Reference design

AN3206

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Doc ID 17406 Rev 3

By routing in this manner, PCB area and cost are wasted. It also requires the model to be

updated to a three-element match, generally making it more difficult to simulate.

An added benefit of using an 0603 inductor over an 0402 inductor is that better Q is

obtainable. The STM32W Ceramic Balun Reference Design uses a Murata LQG18H series

inductor which has a typical Q of approximately 100 at 2.4 GHz. Compare this to the

commonly used LQP15M 0402 inductor, which has a typical Q of approximately 50. This is

worth nearly 0.2 dB. The disadvantage is that it is not possible to get E24 values in 0603

style at these sorts of component values. So a small price is paid on design centering.

As previously mentioned, the balun center tap is fed with a 1.8V DC feed. A decoupling

capacitor is required to define the common-mode RF potential. The balun decoupling

capacitor needs to be as close as possible to the balun pins and with a high quality ground

connection. Its value is chosen to be series resonant with its parasitic inductance at 2.4GHz.

While it would be possible to tune-out longer tracks by using a smaller capacitor value, this

has the effect of making the decoupling more narrow-band, which can impact common-

mode harmonic suppression.

2.1.4

Harmonic filter design

FCC Rule 15.35 allows for a duty-cycle relaxation to the regulatory limits. For EmberZNet

Pro, the relaxation is 6.9 dB.

FCC Rule 15.205 defines 'restricted bands' where harmonics must meet the levels set out in

FCC Rule 15.209. This is defined as -41.2 dBm/MHz up to 12.5 GHz. This applies to the

2nd, 3rd, and 5th harmonics, but not the 4th harmonic.

ETSI EN 300 440-1 requires spurious emissions to be less than -30 dBm/MHz,

1GHz~24GHz, so this becomes the default limit for the 4th harmonic.

Generally, if FCC limits are met it is extremely unlikely that harmonics to 24 GHz would fail

ETSI requirements, so frequencies above the 5th harmonic are not considered here.

The overall requirement is shown in Table 1.

2.1.5

Band-pass filter

ST decided to use a band-pass filter as it is a convenient way to ensure that unwanted

harmonics are filtered out and 50 Ohms adaptation is guaranteed between the balun and

the antenna matching circuitry.

2.1.6

Antenna matching circuit

A matching circuit is needed to adapt the impedance between the 50-Ohm band-pass filter

and the antenna. Ideally a 50-Ohm trace should be enough, but, in reality, antenna

impedance varies with its environment. Once the antenna characteristics as been measured

Table 1.

Harmonic power limits

Harmonic

Frequency

Limit

2nd

4810~4960 MHz

3rd

7215~7440 MHz

4th

9620~9920 MHz

5th

12025~12400 MHz