Development of a Thermal Voltage Converter with Calculable High-Frequency Characteristics

Transcription

1 Development of a Thermal Voltage Converter with Calculable High-Frequency Characteristics Thermal Converter Development Team (Hitoshi Sasaki, Naoko Kasai, Akira Shoji, Hiroyuki Fujiki, Hidetoshi Nakano / AIST) (Koji Shimizume, Kaname Kishino, Shigeru Hidaka / NIKKOHM) High-frequency characteristic of a TVC-input circuit becomes dominant factor in the AC-DC transfer difference at frequencies higher than 00 khz. In order to evaluate the high-frequency characteristics of TC modules up to MHz, a High-Frequency-standard TVC (HF-TVC) has been developed at AIST, in collaboration with Nikkohm Co. Ltd. The HF-TVC is designed such that the frequency characteristic is calculable from the shape and dimension of the input circuit. The thermal converter element (JSTC04) has a simple straight heater configuration, which reduces the parasitic impedance in the heater pattern. The Built-In TEE (Internal TEE) configuration has been used to avoid the effects from parasitic impedance in the input leads.. Type-JSTC04 TC Element Various types of MJTCs have been developed by national standard laboratories to establish primary reference in AC-DC transfer standard. At the AIST, in cooperation with Nikkohm Co., a new thin-film MJTC have been developed as the core component of the ET00 AC-DC transfer standard system. As the thermoelectric effect can be evaluated experimentally by the FRDC method, emphasis in the design criteria was placed to the optimization of high-frequency characteristics, rather than reduction of thermoelectric effects. The structure and appearance of the new MJTC (Type JSTC04B) are shown in Fig. and Fig., respectively. The heater is sputtered on a mm x 8 mm AlN chip, mounted on a polyimide membrane with flip-chip bonding. The Bi/Sb thermocouples are formed on the polyimide membrane supported by an alumina frame as a heat sink. The basic design is the same as the commercial RMS-DC converter device (Nikkohm LP-35), except that the position of the thermocouples are separated from the heater, in order to reduce inputoutput coupling due to stray capacitance. Input-Hi pattern has guard-electrode, which reduces the effect of stray capacitance between the heater and electrode, as described in the next section. Input-Lo pattern has two connecting terminals to realize coplanar return-path to the built-in TEE with minimum lead inductance. By the use of 64 pairs of Bi/Sb thermocouple, sensitivity of 0.6 mv/mw is achieved. In the case a nominal 5V MJTC element operating at 0 ma, the output EMF voltage of 30 mv is obtained. To increase signal-tonoise ration, the nominal output resistance of the MJTC is reduced to 400 Ω from a few kω of conventional design. Thermal response time is adjusted to about 3 sec by choosing proper ratio for heat capacity of AlN chip to thermal conductance across the thermocouples. Due to the good thermal conductivity of AlN substrate, low-frequency dependence is suppressed to less than a few ppm down to 0 Hz. Fig.. JSTC04 Thermal Converter Element. Built-in TEE configuration A picture and a cross-sectional view of a HF-TVC are shown in Fig. 3 and Fig. 4, respectively. The AC-DC difference of a thermal converter is defined at the center-point of a TEE connector, and hence a built-in TEE configuration is used in order to take full advantage of the high frequency performance of the JSTC04 MJTC element. The MJTC element is mounted on a PCB, which connects both Hi and Lo input leads of the MJTC with shortest distance to the reference plane of builtin TEE. Fig. 3 Picture of a HF-TVC. The SMA connector is on the bottom side. To AC/DC Source (SMA Connector) TC element (Type-JSTC04) To UUT (Type N-P) Reference Plane Output Connector Fig. 4. Built-in TEE configuration of HF-TVC. TC element is rotated 90 degree from the actual position. 3. Mathematical Modeling Fig. Structure of JSTC04 Thermal Converter Element - /6 -

2 The frequency characteristic of the HF-TVC is determined by parasitic components in the input circuit, such as lead inductance and stray capacitance, skin-effect, and dielectric loss of the heater. The values of these parasitic components were evaluated using a mathematical modeling of the HF-TVC []. The circuit model of the HF-TVC with heater resistance r is shown in Fig.4. L p and R sk are the parasitic inductance and skin-effect of the leads from the virtual TEE to the heater. C p, C p and R d represent the stray capacitances and dielectric loss between the two electrodes of the heater. between 0kHz to MHz. The actual circuit configuration inside the HF- TVC, and corresponding circuit model is shown in Fig. 5. IN-a (N-P) Chassis IN-b (SMA) PCB Heater Thermopile C in Lp Rsk Cp r Rd C h-c r Ch-t R t Rt Fig. 5. Internal configuration and circuit model of HF-TVC. Fig. 4. Circuit Model of HF-TVC input circuit In addition to the four parasitic components of the input circuit, parasitic capacitance between the heater and the thermopile C h-t can be measured using the LCR meter. However, in the case of the HF-TVC, thermopile pattern is guarded by pattern of the heater, and hence the contribution from the parasitic capacitance is reduced to negligible level. Contribution of the five parasitic components to the ac-dc difference at high frequency range (0 khz MHz) is calculated as; γ γ LR +γ LC +γ CR +γ sk +γ loss γ LR = ( ωl p r) 4. Measurement of L p, R sk To measure the inductance of the lead L p and R sk, the normal JSTC04 element was replaced with a special "SHORT" element. In this element, the cupper film is deposited onto the heater pattern, and has small resistance of about 00 mω. The lead inductance and the skin effect was measured by "L s - R s " mode at 0 ma. The circuit connection inside the HF-TVC, and corresponding circuit model is shown in Fig. 6. γ LC = ω L p C p ( ) r γ CR = 3 ω ΔC γ sk = R sk ( ω) r () L p Rsk γ loss = G( ω)r Fig. 6. Internal connection and circuit model for "L s - R s " measurement. C p C p + C p ΔC C p C p G( ω) R d Here γ represents the frequency dependence of the ac-dc difference of a HF-TVC. The first two terms γ LR and γ sk represents the contribution from the parasitic inductance and skin effect of the lead. These are the dominant components for the low-resistance TVC. The terms γ CR and γ loss represents the effect of stray capacitance and dielectric loss of the heater, which is dominant for the TVCs with higher heater resistance. Since the terms γ CR goes to zero for symmetric case (C p =C p ), the parameter ΔC gives the measure of the contribution from the stray capacitance to the ac-dc difference. The last term γ LC represents the LC resonance due to the parasitic inductance and the stray capacitance, which is independent of the resistance value. Another source of uncertainty arises from the use of N-P plug, to which a test-tvc is connected. As described in section, the N-P plug imitates the half part of the N- RRR tee connector by which the reference plane is defined. Hence the difference in the shape and dimension of the N-P plug and the N- RRR tee connector must be taken as a source of uncertainty in the calibration using the HF-TVC. The uncertainty due to the use of N-P connector may be evaluated experimentally, by adding an extra N-PR connector between HF-TVC and test TVC. 4. Evaluation of parasitic components using LCR meter Some of the parasitic components (L p, R sk, C p, R d ) of the HF-TVC were measured using an Agilent 484A LCR meter in the frequency range The results of the measurement for the two configurations are shown in Fig. 7. The parasitic inductance L p and the change in parasitic resistance R sk were determined to be; Inductance / Resistance L R P sk = 8.7 [nh] = 0.5 f [MHz] f [MHz] Lp(nH) drs(mω) fitting [mω] Frequency (khz) Fig. 7. Result of "L s - R s " measurement for HF-TVC. 4.. Measurement of C p, R d To measure the stray capacitance of the heater C p and dielectric loss () - /6 -

3 component G, the normal JSTC04 element was replaced with a special "OPEN" element without heater pattern. The stray capacitance and dielectric loss were measured by "C p - G" mode at 5 V. The circuit connection inside the HF-TVC, and corresponding circuit model is shown in Fig. 8. Fig. 8. Internal connection and circuit model for "C p - G" measurement. The results of the measurement for the two configurations are shown in Fig. 9. The stray capacitance of the heater C p and dielectric loss component G were evaluated to be; Capasitance / Loss C P = 0.44 [pf] G = 4.5 f [MHz] 8.7 f [MHz] Cp(pF) dg(ns)/0 fitting Cin L p [ns] Frequency (Hz) Fig. 9. Result of "C p - G" measurement for HF-TVC. 4.3 Measurement of C h-t The parasitic capacitance between the heater and the thermopile C h-t was also measured with the "OPEN" element and "C p - G" mode. The circuit connection inside the HF-TVC, and corresponding circuit model is shown in Fig. 0. From the measurement, the capacitance was evaluated to be 83 pf with parallel conductance much smaller than ns. In the case of JSTC04 TC element which have relatively low thermopile resistance (300Ω), the effect to the ac-dc difference at MHz is evaluated using the Eq.(4) to be smaller than 0. ppm at MHz. Rsk (3) C p R d Using the mathematical model and the evaluated values for the parasitic components, the frequency characteristic of the HF-TVC may be evaluated using eq.(). Considering the crudeness of the mathematical model into account, 00% uncertainties (uniform distribution) were assumed for the parasitic inductance L p, parasitic resistance R sk, and the stray capacitance of the heater C p were estimated to be less than twice of the measured value. The asymmetric components of the stray capacitance ΔC were assumed to be ΔC < C p. The dielectric loss component G was estimated to be / of the measured value, because it includes the contribution from the alumina frame which does not contribute to the ac-dc difference. 00% uncertainties (uniform distribution) were also assumed for the dielectric loss. The summary of the evaluated values for the parasitic components, contribution from the evaluated parameters to the ac-dc difference in the case of 500 ohminput HF-TVC are summarized in Table. The contribution from the evaluated parameters to the ac-dc difference in the case of 500 ohm-input HF-TVC is summarized in Table khz khz khz khz khz khz khz LR LC CR Skin Loss Min Max Ave Table Contribution of parameters to ac-dc difference (in ppm) From the results, the frequency dependence in the ac-dc difference of a 500Ω resistor is evaluated as; ( f ) 0 ±(σ ) 0 f or ( f )[ppm] 0 ±(σ ) f [MHz]. (5) The evaluated frequency characteristics of the ac-dc transfer difference for the 500Ω HF-TVC are shown in Fig.. P C ( f ) C rrt P0 = π (4) L p R sk R d C p Ch-t Rt Rt C in Fig. 0. Internal connection and circuit model for measuring input-output coupling. Fig. Estimated frequency characteristic of a 500Ω HF-TVC using LCR meter. 4.4 Summary of evaluation - 3/6 -

4 5. Evaluation using AC-DC difference measurement and series B. 5. Method for evaluation of parasitic components Using the mathematical modeling of the HF-TVC, it is possible to evaluate circuit parameters by least-square fitting of measured ac-dc difference to the theoretical equation. As the parameters R sk and G are both dependent on the frequency, they are expanded to the second order in ω as; ( ω) = αω + α ( ω) = β ω + β ω R sk ω (6) G Hence the over-all frequency characteristic, taking the r-dependence into account, may be determined by the seven parameters, i.e., L p, C p, ΔC, α, α, β, β. The contribution from parasitic capacitance C p, through the γ LC term, does not depend on the heater resistance and hence cannot be evaluated by the least square fitting. The contributions from the other six parameters to the ac-dc difference are summarized in Table 3. L p ΔC α α β β r ω Table 3. Contribution of fitting parameters to γ, dependence on r and ω. Hence, if we can change the resistance and frequency, and measure the relative ac-dc difference with sufficient accuracy, then the parameter can be determined by curve fitting of the data by the following equation.) HF-TVCs 50Ω (A) 50Ω (B) 00Ω (A) 00Ω (B) 00Ω (A) 00Ω (B) 500Ω (A) 500Ω (B) kω (A) kω (B) kω (A) kω (B) Reference TVCs 00Ω (S) 00Ω (S) 500Ω (S) kω (S) Fig.. Measurement Diagram The summary of the measured results are plotted in Fig.3, All the data points are the "relative" ac-dc differences Δγ meas (r,ω) with respect to the reference HF-TVC of 500 ohm, and calculated by taking the mean values between series A and B. The standard deviation of the measurement σ mes was evaluated to be 0.55 ppm. The best-fit curves are also plotted in Fig. 3. The procedure of the curve-fitting will be described in detail in the following section. δ FIT ( L p,δc,α,α,β,β ) = Here, Δγ cal Δγ meas N r N ω N r N Δγ cal r,ω ω I r = I ω = ( r,ω) = γ cal r,ω [ ( ) Δγ meas ( r,ω) ] ( ) γ cal ( r 0,ω) ( r,ω) = γ meas ( r,ω) γ meas ( r 0,ω) Here, the RMS value δ FIT is the measure of the discrepancy between the theoretical value Δ γ cal (r,ω) assumed set of parameters (L p, ΔC, α, α, β, β ) and measured data Δγ meas (r,ω), for all the combination of resistance and frequency. (7) 5. AC-DC Difference measurement Actual measurement was performed using the diagram as shown in Fig.. Six pairs of HF-TVC were prepared, corresponding to the six different heater resistance, i.e., 50Ω, 00Ω, 00Ω, 500Ω, kω, kω. The relative differences between the HF-TVC were measured using the four TC modules as the intermediate transfer standards. As seen from the diagram, the measurement procedure includes a number of "redundant" measurements. For example, 00 ohm HF-TVC and 00 ohm HF-TVC were compared relative to either 00-ohm TC module or 00 ohm TC module. The RSS of differences between the two results can be recognized as the measure of reliability in the measurement. Thus, the estimated standard deviation of the measurement σ mes was calculated by taking RMS for the results of all the closed-loop tests by the following equation. σ mes = () () [ Δγ meas ( r,ω) Δγ meas ( r,ω) ] (8) N Here, the additional factor (/) was introduced taking in to account that the final results will be obtained as the mean of results for series A 5.3 Least-square fitting Fig. 3. Measurement Results In the least-square fitting, the six parameters (L p, ΔC, α, α, β, β ) were scanned in a six-dimensional space. The scanning ranges (min - max) of the each parameter were taken such that all the combination of the parameters which satisfy the following condition should be included the scanning range. δ FIT ( L p,δc,α,α,β,β ) < σ mes (9) The best-fit combination gives the smallest value for δ FIT. [Determination of L p ] If we fix parasitic inductance L p and scan all the other five parameters, the minimal value of δ FIT (L p ) is obtained using eq.(4) as shown in Fig.4. Here, the horizontal axis represents possible values for L p, and the vertical axis represents minimal value of δ FIT for each value of L p. From the results of the evaluation shown in Fig.3, parasitic inductance L p is evaluated as; L p (σ-min) = 0 nh, L p (σ-min) = 0 nh, L p (best-fit) - 4/6 -

5 = 33 nh, L p (σ-max) = 54 nh, L p (σ-max) = 69 nh 6.0 RMS for Curve Fitting (ppm) Skin Effect (mω) s(min) s(min) BestFit s(max) s(max) LCR Frequency (khz) Parasitic Inductance (nh) Fig.6. Least-square fitting for skin effect R sk (ω) Fig. 4. Least-square fitting for L p [Determination of ΔC p ] Similarly, fixing parasitic capacitance ΔC p and scanning all the other five parameters, the minimal value of δ FIT (ΔC p ) is obtained as shown in Fig.5. Here, the horizontal axis represents possible values for ΔC p, and the vertical axis represents minimal value of δ fit for each value of ΔC p. From the results of the evaluation shown in Fig.5, parasitic inductance ΔC p is evaluated as; ΔC p (σ-min) = -.5 pf, ΔC p (σ-min) = -. pf, ΔC p (best-fit) = +/ 0.75 pf, ΔC p (σ-max) =. pf, ΔC p (σ-max) =.5 pf. [Determination of G] The dielectric loss G(ω) can also be evaluated in the same method. Fixing two parameters (β, β ) and scanning all the other four parameters, all the combinations which satisfies the conditions δ fit (β, β )< σ mes, δ fit (β, β )< σ mes are obtained. The estimated G(ω) for the obtained parameters (β, β ) is shown in Fig.7. Here, the horizontal axis represents the frequency, and the vertical axis represents the evaluated skin effect R sk (ω). The lines specified as -s and +s in the figure represents the min/max values of G(ω) which is consistent with the condition δ fit (α, α )< σ mes, and the lines specified as -s and +s represents the min/max values of G(ω) for the condition δ fit (α, α )< σ mes. RMS for Curve Fitting (ppm) Dielectric Loss (ns) s(min) s(min) BestFit s(max) s(max) LCR LCR* Frequency (khz) Parasitic Capacitance Mismatching (pf) Fig. 7. Least-square fitting for dielectric loss G(ω) Fig. 5. Least-square fitting for ΔC p [Determination of R sk ] The evaluation of the skin effect R sk (ω) can be performed in a similar way, except that now we have to consider two independent parameters (α, α ). Fixing the two parameters and scanning all the other four parameters, all the combinations of the two parameters (α, α ) which satisfies the conditions δ fit (α, α )< σ mes, δ fit (α, α )< σ mes are obtained. The estimated R sk (ω) obtained by the obtained parameters (α, α ) is shown in Fig.6. Here, the horizontal axis represents the frequency, and the vertical axis represents the evaluated skin effect R sk (ω). The lines specified as -s and +s in the figure represents the min/max values of R sk (ω) which is consistent with the condition δ fit (α, α )< σ mes, and the lines specified as -s and +s represents the min/max values of R sk (ω) for the condition δ fit (α, α )< σ mes Evaluation of Uncertainty Contribution of the five parasitic components to the ac-dc difference at high frequency range (0 khz MHz) is calculated using the leastsquare fitting method. Once the mathematical circuit model is determined, the method to obtain the "best" fitting is quite straightforward, as shown in the previous section. However, the process to evaluate the uncertainty in the fitting is far more complicated. Thus, instead of determining the uncertainty of γ 500 (ω) through the uncertainty the six parameter, L p, ΔC, α, α, β, β., the possible range (min-max) of γ 500 (ω) was evaluated directly through the least-square fitting process. The result is shown in Fig. 8. The lines specified as - s(mes) and +s(mes) in the figure represents the min/max values of γ 500 (ω) which is consistent with the condition δ fit (L p, ΔC, α, α, β, β )< σ mes, and the lines specified as -s(mes) and +s(mes) represents the min/max values of γ 500 (ω) for the condition δ fit < σ mes. - 5/6 -

6 Fig. 8. Estimated frequency characteristic of a 500Ω HF-TVC. Instead of the "best-fit" value, the "Nominal" ac-dc difference is calculated as the mean value of the -s(mes) and +s(mes), which happened to vary quite linearly with frequency. The uncertainties (σ, σ) are calculated such that it covers all the possible combination (d fit < σ mes, δ fit < σ mes ), using linear boundary. The result agreed with that of the previous section within its uncertainty: ( f ).0 ± 3.0(σ ) 0 f or ( f )[ppm].0 ± 3.0(σ ) f [MHz]. (0) The lines specified as "Nom" represent the average of -s and +s in the figure represents the min/max values of γ 500 (ω) which is consistent with the condition δ fit (L p, ΔC, α, α, β, β )< σ mes, and the lines specified as -s(mes) and +s(mes) represents the min/max values of γ 500 (ω) for the condition δ fit < σ mes. 6. Uncertainty due to Built-in TEE Another source of uncertainty arises from the use of N-P plug, to which a test-tvc (UUT) is connected. As discussed in the previous sections, the N-P plug imitates the half part of the N- RRR TEE connector by which the reference plane is defined. Hence, the difference in the shape and dimension of the N-P plug and the N- RRR TEE connector must be taken as a source of uncertainty in the calibration using the HF- TVC. In the case of HF-TVC with built-in TEE, the uncertainty due to the use of N-P connector dominates the over-all uncertainties below 00 khz. The uncertainty may be evaluated experimentally, by adding an extra N-PR connector between HF-TVC and test TVC. Figure 9 shows an example of results from ac-ac difference measurement of a TC module (SN05004) using a HF-TVC (SN5600) as a reference standard. From the measurement, the effect of the additional N-PR connector (length 35 mm) is evaluated to be < 0.8 ppm up to 00 khz and <.8 ppm up to MHz. Hence, in the case of a TVC with input resistance of 500Ω, the uncertainty due to the use of built-in TEE may be over-estimated be one-half of the effect, i.e., < 0.4 ppm up to 00 khz and <.4 ppm up to MHz. Fig. 9. Change in AC-DC transfer difference: one with a N-PR adopter, and the other without the adopter. 7. Conclusion The frequency characteristic of HF-TVC was evaluated using mathematical model of the input circuit. The circuit parameter was evaluated using two methods, i.e., () direct measurement using LCR meter, and () least-square fitting of measured ac-dc difference to the mathematical model. The frequency characteristic of the ac-dc difference of the HF-TVC was evaluated to be better than. ppm(σ) up to 00 khz and better than ppm(σ) up to MHz. Reference [] I. Budovsky, High- frequency ac-dc difference of NML singlejunction thermal voltage converters, IEEE Trans. Instrum. Meas., Vol. 50, No., pp. 0-05, 00. [] P. S. Filipski, "Experience with high-resistance MJTC AC-DC transfer standards at high frequencies," IEEE Trans. Instrum. Meas., vol. 5, pp.34-39, 003. [3] L. Scariori M. Klonz, D. Janic, H. Laiz and M. Kampik, Highfrequency thin-film thermal converter on a quartz crystal chip, IEEE Trans. Instrum. Meas., vol. 5, pp , /6 -