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1 INTERLLIGENT RF & Microwave Solutions

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Wide Instantaneous Bandwidth Up / Down converters

Dror Regev

Interligent




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Abstract:

Ever growing demand for increased data rate in communications drives

system instantaneous bandwidth to be wider and frequency of operation

higher such that interference with legacy standards can be avoided.

Available test equipment in microwave frequencies such as vector

network analyzer and spectrum analyzer is usually narrow band and

cannot be employed. High Instantaneous bandwidth waveform

generators and scopes are available but cannot directly test at high mm-

wave frequencies hence require frequency converters. Simple or

harmonic mixing for up and down conversions poses test limitations

and impairments that can be mostly overcome by developing wide band

instantaneous up / down converters. This presentation will examine

different approaches for implementing such converters.




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Agenda

• Trends & Test challenges

• Wide Band Converters

• Dynamic Range considerations

• EVM and Impairments

• Wide Band & mm-Wave Impairments

• Converter Frequency Planning




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Trends @ Frequency Domain

Ever growing demand for Data Rate, drives extended BW and

higher carrier frequencies

Wide Band multi-carrier modulation schemes requiring high EVM.

5 GHz 60 GHz E-Band

802.11ac

Wi-Fi

802.11ad

Wi-Gig

PTP

4G/LTE 1.8 GHz 1

60

MH

z 500 MHz

802.11ad

Wi-Gig




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Frequency & BW- Test Challenges • Extended instantaneous BW @ mm-Wave Freq.

• Modern communication systems often requires the specification and characterization of DUT EVM and/or DUT BER.

• Modulated waveforms like OFDM are often required to drive DUT and measure at mm-Wave.

Problem:

Wide band AWGs and Scopes can not test and/or drive at mm-Wave.

60 GHz

802.11ad

Wi-Gig 1.8 GHz

AWG SCOPE




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DUT with Antenna - Test Challenges

• mm-Wave MIMO DUTs with Integrated Antennas

• Requires TX drive and RX measure over the air (OTA) overcoming air path loss Path Loss (68dB loss @ 1meter/60GHz)

• High power& linear drive to be received in DUT OTA and Low Noise measure capabilities are needed

SiBEAM’s 60GHz WHD module(Ali M. Niknejad)

Integrated Antenna

Array




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• Enable WB signal conversion to/from mm-Wave

• Enable OTA test interface when connected to an antenna

• May enable Probe interface, with basic test equipment

• Should have better EVM than DUT and

adequate dynamic range to support needed link budget.

Horn Antenna

mm-Wave

Probe

Frequency Converters – Test Enablers




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Frequency Converters Design Considerations

Considerations:

• Probe or OTA application?

Test link budget should support adequate S/N and/or EVM requirements

• TX, RX or both

Adequate power, NF and/or gain. Channel Directivity?, Dual?

• Characterization (hi performance) or Production (lower performance)

Converter EVM performance

Horn Antenna mm-Wave Probe




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Modulations and S/N

• Complex quadrature modulations are

common

• Data Rate - higher number of bits per

symbol transmitted, drives crowded

constellation points on the IQ plan

• Crowded constellation points on IQ

plan require better system S/N




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Carrier to Noise Ratio Requirements

• OFDM modulations

with multiple carriers

characterized by carrier

to noise ratio (C/N)

• The higher the number

of bits (NOB) per

symbol transmitted, the

better C/N required.




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Dynamic Range of an OTA Example

TX 1 dB out comp. 60GHz of converter : -20dBm

TX Back Off: 8dB

Maximum TX power: -28dBm

TX + DUT Antenna Gains: 20dB

Signal Received in DUT LNA: -28+12-68+8 = -76dBm

Noise Floor @ 1GHz BW: -174+90+6dB(NF) = -78dBm

S/N RX DUT: 2dB

Test case conclusion: Passive converter can not support WB modulated testing OTA.

60GHz TX

Antenna

fLO

Passive

conversion

1 meter

-68dB @ 60GHz

12dB Gain

60GHz RX

Antenna

8dB Gain

Link Budget with Passive Conversion, 1GHz BW @ 60GHz

AWG

Too low OTA

TX Power




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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-symbols

normalized to the

average QAM power

received




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Noise and EVM Both thermal and phase noise, add symbol fluctuations from ideal constellation

point in magnitude and phase.




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Spurious Signal and EVM

When a spur exists

during symbol’s

duration, the different

sub-symbols will be

distorted.

t

A

The Spur will

form a circle around

constellation point

Sub-symbol and Spur presence in time domain:

Spur Effect on EVM:

Constellation Plan

under Spur presence:

Amplitude

Error

Phase

Error




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Amplitude non-linearity and EVM

Signal Mask - Advanced QAM modulations include multiple sub-carriers

(sub-symbols) that can get mixed together to impair adjacent sub-carriers.

4 sub-carrier voltages in

Frequency domain

f

f1 f2 f3 f4

Δf 3

3

2

210

0

)(

vgvgvgg

vgvVi i

iDCout

Assuming Non-Linear output

current of the form:

Non-Linear

terms

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.

)cos()cos(

)cos()cos(

4433

2211

tvtv

tvtvv

Example:




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Amplitude Saturation and EVM

4 sub-carrier voltages in Time domain Example:

t

v

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.

QAM modulation symbols usually have high Peak to Average Ratios during

symbol duration.

Converter should

have high enough

saturation levels such

that transmitted

peaks will not be

clipped.

Amplitude

Peak




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Filtering Amplitude Effect on EVM

Filter Group Delay: Filter Phase transfer function is not linear, especially near “roll-off”

frequencies.

Filters employed in Converters (especially important are the IQ base band filters) impair

both amplitude and phase transfer function. Low Pass Filters (LPFs) are necessary for

rejecting I and Q signal’s alias but usually degrade EVM.

Multi carrier signals,

may encounter different

filter amplitude transfer

functions for the

different carriers.

In-band

Ripple

f f

1 1

Two common LPF topology examples: Butterworth Chebyshe

v




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EVM IQ Mismatches Impairments:

I&Q channels differ on transfer functions in both amplitude and phase, transmit and receive.




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LO Leakage signals will be

direct down converted at the

receiver to I & Q DC offsets and

have a similar effect on EVM.

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

I

Shifted Origin

Vector Origin shift: DC Offset & LO leakage




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EVM Reflection Impairment • In Wide-Band and/or mm-Wave applications, circuits

reflections may become critical EVM impairment.

• RFIC, MMIC interfaces including Antennas, PAs or

LNAs may yield significant ripple degradation.

Transmission

line LNA

Transmission

line PA

RFIC

Conceptual 60Ghz phased array module

Antenna Transmission

lines

Transmission

lines




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Transmission of 2 Circuits Cascaded through TL

a1a

b1a Sa

b2a

a2a

a1b

b1b Sb

b2b

a2b Transmission

line

S11a S22a

S21a

S12a

S11b S22b

S21b

S12b

Lossless Ideal TL IN OUT

And the total transmission from input to output can be derived as :

Where: 𝝓(𝑓) is the

TL electrical length




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Cascaded Transmission Analysis Analysis of 2 cascaded circuits yielded a total transmission of:

However for perfectly matched circuits over the BW of operation, we expect:

Hence, circuits reflection coefficients yielded a transmission

error in both amplitude and phase:




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EVM Reflection Error

The “rotating” vector over BW frequency:

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.




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EVM Reflection Error magnitudes

Phase Error

Amplitude Error

fmin

fmax

This error in vector amplitude and phase is significant and frequency dependent.




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Converter & Frequency Planning

Considerations:

• Convert to IF or to BB IQ?

• Direct conversion or Double/Triple?

• LO signal generation and Harmonics




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Direct Conversion to mm-Wave

• 60 GHz IRM & SSB Sub-harmonic (2nd) MMIC Mixers available

• Image reject performance may not support needed EVM requirements

• Image reject may be improved by calibrating or pre-distorting BB inputs

Challenge: Generation of mm-Wave LO signal at ~ RF/2

• Input Frequency and drive power should be supported by low cost signal

generator

• LO_IN Harmonics at mixer LO input should not impair in-band signals

• Stability in LO chain driving & multiplying




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LO Multiplier Example

• x 4 Multiplier with harmonics evident at the output

• x 3 Harmonic output is significant

• Harmonics if not controlled may create mixer saturation and/or intermodulation

products

Example:

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IMD in Sub-harmonic Direct Conversion Sub-harmonic (2nd)

Mixer Schematic:

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• Non-Balanced design

• Even orders not

suppressed

• IMD22 may impair TX

operation

Conductance of the anti-parallel diode pair

(APDP), peaks twice for each LO cycle and

effectively switches the RF (or BB) signal

@ 2*LO frequency.




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ATT

ATT

2 way

splitter x n x m

LO

mm-Wave Transceiver

RX

Gain

TX

Gain

TX_RFFE

RX_RFFE

TX_IF

RX_IF

LO_IN

LO Multipliers

LO

IF/RF Up/Down Converter • Supports IF testing

• Can employ balanced

mixers

• LO signal generation

challenging as mentioned

• Consider in-band IMDs

• LO=2/3RF risky




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Popular case : M=2, N=1.

32

21

RFLOLO

FFF

LO1, LO2

Generation:

= LO1

= LO2

GHzFF

F RFLOLO 20

32

21

GHzFF

F RFLOLO 20

32

21

1/3, 2/3LO Frequency Plan Approach




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IF @ fRF/3 RF=1LO2+1IF

IM21=2LO2-1IF fRF

WB Signal

IMD21 a

fLO2 =

2*fLO1

fLO1 =

fRF/3

BBI

00/900

BBQ

IF1

IF2

fRF/3

a- Mixer IM21 suppression can be improved to a certain level by

improving Balun Balance

1/3, 2/3LO with IMD21 Impairment




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Summary

• WB & mm-Wave testing can benefit use of frequency converters

• No simple ready solution

• Converter performance and frequency planning is highly important

• Challenging considerations, design and integration

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