“high performance RF capacitors”

[Simon]

In the 51422 Prod Spec v2.0 it states that “high performance RF capacitors” for the X7R decoupling caps should be used.

1. Is High performance RF equivalent to a “High-Q”? If that is correct then what sort of Q value would be appropriate for this application? If this is not correct, how is “high performance RF” specified - (any easy way to communicate this to a manufacturer/supplier)?
2. In looking at ceramic cap datasheets, the maximum frequency usually stated is 1000MHz (even for RF series caps). What frequency range is required for the decoupling caps? If the frequency range is in fact 2.4GHz, then could someone give me an example part number for a cap because I all can turn up is 1000MHz which is less than half 2.4GHz.
3. Also, I’m curious why it’s not stated that the NP0 caps for the xtals and, especially balun should not also be “high performance RF capacitors”. My guess is that class 1 / NP0 is already High performance, or do I need to specify this to the manufacturer as well?

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At the risk of making this thread too complex: what exactly are DEC1 and AVDD? They are related to the radio but I can’t find anything specific on what they are used for or how AVDD is different from VDD. The reason I ask is because the value of the AVDD cap is 100 times less than the other decoupling caps.

[Håkon Alseth]

Hi Simon,

1. by "high performance chip inductor" these requirements have to be met:

For instance the Murata LQP15-series.

2. Decoupling caps should be the class that they are specified to be. Usually, this means X7R for "normal" decoupling, and NP0 (or C0G, which is the same thing) for RF and XOSC.

3. I believe it's stated as NP0 +/- 2% in the datasheet I have (v2.0)

DEC1: This is the decoupling pin for one of the internal regulators. This is the regulator for digital peripherals, such as UART/SPI/TWI/TIMERs etc.

AVDD is similar to VDD_NRF, but it's some added logic internally if you're running on DCDC. If you look at the LDO-power scheme, you can see that this pin is connected to VCC, and for DCDC-power scheme this is fed back to the DCDC-regulator.

Best regards Håkon

[Simon]

Hi Håkon, thanks for that on DEC1 and AVDD. About the caps, will any X7R do for decoupling or do I have to ensure they are special RF caps in addition to the required X7R ? Some manufactures have RF series caps and usually the “SERIES RESONANCE VS. CAPACITANCE” is only expressed up to 1000MHz. So should I be concerned about this and looking at the RF series X7R for decoupling?

[Håkon Alseth]

Hi Simon,

Normal X7R grade caps will work, as they are not located in the RF matching circuitry (ANT1 and ANT2). They are only there to reduce noise on the different VDD lines.

Best regards Håkon

[Simon]

What self resonance frequency requirements need to be met for the RF circuitry caps?

Thanks.

[Håkon Alseth]

Hi Simon,

As long as the 0402 caps are C0G/NP0 grade, they are within our requirements. Since the caps used are in the low pF-area (< 5.0 pF) and of 0402 size, the LC makes it resonante above 2.4GHz.

BR Håkon