CONSIDERATIONS ON THE FRONT- END READOUT FOR BOLOMETERS

Transcription

1 ONSIDEATIONS ON THE FONT- END EADOUT FO OLOMETES LAUDIO ANAOLDI GIANLUIGI ESSINA STEFANO IO (Luca s irthday) 1

2 The Nise surces that affect the S/N f a blmeter: blmeter intrinsic nise Detectr e A L i i Fr i Mi i L i A - The intrinsic nise surce f the blmeter cntribute t the energy reslutin: i = 4K T The pwer spectrum f this nise surce has a white cmpnent, as well as a lw frequency cmpnent.

3 The Nise surces that affect the S/N f a blmeter: lad resistr nise (1) Detectr e A L i i i Fr Mi L i i A - The lad resistr may cntribute t the parallel nise in a appreciable way: L i = 4K L T L The pwer spectrum f this nise surce has a white cmpnent, as well as a lw frequency cmpnent that can be made negligible by design (*). (*):.Arnabldi, et all, IEEE TNS,.49, p (00). 3

4 The Nise surces that affect the S/N f a blmeter: lad resistr nise () The nise cntributin f the lad resistrs is evident. The peratin at 100 K allws t save a factr f almst in energy reslutin. The technlgies (metal film) limits the value f the resistrs t nly 1 GΩ at 10 mk. UOIINO has 60 GΩ at rm temperature fr the warm channels and 60 GΩ at lw temperature fr the cld channels. Tem L The nise is in: 300 fa 100 Hz 10 m 60 G NA L i = 4K L T L 30 G NA 1 G M lmeter i = 4K T 4

5 The Nise surces that affect the S/N f a blmeter: preamplifier parallel nise (1) Detectr e A L i i i Fr Mi L i i A - The gate current cntributes t the parallel nise at rm temperature: A i = qi G The pwer spectrum f this nise surce has a white cmpnent that becmes negligible at temperatures arund 100 K(*). (*):.Arnabldi, et all, IEEE TNS,.51, p.975 (004) 5

6 The Nise surces that affect the S/N f a blmeter: preamplifier parallel nise () arallel nise f the preamplifier gate current is ttally negligible at 100 K. UOIINO has rm temperature perated as well as cld temperature perated preamplifiers. The nise is in: fa Hz Tem m A i = qi G I G Nise * 0.5 p 0.1 <<5 f <<0.01 NA NA 100 M lmeter i = 4K T * This nise is calculated fr the differential cnfiguratin. 6

7 The Nise surces that affect the S/N f a blmeter: preamplifier series nise Detectr e A L i i i Fr Mi L i i A - Series preamplifier nise may have a white and 1/f cmpnent. In ur frnt-end the 1/f cmpnent is negligible(*). Series preamplifier nise is almst negligible in ur experiment, unless the detectr shunting capacitance becmes s large. reamplifier nise e A 3 n Hz Detectr Nise = 4K T = 10 mk Detectr ltage nise = 100 MΩ T 7 n Hz reamplifier nise is cmparable nly t the intrinsic nise f the blmeter. (*):.Arnabldi, et all, IEEE TNS,.49, p. 440 (00). 7

8 eslutin Summary S far we have cmpared the nise effect f the electrnic and lad resistrs with the intrinsic detectr nise. At a first glance it seems that the parallel nise is the parameter t be taken int cnsideratin. It is interesting nw t try t estimate the energy reslutin frm the estimatins cnsidered. The cmparisn between the estimatin and the actual energy reslutin allws t understand if the nise surces cnsidered s far are the nly nes t be taken int accunt. 8

9 The best estimatr (1) At the preamplifier utput we have the signal (t)=us(t), where s(t) is the transfer functin. The utput nise is given by the spectrum N(ω) ((t)=us(t)n(t)). We wuld like t find the ptimal weighting functin, r estimatr fr any time t. Let s call w(t) this functin, able t give the best weight in the interval dt arund t. We gt: U Est = Us(t) 0 [ w(t) α] dt with α the nrmalizing factr. Frm the errr thery we knw that the ptimal weighting functin is the ne the equal the rati f the signal abve the fluctuatin at the time t. Unfrtunately it is nt simple t perfrm the evaluatin in time dmain. It is better t mve t the frequency dmain: 9

10 The best estimatr () If s(t) S(ω) and p(t) (ω) are the Furier transfrm pairs. Frm the arseval therem we have: U Est 1 [ ] [ * ] Us(t) w(t) α dt = US( ω) W ( ω) α = dω π 0 Frm the definitin given t w(t) we cnsider fr W(ω): Frm the abve psitin we get: We have: α = 1 π S( ω) N( ω) 1 U dω Est = = π 0 1 π 1 S( ω) W( ω) = α N( ω) S( ω) dω N ( ω) M * S ( ω) US( ω) α dω N( ω) (N M (ω)=mnlateral pwer spectrum.) 10

11 Our weight is then: U Est 0 The best estimatr (3) 1 [ ] [ * ] Us(t) w(t) α dt = US( ω) W ( ω) α = dω π 0 Frm the definitin given t w(t) we cnsider fr W(ω): Frm the abve psitin we get: Namely: σ σ 1 = π = α = σ 1 π ( ω) 1 W( ω) α S( ω) N( ω) dω = 1 dω = π π 1 S( ω) dω N ( ω) M S( ω) N( ω) N( ω) S( ω) W( ω) = N( ω) 1 π 1 π 1 S( ω) N( ω) (N M (ω)=mnlateral pwer spectrum.) 1 S( ω) N( ω) dω dω dω 11

12 The best estimatr (4) In first apprximatin we can cnsider the signal vltage at the preamplifier utput given by: v(t) = U K The ttal parallel nise is: v e t τ ia tt = il i i 4 ( ω) = U K Therefre, frm the ptimal estimatr we can calculate: v τ 1 sτ, s = jω Where: U FWHM =.355 K v e A Z τ i tt 1 ( ) τ A Z e A e Z A i tt =parasitic detectr shunting capacitance; A =preamplifier pen lp input capacitance; Z = dynamic detectr impedance; K v = energy cnversin gain, usually expressed in µ/me. 1

13 The best estimatr (5) U FWHM =.355 K v e A Z τ i tt 1 ( ) τ A Z e A e Z A i tt Frm the expressin abve a number f cnsideratins can be made: arallel nise effect is independent frm the value f the detectr shunting capacitance; Series preamplifier nise increase its effect n the nise perfrmances at large detectr shunting capacitances. This effect is weighted by the rati f the electrical time cnstant abve the signal rll-ff time cnstant, and by the rati f the parallel nise cntributin abve the series nise. 13

14 Energy reslutin frm the reamplifier and lmeter There is a large dispersin in the characteristics and the energy reslutin f the detectrs f UOIINO. T make sme evaluatin let s cnsider 3 representative detectrs having different range in the impedance and energy cnversin gain. τ is put 300 ms. Detectr name Static Impedanc e (MΩ) Dynamic Imp./ Static Imp. Z / Emergy mv. Gain. µ/me eslutin at 615 Ke FWHM Warm elec. Energy eslutin e FWHM ld elec. Energy eslutin e FWHM lm. Energy eslutin e FWHM We have a large uncertainty in the value f the dynamic impedance at the selected perating pint. 14

15 ery frnt-end rm temperature r cld perated In the previus table the very frnt-end nise as well as the detectr nise has been simulated using the frmula f the ptimum filter fund befre: U FWHM =.355 K v e A Z τ i tt 1 ( ) τ A Z e A e Z A i tt The parameters used fr bth the preamplifier and the detectr are in the phtgraph f the MATLA windw. /300 /

16 Actual detectr perfrmances (1) As can be seen there is a great dispersin in the energy cnversin gain and the value f the detectr static and dynamic impedance, t which the parallel nise is prprtinal. 5 Number f channels E E07 1.0E E08.00E08.40E08.80E08 3.0E E E E E08 5.0E08 lmeter static impedance 16

17 As a cnsequence f the spread in the perfrmances, the energy reslutin spreads ver a large range. ery imprtant: The reslutin at large energies is much wrst, even a factr larger than 5, with respect t small energies. Actual detectr perfrmances () 17

18 Actual detectr perfrmances (3) Unfrtunately fr this presentatin, the analysis f the small energy regin f the spectrum is in prgress and it is nt pssible t investigate any pssible cntributin f the electrnic t the reslutin. What we knw fr the mment is that the average reslutin bserved at lw energies fr the crystals 5x5x5 cm 3 is abut.8 Ke, while fr the crystals 3x3x6 cm Ke. The energy reslutin bserved is much wrst than the detectr intrinsic energy reslutin calculated, but it results als wrst than the limit impsed by the very frnt-end. Fr sure there is anther/ther surce/surces f nise that enter int play in determining the final perfrmance. 18

19 An extra surce f nise There is a surce f nise that is respnsible f the final energy reslutin f UOIINO: Micrphnism 19

20 The last imprtant surce f nise: micrphnism (1) It must be remarked that micrphnic nise that cmes frm mechanical vibratins is affecting the reslutin in ways. The first way is the frictin f the detectr crystal with respect t its mechanical supprts. The frictin creates heat that is detected as nise. Detectr e A L i i i Fr Mi L i i A - HEAT SINK O There is n way t slve the nise prblem cming frm frictin ther than t design an apprpriate detectr supprt: we say that this nise is differential (*). The develpment f a new detectr supprt is in prgress. HEAT SINK (*) G.essina, Supercnducting detectrs and blmeters: achievements and perspectives fr applicatins", 0-1 Nvembre 000, Napli, invited. 0

21 The last imprtant surce f nise: micrphnism () The secnd way micrphnic nise cmes int play is due t the vibratins f the cnnecting link, which mdifies the parasitic capacitance, hence the charge stred n it. Differently that frm the detectr frictin, the vibratin f the link is a cmmn mde nise that can be greatly attenuated by prperly cnnectin apprach. Detectr e A L i i i Fr Mi L i i A - Let s make a step back t understand this prcess. 1

22 Micrphnism: discussin (1) In a classical apprach the detectr signal is readut with a single sided input preamplifier. One leg f the detectr is cnnected t grund, the ther t the readut. The vibratin f the wire cnnected t the preamplifier determines a change f the parasitic capacitance. Since is biased there is a change that is prprtinal t bth the vibrating amplitude and the initial charge stred n it. O cs( ω O t) With large crystal this surce f nise, tgether t the frictin becme the main limit t the energy reslutin.

23 Micrphnism: discussin () If we readut the detectr signal with a differential preamplifier and use a twisted cable the nise generated in the link is the same in bth the cnnecting wires. This disturbance is cancelled at the utput f the differential preamplifier. Althugh the MS nise at the preamplifier is larger the suppressin effect allws t btain extremely gd results. c c - Frictin nise: nt suppressed. Link ibratin: largely suppressed. The residual micrphnism nise f the cnnecting link is given by any pssible nn-unifrmity f the cnnecting link itself. The suppressin efficiency can range between 40 d and 60 d. 3

24 Micrphnism: discussin (3) ery is anther big advantage in using the differential cnfiguratin with a detectr array that cncern crss-talk. With a single sided input preamplifier the cupling between tw clsed detectr may rise the crss-talk that is prprtinal t the parasitic cupling capacitance,. H1 IF THE DETETO IMEDANE IS LAGE ENOUGH: i i1 H 4

25 Micrphnism: discussin (4) The differential preamplifier tgether with the twisted link allws t make the crss-talk signal between tw adjacent detectr a cmmn mde signal that cancel ut at the preamplifier utput. c H1 c - rss-talk highly suppressed. H c c - 5

26 Micrphnism: discussin (5) Let s try t quantify. Suppse that the mechanical vibratins induce a change f the parasitic capacitance f the frm: = [ γimi sin( ωit) ] i We can express the change f the charge n by: Q = I es The current in is: I e s = dq dt I I D /Z es Or, in the frequency dmain: I es (s) = s (s) s (s), s = jω I s i = 1 s = = I es s ( ) i ( ) si s 6

27 7 Micrphnism: discussin (6) Micrphnism: discussin (6) es I I D = i s I s 1 s 1 s ( ) ( ) i i es i s s 1 s s I s I = = = ( ) i 1 ω ω =

28 Micrphnism: discussin (7) If: = [ γi Mi sin( ωit) ] i Then (un-crrelatin): i 1 ω i ( ω ) i i _ i I I D es 8

29 Micrphnism: discussin (8) T simplify cnsider nly a mnchrmatic disturbance: = γ Msin( ω t) S: 1 ω ( ) ω 1. The nise is prprtinal t the fractinal change f the parasitic capacitance;. The nise is prprtinal t the bias level f the detectr; 3. The nise depends n the amunt f the parasitic impedance at the frequency f the disturbance. 9

30 Micrphnism: discussin (9) Nw we have t cnsider that we use the differential cnfiguratin with bth the kind f elctrnics: 1 ( / ) ( ω ( / ) ) ω c c ω( / ) 1 ( ω ( / ) ) - 1 ( / ) ( ω ( / ) ) ω α ( 1 α) The effect f the differential cnfiguratin is given by the term α that in an ideal situatin shuld be equal t 1, but in the real case is slightly less than 1. 30

31 Micrphnism: discussin (9) T simplify cnsider nly a mnchrmatic disturbance: ( / ) ( ω ( / ) ) ω 1 1 ( α) S: ( /)(MΩ) (pf) f ( / ) ( ω ( / ) ) ω

32 ld Electrnic vs m temperature electrnic (1) Detectr ld stage m temperature A 1 - Fridge apacitance sensitive t micrphnic nise: A. A, = capacitance at cld; = capacitance at rm T. A A Detectr apacitance sensitive t micrphnic nise: A. m temperature A - Fridge 3

33 ld Electrnic vs m temperature electrnic () ld Electrnic 1. The nise f the lad resistrs is small;. arallel nise f the preamplifier is ttally negligible; 3. Series preamplifier nise is cmparable with that at rm temperature; 4. Nise due t frictin f the crystal is nt suppressed; 5. It is differential allwing t suppress micrphnic nise f the cnnecting link; 6. The parasitic capacitance is small and the residual vibrating nise is subject t strng attenuatin; 7. The parasitic capacitance is small, signal integratin is minimum and the series preamplifier nise has small effects. m temperature Electrnic 1. The nise f the lad resistr is larger a factr f ;. The nise f the preamplifier is small, but nt negligible; 3. Series preamplifier nise is cmparable with that at rm temperature; 4. Nise due t frictin f the crystal is nt suppressed; 5. It is differential allwing t suppress micrphnic nise f the cnnecting link; 6. The parasitic capacitance is large and the residual vibrating nise is larger a factr f almst ; 7. The parasitic capacitance is maximum, signal integratin is maximum and the series preamplifier nise may have nn negligible effects. 33

34 ld Electrnic vs m temperature electrnic (3) ld Electrnic Lad resistr effect at cld m temperature Electrnic 0.5 x (Lad resistr effect at rm T.) reamplifier effect at cld 0.5 x (reamplifier effect at rm T.) Link vibratin effect at cld 0.5 x (Link vibratin effect at rm T.) rystal frictin: nt affected = rystal frictin: nt affected 34

35 nclusins (1) The cmparisn between the expected energy reslutin and the measured ne indicates that, at the mment, the main surce f nise is micrphnism. Micrphnism acts n tw independent channels: the crystal and the thermistr cnnecting wires. It is very difficult t determine which channel is the dminant (an hazardus is t say that since the detectr with cld and warm electrnic in UOIINO have cmparable reslutin, the main surce is crystal vibratin). A study is in prgress, with impressive results, t imprve the vibratin rejectin cming frm the crystal supprt. We expect that micrphnic nise culd imprve much in UOE. 35

36 nclusins () If we suppse the micrphnism nise cming frm mechanical supprts will be reduced, the cld stage may allw a factr f tw imprvement in cntributin t the reslutin that cmes frm the electrnic. At present at large energies, the energy reslutin wrsens a factr f abut 5 with respect t the lw energy regin. This effect have different interpretatins: residual drift, psitin dependence f the interactin pint in the crystal, etc. If the interest f UOE is nly in ββ decay, there is negligible advantage in using the cld electrnic, if we believe that the limit in the reslutin will remain the present ne. In this scenari nly the lw energy regin f the spectrum may take advantage frm the cld electrnic. 36

37 nclusins (3) A prgress has been made in the cld electrnic study. With respect t the characteristics declared in the UOE prpsal (*), very imprtant parameters has been imprved: The nise at lw frequency is cmparable t the white nise. It was imprved by a factr f better than 4. wer dissipatin needed t bias 1000 cld differential JFET buffers is nw between 10 mw and 0 mw (the pwer that is nw needed t run the 1 channels f UOIINO). (*).Arnabldi, et all, Nuc. Instr. and Meth. in hys. es. A,. A518, p.775,

38 nclusins (4) st increase f the cld electrnic JFETs OADS (special assembly) Metallic OXES Ttal 3 K 5 K 7 Ke 35 K 38

39 nclusins (5) eliability (1): JFETs T find a failure in a cld stage we have t cnsider the activity f Hall A, Hall and MIETA in Milan ver almst 0 year. We have fund nly ne JFET brken in MIETA after a wild test made t study the cnnecting link. N failure has ever been bserved during a run. 39

40 nclusins (6) eliability (): wiring In the first run f UOIINO we had many failures in the cnnecting link at the start-up and after many, necessary, heat-up f the fridge. After this negative experience we have studied accurately the system cnnectin btaining excellent results: in the secnd run f UOIINO n failure has been bserved after the start-up and the many system heat-up. Wrk is in prgress in the study f a new cnnecting link that will give even mre degree f reliability. 40