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PRODUCT REPORT High-Frequency Filters The Benefits of HF Filters If you’re an average end user planning to set up your own Ku band satellite reception system you simply get your antenna, LNB, receiver and coax cable to connect LNB and receiver – no need to worry about anything else. If, however, you’re the kind of satellite enthusiast who always wants to dig a little deeper, or if you run a professional cable head-end or even a satellite uplink station, then you might need some more equipment, such as high-frequency filters. The market for those accessories is rather small, and this is why only a handful of manufacturers can actually supply such filters. MFC (Microwave Filter Co., Inc.) is one of them – a 4 company that specialises in filters and optional equipment for the high-frequency range between 5 Hz and 50 GHz. MFC’s product portfolio includes waveguides, dielectric resonators, frequency-separating filters, standard filters, load resistors (frequently called ‘dummy loads’), adapters and all accessories that come with those items. Demand is particularly high for C band filters, because this is where interference frequently occurs and – more importantly – the right filter can work wonders in eliminating such interference. High-frequency filters are mostly used for eliminating unwanted signals. More often than not, such interfering signals cannot only be noticed on a single frequency, but also have a nega- 1. A sample spectrum: the signal level is high over a great frequency range, no filter is used. 2. Using a high pass filter: only frequencies above the cutoff frequency pass the filter, low frequencies are attenuated substantially. 3. Using a low pass filter: only frequencies under the cut-off frequency pass the filter, high frequencies are attenuated substantially. 4. Band-Pass filter: combining both a high pass filter with a lower cut-off frequency and a low pass filter with a higher cut-off frequency. The result is that the centre band will pass the filter with minimal attenuation. 5. Band-Rejection filter: in this case a low pass filter with a low cut-off frequency is combined with a high pass filter with a high cut-off frequency are combined. The result is that the centre band is attenuated substantially. 1 2 3 5 172 TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine — 11-12/2013 — www.TELE-audiovision.com
■ Example with a UHF filter of a pay TV operator: The left picture shows the whole CATV spectrum without any filter. The right picture shows the result of using a low pass filter with a cut-off frequency of 296 MHz. ■ The new catalogue by MFC gives an extensive overview of all available filters made by MFC. The catalogue can also be downloaded directly from their website: www.microwavefilter.com tive impact on neighbouring frequencies. In addition, receivers and other active elements within the system are at risk of malfunctioning due to interference. The trick now is to filter out those unused frequency ranges that carry the interfering signals. Existing signals can be filtered in a number of different ways. For one, it is possible to filter out signals above and/or below a certain specified frequency. Low-pass and high-pass filters are used to that end. A low-pass filter allows all frequencies below the cut-off frequency, while a high-pass filter lets through all frequencies above a set cut-off frequency. Unwanted frequencies that are outside the cut-off frequencies are highly attenuated, whereas the target frequency range comes through with minimal attenuation. Now if you combine a highpass filter with low cut-off frequency and a low-pass filter with high cut-off frequency it is even possible to only allow a single frequency range through the filter setup. The correct term for such a configuration is band-pass filter. If, on the other hand, a low-pass filter is used in conjunction with a high-pass filter that has a higher cut-off frequency, only the centre frequency space is filtered and what we get is a socalled reject filter. Then again, what’s the use of all those filters? To start with, they allow providing individual frequency bands to different receivers without those receivers having to share frequency bands. SCR (Single Cable Routing) distribution setups, for instance, make use of this approach, with up to eight receivers having independent access to all satellite channels via a single cable that is led from one wall outlet to the next. In such a configuration, each receiver is assigned a dedicated frequency band with a central router modulating the required transponder onto the corresponding frequency band. Network operators, on the other hand, use filters in analog CATV networks as well to make sure customers with less expensive subscriptions cannot receive premium channels. Those channels are usually transmitted on higher frequencies and a sealed low-pass filter at the transfer point just outside the house or apartment prevents those subscribers from watching channels they don’t pay for. The most important reason for installing filters, however, can be found in the fact that neighbouring signals are generally prone to interference from each other. Unlike the number of different applications and uses sharing the same resources, the frequency range cannot be increased at random and has to be accepted as a given, with all its capacity constraints. Even very strict technical regulations and mandatory frequency charts cannot do much in terms of interference prevention. A prominent everyday example is interference in the DVB-T/T2 and ATSC range caused by LTE signals. As far as the regulator is concerned, all applications should work side by side in the frequency spectrum without doing harm to each other by using only those frequencies that have been specifically sat aside for each application. We all know too well, however, that in the real world it’s often an entirely different story. Generally speaking, highfrequency interference can by caused by a number of different phenomena. As far as receivers are concerned: • Interference from neighbouring frequencies • Interference in the IF (intermediate frequency) • Interference in the LO (local oscillator) frequency Interference can also be caused at the transmitting end: • In addition to the desired emission frequency, neighbouring frequencies may be affected by unwanted emissions that are caused by the modulator. • Harmonics emissions • Interference caused by intermodulation When it comes to selecting an appropriate filter it is paramount to understand all parameters given by the manufacturer. Listed below are the most important of them: • Attenuation Attenuation is measured in decibels and indicates the level by which the input signal is decreased. To find out the exact attenuation the signal level is measured first at input and then again at output, with the resulting difference in decibels (dB) being the achieved attenuation. • Bandwidth This parameter indicates the bandwidth of a bandpass filter, that is to say the frequency range that passes through the filter with a relative insertion loss of 3 dB or less. • Cut-off frequency This is the frequency that triggers either the high-pass New High-Frequency Filters by MFC for the C-Band Model 18253 - C-Band (INSAT) Transmit Reject Filter • This TRF provides deep rejection of the transmit band with minimal effect on the receive band. • Ideal for INSAT and other Region-Specific Receive Applications • Alternate Flange Configurations are Available Upon Request Pass band 4.5 - 4.8 GHz (C-INSAT Downlink) Insertion Loss 0.50 dB Max VSWR 1.30:1 Max Reject Band 6.725 - 7.025 GHz (C-INSAT Uplink) Rejection 80 dB Min Operating Temperature Range -10°C to +60°C Flanges CPR229G Dimensions 3.95” x 3.88” x 2.75” (100mm x 98mm x 70mm) Finish Gloss White Lacquer Model 18323 - C-Band (INSAT) Receive Reject Filter • Same as before but rejection of the receive (Downlink) band Passband 6.725 - 7.025 GHz Insertion Loss 0.10 dB Approx. VSWR 1.22:1 Max Reject Band 4.5 - 4.8 GHz Rejection 80 dB Typ Flanges CPR137/CPR137G Dimensions 5.00“ x 2.69“ x 1.94“ (127mm x 68mm x 49mm) Finish White Lacquer Model 18506 - Multi-Purpose C-Band Transmit Filter • This Uplink filter not only rejects the entire receive band (below 4.2 GHz), but it also rejects transmissions from other potential sources of interference etc., that RRFs do not. • Ideal for use in high-density transmit paths, like: Wireless Services (Point-Multipoint) 4.55 - 4.9 GHz Maritime & Aeronautical Radio Navigation 4.2 - 5.6 GHz Broadcast Auxiliary Services 6.95 -7.15 GHz • Ideal for all “standard band” C-Band Uplink Applications • Easy bolt-on installation and no power supply required Passband 5.925 - 6.425 GHz Passband Loss 0.3 dB Max Passband Return Loss 17.7 dB Min Rejection 50 dB Min @ 5.625 GHz 40 dB Min @ 6.725 GHz Power Rating 400 Watts Flanges CPR137F Dimensions 9.50” x 2.69” x 1.94” (241mm x 68mm x 49mm) Finish Gloss White Lacquer 174 TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine — 11-12/2013 — www.TELE-audiovision.com www.TELE-audiovision.com — 11-12/2013 — TELE-audiovision International — 全球发行量最大的数字电视杂志 175
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