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How to remedy/reduce intertransmitter intermodulation

In order to reduce the possibilities of intermodulation in a transmitter output, the key method is to prevent the emitted energy from other transmitters from reaching the output.

How to remedy/reduce intertransmitter intermodulation

In order to reduce the possibilities of intermodulation in a transmitter output, the key method is to prevent the emitted energy from other transmitters from reaching the output.

 

This can be achieved by means of:

 

a) Isolators

b) Pass-reject / reject-pass filters

c) Notch filters

d) Band-pass filters

 

 

a) Isolators

 

a) Isolators

 

An isolator is a 3 port device that allows the signals to pass on one direction – typical insertion loss 0.3–0.4 dB – and attenuates signals in the reverse direction – typically 25–40 dB.

 

The isolator – or circulator – is a ferrite device in which the ferrite is premagnetized by permanent magnets. The magnets generate an electron rotation in the core that will permit transmission in the frequency range in which the circulator is tuned whereas considerable attenuation occurs in the reverse direction.

 

Another property of the circulator is that if the antenna or the antenna cable fails, the output from the transmitter will be "dumped" in the dummy load, thus preventing destruction of the output transistors.

 

isolator

 

A disadvantage of the circulator is that it can generate harmonics. So a low-pass filter should always be inserted between circulator and antenna.

 

Circulators are the only appropriate devices if the transmitter frequencies are closely spaced.

 

b) Pass-reject / reject-pass filters

 

b) Pass-reject / reject-pass filters

 

A pass-reject filter (reject-pass filter) is a device that permits one frequency (frequency band) to pass and attenuates or blocks another frequency (frequency band).

 

The device can be made from high Q quarter-wave-length resonators (approx. 100 mm diameter) where the frequency spacing between pass and reject may be as low as 250 kHz in the VHF frequency range. A filter of this type can provide about 25 dB isolation.

 

If the spacing is about 4 MHz or more, a filter made with helical resonators can be used. This filter is physically much smaller than the quarter-wavelength resonator filter. Due to the increased spacing between the pass and reject frequencies, a filter of this type can provide about 50–80 dB isolation, depending on the number of helical resonators.

 

Both types permit a reject frequency higher than (pass-reject type) or a reject frequency lower than (reject-pass type) the pass band.

 

c) Notch filters

 

c) Notch filters

 

A band-reject filter is a filter attenuating a single frequency (narrow frequency band), permitting both higher and lower frequencies to pass. If high Q quarter-wavelength resonators are used, the spacing between the 1.5 dB points of the reject band may be less than 1 MHz wide for about 25 dB attenuation at the reject frequency in the VHF frequency range.

 

If helical resonators are used in the filter, the spacing between the 1.5 dB points of the reject band will be about 5-6 MHz wide for about 45 dB attenuation at the reject frequency in the VHF frequency range.

 

d) Band-pass filters

 

d) Band-pass filters

A band-pass filter is a filter that permits a single frequency (frequency band) to pass and attenuates frequencies above and below the pass range. If high Q resonators are used, attenuation at 5 MHz from the pass frequency will be about 25-35 dB depending on the accepted insertion loss at the pass frequency.

 

If helical resonators are used in the filter, the result is also an efficient band-pass filter, but with a considerably wider pass range and less steep profiles on the filter.

 

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