the F-5 filters - 18 dB. These parameters are suitable for the applications.
Band-stop filters not only reduce reflections but also eliminate interferences coming from various transmitters operating close to antenna systems. Strongly attenuating their carriers, the filters eliminate possible intermodulation in antenna amplifiers caused by strong unwanted signals. Numerous base stations, cellular BTSs, local FM transmitters (up to several kW) are the typical sources of the unwanted signals.
This is the task of band-stop filters connected between the antenna and the amplifier (or being an integral part of the amplifier).
An example solution of band-stop filter for FM band (88-108 MHz) is shown below. The basic principle for the project has been minimum 30 dB attenuation of the unwanted signals. The best choice is 3-stage Chebyshev filter:
The picture below shows the practical implementation (C8 is SMD type - not visible here).
The A-130 antenna amplifiers are equipped with such filters, so they can be used in antenna installations operating close to local FM transmitters. Below - characteristics of the amplifier.
Other suitable type of filter that can be used for suppressing FM interferences is Cauer (or elliptic) filter.
This kind of filters is sharper than all the others, but the tolerances of LC elements have to be much smaller than in the case of Chebyshev filters. It also show ripples on the whole bandwidth.
Below- characteristic of band-stop Cauer filter (88-108 MHz, 40 dB).
This kind of filters is sharper than all the others, but the tolerances of LC elements have to be much smaller than in the case of Chebyshev filters. It also show ripples on the whole bandwidth.
Below- characteristic of band-stop Cauer filter (88-108 MHz, 40 dB).
In manufacturing practice it is difficult to select capacitors precisely, there are usually chosen values from the IEC 60063 preferred number series (E12 or E24). The following diagram depicts the characteristics obtained with capacitors from the series.
It is clear that only E24 series ensures proper results.
The schematic diagram of FM band-stop filter (75 ohm input/output impedance):
The implementation shown above (Chebyshev filter) has been based on conventional components (lumped parameter system), the second solution (Cauer filter) involves microstrip technology (distributed parameter system).
The latter implementation is cheaper in mass production and eliminates the need of time-consuming adjustment.
After calculating the inductive components, we got the following PCB layout:
MASA=GROUND; We=In; Wy=Out
The latter implementation is cheaper in mass production and eliminates the need of time-consuming adjustment.
After calculating the inductive components, we got the following PCB layout:
MASA=GROUND; We=In; Wy=Out
Electromagnetic analysis of the layout proves correctness of the solution:
The differences between the above graph and the specifications are of no significance.
For the computer simulation there were assumed the following parameters:
For the computer simulation there were assumed the following parameters:
- One-sided PCB.
- Epoxy-fiberglass substrate, relative permittivity: Er=5.
- PCB thickness: 1.5mm.
- Copper clad thickness: 0.035 mm.
- Dielectric loss factor: 0.04.
The picture of the physical prototype...
...and the characteristic taken on a wobbuloscope:
There is only needed a minor correction of frequency characteristic (to the left). The general requirements have been met - the FM band is effectively suppressed.
The discussion concerned use of RF filters in individual (or small community) antenna installations for filtering unwanted signals. Similar filters are also used in antenna diplexers/multiplexers, in band/channel amplifiers etc.
