RF Matching Guidelines for WIFI

WLAN RF Transceivers

Qualcomm WCN 3660 [15]

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2.4 GHz Matching Procedures and Recommendations

2.4 GHz RF Path [15]

Transceiver

2.4 GHz BPF2.4 GHz / 5 GHz

Diplexer

PDET5 GHz

Coupler

3


Step1. Measure chip output performance. Do not

measure at the RF I/O directly, as it is not matched [7]

Start with Rx and match to 50 Ω impedance.

Performance goal at RF launch:

Once Rx match is close to 50 Ω impedance, change to Tx mode and fine tune for Tx performance to acceptable level

Transceiver

Z1

Z2

RF launch

port here for

measurement

4


RF launch example [7]

It is very important to use a short RF launch with

sufficient ground. 5


General S-parameters measurement needs two ports,

one port is source, and the other is load.

Nevertheless, for the situation, RX S-parameters

measurement needs only one port. Because the only

port is source, and transceiver(must be powered on) is

just the load.

Transceiver

Z1

Z2

RF launch

port here for

measurement

SourceLoad

6


Indeed, to check Tx S-Parameters needs to remove

transceiver(due to reference plane). But this requires a

lot of hard work.

Transceiver

Z1

Z2

Port 1 Port 2

S11

Chip 2.4 GHz

I/O Pin

Hence, for Tx Mode, just start with initial LC value and

fine tune for Tx performance such as EVM and power.

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Rx S-Parameters for 2.4 GHz [7]

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Step2. Tune filter matching values for 50Ω matching [7]

Start with the recommended matching values from the

manufacturer’s filter data sheet.

Measure return loss and insertion loss for filter and

diplexer with their matching values:

Place two RF launches (pigtails) at ports 1 and 2.

2.4 GHz BPF 2.4 GHz / 5 GHz

Diplexer

5 GHz

Port 1 Port 2

Matching

9


The input / output impedance of filter and diplexer should be

broadband and close to 50 Ω. Otherwise, the frequency

response of filter and diplexer will be unexpected, and the

loss and outband rejection will become poor.


diplexer with their matching values (cont.) :

The insertion loss (S21) from port1 to port 2 should be as

expected from filter and diplexer specifications. The example

below shows ~ 2.3 dB of loss [7].

Make sure insertion loss (S21) is as low as possible because

the final achievable performance will vary based on

the path loss of the front-end components.

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diplexer with their matching values (cont.) :

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Step3. Combine filter matching and RF I/O matching,

and then perform fine-tuning.

Please keep in mind that PDET (Power Detrctor) is RF

path as well, which affects EVM and TSSI. Hence,

reserve match networks in PDET trace in advance.

PDET

Coupler

Matching

12


Because filter matching is well controlled in step 2 by

S-parameters check. Thus, start with tweaking

transceiver and PDET matching values by measuring

TX performance at first.

If TX performance is still not acceptable by tweaking

transceiver and PDET matching values, return to step2

until TX performance and Rx S-Parameters are both

acceptable.

Transceiver

2.4 GHz BPF2.4 GHz / 5 GHz

Diplexer

PDET

5 GHz

Coupler

Matching Matching

Matching

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5GHz Matching Procedures and Recommendations for

FEM

5 GHz RF Path [7]

Transceiver

2.4 GHz / 5 GHz

Diplexer

2.4 GHz

FEM

RxPDET

14


FEM

5 GHz RF Path (cont.)

Transceiver

2.4 GHz / 5 GHz

Diplexer

2.4 GHz

FEM

RxPDET

15


FEM

Transceiver

Z2

Z3

RF launch

port here for

measurement

Z1

Step1. Measure DA output performance. Verify the RF

performance coming out of the chip is in line with the

transceiver specification. Adjust matching components

if necessary to achieve acceptable performance.

Because WIFI 5 GHz has wider RF bandwidth from 5170

to 5835 MHz (~15% fractional BW), it's better to reserve

at least three components(T-type or pi-type) for match

networks to achieve broadband matching.

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FEM

Step2. Measure and adjust the FEM match networks.

In order to accurately measure S11 of the FEM, the FEM

must be powered on.

Setting the output power of the network analyzer too

high can damage the FEM. Set the network analyzer to 0

dBm or lower.

Matching FEM

Port 1 Port 2

Rx

S11

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FEM

Step3. Measure S11 and path loss after FEM

2.4 GHz / 5 GHz

Diplexer

2.4 GHz PDET

Port 1

Matching

Port 2

S11

Using a 2nd RF launch or an RF switch connector, load

port 2 and measure S-parameters [7].

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FEM

Step4. Measure system performance:

If the results are worse than expected, return to steps 2

and 3 for further tuning. 19


FEM

Step5. Measure 5G Rx input impedance.

Matching 2 FEM

Port 1

Rx

S11

Transceiver

Matching 1

Because matching 2 is well controlled in step 2. Thus,

start with tweaking matching 1 to achieve 50Ω.

If necessary, to adjust matching2 further to to achieve

50Ω. But need to return to step 4 to check system performance is still acceptable or not [7].

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FEM

Step6. Test 5G sensitivity.

See the transceiver device specification for the

expected sensitivity results.

If the results are worse than expected, return to step 5

and adjust the Rx matching components.

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Reference

[1] CHALLENGES IN DESIGNING 5 GHZ 802.11AC WIFI POWER AMPLIFIERS, RFMD

[2] WCN3660 EVM Degradation Issue Technical Note, Qualcomm

[3] SE5516A: Dual-Band 802.11a/b/g/n/ac WLAN Front-End Module, SKYWORKS

[4] 802.11ac Technology Introduction White Paper, RHODE & SCHWARZ

[5] WLAN IEEE 802.11ac Wide bandwidth high speed 802.11ac technology and testing,

RHODE & SCHWARZ

[6] ACPF-7024 ISM Bandpass Filter (2401 – 2482 MHz), AVAGO

[7] WCN36x0(A) RF Matching Guidelines, Qualcomm

[8] MCS Index for 802.11n and 802.11ac Chart

[9] Sources of Error in IQ Based RF Signal Generation

[10] Integration Aids 802.11ac Mobile Wi-Fi Front Ends

[11] Mini filters for multiband devices, TDK

[12] QCA61x4-1 RF Matching Guidelines, Qualcomm

[13] QCA61x4, QCA937x, QCA65x4 Design Guidelines/Training Slides, Qualcomm

[14] QCA6164-1 with External 2.4G and 5G RFFM Reference Design Example, Qualcomm

[15] WLAN/BT/FM Training Using WCN3660, WCN3660A, or WCN3680, Qualcomm

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RF Matching Guidelines for WIFI

Engineering

Transcript of RF Matching Guidelines for WIFI