|
|||||||||||||
|
|
|
||||||||||||
|
|
|||||||||||||
|
Syllabus Sections:- Routes of entry 6C1 41 Understand that amateur transmissions can enter audio stages via long leads or other interconnections. Not only can signals enter equipment from antenna leads but also via speaker leads in fact any interconnecting cable between one part of equipment and another can and sometimes does act as an antenna bringing the unwanted signal from your RF transmission into audio equipment. In addition to audio equipment, an alarm system also has long leads interconnecting items and these leads too can act as an antenna. Both audio and alarm systems are intended to amplify small electrical signals and can therefore contain high gain amplifiers which can be prone to interference from almost any RF source.6C1 41 continued Understand that any semiconductor or diode junction within an electronic device can rectify unwanted RF. In the Intermediate course you were introduced to
the crystal set. The crystal set has a simple diode to
rectify the RF signal. A diode is a PN junction. Because the
PN junction can rectify the RF in a crystal set it can and
sometimes does in a TV or radio receiver. This is similar to
the "Rust Bolt Effect" where there are two dissimilar metal
in contact with each other. ------------------------------------------------------------------------------------ 6C2 41 Understand that many TV mast-head amplifiers are wide band devices and can suffer from cross-modulation and overload causing intermodulation and blocking and may also overload the TV. To remind
you Cross modulation is where two or more signals present in
an amplifier which modulate one another, giving additional
outputs which are the sum and difference between the signals. The TV mast-head amplifiers used by many who receive TV via an antenna are probably one of the most difficult areas of EMC to remedy. The broadband nature of the mast-head amplifier will readily suffer from overloading which in turn can overload the TV. There can also be cross-modulation where the strong RF signal from your equipment enters the amplifier and then gives the result of varying the "gain" of the TV signal in sympathy with your RF signal. Cross-modulation in the RF stage of a TV or Broadcast Receiver. When an RF stage of a broadcast radio is presented with both :-
the amateur signal will very often be higher in amplitude than the broadcast signal and the amateur signal may modulate the broadcast signal or it can desensitise (also known as blocking) the RF stage so that the broadcast signal cannot get through. If the RF stages are presented with a strong local FM signal, there is a constant carrier and the broadcast receiver may be desensitised (blocked) as whilst the FM transmission is there as the FM signal impresses its modulation on the wanted signal and overloads the receiver. If the RF stages are presented with an AM signal then the modulation may be heard when the gain of the RF amplifier in the receiver change as the AM signal impresses it modulation on the wanted signal. If the RF stages are presented with a CW signal then they may hear a "ticking" in the receiver as the Cw signal impresses it modulation on the wanted signal. The cure ? Cross modulation can usually be prevented by installing some form of HIGH pass filter to keep the amateur signals away from the TV or radio input if you have an aerial going to the radio but very often it is just a whip aerial. ----------------------------------------------------------------------------------- 6C3 42 Understand that amateur radio transmissions can be picked up by the intermediate frequency stages of TV and radio receivers. The IF stage frequency in a TV is often about 36 MHz +/- thus an amateur transmission on the 18MHz band could be doubled in the TV and present patterning on the TV screen or some other visual interference. In an FM broadcast radio receiver the IF frequency is about 10.7MHz. This could then be susceptible to direct pick-up from the 10MHz band and the third harmonic of the 3.5MHz band. See also page 90 under the picture of the ferrite
Fig 13.12.-------------------------------------------------------------------------------------------------------------- 6C3 42 continued Understand the potential for image frequency interference to analogue and digital radio. Image
Frequency Interference is also known as Second Channel
Interference See also page 90 central column.
-------------------------------------------------------------------------------------------------------------- 6C3 42 continued Understand that television receivers and most broadcast radio receivers employ superheterodyne circuits and recall some typical frequencies used in radio and television receivers. Medium Wave radio broadcast 526 - 1606kHzVHF FM radio broadcast 87.5 - 108MHz VHF DAB radio broadcast 174 - 230MHz TV broadcast 470 - 790MHz Radio IFs typically 455 - 500kHz and 10.7MHz NOTE: Current digital TV receivers use a variety of Intermediate frequencies between 4 and 39MHz Please be aware that the above frequencies need to be know as they are not available on the paperwork in the exam !! If the intermediate stage is badly screened then you can get amateur radio transmissions break through into TV and radio receivers. Often manufacturers are not making equipment that has sufficient screening to take account of such amateur transmission in fact there can be no screening at all! If you are operating using AM it is likely that the person using the TV or radio will be able to hear what you are saying. If you are using CW then the breakthrough will be minimal and the person using the equipment would not be able resolve what it is. With SSB your voice will be less intelligible than AM. There can also be pick-up in the audio stages of TVs and radios where the first audio amplifier stage acts like a detector and demodulates your transmission. The IF frequencies that are used in TVs and radios are generally 33 - 40MHz and whilst we do not have any transmission on those frequencies allocated to the amateur bands it could be that the power of RF signal could break through into the IF stage. The UHF radio frequencies used for the for the channels 21 to 68 are between 470 to 854MHz. Whilst again there are no amateur transmissions in those bands but high powered RF can "shock excite" the first RF amplifier in the TV tuner and cause cross modulation by virtue of shear breakthrough so we need to keep out any signals above and below the TV bands by the use of high and low pass filters.Further in a TV set all the video frequencies that come out of the video demodulator will be in the range 0 to 5MHz and this means that lower amateur bands eg 1.8MHz and 3.5MHz signals could again break into the video amplifiers and that would cause coloured herring bone effect on the screen. THE CURE for TVs Keep the RF signals out of the TV for all of those problems. You would therefore suggest to the neighbour that as you work HF bands that they fit a HIGH pass filter at the aerial socket on the TV so that it would allow the high TV frequencies to pass into the UHF tuner. It is difficult to add screening to any TV as you have to go inside the TV and if anything went wrong the neighbour would think that it was your fault. You cannot re-engineer the TV so you are reliant upon the manufacturers to do a good job in the first place. You would have of course have checked that it was your signals causing the problem by reducing power to such an extent that the problem went away - then fit the filters and increase power again to a level where no problem occurs with the TV. THE CURE for radios With radios, audio break through can occur into the IF stage - on Medium Wave the IF is 455 to 500kHz and on the FM band it is usually 10.7MHz - BUT thankfully breakthrough is rare.
-------------------------------------------------------------------------------------------------------------
|
|||||||||||||
|
|
|||||||||||||
|
|
|||||||||||||
|
|
|||||||||||||
