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11. Interference

Definitions

  • Interference may be due to faults and/or deficiencies in either the transmitter or in the receiver or both
  • Blocking happens if the receiver is situated close to a transmitter when the transmitter Is operating
  • Key clicks are generally undesired "clicks" or "thumps" generated by a CW transmitter as the key is put down or let up.
  • Chirp is a signal in which the frequency increases (up-chirp) or decreases (down-chirp) with time.
  • A braid-breaker is a filter that prevents television interference
  • Ferrite bead or ferrite choke is a passive electric component that suppresses high frequency noise in electronic circuits.

Diagrams

Circuit diagrams

  • Key click filter

  • PI-network circuit

  • Braid breaker

Notes

Receiver problems

  • Blocking
    • if the receiver is situated close to a transmitter
    • receiver is blocked when the transmitter is operating
    • receiver, although tuned to another frequency, becomes overloaded and it will no longer 'hear' the wanted station
  • Second channel interference
    • Most receivers are of the superheterodyne type and reply on mixing
    • for a given Local Oscillator frequency and Intermediate Frequency there will two frequencies
    • frequency of the unwanted station will be 2X the IF away from the wanted frequency
    • RF stage of the receiver should be designed to reject the 'second channel'
  • Local Oscillator Harmonics
    • If the LO in the receiver generates harmonics these will cause unwanted mixing products
    • unwanted stations could be received
  • Detection
    • If the receiver or Hi-Fi is not well screened then the RF from a nearby transmitter may enter and be detected by any non-linearity in the audio stages
  • IF Breakthrough
    • If a receiver has an Intermediate Frequency in an Amateur Band then any transmission on that frequency might be received where ever the receiver is tuned.
    • very important that the high gain IF stages of a receiver are well screened

Resolution

  • ensure that the receiver does not get an extremely strong signal from the Amateur Transmitter
  • TV / VHF receivers: ensure that the outer of the coax cable does riot act as an aerial for the amateur transmissions
  • A filter (braid breaker) can be connected in the coax near to the TV or hi-fi
    • screen is then no longer connected directly to the receiver
    • loss at UHF TV and VHF hi-fi frequencies is very low
    • This filter should be fitted about one Metre along the cable from the receiver
    • Signals picked up on the outer of the cable can also be much reduced by winding it several times through a ferrite ring

Deficiencies

  1. Receiver deficiencies
  • internally
    • add inductance to the base circuit of a transistor
    • passing the base connection through a small ferrite bead
  • externally
    • A rejector circuit is a parallel tuned circuit and at its resonant frequency it is a high impedance. Therefore it is connected in series with the aerial wire
    • The acceptor is a series tuned circuit and it will have a low impedance at resonance. It is therefore connected across the aerial and earth connection to '”short circuit1” the unwanted frequency
  • Transmitter deficiencies
    • A Morse (CW) transmitter, although simple, is capable of producing clean harmonic free transmissions
    • Morse key is up: no output
    • Morse key is pressed: a full output until it is released and the output drops abruptly
    • sudden changes (arrowed) will generate clicks that are rich in harmonics
    • annoying clicks will be heard, not only on the transmitted frequency, but also on its harmonics
    • solution:
      • wave shape must be rounded to remove those harmonic laden sharp corners
      • a key click filter comprises of a coil, capacitor and a resistor wired into the Morse key
        • coil prevents a sudden build up of current when the key is pressed
        • decay of the current is prolonged by the capacitor and the resistor when the key is released

Types of Interferences

  1. Harmonic transmission
  • all waveforms apart from sine waves contain harmonics
  • important to ensure that any harmonics produced by a transmitter are not actually transmitted
  • harmonics must not reach the aerial
  • Harmonic reduction
    • A PI-network is connected between the aerial socket of the transmitter and the aerial wire
    • very effective in reducing harmonics:
      • 4X better than a simple parallel tuned circuit for reducing the second harmonic
      • 9X better than at the third harmonic
    • frequency multiplier stages of the transmitter should be carefully screened to ensure that no 'unwanted' frequencies escape
    • Fit a low pass filter in the transmitter output
    • important to have good earthing and de-couple all the power leads
  • Over modulation
    • must be avoided in an amplitude modulated transmitter
    • transmitter should be designed so that over modulation cannot occur
    • an indicator should be fitted to show when 100% modulation is being approached
  • Parasitic (or spurious) Oscillations
    • Certain stages of a transmitter may break out into self oscillation at a an apparently random frequency
    • Low Frequency
      • Amplifiers may burst into oscillation that cannot be sustained
      • Transistors that are used in Radio Frequency amplifiers usually have an unwanted, high gain at audio frequencies
    • Signal Frequency
      • Amplifiers may oscillate at the frequency they are intended to amplify
      • an avoided by ensuring adequate screening
      • avoided with a change in the component layout
    • High Frequency
      • usually at VHF
      • Valve power amplifiers suffer from this problem
      • resolved by adding a low value resistor or a small coil at the grid tag
      • difficult to see which of the components have "unintentionally" formed a tuned circuit at VHF
      • even a straight piece of wire has some inductance at VHF
  • Chirp
    • A Morse transmitter should be designed to maintain a constant frequency
    • If the Variable Frequency Oscillator (VFO) or crystal oscillator is not adequately buffered or the power supply adequately stabilized then the frequency may shift during the "key down" period

Unwanted frequencies

  • All the unwanted products must be attenuated to ensure that they do not appear at the output to the aerial
  • Ensure unwanted frequencies do not leak out
  • Also possible that unwanted frequencies are being produced by non-linear devices outside the transmitter