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EDR Differential EVM

RFmx Bluetooth 19.1 Help

Edition Date: October 2019

Part Number: 377889C-01

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RFmx Bluetooth 19.1 Help
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Differential Error Vector Magnitude (DEVM) quantifies the modulation signal quality of the differentially encoded modulated Bluetooth Enhanced Data Rate (EDR) packets. The EDR packet with 2Mbps data rate uses the pi/4QPSK modulation and the EDR packet with 3Mbps data rate uses the 8DPSK modulation.

As per the Appendix C, Part A, Volume 2 of the Bluetooth Core Specification v2.1+ EDR, the differential error vector is defined as the difference between the vectors representing consecutive symbols of the transmitted signal. The magnitude of the normalized differential error vector is called the DEVM. The DEVM measurement is computed on 50us blocks (50 symbols) in the EDR portion of the packet after compensating transmitted symbols for carrier frequency error and for ideal carrier phase changes.

To get integral number of 50us blocks, set the Payload Length Mode property to Manual and set the spec defined payload lengths. If the number of symbols in the EDR portion of the packet is not an integral multiple of 50us, then extra symbols will be discarded from the computation. As per section 4.5.11 of the Bluetooth Test Specification RF.TS.5.1.0, the payload length requirement for EDR packets and the corresponding number of DEVM blocks are as given in the following table.

# Packet Type Payload Length (Bytes) Number of DEVM Blocks
1 2-DH1 31 3
2 2-DH3 356 29
3 2-DH5 656 53
4 2-EV3 58 5
5 2-EV5 358 29
6 3-DH1 11 1
7 3-DH3 536 29
8 3-DH5 986 53
9 3-EV3 88 5
10 3-EV5 538 29

Table 1 - Number of DEVM blocks for standard defined payload length for EDR packets

DEVM Measurement Principle

Figure 1 DEVM Measurement Principle

To perform the DEVM measurement, the EDR packet is compensated for the initial frequency error ωi. In addition, a square-root raised cosine measurement filter with a roll-off factor, α of 0.4 and 3 dB bandwidth of ±500kHz is applied to the EDR portion of the packet. The output of the measurement filter is then portioned into 50us non overlapping blocks starting from the first nominal sync symbol following the reference sync symbol. For each 50 us block, the tester calculates the sampling phase ε0 and frequency error ω0 for the RMS differential error vector magnitude (DEVM) for the block. These results are then used to calculate DEVM for each symbol in the block.

RMS DEVM is computed as:

The Symbol DEVM is computed as:

where

Qk(ε, ω) is the compensated sequence of the kth symbol where the ideal phase changes have been removed and ε(phase) and ω(frequency) are chosen optimally to minimize the DEVM

Ek(ε, ω) is the differential error sequence of the kth symbol and it is defined as {Qk(ε, ω)—Q(k-1)(ε, ω)}

N is the number of symbols per block (50 symbols)

ε0 is the sampling phase error used to compute symbol DEVM

ω0 is the sampling frequency error used to compute symbol DEVM

The corresponding results under ModAcc measurement are Peak RMS DEVM Maximum (%) and RMS DEVM Mean (%)

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