Espressif ESP8266 Series System Description page 17

Analysis:
Ripple has a strong impact on the performance of RF TX. It should be noted that ripple must
be tested when ESP8266EX sends normal packets. The ripple increases when the power gets
high. Generally, the ripple should be <80 mV when sending 11N MCS7 packets; the ripple
should be < 120 mV when sending 11B packets.
Solution:
Add a 10 μF filter capacitor to the branch of source circuit (ESP8266EX AVDD pin). The 10 μF
capacitor should be adjacent to the VDDA pin.
Q: The power ripple is small and the TX performance of the chip is poor.
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Analysis:
Besides power supply ripple, poor RF TX performance could also be due to the choice of a
wrong crystal. For example, the crystal has low quality-factor or the crystal frequency offset is
too big — when the frequency offset is more than ±40 ppm, ESP8266EX will not work
properly and the performance degrades. Sometimes, the crystal could also be corrupted by
other interfering signals, such as output digital signals or input signals. Another possible issue is
the coupling from inductors or antenna to the crystal. Usually the crystal can filter such
interferers but a large enough amplitude may affect performance.
Solution:
This problem is caused by improper layout and can be solved by re-layout. See Chapter 1.5 for
details.
Q: When ESP8266EX is sending data packages, the power value tested by an instrument
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is much higher or lower than the target power value, and the EVM is relatively poor.
Analysis:
The difference between the power value tested by an instrument and the target power value
can be caused by the impedance mismatch of the chipset RF pin to the antenna.
Solution:
Reserve a π type circuit on the RF wiring which allows match of the resistance with the
antenna, so the resistance from the chipset RF pin to antenna approaches 39-j6 Ω, which
usually gives the best performance.
Q: TX performance is not bad, but the RX sensitivity is low.
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Analysis:
Good TX performance means proper RF impedance matching. External coupling to the antenna
can influence the RX performance. For instance, the crystal oscillator signal's harmonics could
couple to the antenna. If ESP8266EX serves as slave device, there will be other high frequency
interferers or high speed digital signal interference sources on the board.
Solution:
Keep the antenna away from crystal oscillators and keep RF trace lines away from high
frequency signals.
Espressif 14 27 /
ESP8266EX
2016.06