[Imported] Duplexers and other Filters

[sbeadle]

>>Message imported from previous forum - Category:Trio Data Radios<<
User: joelw, originally posted: 2018-10-17 21:20:55 Id:108
This is a re-posting from the obsoleted (October 2018) "Schneider Electric Telemetry & SCADA" forum.

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**_jdale:
Here's a bit of information on the duplexers sold by Trio._**

The TBUMDUPLXBP4XXCOA duplexer is mounted in a 19" rack (2 RU tall) . I've attached a data sheet for it, though that datasheet includes all the other Trio accessories as well.

Duplexers are mechanical filters which allow both a transmitter and receiver to be connected to a single antenna, with both operating at the same time. Alternately, two simplex (single frequency) radios can be connected to one antenna in the same way, as long as the two channels used are about 5 MHz apart. The purpose is to allow a radio to act as a full duplex repeater, meaning it can receive a signal and re-transmit it with only a few millisecond delay. (to demodulate then re-modulate the data) This requires the duplexer to be able to separate out the two signals eg a transmit signal going up to the antenna at +37 dBm (5 watts) and a receive signal coming from the antenna at -88 dBm. That's a huge difference in power. The duplexer has to be able to filter the transmitter's signal to keep it out of the receiver or the receiver would be deafened (radio people say it's been de-sensed) by its own transmitter.

The band pass duplexer will allow a small range of frequencies around its tuned frequency (there's both a High side and a Low side) to pass through, and then about 5 MHz away there will be a notch that blocks as much of the opposite channel as possible. Away from the fairly narrow pass-band of this duplexer it will also reject all other signals to some degree, providing some additional protection from nearby transmitters. See the attached filter response curves. The high and low frequency sides are both shown overlapping. Beyond the filter's pass-band region the rejection rises fairly rapidly, to a maximum at the notch frequency.
 zt/wzcdsoglgdj2.jpg "")
Duplexers for the 400-500 MHz band tend to use a 5 MHz split between the two channels. (at lower frequencies the split can be less, at higher frequencies it must be more - it's a percentage of the frequency in use) Such filters are never perfect - if the frequencies are too close together it's very hard to pass one with minimal loss while blocking as much of the other as necessary. There is always some loss even when a duplexer is properly tuned, about 1.5 or 2 dB in the pass band. This must be added to the filter loss area in a Pathloss link when testing paths.

A Trio QB or QH must include a duplexer, even in the (somewhat rare) cases where the radio is only operating in a half duplex mode. (Tx OR Rx, not at the same time) This is because the QB and QH do not have an option for a transmit/receive switch. Such a switch is included in all half-duplex radios. It is typically a FET type device, though in very old radios a relay was used.

There are many other kinds of filters which can be used in radio systems. A common type is the notch filter. (example attached) This may for example be placed in line with the coax cable to the receiver to block out one specific very strong interfering signal. That signal must be some distance away from the signal you want to pass. The closer together the two are, the higher the loss will be in the desired signal.

There are also stand-alone band-pass filters which do the opposite - just pass one frequency and reject all others. A single-cavity band-pass duplexer will pass one specific frequency and block all others. A multi-cavity filter (several in a row) is required to pass a range of frequencies. A good example of this is the model 17965 made by Microwave Filter Company, which passes all of the 902-928 MHz ISM band but blocks signals outside that range.

Attached file: (editor/bn/3miclnwi8500.pdf), SE-DataSheets-Trio-Wireless-Accessories-letter-TBU-v2.pdf File size: 435168

Attached file: (editor/zt/wzcdsoglgdj2.jpg), Bandpass Filter Response Curves.jpg File size: 174887

Attached file: (editor/g1/pd8dj58tcrsh.pdf), Sinclair Notch Filter FR30107-3-DI.pdf File size: 298975

Attached file: (editor/uf/26r53hyodg5e.pdf), Sinclair Band Pass Filter FP30107-3-DI.pdf File size: 260801