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