Using A Bluetooth Module vs A Chip Based Design

Bluetooth OEM modules have usually gone through all the testing so you don’t have to.  You can slap one on your PCB and your ready to go, often with a faster development route too.  The catch is that it costs more in production than using the discrete components instead.  It can be even more frustrating that the Bluetooth IC the module uses is capable of running the embedded software your application needs, but you have to use a separate microcontroller because either the Bluetooth module implementation doesn’t allow it or if you do you would break the Bluetooth stack certification, requiring re-testing.

As a general rule of thumb, if your production volume is likely to be say 10k or 20k then going with a module is often an overall cheaper choice once your factor in the testing costs.  However if your production volume is likely to be say 100k then the testing costs will typically be overcome by the savings in production cost.

The Cause

Until relatively recent times digital PCB design (and especially when prototyping) could be viewed as simply a means to electrically interconnect components and unless you designed RF circuits there was little else to worry about. However the PCB itself, or the means of connecting the components used (i.e. prototyping), is now is a very common cause of a loss of signal integrity. The reason is mainly due to the rise and fall times of output signals having decreased as devices are designed to operate faster and faster and to use smaller and smaller silicon manufacturing processes. This problem is not actually due to the operating frequency of a device or the frequency at which a signal is changing, it is due to the speed at which a signal output changes state from high to low and low to high. A signal doesn’t instantaneously change from high to low or low to high, it takes a certain amount of time which will be specified as the rise and fall time in a devices data sheet. Previous signal rise and fall times of many 10’s of nano seconds have now become times measured in just a few nano seconds or for many devices they are measured in pico seconds.

So you may be thinking, this can’t possibly be an issue for me, my board is only operating at a few MHz and I’ve even slowed my data bus down to a few KHz. Well unfortunately that doesn’t matter. If you work with a DC signal the only thing you really care about in a wire or PCB track is its resistance, which for short lengths will be close to zero. However, when using that wire or PCB track with a fast AC signal it starts to behave like a capacitor and inductor. Capacitors and inductors exhibit resistance to alternating current called reactance. The impedance of the wire or track is the vector sum of resistance and reactance, essentially the total resistance seen at a particular frequency. What happens when you send a signal with a fast rising and falling edge down a wire or PCB track, if the impedance of the gate driving the wire or track isn’t exactly the same as the one receiving the it, is it that some of the pulse bounces (literally) back to the driving gate. As there is still an impedance mismatch, the signal continues to bounce between the two until it finally dampens out. This bouncing becomes worse as the speed of signal rise and fall times increases. Basically, the faster rise and fall times of signals from modern semiconductors combined with wire or PCB trace inductance and capacitance causes noise signals of a greater magnitude than before. Greater magnitude means the bouncing signals can reach the threshold voltage required for the receiving device to ‘see’ another clock pulse, or an incorrect data level at the moment it is sampling the data line.  The solution is to design your PCB to use impedance controlled tracks on these clocked connections.

Single Connections

Download the free Saturn PCB Design Toolkit – its a great tool for this.  Use it as follows:

Select Conductor Impedance

Select Imperial or Metric (you can select later and it will auto convert all values)
Set copper weights and plating thickness (e.g. 18um copper + 18um plating = 35micros (1oz))
Select internal or external layers (typically microstrip or embedded microstrip)
Enter substrate options (prepreg dielectric constant – typical PCB’s are FR4)
Enter track width in conductor width
Enter prepreg height in conductor height (the distance between the copper layers excluding their thickness)
Click ‘solve’
Now adjust the track width until you get the impedance (Zo) you need.  This is the thickness to make your tracks

100ohm Single Track Impedance (General signals, SPI bus etc)

General signals, SPI bus, etc will generally perform well with a 100ohm track Impedance.  Some examples track widths with a GND plane under the tracks:

2 layer 1.6mm PCB (1.48mm FR4) = 0.61mm (100.3150ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 0.27mm (100.2273ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.12mm (100.9915ohms), or if too small then 0.15mm = (95.0631ohms)

4 layer PCB Train – Internal to Internal (0.99mm FR4) = 0.4mm (99.8636ohms)

4 layer PCB Train – Internal to External (0.22mm FR4, layer 2-3) = 0.05mm – not possible

4 layer PCB Train – Internal to External (1.245mm FR4, layer 1-3) = 0.49mm – not possible

75ohm Single Track Impedance

Also relevant to 75ohm radio antenna connections.Example track widths with GND plane under track

2 layer 1.6mm PCB (1.48mm FR4) = 1.29mm (75.0457ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 0.6mm (74.7938ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.3mm (74.8156ohms)

50ohm Single Track Impedance

Also relevant to 50ohm radio antenna connections.  Example track widths with GND plane under track

2 layer 1.6mm PCB (1.48mm FR4) = 2.65mm (50.1165ohms)

4 layer PCB Pool – Internal to Internal (0.71mm FR4) = 1.25mm (50.0581ohms)

4 layer PCB Pool – Internal to External (0.38mm FR4) = 0.65mm (49.9609ohms)

Differential Pairs

Download the free Saturn PCB Design Toolkit – its a great tool for this.  Use it as follows:

Select Differential Pairs

Select Imperial or Metric (you can select later and it will auto convert all values)
Set copper weights and plating thickness (e.g. 18um copper + 18um plating = 35micros (1oz))
Select internal or external layers (typically microstrip or embedded microstrip)
Enter substrate options (prepreg dielectric constant – typical PCB’s are FR4)
Enter track width in conductor width
Enter clearance between tracks in conductor distance
Enter prepreg height in conductor height (the distance between the copper layers excluding their thickness)
Click ‘solve’
Now adjust the track width and spacing until you get the impedance (Zdifferentail) you need.  This is the thickness to make your tracks

USB 90ohm Differential Pair Track Impedance

USB 2.0 requires 90ohms differential impedance (max 45ohms per track)

Max trace-length mismatch between High-speed USB signal pairs should be no greater than 3.81mm.

Example track widths with GND plane under track

4 layer PCB Pool – Internal to External (0.38mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.38mm track width = 90.525ohms Zdifferential
35um copper, track spacing 0.2mm and 0.43mm track width = 90.174ohms Zdifferential

To stick with the max 45ohms per track (not practical for many designs):
35um copper, track spacing 1.4mm and 0.75mm track width = 89.118ohms Zdifferential

4 layer PCB Pool – Internal to Internal (0.71mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.61mm track width = 90.385ohms Zdifferential

4 layer PCB Train – Internal to Internal (0.99mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.8mm track width = 90.156ohms Zdifferential

2 layer 1.6mm PCB (1.48mm FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 1.12mm track width = 90.184ohms Zdifferential

10/100Mbps Ethernet 100ohm Differential Pair Track Impedance

Ethernet requires 100ohms differential impedance (max 50ohms per track)

Example track widths with GND plane under track

4 layer PCB Pool – Internal to External (0.38mm height – FR4 thickness to GND plane)

35um copper, track spacing 0.15mm and 0.3mm track width = 100.462ohms Zdifferential
35um copper, track spacing 0.2mm and 0.34mm track width = 100.923ohms Zdifferential

To stick with the max 50ohms per track (not practical for many designs):
35um copper, track spacing 2.5mm and 0.65mm track width = 99.835ohms Zdifferential
If you use 1 of the plane layers with GND above and below (0.71mm & 0.38mm from track to GND planes) you get:
35um copper, track spacing 1.5mm and 0.42mm track width = 99.0.16ohms Zdifferential

Ensure Your PCB IS Made With The Right Stackup

Its a good idea to include the required stackup on one of your copper layers to ensure it is used.  Something like this:

http://www.eecs.umich.edu/~prabal/projects/hijack/

Good for temperature ranges of -200 to +600ºC.

Temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. They are usually made of platinum and often called platinum resistance thermometers (PRTs).  They are often more suitable than thermocouples in industrial applications below 600 °C due to their higher accuracy and repeatability.

PT100 & PT1000

The most common type of resistance temperature sensor used in industry

100 ohms and 1000 ohms at 0°C respecitvely, increasing with increasing temperature.

Useful resources:

http://www.iqinstruments.com/iqshop/technical/pt100.html

Thinking of having your product manufactured in China, India, or another low labour cost country? This can be a very cost effective route for electronic product manufacture, but you should bear in mind that these countries are not always the best choice with the new environmental producer regulations coming into force in Europe and maybe also your companies own environmental policies. Many products can be cost effectively manufactured in the UK and there are several European countries, such as Bulgaria and Poland, that also provide very low cost manufacturing choices, without the higher transportation financial and eco costs of further afield countries. Remember also to factor in your costs of dealing with offshore manufacturers. Any production cost savings need to cover your own companies costs of visiting distant manufacturers, both in terms of staff time and travel costs. If your company is environmentally aware and views carbon footprints as important you also need to factor in that many low cost manufacturing counties rely on fossil fuels for the bulk of their power generation, dramatically increasing the carbon footprint of their manufacturing processes.

Cable looms, electro-mechanical assembly, panel and cabinet wiring & full product build

Complete product manufacture. Highly recommended by some of our clients.

Complete product manufacture. Highly recommended by some of our clients.

Surface mount PCB assembly. Lovely people to work with

Complete product manufacture

Through hole PCB and cable loom manufacture. Great guys to work with.

When selecting or designing your enclosure you should consider the recycling impact of the enclosure at the end of your products life. Metal enclosures are generally very good from a recycling point of view, as with increasing raw material costs it can be very economic to melt down the metals and recycle them. For instance, using recycled Aluminium can cost as little as 10% of the cost to mine new aluminium from the ground. If you will use a plastic enclosure bear in mind that many plastics can be recycled at the end of a products life, but only if each part comprises of just one plastic type. Using moulded rubber sections for example, whilst fashionable, usually means that the part cannot be recycled economically and has to be sent to land fill. Using a label glued to a plastic part will also usually make it worthless and consigned to landfill.

Shenzhen Lian Lu Xing Technology Co., Ltd

Kingfung Internations Ltd

SCS Circuits

Anke Circuit Technology Ltd

Hampoo

Assemblink

China Shenzhen

Dongguan Shenan Electronics Co., Ltd

HongKong Credit PCB Technology Co., Ltd

CG Global – We met Henry Ng from CG Global at a UK trade show in 2010 and liked his companies keen approach.

Seal Group

Daywell mould Co Ltd

SHOUJU Industrial Limited

Xiamen(beijing,suzhou)Technology Co.,LTD

Suzhou Kaibao Electric Co., Ltd

SuNPe Prototype

Jing Cheng Technology Group(HK) Ltd

If you ever need to do surface mount rework, remove surface mount components, etc, etc, this tool is simply brilliant.  The most brilliant thing about it is the price – for around GBP 40.00 you get a tool that used to cost several hundred pounds.  Indeed many still do and we’re not saying this tool is better than the high priced models.  However for this price buying the tool from eBay is a no brainer and once you receive it (typically from China) it will quickly become one of those indispensable tools you’ll wonder how you ever did without.  Get over the price concern, it is too cheap but it does what it says on the tin and it does it well!