| ADVANCED LESSON 56 | |
| LEARNING OBJECTIVES and NOTES | |
| EMC - Routes of entry into TV and Radio 1 | |
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7a.1 Understand that amateur transmissions can be picked up by the
intermediate frequency stages of TV and radio receivers and identify
related amateur transmissions. Understand that television receivers and most broadcast radio receivers employ superheterodyne circuits and recall some typical frequencies used in radio and television receivers; i.e. 470-854MHz TV RF 33-40MHz TV IF, video baseband 0-5MHz Radio IFs typically 455-500kHz and 10⋅7MHz. Understand the potential for second channel (image frequency) interference. One source of radio breakthrough into TV sets is via the IF stages. Nearly all radios and TVs work on the superhet principle where the incoming radio waves are converted to a common frequency called the IF frequency. These are often high gain stages and so any any signal on an IF frequency if it gets into the IF stage could cause serious problems. IF stages are often determined by manufacturers of TVs and radios. the introduction of digital TV has had an effect on IF stages in TV receivers. TVs often have an IF in the region of 33 to 40MHz. The second harmonic of 18MHz would fall directly in this range. Tv transmissions take place between 470 - 854 MHz. This falls outside the 70cm band as does the second harmonic. However, very strong 70cm (and other frequencies) could cause cross modulation in the mixer stage which would result in damage to the picture. In a TV the output of the video demodulator is 0-5MHz. This could be open to amateur transmitters operating in the 1.8 and 3.5MHz bands. For MW and FM radios: 455-500 kHz and 10.7MHz The new 472-479 kHz allocation could cause interference to radios with an IF in the region 450 to 500kHz. The 10.7MHz IF is close to the 10MHz amateur band and the third harmonic of 3.567MHz is 10.7 MHz. The 10.7MHz IF is very close to the second harmonic of 5.354, one of the new frequencies allocated under the NOV licencing arrangement for 5MHz. In addition these IF frequencies are susceptible to second channel (image) interference. A classic case of second channel interference is the reception of a 160m signal on the MW band of a domestic receiver with a 455kHz IF. For example a station is received on 1517kHz, but there is second channel interference on 1972kHz in the 160 amateur band. This is what is happening: LO signal 1517-Rx sig at 1063 = IF of 455kHz But a strong signal on 1972-LO on 1517 = IF of 455kHz |
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7a.2 Recall that amateur transmissions can enter the RF stages and cause cross modulation and/or blocking. Recall that cross modulation occurs when strong varying transmissions (e.g. AM, SSB or CW signals) impresses its own modulation on the wanted signal. Recall that blocking (also known as desensitisation) occurs when strong constant transmissions (e.g. f.m. signals) cause the radio or television to be overloaded. Amateur signals can enter the RF stage of a TV by pick up from the antenna, but more likely pick up from the shield of the coax cable. These can be at any frequency. The RF stage will act as a mixer for AM, SSB and CW signals, mixing with the TV or radio frequency and producing new frequencies that will cause interference. Strong FM signals with a constant amplitude can cause the early stages of the receiver to be overloaded, blocked or desensitised. This will prevent the wanted signal progressing through the receive |