Reflection EVM Impairments in Wideband 60GHz and E band designs

Transcription

1 Reflection EVM Impairments in Wideband 60GHz and E band designs Dror Regev

2 About Presto Engineering Leader in Integrated Test & Product Engineering and Back-end Production services Service hubs in USA, Europe and Israel Jan/12: Acquisition of ITH operations ~100 WW team expert in: Test Engineering (Test HW and SW) Qualification & Reliability Failure Analysis Special focus in RF testing 2

3 Agenda Error Vector Magnitude (EVM) RF frequencies and narrow band considerations: o EVM Total Noise Effects o Spurious Impairment EVM o Amplitude linearity EVM impairment o Phase linearity EVM impairment o DC Offset & LO Leakage EVM Effects o IQ Amplitude and Phase EVM impairments mm-wave and/or wideband additional considerations: o Cases of increased interest o Reflection errors

4 EVM Introduction Error vector measures the distance on the IQ plan between the ideal constellation point of the symbol and the actual point. It can be measured in db or % of the ideal sub-symbol 4

5 Thermal Noise and EVM For symbol s duration: Thermal Noise reflects random fluctuations in sub-symbol s amplitude. These fluctuations are normally distributed. Q A t = Q t 2 + I t 2 + TN(t) Thermal Noise Fluctuations in Symbol s Amplitude I Thermal Noise 5

6 Phase Noise and EVM Phase Noise reflects random fluctuations in the sub-symbol s phase. Q For symbol s duration: φ t = tan 1 Q t I t + PN(t) Phase Noise PLL Phase Noise over Frequency: Carrier Loop BW Reference Noise Phase Noise Symbol Fluctuations VCO Noise f φ(t) I 6

7 Total Noise and EVM Q The total sub-symbol noise uncertainty will form a cloud in the IQ constellation Plan. Thermal and Phase Noise Fluctuations in the Sub-Symbol s Constellation Plan I Since noise is stochastic these EVM errors can not be calibrated. Different averaging techniques may be implemented but will lengthen EVM test time. 7

8 Spurious Signal and EVM When a spur exists during symbol s duration, the different sub-symbols will be distorted. Spur Effect on EVM: A Sub-symbol and Spur presence in time domain: Amplitude Error Phase Error t The Spur will form a circle around constellation point Constellation Plan under Spur presence: 8

9 Amplitude non-linearity and EVM Advanced QAM modulations include multiple sub-carriers (subsymbols), hence it is fairly complicated to predict linearity EVM analytically. 4 sub-carrier voltages in Frequency domain Example: f = 1 T = 1 Symbol Duration f 1 f 2 f 3 f 4 Δf f Assuming Non-Linear output current of the form: i out ( V g 0 DC v) g v v cos( t) v 1 v cos( t) v 3 2 v cos( t) 4 2 g cos( t) v 2 g i v g i 3 Non-Linear terms v 3 At Base Band frequencies, both squared (like IP2) and cubic (like IP3) terms contribute intermodulation products at the original sub-carrier frequencies and distort sub-symbols. At RF frequencies, it is the cubic term that generates intermodulation products. 9

10 Amplitude Saturation and EVM QAM modulation symbols usually have high Peak to Average Ratios during symbol duration. 4 sub-carrier voltages in Time domain Example: v Amplitude Peak t Test equipment needs to have high enough saturation levels such that transmitted peaks will not be clipped. Another known saturation effect is dependency of transmission phase in input/output power level. This power to phase dependency will also distort the symbol at high power. Pre-distortion techniques may be available to negate some of these effects. 10

11 1 Filtering Amplitude Effect on EVM Filters are common in test instruments and especially important are those employed at IQ base bands. These Low Pass Filters (LPFs) are necessary for rejecting I and Q signal s alias but have the potential of degrading EVM. Two common LPF topology examples: Chebyshe v In-band Ripple f 1 Butterworth Multi carrier base band signals, may encounter different filter amplitude transfer functions for the different carriers. Since filter in-band ripple or BW roll-off can be measured, their effects may be mostly compensated at system level. f 11

12 Filtering Phase Effect on EVM Filters have a transfer function of the form: H jω = H jω e jθ(ω) Where the frequency dependent amplitude is given by: H(jω) θ(ω)- Phase transfer function should be linear over frequency to support phase accuracy of different sub-symbols. Group delay is defined as: τ ω = θ(ω) ω and will be constant for a linear phase filter. 12

13 Filter Group Delay & EVM Amplitude H(jω) and phase θ(ω) transfer functions are related, hence Group Delay τ ω is also amplitude dependent. Qualitative LPF Amplitude and Group Delay example: H(jω) Amplitude τ ω Amplitude & Group Delay both change at filter s BW edges. Group Delay BW Edge f Change will depend on Filter s type and order Hence at filter s BW roll-off frequencies, Phase transfer function is not linear. Choosing LPF with BW wider than signal s BW is usually not practical as it degrades filtering. These phase nonlinearities are measurable and their effects may be compensated. 13

14 Vector Origin shift: DC Offset & LO leakage I and/or Q offsets in the DC level will skew the origin of the IQ constellation plan. The effect is a constant error vector added to all constellation points as seen below: Q Shifted Origin I LO Leakage signals will be direct down converted at the receiver to I & Q DC offsets and have a similar effect on EVM. 14

15 IQ Amplitude Mismatch EVM Impairment I and Q gain offsets or different amplitude ripple performance, will degrade EVM. The different amplitude transfer functions will shift all constellation points as shown: A Q A I Q A I = H I (jω) *I A Q = H Q (jω) *Q I Amplitude IQ mismatch can generate both amplitude and phase errors 15

16 IQ Phase Mismatch EVM Impairment I and Q phase transfer functions may differ at all or some of the frequencies effectively skewing the ideal 90 0 phase between I and Q degrading EVM. The different phase transfer functions will shift all constellation points as shown: Q θ ε (ω)=θ I (ω)-θ Q (ω) I Phase IQ mismatch can generate both amplitude and phase errors 16

17 EVM Reflection Impairment The above impairments sufficiently cover narrow band and lower than 6GHz applications they may not be sufficient for wideband mm-wave applications. In Wide-Band and/or mm-wave applications, circuits reflections may become critical EVM impairment. RFIC interfaces including Antennas, PAs or LNAs and such may yield significant reflection EVM degradation. 17

18 Relevant mm-wave Interfaces Transmission line LNA 60GHz Antenna 60GHz Antenna PA mm Wave RFIC Example: Vubiq 60GHz micro-modules Transmission line mm Wave RFIC 18

19 Phased Array 60GHz Interfaces SiBEAM s WHD module(ali M. Niknejad) Conceptual 60Ghz phased array module Antenna Transmission lines RFIC Transmission lines 19

20 Transmission of 2 Circuits Cascaded through TL a 1a b 1a S a b2a S 21a a 2a Transmission line 1 φ f a 1b b 1b S b b2b S 21b a 2b IN S 11a S 22a Lossless Ideal TL S 11b S 22b S 12a 1 φ f S 12b OUT And the total transmission from input to output can be derived as : S 21T = S 21aS 21b φ f 1 S 22a S 11b 2φ f Where: φ(f) is the TL electrical length 20

21 Cascaded Transmission Analysis Analysis of 2 cascaded circuits yielded a total transmission of: S 21T = S 21aS 21b φ f 1 S 22a S 11b 2φ f However for perfectly matched circuits over the BW of operation, we expect: S 21Tideal = S 21a S 21b φ f Hence, circuits reflection coefficients yielded a transmission error in both amplitude and phase: S 21T mismatch error = 1 1 S 22a S 11b 2φ f 21

22 EVM Reflection Error S 21T mismatch error = 1 1 S 22a S 11b 2φ f The rotating vector over BW frequency: S 22a S 11b 2φ f = S 22a (f) S 11b (f) S 22a f + S 11b f + 2φ f Vector Amplitude Vector Phase For narrow band circuits connected by electrically short transmission lines, this mismatch error resulting from the frequency dependent vector may be assumed constant within BW. In wide band circuits, reflection coefficients change amplitude and phase within BW and moreover if connected by a long TL, the total error further changes within BW. 22

23 EVM Reflection Error magnitudes S 21T mismatch error = 1 1 S 22a S 11b 2φ f A(f) θ f error denominator = 1 S 22a S 11b 2φ f S 21T mismatch error = 1 A(f) θ f = 1 A(f) θ(f) f max 1 f min Phase Error Amplitude Error 23

24 Reflection EVM Discussion REVM is function of the magnitude of the product S 22a S 11b!. Phase change of this term within Band Width is also of high importance as well as the additional electrical length stemming from the connecting TL, S 22a of PAs is known to be very poor (>0.5) and it s phase change within BW may be significant. Matched with a reasonable antenna S 11b of >0.33 that also change it s phase within BW through a transmission line electrical length, may yield in some frequencies up to: +/- 20% amplitude error and +/- 11 phase error. This maybe the most significant error in the system. 24

25 Summary 60-90GHz wide band applications and devices are predicted to become more common in the near future. Low frequency narrow band treatment of these devices may not be sufficient for such devices. Reflection EVM is a significant source of error that must be considered in such cases. 25