Mechanical Design for good RF Performance

Created; 09/02/2015, Changed; 19/02/2024, 06/02/2024

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Bakelite radio, Ultra MW/LW, built in loop aerial. Purchased in 1954 for £16, pictured working in 2009 when I last turned the radio on.

The heater current is supplied by a resistor and a negative temperature coefficient resistor (NTC) in series with the mains supply.  This is evident because the Ultra power light (that would be powered from the valve's heater power supply) initially comes on, then dims, then comes up bright just before the sound comes through, finally the light returns to a medium intensity.  Small valve radio's like this usually have a live chassis (connected to live, or neutral mains supply wire).  This radio does not suffer with audio mains hum, that others like it invariable do, though the supply voltage mains ripple would be high. 

The sound reproduced is warm mellow, as if there were a large loudspeaker and substantial baffle (Valve sound, which I think, was developed by Philips in the 1930s).  The inherent triode valve distortion was used to create sub-harmonic distortion that gives the impression that the radio has a better low frequency performance.

The circuit is identical to a 240V AC/DC model radio, although the rating plate on this radio specifies AC mains electricity supply only.  A lot of these cheaper radios had a heater transformer, but the most basic types like this one just uses series resistors to power the valve heaters.  The main input reservoir/smoothing electrolytic capacitor could explode, because of its age and because it has not been used regularly.  Old capacitors do not have a safety pressure relief vent, unlike modern electrolytic capacitors.  These live chassis radios are not safe, so be careful not to touch any metal in contact with the chassis, such as a screw within the control knobs.

The Wireless Museum in Dulwich, West London is well worth a visit, I visited in the mid 1990s.  They had working self-seeking Radio, I had never seen one of those working before my visit.  Working 405 line TVs, World War One valve manufacturing.  The components are so fragile because they were cheap at the time, but the museum manages to maintain these Radios.

Electromagnetic Compatibility;

There are a number of strategies for minimising radio emissions and susceptibility to radio emissions.  I intend discussing these points using various examples and small detours into other areas of electronics.  It is necessary to separate circuit function, stability and electromagnetic comparability, though the symptoms can be similar.  These things can interact with each other.

Similarly to the radio with PCB design; Contain any susceptibility or emissions locally (to the PCB) by;

Filter (bandwidth limiting)

At the boundaries, which are the input and output signals and connectors, use passive filtering, LC or RC, as opposed to active filtering, this is very important.  Also keep the bandwidth limited to what is required throughout the circuit.  Avoiding high Q filter circuits, as they can ring at some frequencies (resonate).

Transistorised Hi-Fi often used to suffer with susceptibility to lights and other thing being switched on or off, producing radio interference and causing clicks to be heard on the Hi-Fi.  Police and Taxi radio which used VHF band would "break through" on transistor radios.  The problem was that a bipolar junction transistor could rectify radio interference and thereby act as a detector (diode).  A fix that Electronics technicians, in the 1970s, used with amplifiers, was to fit a 100nF ceramic capacitor between the Base and Emitter pins of the first transistor.  I have also seen 100pF capacitor fitted similarly built into a design for an Eagle amplifier.

Positioning of components is also important.

 Small signals --- Switch mode driver IC --- Switching power and Inductor --- Input and output capacitors --- Filter section --- Input/Output connections. 

Adding a metal case makes electromatic emissions worse --- this is usually resolved by adding more earth points and greater distances between the inductors and the metal case. 

The above PCB is for a switch mode power supply for LED driving.  The design has a number of features to achieve low electromagnetic emissions.  (AL-0022-01C) created using CADSTAR 16 express.  The design is most certainly over filtered and is the best way to design because it is easy to reduce the size and costs of filtering rather than to add it later with a good PCB layout that was not designed to have those additional parts.  On the other hand, don't over design in functions that you probably won't need later.

Spot the omission - the circuit can have PWM driven to control the LED intensity without changing the LED's colour.  The control both isolates the LED using a transistor and also turns the regulator off during the off period.  The usual mechanical dilemma applies and there is no space to do what is desired and there is little or no overvoltage protection.  The solution is you have got to talk to the mechanical designers and stylists (the later people can stitch you up) both press your point and offer work-around's and discuss the problems with those or express the compromises. 

This sort of function can be placed on a PCB with other RF and signal circuits and although the power planes should not be split I've seen it done and the engineers who did that were not happy, but it was expedient.  In this case, the input output and power would be connected on the left, but an isolation to the planes put around the other three sides.  But if you can put the power supply block on corner edge and away from sensitive circuit, that is better.  Put very sensitive circuit on cable and coupled through common mode choke then bonded to a metal case is very good.

An instrument I designed called a polarimeter required the motor encoder light source and display all be placed in one end of the instrument, but the photodetector amplifier board placed in the other end of the instrument.  This would have made the instrument misshaped.  The motor and encoder were placed near the detector, where place together but away from the display.  The instrument worked better than expected was very successful, but when improvement was required the design layout now prevented that from occurring.  The options remained were to enclose the amplifier within a mu-metal box, the metallised screening of the instrument was otherwise good.  The design idea was developed over a 20-year period, but when an instrument was envisaged that was developed over a period of two years.  In the meantime, other more manually operated electronic polarimeters were developed or improved.

EMC and compromise might be expedient - this works, but is not recommended.  This sort of function can be placed on a PCB with other RF and signal circuits and although the power planes should not be split I've seen it done and the engineers who did that were not happy, but it was expedient.  In this case, the input output and power would be connected on the left, but an isolation to the planes put around the other three sides.

If you can put the power supply block on corner edge and away from sensitive circuit, that is better.  Put a very sensitive circuit on ribbon cable to a PCB and whatever sensor you are using mounted on that PCB is very good. Connecting the ribbon through common mode choke then bonded to a metal case is very good.  But also a star point but no common mode choke is, once again, not recommended and is also very good.

I can think of an example where if a motor and encoder were moved to where the microprocessor was at the other end of an instrument away from the detector, that in every respect the instrument should have worked better.  But there was a styling constraint that prevented that happening originally.  It was already an excellent instrument, and it had been improved over time and always was much more sell-able than had been anticipated.  Later a desire was expressed to develop another but higher performing instrument, but such was not going to be possible without the addition of a mu-metal screen the re-arrangement of some electronic and software, but the design time was not available.  In the end those things were understood, I made sure of that.

If necessary, a capacitor may be placed between the 0V plane and the connection to chassis to provide DC isolation (ensure that this part meets any standard for the industry and application).  But using the mounting points as shown this could increase emissions a solution is to fit a zero ohm link that can be removed but firstly add more mounting points and work on other things before resorting to isolating any mounting points.  RF emissions in the Near field are not predictable, and a metal enclosure for a power supply can make them worse.  Firstly, though, ensure that the controller control loop is stable that is optimally compensated or a little over damped.  If PWM for LED brightness control, ensure that the control loop does not RAMP but switches between on and off (this also ensure that the LED colour is stable).

Home made Oscilloscope based on a Practical Wireless design of about 1973.

I worked on the design over a number of years, improved it greatly.  By 1977 it was evident that I had learnt a lot, but this was never going to be a particularly good Oscilloscope primarily because of the low intensity of the CRT, and I stopped working on it.  The bandwidth is increased from less than 10KHz to over 500KHz, High voltage CRT supply was changed to a high frequency type, low noise, sine wave, with proportional control, to eliminate Z-modulated mains-hum, and the voltage increased. Input transformers relocated to under the chassis back left as far away from the CRT as possible to eliminated X and Y axis modulation.  Improved trigger and time base. Grounded everywhere (approximately because I was quite in-experienced), and all modification made with recycled parts.  The next improvement would have been to fit a larger transformer, which would have had to be nearer the CRT, and I had some Mu metal to screen that, as everything was under stress.

Grounding strategy - Using one point (star ground) or grounding everywhere.

A star point, connection to a single point on a chassis, tends to give very good low noise performance, but there will be resonant frequencies and this is very difficult to deal with.  Alternatively grounding everywhere can give a higher noise level, because there may be varying current across the chassis that inject low level voltage into the circuit, but resonances are likely to be a lot less this configuration is currently more in favour.  I recommend that you pick and choose, but generally go with bonding everywhere.

Stereo Amplifier with tone controls, balance and volume pre-amplifier.

About 5+5W into 4-16 ohms I believe?

Mullard amplifier - these modules or reference designs were very widely used classic transistor designs.  They use a minimal number of cheap but good quality components, no 0V plane on the PCBs but good bandwidth limiting and star point 0V strategy.  I continued with that star point strategy in connecting the modules having tried grounding everywhere unsuccessfully so under guidance of my manager at my place of work, I would have been about 18.  The amplifier works very well.  One of the power amplifier's is not fitted and there are some disconnected wires.

The output stage is likely to be a push-pull NPN/PNP transistor output stage, probably with the output transistors base bias provided using bootstrap from the loudspeaker.  There is a snubber network across the loudspeaker in this case it is called a Zobe network and ensures that the amplifier works predictably despite the inductance of the loudspeaker (the resistor's value is equal to the loudspeaker's resistance and the capacitor is 100nF presumably balancing the speaker inductance).  The pre-amplifier may be a module LP11 cut in half and stacked to fit in the space.

The mains transformer is a spilt bobbin type, designed that way to minimise electrostatic noise coupling between the primary and secondary windings.  I was very unaware of safety with mains, as is evident but is typical of hobby electronics.  For example, earth bond should not support anything but just carry the earth conductor and use crimped connections bolted with anti-shake washer connections - or just use an external mains power supply.  There are plenty of used power supplies and other components that can be re-used. 

For example, Radio's like the Ultra above, have a steel chassis which is signal ground (0V).  When ever 0V (possibly connected to mains live or neutral) is required, there is a short connection to a nearby point to chassis.  My homemade oscilloscope is also bonded everywhere (approximately - I was inexperience).  Therefore, neither design use much star point for grounding and both work well.  A strategy of using differential signals when bonding everywhere, like you would have to with Star point anyway, also improves the outcome.  You can place common mode chokes between PCBs and provided any common mode current due to voltage differentials across the metal work that would flow within the common mode chokes is less than the rated saturated current, there will be benefit.

Pictured below is an RF PCB layout for a very low power crystal oscillator and Phase Locked Loop of a microcontroller.  The PCB layout is an improvement on another Motorola (Freescale) example that used star point and made a point of having no 0V plane under the crystal oscillator section of the board.  But it still is not an optimal adherence to the rule of RF design should be bonded to 0V everywhere though it no doubt works, but I don't recommend it.  This design has completed the 0V guard track to form a ring around the crystal but only has one connection to the 0V plane - I don't see any merit in laying out the oscillator this way, but I have used an earlier similar reference design successfully.  Do ensure that there is an 0V plane as the next layer below the routing plane shown and that it is not fragmented, and don't place any other tracks under the crystal to prevent 0V return currents flowing under the crystal - the main point made in most applications notes. (Page 21)

This application also suggests star point for the analogue systems, but it works fine with the 0V supplies taken by the shortest path to 0V plane and the supply decoupled and connected to the supply pin via a small series resistor, instead.  Not only is this approach I suggest correct, but it works.  On some occasions, though, you do have to follow the application note - particularly when there is a power supply start up order and the RC network on the analogue supply makes the order of start up wrong or that it simply won't work as well any other way.  The latter case comes about because (my theory) star point produces very low noise frequency bands and tuned notches as I say elsewhere, provided a tuned notch is avoided this may be the only way to achieve the performance required of some Delta Sigma or Sigma Delta A/D converter types.

A tip given to me for when it is necessary to use star point in an RF environment and that is adding a bead choke - this is a low Q constructed choke using a soft magnetic material such as used in mains filters.  Alternatively, add series resistors and keep the signal paths short and like a transmission line grouped together, following the same path.  Also, common mode chokes intended for filtering are similarly low Q and can be placed at the output or the input of board to board ribbon or multi wire cables and in that way each board can be connected to the chassis.

Examples of enclosures and magnetics;

Grounding everywhere (connecting 0V to metal work and bonding everywhere, not specifically earthing) is best, but grounding at one point has been common practice since 1960 until the turn of the century.  Power Inductor can induce current into the enclosure, so keep them some distance from metal enclosures.  It will be found that grounding at many points does not work in small enclosures and the workaround was to ground at one point where the power signals came in - So I use some star point if really necessary but not as a starting point. 

Keep some distance between wound components to minimise the coupling between them.  Some power input soft common mode chokes are wound with the former in a horizontal plane, whereas power chokes are wound with the former in a vertical plane in order to somewhat null and thereby minimise the coupling between them.  It may be possible to have three wound components in the three possible planes, but it is better to put distance between those components you can use both approaches.  The PCB layout for the LED PSU above shows wound components that are wound in the same plain, except for input bead chokes.  In that design it is not possible to source ideal wound components, but a design like this still can achieve very low radio emissions.  In the PCB design, I have phased the input and output chokes to behave as a common mode choke if there is coupling between them, but I have not experimented to see if there is a measurable benefit in doing that?

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