Maximum PDM clock frequency supported by nRF5340

[Ankit Mallick]

Hi,

We wanted to implement nRF5340 with our ultrasonic microphone SPH0641LU4H-1 (https://www.knowles.com/docs/default-source/model-downloads/sph0641lu4h-1-revb.pdf?Status=Master&sfvrsn=bdc077b1_4) to sample 80kHz data. To understand the capabilities, can you support us with -

1. Can we achieve PDM clock frequency of 4.8 MHz to run the sensor in ultrasonic mode?

2. Can we sample 80kHz data at a ratio of 64? If not, what is the maximum sampling rate we can achieve?

3. How much RAM and flash will be acquired to store 80 kHz data for 1 second?

Thanks & BR,

Ankit M

[Ankit Mallick]

Thank you for your valuable input on the PDM part.

I have a separate enquiry regarding the BLE range provided by NRF5340. I happened to go by your blogs from 2 years back on NRF52840 range testing and have used the same IC in one of our products, but dont seem to find enough data now on NRF5340's max BLE range at 3dBm.

Can you help to understand how much range can I achieve with NRF5340 at 3dBm power in LE_CODED 125kbps mode?

Is there any calculator or equations to find out the same?

[Susheel Nuguru]

Ankit Mallick said:

Can you help to understand how much range can I achieve with NRF5340 at 3dBm power in LE_CODED 125kbps mode?

Ankit, 

I tested and wrote the blog, that you are mentioning above. Unfortunately, we did not test this with nRF53 yet. 

But we can try to calculate this, as we know that " every 6-dBm increase in output power doubles the possible distance that is achievable"

On nRf52840, you have these radio sensitivity. 

• -103 dBm sensitivity in 125 kbps Bluetooth low energy mode (long range)
• -20 to +8 dBm TX power, configurable in 4 dB steps

On nRF5340. 

• -104 dBm sensitivity in 125 kbps Bluetooth Low Energy mode (long range)
• -40 dBm to +3 dBm configurable TX power

There is a slight improvement in the RX sensitivity in nRF53 but a bit of less TX power. 

So if you do the same test I did with same TX power as nRF52840 and measuring distance, you will get slightly more distance on nRF5340 just because of slightly better radio rx sensitivity. 

But since you cannot do the 8 dBm TX power on nRF5340, you wont be able to do that test on it to compare it with nRF52840.

[Ankit Mallick]

In one of your blogs, you mentioned reference to this link (https://www.electronicdesign.com/technologies/communications/article/21796484/understanding-wireless-range-calculations) which elaborates the calculations used to measure distance based upon the receiver sensitivity and transmit power.

Considering these equations, I am getting these values - 

Frequency (MHz)	2450
Transmit Power (dBm)	3
Transmit Power (mW)	1.995
Receiver sensitivity (dBm)	-104
Antenna Gain (dBi)	0
Fade Margin (dB)	0
Maximum Path Loss (dB)	107
Distance (kM)	2.182

May I consider the antenna gain as 0dBi for I have not decided any antenna so far and what can I put the fade margin as, since in practical case, it will never be 0?

Also, please verify whether this distance is okay to consider or not?

Thanks,

Ankit

[Susheel Nuguru]

The theoretical distance might be about right with 3dBm TX power and the given sensitivity but in real life scenario, I think it is hard to get any meaningful connection after 1.5 kms unless you have an exceptionally good antenna with high gain. But this antenna gain needs to be on both ends of the connection, also the peer you connected to. This is because the connection will continue to exist if both peers are able to exchange packets at those distances, not just one.