Does my board qualifies as high speed PCB?

[leolib2004]

Hi,

I am designing a wireless audio generator which has a sam s70 device (running at 300 Mhz from an external 12 MHz crystal), a CS42L51 audio codec, a ATWINC1500-MR210PB, audio jack connector, usb connector, Leds and switches.
The interfaces will be: SPI for the wifi module, I2C for the audio codec, I2S for the audio Codec (including a 12 MHz clock line for MCLK), USB directly and close to the sam device and the SW programmer connection.

I am aiming at a 2 layer pcb design as it is less expensive and to reduce possible issues of 4 Layer design as it would be my first time doing one.
However, I started to worry about possible issues like crosstalks and ringings...

Would it be mandatory for my design to use 4 layers and to consider controlled impedance traces? How are the odds that it could work with 2 layers and maybe later, to reduce its size, I could add 2 more layers?

Thanks.

 

[D.A.(Tony)Stewart]

It is mandatory to use guarding methods on high impedance inputs and ground interleaved tracks ( (stripline) on 300MHz signals.

Wifi may require shielded lid to improve sensitivity to <80 dBm.

Crosstalk can be defined by track coupling impedance ratios and depends on your specs.

 

[leolib2004]

What do you mean with high impedance methods? The 300 MHz are generated inside of the SAM with a PLL...
The wifi module already comes with a shield...

Is it that crazy to try to do this in a 2 layers pcb?

 

[marce]

Yes...
4 layers would be far better.

 

[dehati]

One of the things that I will be careful is the loop area of the return current of all the high speed signals. This has the potential to create EMI and cross talk issue. 2 Layer board may work provided, you can visualize the return path of all critical signals and make sure that the loop area formed between the signal and the return current is minimal. If you can not, go with 4 layer. The cost of PCB is not too high and it keeps the routing clean ( all ground connections drop on ground layer). See the section of book for guideline on 4 Layer, 6 Layer or 8 Layer PCB - http://referencedesigner.com/books/si/power-plane-ch6.php.

 

[D.A.(Tony)Stewart]

You can use a 2 layer board with interleaved ground signals or guarding methods with ground planes, but as in TV tuners, you may need to fabricate a tin-plated brass lid (or purchase) to reduce E field crosstalk and rectification of RF onto input protection diode, causing modulated DC bias or added noise to high impedance inputs.


See www.Tech-etch.com for examples.

Prudent application of ferrite beads and RF caps can reduce this.

 

[dehati]

Tech-etch is really good in providing cost effective solutions. If you need any kind of shielding in US they are the first guys that come to my mid. However, the 4 layer solution could be more cost effective, provided it is done properly. Shielding is often done as an added protection for RF circuits.

 

[leolib2004]

Thanks guys. I am currently filling the whole bottom, except from the part that is located in the same place the wifi module is located on top, with ground and I have two islands on top. 1 in the part that the LDO is located, for heat dissipation, and the other where the audio codec is. Apart from that everything is signals or Vdd on top. Would it be better if I have every little empty spot tied to ground on top? I tried to use the lesser vias I could and doing the lines that connects the ARM and the codec with 2 or 0 vias. I also tried to respect a gap of at least 3 times their width for them.
Thus, I used several vias for the interconnection between the GND islands and bottom.