Advanced RF Measurement

Techniques with SW-based Modular

Signal Analyzers

Andreas Gustafsson

National Instruments

Modern RF Measurement System

Software for Wireless Standards

(802.11 a/b/g/n, GSM, EDGE, WCDMA,

RFID, WiMAX,

GPS, and so on)

Modular Hardware Architecture

What are Vector Signal Analyzers (VSAs)?

• Convert time domain to frequency domain

• Both Amplitude and phase

Amplitude

(power)

Time Domain

Measurements

Single vs. Multi-Stage Architectures

Single-Stage Downconversion

1-stage = 1 mixer (PXIe-5663)

Multi-Stage Downconversion

ADC

3-stage = 3 mixers (PXIe-5665)

Benefits

- Faster Tuning

- Simpler design = savings on

hardware

Benefits

- Image rejection

- Improved Performance

Modular Architecture

- The 5622 is a 150 MS/s 16 bit digitizer

- The 5603/5605 downconverter allows for

frequencies up to 3.6/14 GHz and features a

three stage super heterodyne architecture

- The 5653 oscillator provides industry leading

phase noise performance

Architecture of a 4-channel VSA

ADC

ADC

Rx0

Rx1

ADC Rx2

ADC Rx3

DDC

16

Shared Local

Oscillator Shared ADC

Sample Clock

I

Q

DDC

16 I

Q

DDC

16 I

Q

DDC

16 I

Q

10 MHz

OCXO

• Shared clocks

• Local oscillator

• ADC clock

• NCO on DDC

• Example shown

with PXIe-5663

NI PXIe-5665

High Performance Vector Signal Analyzer • 20 Hz to 3.6 or 14 GHz frequency range • Phase noise -129 dBc/Hz at 10 kHz offset • Average noise floor: -165 dBm/Hz @ 1 GHz • TOI - +24 dBm • WCDMA ACLR: -80 dBc • 0.1 dB Amplitude Accuracy • .33% 256 QAM EVM

5622

5603

5653

ADC RF In DDC/F

FT

Achieving Improved VSA Measurements ① Calibration

a) System Calibration, Cal tone

b) IF Equalization

② Dynamic Range and Noise Floor

a) Resolution Bandwidth

b) IF Filters

c) Instantaneous Bandwidth

d) Min ACPR

e) pre amp

f) noise correction

③ Speed –

a) Phase noise tuning

b) RF list mode

5622

5603

5653

ADC RF In DDC/F

FT

Achieving Improved VSA Measurements ① Calibration

a) System Calibration, Cal tone

b) IF Equalization

② Dynamic Range and Noise Floor

a) Resolution Bandwidth

b) IF Filters

c) Instantaneous Bandwidth

d) IF gain

e) pre amp

f) noise correction

③ Speed –

a) Phase noise tuning

b) RF list mode

Challenges – Calibration

• Temperature drift

• Regularity of calibration

• Individual module calibration

• Use of external high precision

instrumentation

1 2 3

a) Solution – Factory & Self Calibration

• Factory Calibration on each module:

Calibrated for accurate frequency and amplitude response at manufacturing • RF Response – Frequency and temp

compensation

• IF Response – response of IF filters is measured

Ships with calibration certificate verifying NIST-traceable accuracy

• Self Calibration –

NI 5603 measures the reference source

Compares the resulting measurements to a value stored in the NI 5603 EEPROM

Tip #1

Solution – Use of Cal Tone

Built in high precision cal-tone calibrates the down-

converter

NI RFSA driver combines the calibrated values for a

system calibration

5622

5603

5653

ADC RF In DDC/FF

T

Cal tone

Tip #1

b) Use of equalization with calibration

• Equalization is used to compensate for non linear components of filters (group delay)

• Envelope Delay technique Apply information to a carrier (AM modulated) and measure how much the information is delayed using a phase detector working at the modulation frequency (612.5MHz)

• Results in better EVM measurements

Tip #2

WCDMA EVM Improvements with Equalization

No equalization

Equalization

5622

5603

5653

ADC RF In DDC

Achieving Improved VSA Measurements ① Calibration

a) System Calibration, Cal tone

b) IF Equalization

② Dynamic Range and Noise Floor a) Resolution Bandwidth

b) IF Filters

c) Instantaneous Bandwidth

d) IF gain

e) pre amp

f) noise correction

③ Speed –

a) Phase noise tuning

b) RF list mode

Challenge – Measuring Low Level Signals

• ADC resolution determines floor on achievable dynamic

range

• Larger signals blocks / masks the weak signal which may

get quantized with only a few bits and end up in the noise

a) Resolution Bandwidth (RBW)

Effects of lowering resolution bandwidth: Increases frequency resolution (# of spectral lines)

Lowers DANL (Displayed Average Noise Level)

• Increases dynamic range – narrowband signals only

Increases acquisition time.

Tip #3

RBW=10kHz RBW=300kHz

b) Use of Appropriate IF Filters

• Some VSAs support multiple IF filter options

Through - Provides a nominal 50 MHz bandwidth

• Advantages - Speed

Narrow (300 KHz) - Provides a nominal 300 kHz

• Advantages – Lower noise floor

f1 f2 f3

FFT1 50 MHz Filter

300 kHz Filter

FFT1

5622

5603

5653

ADC RF In FFT

Tip #4

c) Use of Instantaneous Bandwidth

• The bandwidth in which all frequency components can be simultaneously captured and analyzed.

• Increase in measurement speed for large spans

• IF path selected based on instantaneous BW

• Resolution bandwidth applies after the ADC and instantaneous bandwidth applies before the ADC

Span

IBW

FFT1 FFT3 FFT2 FFTN ……..

Total Measurement Time

Tip #5

d) IF Gain Optimization

• Adjusts variable attenuators and IF gain to optimize complete range

of ADC

• Applies only to narrow bandwidth IF filter (300 KHz)

• Affects noise floor but not linearity

5622

5603

5653

ADC RF In FFT

Tip #6

Effect of IF Gain on Adjacent Tones

ADC

1V pk to pk (16 bits)

ADC

ADC IF Gain Applied

Bandwidth No IF Gain 1V pk to pk (16 bits)

1V pk to pk (16 bits)

Noise Floor and Time Effects of IF Filters

• 5-6dB improvement with 300 KHz filter Path + IF Gain

• Takes longer to sweep with the 300 KHz filter path

• Adjacent tones can be detected

e) Use of optional pre amplifier

• NI 5603/5605 includes

Switchable 15 dB preamplifier before the first mixer

Mechanical step attenuator

Solid state step attenuator

• 15 dB pre-amp offers lower noise floor

5622

5603

5653

ADC RF In FFT

Tip #7

Pre-amplification

• NI 5665 features an optional built in pre amplifier

• ANL of -165 dBm/Hz with pre amp

f) Noise Correction

1. Disconnect input – connect to terminator and measure noise floor

2. Store measures results in cache

3. Connect input and make measurement

4. Subtract stored values from measurement to get noise corrected data

RF In

Disconnect

RF In

Noise Corrected Data

Tip #8

ACP Measurements with Noise Correction

Without Noise Correction

• ~64 dBc ACPR

With Noise Correction

• ~70 dBc ACPR

Note: Perform noise correction when any parameter such as

frequency or amplitude changes.

5622

5603

5653

DAC RF In FFT

Achieving Improved VSA Measurements ① Calibration

a) System Calibration, Cal tone

b) IF Equalization

② Dynamic Range and Noise Floor

a) Resolution Bandwidth

b) IF Filters

c) Instantaneous Bandwidth

d) IF gain

e) pre amp

f) noise correction

③ Speed –

a) Phase noise tuning

b) RF list mode

Tuning Time

Mode Tuning Speed

(1 GHz Step)

Frequency

Accuracy

Phase Noise (10

KHz)

Normal 9.3 ms 0.1 ppm

-133 dBc/Hz

Fast 2.3 ms 1 ppm

-130 dBc/Hz

Tip #9

RF List Mode vs. Traditional Method

Freq 1

Deterministic Triggers

Freq 2

SW Call

Freq 1

SW Call 2 SW Call 1

Traditional Software List

RF List Mode

Freq 3

Freq 2 Freq 3

SW Call 3

∆t

Time savings

RF List Mode

• A way to dramatically improve test time for

stepped or swept measurements

Tip #10

Deterministic Scanning • 32,000 configuration scan list

• Internal and external trigger connections available

Scanning Support

Start

Switch:

Connect next channel in scan list

Switch:

Output Trigger

Instrument:

Input Trigger

Instrument:

Perform Measurement

Instrument:

Output Trigger

Switch:

Input Trigger

Summary

• Modern Signal Analyzers based on a SW and

modular approach are very powerful when some

challenges are overcome:

Calibration: New techniques enhance phase and

amplitude accuracy

Dynamic Range: IF path selection, gain and noise

correction can greatly improve D.R. and noise floor

Speed: Use techniques to improve measurement

speed such as RF list mode and tuning time options