ADVANCEDTEMPLATE.MZT

ADVANCED LESSON 40

LEARNING OBJECTIVES and NOTES
Transmitter Interference 1
4h.1 Recall the effect and the importance of minimising drift. You have seen in previous lessons that careful attention is paid to the stability of frequency generators, particularly VFO's. If these drift, they can cause a number of problems: The signal can move out of the other stations filter, they then follow you by adjusting their VFO. You readjust your VFO and so on. The two signals can drift up or down the band causing problems to other stations. Your signal drifts, the other station uses his/her RIT to follow you and you finish up on two frequencies taking up more space on the band Your signal could drift out of the band causing illegal operation on transmit Ways of improving stability: Use temperature compensating capacitors to reduce the effect of temperature on frequency. Build your VFO into a solid box, with solid wiring Use a voltage stabiliser between the 12v supply and the VFO supply Use buffer stages to prevent the following stage pulling the VFO off frequency Screen the VFO from other parts of the circuit Have adequate ventilation to prevent the build up of heat. Use frequency synthesisers to maintain a constant frequency Use a good slow motion drive with no back lash Use a crystal oven to maintain a constant temperature.
4h.2 Recall the cause and effect of ‘chirp’ and identify suitable remedies. Recall the cause and effect of ‘key clicks’ and the shaping of Morse keying waveforms. Causes of chirp Chirp from a CW transmitter is caused by an oscillator that changes frequency slightly every time a dot or a dash is sent. It sounds like a bird chirping. The signal is harder to copy; will interfere with nearby stations and if a narrow band filter is used the signal can move out of the pass band altogether. Remedies for chirp use a voltage stabiliser to supply the oscillator as variation in voltage as an oscillator draws more current on key-down is one of the causes of chirp use a buffer stage after the oscillator and loose coupling between these two stages to prevent the buffer pulling the oscillator off frequency. leave the oscillator running permanently and key the buffer on and off. Reduce the level of feedback in the oscillator stage.
Causes of key clicks These are caused by having dots and dashes with a very fast rise and decay. The effect can be either short distance interference because of radiation from the keying leads, or long distance interference because the square wave from the keyer produces multiple sidebands that can spread over a whole amateur band and even further. Remedies for key clicks Change the shape of the keyed dots and dashes. This is achieved by placing a choke in series with the key to increase the rise time and by placing a capacitor and resistor conected in series to in parallel with the key to increase the decay time. Key low power parts of valve circuits such as the screen grid rather than the anode Key earlier stages rather than later stages.Avoid keying the oscillator stage.
4h.3 Understand ways to avoid generating harmonics (e.g. use of push-pull amplifiers, use of inductive coupling between stages, avoiding high drive levels). Harmonics are multiples of the operating frequency. So, a transmitter on 7.00 MHz will produce harmonics on 2x7=14 MHz (1st Harmonic) 3x7=21 MHz (2nd Harmonic) 4x7=28 MHz (3rd Harmonic) 5x7=35 MHz (4th Harmonic) 6x7=42 MHz (5th Harmonic) If these are not removed they will result in multiple transmissions both within and outside of amateur bands. These can be reduced in a number of ways: use push-pull PA stages. These tend to reduce the level of even harmonics (2nd, 4th, 6th etc.) use class A amplifiers for SSB and low power generated AM. use inductive coupling between stages - in single band transmitters these can often be tuned to act as band pass filters for the operating frequency, thus attenuating harmonics. Do not over drive a stage as this will lead to an increase in the strength of harmonics keep connections as short as possible Screen each section Screen all leads Filter out RF from DC leads by using decoupling capacitors Also see the section of low pass filters for removing harmonics.
Recall that transmitters may radiate unwanted mixer products and identify suitable remedies. Mixers produce many outputs other than the intended frequency. For example a harmonic of a transmitter VFO frequency could mix with an incoming signal to produce an unwanted frequency. For example a 5.2MHz crystal oscillator might be designed to mix with a 9.0 MHz oscillator to produce an output for transmitter of 14.2MHz (remember 9 + 5.2 =14.2) However, the first harmonic of the 5.2MHz oscillator would = 5.2 x 2 = 10.4MHz. This would mix with a 9.0MHz signal to give 10.4 + 9 = 19.4MHz and 10.4 - 9= 1.4MHz These can get into the amplifier stages and be broadcast. These are not related to the desired frequency and can be above or below the operating frequency. methods of reducing unwanted mixer products: To remove mixer products: Select VFO and IF frequencies that produce mixer products well away from the desired frequency Keep the level of drive of VFOs and RF stages to the mixer as low as possible to reduce mixer products. Use mixers designed to reduce the number of frequencies produced such as double balanced mixers and balanced mixers..
Understand the use of low and band pass filters in minimising the radiation of unwanted harmonics and mixer products. We have already looked at the design of low and pass band filters to remove unwanted harmonics and unwanted mixer products. The low pass filter will attenuate all frequencies above a certain frequency. These are placed between the PA stage and the antenna output. They will remove harmonics and mixer products above the desired frequency. Band pass filters will remove harmonics and mixer products above the desired frequency and below the desired frequency only allowing through signals in the band pass. On a practical note if you run the output from a transmitter into a dummy load without a filter there will be more power shown than when a filter is used. This is because the filter is removing some RF energy from the harmonics and unwanted mixer products. This is perfectly correct as you do not want to radiate these unwanted signals.