Designing a 2 GHz to 10 GHz Vector Reflectometer

Jonathan Klein, University of Alaska, Fairbanks

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A vector reflectometer measures reflection as a function of frequency.

A vector reflectometer measures the ratio between an injected and reflected signal.

Vector (magnitude and phase) measurements help extract phase shift and improve calibration over scalar measurements.

Measuring this across a range of frequencies is useful.

Z0

ZLV1

I1 b1

VS

S11=b1a1

a1

a1=V s

2√Z0b1=

V s

2√Z0

(Z L−Z0)

(Z L+Z 0)

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Reflection measurements are useful for microwave design..

Sometimes you want low reflections (matched impedance).

And sometimes you want high reflection (filtering).

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I’m building one because microwave equipment is big and/or expensive..

Copper Mountain R140, 85 MHz to 14 GHz, $5495www.coppermountaintech.com/products/11/R140

Keysight N9916A FieldFox Handheld Microwave Analyzer, 14 GHz, $16370

HP 8510B, 45 MHz to 26.5 GHz Network Analyzer

$1495 + $300 shipping

(used, “GREAT” condition, missing sweeper source)http://www.ebay.com/itm/HP-8510B-8515A-NETWORK-ANALYZER-AND-S-PARAMETER-SET-45-MHZ-26-5-GHZ-/112317940738

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Block diagram of reflectometer

DeviceUnderTest

ADC 1

ADC 2

RF Synth

LO Synth(RF+45.01 MHz)

HMC311Amplifier

HMC424Variable Attenuator Filter Bank

Narda 30357A, 30 dBDirectional Coupler

HP 87300B, 10 dB Directional Coupler

LTC5548Mixer

AD9864IF Digitizer

-10

dB

LMX2592Synthesizer

-10 dB

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RF Design – synthesizer

I designed the synthesizer boards around the TI LMX2592 wideband synthesizer with integrated VCOs.

A filter bank cleans up the harmonics, then a variable attenuator levels the output amplitude.

HMC424Variable Attenuator

HMC321, filters, HMC321 Filter Bank

LMX2592Synthesizer

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RF Design – I/Q demodulators2-10 GHz goes in, 29 kS/s goes outI/Q demodulator boards use a mixer to downconvert the RF signal to a 45 MHz IF.

The 45 MHz IF is mixed down to 3.25 MHz then sampled and decimated to 29 kS/s.

The demodulator boards share a common clock and produce simultaneous I/Q samples.

ADC

samples:AD9864ADC

LTC5548Mixer

LO in

RF in

IF in

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I cheated on the demodulator and used fancy chips to save time..

The ADC has an integrated LNA, mixer, decimation filter, and PLLs for the clock and IF.

The LTC5548 has integrated baluns, an LO amplifier, and a multiplier.

Figure 1: AD9864 block diagramhttp://www.analog.com/media/en/technical-documentation/data-sheets/AD9864.pdf

Figure 2: LTC5548 block diagramhttps://cds.linear.com/docs/en/datasheet/5548f.pdf

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RF Design – other modules

above: HMC311 ampleft: directional couplersbelow: power splitter for LO distribution

above: IF and ADCclock distribution boardleft: synthesizer clocksplitter board

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Digital design – TI BeagleBone Black

The VNA is run on a TI Sitara AM3358 ARM Cortex-A8 processor, with dual 200 MHz MIPS coprocessors.

The MIPS coprocessors stream samples from multiple ADCs simultaneously into shared memory with the Cortex-A8.

The Cortex -A8 runs Linux, it spawns hardware drivers for the synthesizer and demodulator boards that manage SPI communication as well as low speed IO.

Each of the hardware drivers communicates over the network to a VNA control program, which does not necessarily need to run on the BeagleBone.

Custom “cape” for BeagleBoneblack development board

4x headers for demodulator boards

2x headers for synthesizer boards

VNA controlled over Ethernet

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The VNA was designed and built with free and open source software.

VNA calibration and plotting uses the scikit-rf python library.

All circuit boards were designed in KiCad, an open source EDA tool.

All boards were hand assembled low temperature tin-bismuth solder paste, stencils, and a hot air rework station…

Layout of synthesizer PCB in KiCad

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Initial measurements - SF-SF50+ SMA barrel looks lumpy?

Left: Mini-Circuits SF-SF50+ SMA barrel

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The reference path is different from the reflected path, we need calibration!

DeviceUnderTest

ADC 1

ADC 2

RF Synth

LO Synth(RF+45.01 MHz)

HMC311Amplifier

HMC424Variable Attenuator Filter Bank

Narda 30357A, 30 dBDirectional Coupler

HP 87300B, 10 dB Directional Coupler

LTC5548Mixer

AD9864IF Digitizer

-10

dB

LMX2592Synthesizer

-10 dB

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Calibration is important for vector network analyzers/reflectometers!

b1M

a1M 1 a1A

S11AESF

ERF

EDF

b1A

one-port VNA error model

Equations from Handbook of Microwave Component Measurements: with Advanced VNA Techniques. [1]

b1M

a1M

=

One port calibration is possible by measuring three known standards(commonly short, open, and load) then using them to solve the error terms. EDF - directivity error ESF – source match errorERF – reflection tracking error

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Calibration solution - short-open-load calibration!

Measure known reference standards,

then use those measurements

to calculate error terms.

SDR-Kits SMA SOL Calibration Kit

SDR-Kits Calibration Kithttp://sdr-kits.net/VNWA/Rosenberger_Female_Cal_Standards_rev4.pdf

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Calibration standards are not ideal, so we apply a generalized solution

Where

ГAO = actual reflection coefficient of open

ГAS = actual reflection coefficient of short

ГAL = actual reflection coefficient of load

ГMO = measured reflection coefficient of open

ГMS = measured reflection coefficient of short

ГML = measured reflection coefficient of load

EDF – directivity error (directivity of directional coupler)

ESF – source match error (mismatch at VNA DUT port)

ERF – reflection tracking error (mismatch between reference path and reflected path)

Generalized three-term solution for VNA one port open-short-load calibration [1]

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Calibration results, error terms!

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Let’s try measuring the barrel again..

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Results: the reflectometer works reasonably well!

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Future work: two port measurements, extended frequency range..

Future plans for the network analyzer include:● two port measurements

● extending operation out to 14 GHz

● increasing the sweep speed.. (currently about 100 ms/point)

● replacing off-the-shelf couplers and splitters with distributed designs

● reducing the bill of materials cost, de-modularizing design.

Multiplier and switches to increase frequencyrange to 14 GHz

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Conclusion: the reflectometer works out to 10 GHz!

● The vector reflectometer that works reasonably from 2 GHz to 10 GHz

● The design should scale to two port measurements, work is in progress.

● Total prototype cost is ~$1200 for a two port VNA, next revision could be about half that price.

Source code and design files are available at:

https://github.com/loxodes/vnaThe vector reflectometer prototypeneeds an enclosure..

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Bibliography

[1] Joel P. Dunsmore (7 August 2012). Handbook of Microwave Component Measurements: with Advanced VNA Techniques. Wiley.

[2] David M. Pozar (5 February 2004). Microwave Engineering. Wiley.

[3] Henrik Forsten (2 August 2016). Cheap homemade 30 MHz - 6 GHz vector network analyzer. http://hforsten.com/cheap-homemade-30-mhz-6-ghz-vector-network-analyzer.html