What Not to Do !

This page shows you what not to do when dealing with circuits at Radio Frequencies, I have illustrated it from various Student's projects that went wrong .

For convenience we will take Radio Frequencies to mean any circuit working above about 50kHz

If you are using 433MHz Wireless modules for a digital link DO NOT even consider using either Veroboard or a Solderless breadboard USE a Printed Circuit Board . Why ? Because the basic construction of a solderless breadboard is a bank of capacitors of around 10 - 20 pF this capacitor has a shunt reactance of < 50R so effectively causing the RF power to be lost and not available to the aerial .

This item is a student's 1GHz radio transmitter .

This project has several problems , due to the choice of components requiring a mixture of through hole & surface mount techniques . If you study the photograph you will notice firstly the total lack of any decoupling capacitors on the supply rails . The MMIC amplifiers are soldered correctly to the board as intended for the frequency of operation BUT the tracks connecting them are not 50 R microstripline so there is an impedance mismatch present , then we have the DC blocking capacitors between the stages these are wire leaded types with 5mm long leads ( that's an inductor at 1GHz ), this together with the 10mm long earth tracks from the MMIC's to the veropins which eventually connect to the groundplane , these will also reduce the gain together with the risk of instability occuring given the bandwidth of the MMIC's chosen ( DC - 6GHz ). The Voltage Controlled Oscillator ( Silver box with 8 legs )is a through hole device it is mounted above the PCB on it's connecting pins , note there is no voltage regulator or decoupling of the supplies or tuning voltage are present . The output of the mixer should be 50R microstripline to the mixer , the earth connection from the VCO is nearly 20 mm long, the same long earth path for the mixer module also occurs .The inductors in the supply to the MMIC's have 3 mm long leads so that they are not the intended value and also can lead to instability by acting as an Aerial given the high gain for the three cascaded amplifiers this is also brought about by the use of conventional wire ended resistors used to set the supply of the MMIC's allowing for radiated signal to be coupled from stage to stage . The power supply leads for ALL the various modules all behave as Aerials so that unless the student is testing the project in an EMC or Anechoic chamber it will amplify the local Wlan in the lab , the local Cellphone transmitters ( Both handset & Base station ) picked up on the leads due to the lack of decoupling components !

This project is a cascaded Wilkinson splitter / combiner for 2.4GHz

As can be seen the student has cased up the PCB in a small diecast metal box but due to the way that they have added the earthing ( using lengths of thin wire , just think of the Inductance at this frequency !) for the semi - rigid coax to the boards they have negated some of the benefits of using the coax , a better solution would to be have used coaxial edge transitions ( using bulkhead types as required ) and plug & socket extension lead (s) to allow for the fact that the diecast box walls are not parallel to each other this would then maintain system impedance for all the RF paths and provide correct earthing but the transmission lines may have to be extended to ensure that the edge transitions do not short out the folded lines for ports in the middle , again this can be solved at the modelling stage prior to producing the PCB .

This page will be added to as & when students request help on a particular subject

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This page last updated 9th May 2011