Designing a 2 GHz to 10 GHz Vector Reflectometer. Jonathan Klein, University of Alaska, Fairbanks

Transcription

1 Designing a 2 GHz to 10 GHz Vector Reflectometer Jonathan Klein, University of Alaska, Fairbanks

2 A vector reflectometer measures reflection as a function of frequency. a1 Z0 A vector reflectometer measures the ratio between an injected and reflected signal. I1 b1 V1 VS Vector (magnitude and phase) measurements help extract phase shift and improve calibration over scalar measurements. Measuring this across a range of frequencies is useful. 2 a1 = Vs 2 Z0 S11 = b1 a1 b1 = V s (Z L Z 0 ) 2 Z 0 (Z L +Z 0) ZL

3 Reflection measurements are useful for microwave design.. Sometimes you want low reflections (matched impedance). And sometimes you want high reflection (filtering). 3

4 I m building one because microwave equipment is big and/or expensive.. Copper Mountain R140, 85 MHz to 14 GHz, $ Keysight N9916A FieldFox Handheld Microwave Analyzer, 14 GHz, $16370 HP 8510B, 45 MHz to 26.5 GHz Network Analyzer $ $300 shipping (used, GREAT condition, missing sweeper source) 4

5 Block diagram of reflectometer LMX2592 Synthesizer Filter Bank HMC424 Variable Attenuator HMC311 Amplifier Narda 30357A, 30 db Directional Coupler RF Synth HP 87300B, 10 db Directional Coupler -10 db LO Synth (RF MHz) AD9864 IF Digitizer LTC5548 Mixer -10 db ADC 1 ADC 2 5 Device Under Test

6 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. 6 LMX2592 Synthesizer HMC321, filters, HMC321 Filter Bank HMC424 Variable Attenuator

7 RF Design I/Q demodulators 2-10 GHz goes in, 29 ks/s goes out I/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: LTC5548 Mixer AD9864 ADC IF in 7 RF in LO in

8 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 diagram 8 Figure 2: LTC5548 block diagram

9 RF Design other modules above: HMC311 amp left: directional couplers below: power splitter for LO distribution 9 above: IF and ADC clock distribution board left: synthesizer clock splitter board

10 Digital design TI BeagleBone Black The VNA is run on a TI Sitara AM3358 ARM Cortex-A8 processor, with dual 200 MHz MIPS coprocessors. 4x headers for demodulator boards 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. 10 Custom cape for BeagleBone black development board 2x headers for synthesizer boards VNA controlled over Ethernet

11 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 11 Layout of synthesizer PCB in KiCad

12 Initial measurements SF-SF50+ SMA barrel looks lumpy? Left: Mini-Circuits SF-SF50+ SMA barrel 12

13 The reference path is different from the reflected path, we need calibration! LMX2592 Synthesizer Filter Bank HMC424 Variable Attenuator HMC311 Amplifier Narda 30357A, 30 db Directional Coupler RF Synth HP 87300B, 10 db Directional Coupler -10 db LO Synth (RF MHz) AD9864 IF Digitizer LTC5548 Mixer -10 db ADC 1 ADC 2 13 Device Under Test

14 Calibration is important for vector network analyzers/reflectometers! One port calibration is possible by measuring three known standards (commonly short, open, and load) then using them one-port VNA error model to solve the error terms. a1m 1 a1a EDF - directivity error ESF source match error ERF reflection tracking error EDF ESF b1m ERF b1m a1m b1a = Equations from Handbook of Microwave Component Measurements: with Advanced VNA Techniques. [1] 14 S11A

15 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 Kit 15

16 Calibration standards are not ideal, so we apply a generalized solution Generalized three-term solution for VNA one port open-short-load calibration [1] 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) 16

17 Calibration results, error terms! 17

18 Let s try measuring the barrel again.. 18

19 Results: the reflectometer works reasonably well! 19

20 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 frequency range to 14 GHz 20

21 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: 21 The vector reflectometer prototype needs an enclosure..

22 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. 22