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Interference in receivers caused by powerful signals outside the receiving frequency

The duplex filter and its functions

Interference in receivers caused by powerful signals outside the receiving frequency

The duplex filter and its functions

 

One of the most widely used filters when coupling transmitters and receivers working simultaneously but at different frequencies is THE DUPLEX FILTER.

 

A typical receiver for 465 MHz can receive a signal of 0.3 μV or –117 dBm at approx. 12 dB noise suppression (or expressed as 0.3 μV for 12 dB SINAD).

 

It is assumed that the receiver is used with a transmitter of 25 W or 10.log

 

intermodulation-01

 

This is illustrated in the graph below:

 

intermodulation-02

 

Adjacent-(channel)

 

Adjacent-(channel)

 

If sketching in the adjacent-(channel) selectivity of the receiver, which is 80 dB increasing by approx. 5-8 dB per MHz moving away from the receiver frequency, it can be seen that approx. 50-55 dB attenuation is required to prevent the transmitter from over-loading the receiver.

 

This attenuation can be achieved with a reject-pass filter with approx. 55-60 dB attenuation and an insertion loss of approx. 1 dB.

 

This system would be applicable if the transmitter only emitted 460 MHz, in other words was an ideal transmitter.

 

intermodulation-03

 

 

Unfortunately, this is never the case. Regulations stipulate that noise and spurious signals must be below -36 dBm or 80 dB below the 25 W output (43 + 10 . log output) or (44-80) dBm = -36 dBm.

 

If inserting this limit in the graph below, it can be seen that the –117 dBm large signal is "stifled" by a sideband noise which is approx. 50-60 dB stronger.

 

 

intermodulation-04

 

Overloading

 

Overloading

Consequently, the transmitter must be fitted with a band-pass / band-reject filter with an attenuation of 60-70 dB.

 

The ideal situation in which both transmitter and receiver operate satisfactorily at the same time is shown here:

 

intermodulation-05

 

 

As is evident from the graphs the receiver is now protected against overloading from the actual transmitter frequency by a band reject/band-pass filter and from the transmitter sideband noise by a band-pass/band-reject filter.

 

If these filters are combined in one filter, this filter is a Duplex filter.

 

 

intermodulation-06

 

Band-pass / Band-reject filter

 

Band-pass / Band-reject filter

 

If several transmitters operate in the area, which is often the case, it may be necessary to add additional filters in front of the receiver to prevent intermodulation.

 

 

intermodulation-07

 

If another transmitter is inserted in the graph with an output of 0 dBm, a transmitter of +44 dBm but with a separation attenuation of approx. 44 dB, it can be seen that it may interfere with the receiver.

 

 

intermodulation-08

 

 

This transmitter must be attenuated approx. 20 dB, which can be achieved with a band-pass filter in front of the receiver.

 

This is shown in the graph below:

 

 

intermodulation-09

 

It can be seen that this band-pass filter will also provide additional isolation to this and other adjacent transmitters.

 

Another solution would be to insert a band-reject filter.

 

 

intermodulation-10

 

 

The graph illustrates that only a narrow frequency band will be attenuated – everything else passes without noticeable attenuation. In this case the band-reject filter provides approx. 50 dB attenuation of transmitter II – a greater attenuation than the one provided by the band-pass filter.

 

However, the band-reject filter does not provide the same protection against unwanted frequencies as the band-pass filter.

 

Low-pass / high-pass filters

 

Low-pass and high-pass filters will be well-suited for combining transmitters/receivers in various bands on the same feeder and/or antenna. Since the frequencies used are far apart the filters need not have the same steep profile as duplex filters. This means that these filters can often be made from more simple LC circuits, and as a result low-pass / high-pass filters are both smaller and often cheaper than high Q filters.

 

Another important function may be to prevent direct signal pick-up in the intermediate frequency of the receiver and, equally important, to reduce the sensitivity of the receiver to 2nd order intermodulation products.

 

 

But the primary task of the low-pass filter, in particular, is related to the transmitter. Here it may contribute to reducing the harmonics from the transmitter and attenuate any harmonics generated in any circulators or isolators.

 

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