32Mhz external crystal is used for 2.4G transfer and receiver?

[james]

Hi,

    We have a product which was developed with nRF52810.We found Samsung cellphone M31 couldn't  scan the broadcast of nRF52810 with nRF Connect, but ther cellphone  could scan and connet it with nRF connect. Samsung cellphone could scan the broadcast of nRF52810 and connet it after we changed 32Mhz +-20ppm crystal to 32Mhz +-10ppm crystal. We want to know 32Mhz crystal can affect 2.4G transfer or receiver, and if it does how can 32Mhz crystal affect 2.4G transfer or receiver.

[ketiljo]

The 32 MHz crystal can indeed affect the connection. What's the specification of the crystal and the load caps you are using with it?

If the frequency isn't within spec, you may experience problems with the connection to some devices. A typical error is when the crystal is incorrectly loaded so that the frequency so the crystal is pulled up or down.

[james]

We used 32Mhz +-20ppm 12pF crystal and 12pF for C1,C2  before we found that problem.We realize we may use the wrong parameter for C1,C2  after you mentioned the load caps.So we used 18pF for C1,C2 after we calculated the load caps with load caps equation(according to the figure 20 ).

18pF may not be the best parameter for C1,C2 as we not sure value of PCB cap,we assume PCB cap is 2pF.But after that  the samsung cellphone could scan and connect our product.

Now we get two questions:

1. Is our calculation  right for C1,C2? 

2. We had produced tens of thousand of that product, can we match the load caps by software as it will be big loss to change the C1,C2 for us

.

[ketiljo]

For a crystal with CL = 12 pF, you can use 18 pF caps. Calculate as follows: 

Ccap = CL *2 - Cpin, where Cpin = 4 pF. 

Round down to the nearest standard value to account for the PCB capacitance. The PCB capacitance is the order of  ~1 pF.

Unfortunately, there's no SW fix for this on this device. 

[james]

Ok we will change the caps.

We want to make sure it won't happen again, so we plan to test 2.4G carried frequency in broadcast channel with frequency spectrograph for every product. Can  frequency spectrograph detect the  frequency offset of 2.4G carried frequency?

The screen shot mention "relationship between crystal frequency and Tx carried frequency is 1:1" . It means Tx carried frequency offset 100khz if crystal frequency offset 100khz, am I right?

If it's true, we guess we can tune  crystal frequency to fix the  frequency offset of 2.4G carried frequency.can we do that?

[ketiljo]

You don't need to test each and every device this way but it's a good way to test the prototype for the correct load cap value. But you should test the radio on each product to make sure the load caps are correctly mounted etc. Thus can be a simple connection test.

The offset in the 32 MHz crystal frequency scales with the ratio between the carrier and the crystal freq. The difference in ppm is the same, but not Hz. For example, If you have a 30 ppm error in the 32 MHz clock, the carrier will have the same offset.  This corresponds with a 960 Hz offset for the 32 MHz clock but a 72 kHz offset for the carrier. 

The frequency of the crystal can only be changed of you change the load caps for the crystal. There's no way to change it by SW.

[james]

Thank you.It helps me a lot.