Specifications

Bluegiga Technologies Oy
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10 Design Guidelines
This chapter shows briefly the most important points to consider when making a design with WT32i. Please
refer to the DKWT32i datasheet for detailed description of the development board design.
10.1 Audio Layout Guide
10.1.1 EMC Considerations
To avoid RF noise coupling top the audio traces it is extremely important to make sure that there aren’t GND
loops in the audio traces. Audio layout can not be compromised. RF noise that couples to audio signal lines
usually demodulates down to audio band causing very unpleasant whining noise.
Noise couples to signals lines either through a parasitic capacitance or by coupling to a loop. The noise that
couples to a loop is proportional to the area of the loop and to the electromagnetic field flowing through the
loop. Thus the noise can be minimized in two ways. Minimizing the field strength flowing through the loop by
placing the signal lines far from the RF source or most importantly minimize the size of the loop by keeping
the trace as short as possible and making sure that the path for the return current (usually GND) is low
impedance and follows the forward current all the way as close as possible. GND vias must be placed right
next to the any component GND pins and solid GND plane must follow the trace all the way from start to the
end. When using fully differential signals they should be routed as differential pairs, parallel and symmetrically.
Typical RF noise with Bluetooth
• How the BT noise couples to audio?
-1.6kHz noise
1 Bluetooth slot = 625us
1 ÷ 625e-6 = 1.6kHz
- 320Hz noise
5 slot packet (A2DP profile)
1 ÷ (5 × 625e-6) = 320Hz
(5)
* RF takes an amplifier out from it’s linear region causing demodulation of RF
down to audio band.
(6)
RL
EM field couples to a
loop
RL
Noise couples through a
parasitic capacitance
Figure 26: Noise coupling schemes
10.1.2 Choosing Capacitors and Resistors
Metal film resistors have lower noise than carbon resistors which makes them more suitable for high quality
audio.
Non-linearity of capacitors within the audio path will have an impact on the audio quality at the frequencies
where the impedance of the capacitors become dominant. At higher frequencies the amplitude is not
determined by the value of the capacitors but at the lower frequencies the impact of the capacitors will be
seen.
Ceramic capacitors should be X5R or X7R type capacitors with relative high voltage rating. The higher the
capacitance value, the lower is the frequency where the non-linearity will start to have an impact. Thus it is not
a bad idea to select the capacitors value bigger than necessary from the frequency response point of view.
For optimal audio quality the best selection is to use film capacitors. Film capacitors have excellent linearity
and they are non-polarized which makes them perfect choice for using in audio path. The drawback of film
capacitors is bigger physical size and higher cost.
Figure 27 shows a modulation distortion measurement when using different type of capacitors in the audio
paths. Modulation distortion measures the amount of distortion between two closely located sine waves. The