Design and Evaluation of a HELA–10 Based FEE with 3–State Switched Calibration R.H. Tillman∗ April 25, 2013

Contents 1 Introduction

2

2 Design

2

3 RF 3.1 3.2 3.3

Chain Testing Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 Calibration Demonstration

∗ Virginia

8 8 8 10 10

Tech. E–mail: [email protected]

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1

Introduction

This report documents the design, testing, and demonstration of new front end electronics (FEE) for use in an interferometer calibrated for absolute flux measurement. The FEE implements a 3–state switched calibration similar to that used by the Large aperture Experiment to detect the Dark Ages (LEDA) and Experiment to Detect the Global EoR Step (EDGES) [1, 2]. Section 2 summarizes the the design of the FEE. Section 3 presents the laboratory testing of the FEE’s RF characteristics. Section 4 demonstrates the calibration capabilities of the FEE under laboratory conditions.

2

Design

A block diagram of the FEE is shown in Fig. 1. The design and selection of components for the individual blocks is documented in [3–6]. Two Mini–Circuits Laboratories (MCL) HELA–10 amplifiers are used as a pre–amplifier and a line driver, and were selected for their exceptional linearity ratings [3, 7, 8]. A summary of the remaining blocks follows: • Relays – Mechanical relays configured in a balanced arrangement provide the switching required for the calibration. Mechanical relays were selected over an IC switch because of their low loss and noise characteristics [5]. • CB & FM – Two series resonant LC notch filters designed to trap the strong signals from Citizen Band and FM radio at 26.5–27.5 and 88–107 MHz, respectively [6]. The loss in the filters is small (∼ 1 dB) and thus they have been placed before the preamplifier. • 30–90 MHz – This filter serves to remove intermodulation products generated by the preamplifier to enter into the line driver [6]. Additionally, the filter is absorbitive (i.e. a diplexer) to prevent reflections between the two amplifiers from corrupting the desired signals. • Noise Source – The calibrated noise source is provided by a Mercury1 SM4 noise diode2 . • Bias and Control – The switch state is determined by the DC voltage level provided to the input of the FEE. Additionally this block regulates the DC voltage to the levels required by the other blocks. Figures 2 and 3 show the RF chain and the Bias and Control sections of the FEE, respectively. The calibration state is determined by the DC bias voltage as follows: DC Bias 14 V 16 V 18 V

Switch State Antenna Noise Source – Off Noise Source – On

Figure 4 shows the layout of the FEE. The FEE was fabricated using ExpressPCB’s ProtoPro 4–Layer service. The fully populated FEE is shown in Fig. 5, and a bill of materials (BoM) is shown in Table 1. 1 Previously

Micronetics.

2 http://rf.mrcy.com/RF_Components/Noise_Diodes.html

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Figure 1: Block diagram of the FEE.

Figure 2: FEE’s RF chain.

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Figure 3: FEE’s bias and control network.

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5 Figure 4: Layout of FEE. From top to bottom, the layers are Top Copper, Ground Plane, Power Plane, and Bottom Copper. The top silk screen layer is shown on the ground and power planes for reference.

Figure 5: FEE after population.

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Part ID ATTN1 ATTN3 Bypass Caps RF Cs

D1 D2 J3-J5,J8 J6,J7 Bias L RF Ls

R1,R4,R6 R11 R16 R2,R5 R3 R7-R10,R12-R15 RLY1,RLY2 U1,U2 U3 U4 U5 PCB

MFG Mini-Circuits Mini-Circuits AVX AVX AVX AVX AVX AVX Micronetics Diodes Inc Sullins Linx TDK Epcos Epcos Epcos Epcos Epcos SEI SEI SEI SEI SEI SEI Omron Mini-Circuits TI TI TI ExpressPCB

MFG Part # PAT-6 PAT-30 06035C104KAT2A 08052U560GAT2A 8052U680JAT2A 08052U101JAT2A 08052U131JAT2A 08051U161JAT2A SM4 1N4148W-7-F PPTC021LFBN-RC CONSMA001 MLZ2012E4R7M B82498F3180J B82498F3680J B82498F3820J B82498F3101J B82498F3151J RNCF0805BTC1K13 RNCS0805BKE1K00 RNCS0805BKE825R RNCS0805BKE332R RNCS0805BKE75R0 RNCS0805BKE10K0 G6ZU-1F-DC9 HELA-10 LM2940CSX-12/NOPB LM2940CSX-9.0/NOPB LM339ADR

Table 1: BoM for the FEE.

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Dist Mini-Circuits Mini-Circuits Mouser Mouser Mouser Mouser Mouser Mouser Mercury Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Digikey Mouser Mini-Circuits Digikey Digikey Digikey ExpressPCB Totals

Unit Price 2.95 2.95 0.035 0.46 0.35 0.35 0.35 0.35

Quantity 20 20 100 20 20 20 20 20

Total Price 59 59 3.5 9.2 7 7 7 7

0.156 0.262 2.4 0.0866 0.702 0.702 0.702 0.702 0.702 0.551 0.523 0.523 0.523 0.523 0.3162 6.34 15.95 1.578 1.578 0.32 195 43.9348

10 50 25 50 20 20 20 20 20 20 10 10 20 10 50 10 10 10 10 10 1 840

1.56 13.1 60 4.33 14.04 14.04 14.04 14.04 14.04 11.02 5.23 5.23 10.46 5.23 15.81 63.4 159.5 15.78 15.78 3.2 195 813.53

A few layout and component selection errors were found during population and initial testing. These errors are summarized below, along with the fixes that were implemented: • The pads of the first diplexer component were connected together. The trace was scratched out with an x-acto knife. • R10 was not connected to the DC bias plane. The connection was made externally using spent solder wick. • The ground pads of the attenuator were not connected to ground. The connection was made externally using spent solder wick. • The diode D2 was not rated for the high current draw. It was replaced by another inductor identical to L22 that can. • The originally selected comparator, TI’s LM339ADR, was unable to drive the ∼ 200 Ω input impedance of the mechanical relays due to current limitations. It was replaced by an Advanced Linear Devices’ ALD4302SBL.

3

RF Chain Testing

This section presents the characterization of the FEE’s RF chain, consisting of the amplifiers, filters, and balun components. Measurements were made of the FEE’s gain, linearity, and noise characteristics.

3.1

Gain

Figure 6 shows the measured gain and expected FEE gain, GF E . The passband reasonably matches the expected form, but with a couple differences. The notches differ from expected, but within the bounds of the Monte Carlo simulation in [6]. Additionally, there is notable deviation from the expected at higher frequencies. The high frequency deviation was also seen in [6], and a comparison of the previous and current results is shown in Fig. 7. The original diplexer component values were used to see if board parasitics were a significant factor in the previous work. The current diplexer response is improved over the previous result, indicating parasitics were partially responsible. If this response is found to be unsatisfactory in later measurements, a frequency translation technique that was explored in [6] may be considered.

3.2

Linearity

Figure 8 shows the linearity measurements made of the FEE. Measurements were made with a Rhode and Schwarz FSH3 spectrum analyzer, and the input signals were provided by and an Agilent E4438C and a Stanford Research Systems DS345. Measurement of the 1 dB compression point (P1dB ) was made at 50 MHz, and is 11 dBm input referenced. The expected P1dB for two cascaded HELA–10 amplifiers is only 6 dB, so the attenuation induced by the impedance matching transformers and relays is slightly beneficial here.

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Figure 6: Measured (solid) and expected (dashed) GF E .

Figure 7: Measured (solid) and expected (dashed) diplexer gain. The red curve is the measured response from the demo diplexer in [6].

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Figure 8: Summary of linearity measurements made of the FEE. The difference in the first order terms is due to the different frequencies at which various measurements were made. Measurement of the input second order intercept point (IIP2) was made at 35 MHz to ensure the second harmonic at 70 MHz was within the passband. The extrapolations of the linear and second order responses are also shown in Fig. 8, and IIP2 is about 29 dBm. Finally, the input third order intercept point (IIP3) was measured with two signals at 29.7 and 30.3 MHz, as the DS345 has a maximum output frequency of 30 MHz. The measured IIP3 is at 12 dBm, however the third harmonic levels measured increase less than a cubic term, so this 12 dBm is more of a lower limit on the IIP3 than a hard number.

3.3

Noise

Figure 9 shows the FEE’s measured and expected noise temperature, TF E . The measurement was made by connecting a matched load to the antenna terminals and measuring the output power spectral density (PSD). Very good agreement is seen between the measured TF E and the expected value derived from the component datasheets and theory.

4

Calibration Demonstration

This section describes and demonstrates the FEE’s calibration procedure. Considering a constant FEE noise temperature, TF E , such that the output PSD at each switch position is: Pant = kGF E ((1 − |Γant |2 )Tant + TF E )

(1a)

PL = kGF E (Tamb + TF E )

(1b)

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Figure 9: Measured (solid) and expected (dashed) TF E . Pcal = kGF E (Tamb + Tcal + TF E )

(1c)

Where Pant , PL , and Pcal are the PSDs when the switch is connected to the antenna, cold noise source, and hot noise source, respectively, k is Boltzmann’s constant, Tant is the antenna temperature, Tamb is the ambient temperature, and Tcal is the noise source’s excess noise temperature. The factor 1 − |Γ|2 is commonly referred to as the impedance mismatch efficiency (IME). The calibrated temperature, T3p , comes from subtracting (1a) and (1c) from (1b) and dividing, and is independent of GF E and TF E :   1 Pant − PL T3p = T + T (2) cal amb (1 − |Γant |2 ) Pcal − PL Calibration by (2) requires a priori knowledge of Tcal . The SM4 noise diode’s output PSD was measured at J7 before adding the 30 dB pad between the noise diode and the impedance matching transformer T4. The calibrated noise temperature is then the noise temperature presented to the relays terminals, found by Tcal = GAT T N (TSM 4 + Tamb (1 − GAT T N ))

(3)

where GAT T N is the gain of the pad and T4, TSM 4 is the SM4’s measured noise temperature, and Tamb ≈ 297K. Figure 10 shows the measured Tcal and its expected value. The measured temperature is a little higher than expected, but its spectral variation is still small, which is why the SM4 was selected. For the lab demo, a matched 150 Ω load at room temperature (∼297 K) was connected to the FEE’s antenna terminals. The measurements were made with the FSH3 and three MCL ZFL–500 SMA connected amplifiers acting as an analog receiver. 11

Figure 10: Measured Tcal . The top curve, labeled “Before Pad”, is the SM4’s noise temperature. The bottom curve, “After Pad”, is Tcal according to (3). Figure 11 shows the measured noise temperatures at each switch state, and Fig. 12 shows the calibrated noise temperature using (2) and Tcal from Fig. 10. Despite the low integration time, the calibration still takes the input signals, on the order of 101 3 K, and obtains calibrates to within an order of magnitude of the expected result. A longer integration test will be performed once the control board (currently being fabricated by ExpressPCB) has been completed.

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Figure 11: Measured noise temperature in the three switch states. RBW=100 kHz, 1 ms integration.

Figure 12: Calibrated noise temperature. The dashed line is 297 K.

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References [1] L. Greenhill, D. Werthimer, G. Taylor, and S. Ellingson, “A broadband 512-element full correlation imaging array at VHF (LEDA),” in Electromagnetics in Advanced Applications (ICEAA), 2012 International Conference on, 2012, pp. 1117–1120. [2] A. E. E. Rogers and J. D. Bowman, “Absolute calibration of a wideband antenna and spectrometer for accurate sky noise temperature measurements,” Radio Science, vol. 47, no. 6, 2012. [Online]. Available: http://dx.doi.org/10.1029/2011RS004962 [3] R. Tillman and S. Ellingson, “Design and evaluation of a HELA-10 based FEE for HF/VHF radiometry,” Tech. Rep. 2, Jan. 2013. [Online]. Available: https: //filebox.vt.edu/users/hankt5/Public/ [4] R. Tillman, “Line driver selection,” Tech. Rep. 3, https://filebox.vt.edu/users/hankt5/Public/

Feb. 2013. [Online]. Available:

[5] ——, “RF switch comparison for a 3–state calibrating FEE,” Tech. Rep. 4, Mar. 2013. [Online]. Available: https://filebox.vt.edu/users/hankt5/Public/ [6] ——, “Filter design for a 3–state calibrating FEE,” Tech. Rep. 5, Mar. 2013. [Online]. Available: https://filebox.vt.edu/users/hankt5/Public/ [7] “HELA-10: High IP3, wide band, linear power amplifier,” Appl. Note AN-60-009, Mini–Circuits Laboratories, July 2008. [8] “Surface mount high IP3 monolithic amplifier HELA–10,” Datasheet, Mini–Circuits Laboratories, 2008.

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