NXP Semiconductors
JN-RM-2079
QN9090 module development reference manual
JN-RM-2079
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2020. All rights reserved.
Reference manual
Rev. 1.0
— 17 Jan 2020
9 of 31
production there can be variations in PCB fabrication from lot-to-lot which can degrade
performance.
The use of a correct substrate like the FR4 with a dielectric constant of 4.4 will assist you
in achieving a good RF design.
While no special measures are required for the board design, it is recommended that
Class 1 tolerances be used.
4.3 RF circuit topology and matching
NXP always recommends that designers start by copying the existing NXP reference
design. This applies to both the circuit portion (schematic) of the design, and the PCB
layout. For all RF designs, particularly for designs at frequencies as high as 2.4 GHz, the
PCB traces are a part of the design itself. Even a very short trace has a small amount of
parasitic impedance (usually inductive), which has to be compensated for in the
remainder of the circuit.
What may seem like a minor change to the layout, or what would certainly be a minor
change at a lower frequency of operation, can actually be a significant change at 2.4 GHz.
For example, we may consider that a metal trace on a PCB such as the QN9090-001M1x
modules is approximately 0.8 nH per mm. At lower frequencies, this would have no
impact, but at 2.4 GHz this would have a significant impact in any matching circuits.
The circuits used on the NXP reference designs are all tuned and optimized on the
actual layout of the reference design, such that the final component values take into
account the effects of the circuit board traces, and other parasitic effects introduced by
the PCB. This includes such issues as parasitic capacitance between components,
traces, and/or board copper layers, inductance of traces and ground vias, the non-ideal
effects of components, and nearby physical objects.
The layout of the RF portions of QN9090 based modules is critical. It is important that the
reference designs are strictly adhered to, otherwise the following may occur:
•
Reduction in RF output
•
Excessive spurious RF outputs leading to RF compliance issues
•
Unacceptable power slope across the full channel range
•
Poor range
•
Reduced Rx sensitivity
4.4 Transmission lines
Transmission lines have several shapes such as microstrip, coplanar waveguide,
and strip-line. For BLE applications built on FR4 substrates, the types of
transmission lines typically take the form of microstrip or coplanar waveguide
(CPW). These two structures are defined by the dielectric constant of the board
material, trace width, and the board thickness between the trace and the ground.
