How NOT to make microstrip loss measurements

1. How NOT to make microstrip loss measurements Characterizing losses with very crude scale models

2. Outline • Need and design of a loss experiment • Implementing the design very crudely • Improvements on original set-up • Results from successive designs

3. The need to characterize losses in microstrips • Needed as conduction lines from antennae to detectors • Observation bands very broad – need to know frequency response of losses • Feasibility of using microstrips as conduction lines to be tested

4. Microstrip circuit layer 0.3 mm Substrate 1.0 mm Ground plane 0.3 mm Supporting layer ~ few mils Designing a loss experiment: Start with the crudest possible scale model (x10) working around 10 GHz: • Shine microwaves in the X-band (8-12 GHz) on one end of a copper wire, and detect at the other end. • Use the exact setup as the cryogenic W-band circuit, all dimensions scaled x10:

5. Crude scale model design: Several different lengths of microstrip needed for loss/wavelength measurement • Achieve this with meanders Network of meandering conduction striplines needed in final design • Possibility of characterization of losses due to meanders in crude model I

6. Implementing the design Choice of material: • Circuit board for ground plane and support layer • Copper layer on circuit board (~15 mm) • Mylar gauge-10 ( 0.1 mil or ~3 mm) • Copper wire (3 mils) • Use spray-glue to attach board, mylar and wire – difference in thickness < 1 mm

7. Set-up Slot antennas Cut in Ground plane (on other side) Copper ground plane Copper wire Pc board Dielectric (substrate) X-band waveguides (To network analyzer) Mylar (dielectric)

8. Crude Model I Slots Cu wire meanders Mylar sheet on Cu

9. Set-up in the microwave lab Network Analyzer Metal side of circuit faces waveguides Waveguides held in place manually

10. Model I and II Results

11. Results from Crude Model I Spectacular Failure! • Signal from microstrip indistinguishable from noise • Received power goes up with waveguides in air (!!), implying that: • Microstrip set-up is suppressing power!!

12. Reasons for failure • Near-complete mismatch in impedances • Not careful about maxima of field at waveguide-microstrip transition • Waveguide and ground plane shorting? Possible Solutions: • Cut waveguide to “resonant length” • Remove metal at waveguide-microstrip transition Fails again! – Signal still suppressed by microstrip set-up

13. Crude Model II Need a resonant mechanism at transition perhaps? • Most natural resonance: slot in ground plane • Dimensions of slot? • Length fixed by resonant frequency • Width < length/2 for effective resonance/matching • Ground plane, substrate as before • Try shining microwaves from the dielectric side of the setup, to reduce radiative parasitics

14. Model II: set-up Slot antennas Cut in Ground plane Copper wire Mylar (dielectric) Copper Ground plane Pc board Dielectric (substrate) X-band waveguides (To network analyzer)

15. Crude Model II Replica of Model I, with different slot separations Substrate side Metal side

16. Set-up Substrate side faces waveguides

17. Crude Model II results • Signal from stripline > waveguides in air by upto 5 dB • Signal > Noise – phew! • Difference between signals from different lengths not significant (~1 dB) • Need better resonanating slot/less mismatch • Signals more stable and REPEATABLE (to ~2 dB)\ • Stability achieved by using dielectric side of the setup

18. What next? • Can tune wg-stripline transition better – extend microstrip beyond slot by l/4 for maxima at transition • Need to establish consistency and repeatability – measurements should not change significantly with weather! • Set-up clumsy and time-consuming – do away with spray glue? • Another consistency check – reflection measurements

19. Microstrip extended by l/4 Model III

20. 5 dB / Ref –35 dB 10” 5 dB / Ref –35 dB 8” 10 dB / Ref –50 dB 12” 5dB / Ref –25 dB Model III Results

21. Dielectric side Metal side slots Tape to hold down Cu wire

22. Model III: Final Answer? • Signal stable to ~1dB, but not absolutely repeatable (repeatable to ~ 1 dB) • New discovery: quality of SMA connector transitions affect signal to noise and signal stability, as well as repeatability • What about wg-to-wg crosstalk across circuit? • Do we understand all the “resonances”? • Numerous resonances in reflection measurements – mechanism and effect on transmission?

23. Trivialities: get new SMA connectors, handle carefully, stabilize and make setup hands-free Need to characterize ALL transitions: 1. SMA / cable to VNA / waveguide 2. Waveguide to slot 3. Slot to microstrip Need to eliminate or place limits on waveguide crosstalk Need to have slots away from board edge Forget meanders for now – work with straight length differences More improvements - Onto Model IV

24. Model IV setup Eccosorb inserted here Waveguides Waveguide holders Clamps for increased stability

25. Characterizing transitions and crosstalk • Attempt to characterize ONLY ONE effect at a time • Start with non-clad substrate board measurements • Add ONE component at a time: ground plane, slot, substrate and microstrip, in that order • Check for repeatability regularly • Use eccosorb for minimizing crosstalk if characterization is complicated

26. Base –14 dB 1 dB/div Model IV Results: S11

27. Base –40 dB 5 dB/div S21 from 2”, with & without eccosorb

28. 2 dB/Ref –66dB Red: w/ Purple: w/o ecc, w/wire 10dB/Ref –90dB Red: w/ Purple: w/o ecc, w/o wire S21 from 8”, with & without eccosorb/wire

29. S21: comparison across lengths

30. Results from Model IV • All measurements now repeatable • Significant difference between signal and noise • Noticeable difference between different lengths of microstrip – enough to crudely estimate loss per unit wavelength!! • One extra resonance still present.. • Problems with higher frequency (11-12 GHz) persist – can live with it

31. Conclusion from scale model tests • Successfully demonstrated ability to estimate losses in microstrips • Sensible to repeat this effort in W-band at 4K • Higher frequencies a problem – fortunately, cut-off frequency known • Characterization necessary for all transitions?

32. Preparation for loss tests at 100 GHz • Need to scale all lengths down by a factor of 10 • Need the mechanism to fit inside available dewar • Need to be able to control relative movement of circuit and waveguides repeatably from outside the dewar • Need to use small waveguide section to reduce losses • Have all parts ready except rotary feed-through mechanism • VNA calibrated successfully with waveguide parts for the W-band

33. Schematics of the loss test Parts to be added to dewar for loss test Waveguide sections to be added here Waveguide bends Second L-bracket set Brass waveguide section Circuit holder mechanism Supporting L-brackets Dewar cold plate

34. Preparation for loss tests at 100 GHz + Brass waveguide section Microstrip circuit fits in here Main L-bracket Circuit holder Circuit holder slides through here Supporting brackets with wg section

35. Preparation for loss tests at 100 GHz Waveguide bends to be attached here Entire structure mounted on cold plate of dewar

36. Lessons from the exercise • Do not place too much trust in instruments – do not treat as black-boxes • Attempt to characterize every instrument, irrespective of how “nice” it seems – they’re ALL mischievous • New Murphy’s Law: If something cannot go wrong, stars will misalign themselves to make it possible: 1. Sparks fell on our improved models 2. Precious mylar got lost in random lab-shift 3. What next – black hole gobbles up circuit?

37. Credits • Startegy thought out by Dan Van der Weide, Peter Timbie, Shafinaz Ali and Rashmi Pathak • Very useful help, suggestions and explanations from Rashmi Pathak • Occasional grumble, ramble and random disruptions by Siddharth