P UBLISHED
BY
IOP P UBLISHING
FOR
SISSA
R ECEIVED: June 23, 2009
R EVISED: September 4, 2009
ACCEPTED: September 25, 2009
P UBLISHED: December 29, 2009
T HE P LANCK L OW F REQUENCY I NSTRUMENT
LFI 30 and 44 GHz receivers Back-End Modules
a Dpt.
Ingenierı́a de Comunicaciones, Universidad de Cantabria,
Plaza de la Ciencia, 39005 Santander, Spain
b
Instituto de Fı́sica de Cantabria, CSIC-Universidad de Cantabria,
Avda. de los Castros s/n, 39005 Santander, Spain
c Departament de Teorı́a del Senyal i Comunicacions, Universitat Politécnica de Catalunya,
Barcelona, Spain
d Mier Comunicaciones S.A.
La Garriga, Barcelona, Spain
e Jodrell Bank Centre for Astrophysics, University of Manchester,
Manchester, United Kingdom
f Università degli studi di Milano, Department of Physics
Via Celoria 16, Milano, Italy
g
INAF/IASF - Bologna
Via P. Gobetti 101, 40129 Bologna, Italy
E-mail: artale@unican.es
A BSTRACT: The 30 and 44 GHz Back End Modules (BEM) for the Planck Low Frequency Instrument are broadband receivers (20% relative bandwidth) working at room temperature. The signals
coming from the Front End Module are amplified, band pass filtered and finally converted to DC
by a detector diode. Each receiver has two identical branches following the differential scheme of
the Planck radiometers. The BEM design is based on MMIC Low Noise Amplifiers using GaAs PHEMT devices, microstrip filters and Schottky diode detectors. Their manufacturing development
has included elegant breadboard prototypes and finally qualification and flight model units. Electrical, mechanical and environmental tests were carried out for the characterization and verification
of the manufactured BEMs. A description of the 30 and 44 GHz Back End Modules of Planck-LFI
radiometers is given, with details of the tests done to determine their electrical and environmental
performances. The electrical performances of the 30 and 44 GHz Back End Modules: frequency
response, effective bandwidth, equivalent noise temperature, 1/f noise and linearity are presented.
K EYWORDS : Space instrumentation; HEMT amplifiers; Microwave radiometers; Instruments for
CMB observations
1 Corresponding
author.
c 2009 IOP Publishing Ltd and SISSA
doi:10.1088/1748-0221/4/12/T12003
2009 JINST 4 T12003
E. Artala,1 B. Aja,a M.L. de la Fuente,a J. P. Pascual,a A. Mediavilla,a
E. Martinez-Gonzalez,b L. Pradell,c P. de Paco,c M. Bara,d E. Blanco,d E. Garcı́a,d
R. Davis,e D. Kettle,e N. Roddis,e A. Wilkinson,e M. Bersanelli, f A. Mennella, f
M. Tomasi, f R.C. Butler,g F. Cuttaia,g N. Mandolesig and L. Stringhettig
Contents
Introduction
1
2
Design of the Back-End Modules
2.1 Low Noise Amplifiers
2.2 Band Pass Filters
2.3 Diode Detectors
2.4 DC Amplifier
2
2
3
4
6
3
Manufacturing
3.1 Elegant breadboard prototype
3.2 Qualification and Flight Model units
7
7
8
4
Electrical characterization tests
4.1 Frequency response and effective bandwidth
4.2 Equivalent noise temperature
4.3 Stability: 1/f noise
4.4 Linearity
10
11
12
13
14
5
Verification tests
5.1 Vibration and thermal vacuum
5.2 Electromagnetic compatibility
15
15
17
6
Conclusions
18
1
Introduction
The Low Frequency Instrument (LFI) is one of the two instruments of the ESA Planck satellite.
The objective of the satellite is to achieve precision maps of the Cosmic Microwave Background
with a very good sensitivity to observe sky temperature anisotropies [1]. LFI contains very high
sensitive receivers based on cryogenic radiometers with their Front-End Modules (FEM) operating at 20 K [2]. LFI covers three adjacent frequency bands centred at 30, 44 and 70 GHz, with a
fractional bandwidth of 20%, and uses cryogenic very low noise amplifiers with Indium Phosphide
(InP) High Electron Mobility Transistors (HEMT) in the FEM. The Front-End Modules are physically and thermally separated from the Back-End Modules by over a meter of copper and stainless
steel rectangular waveguide. The signals originating in the FEM are then further amplified and
finally detected in the BEM that operate at 300 K.
Planck-LFI radiometers are based on a differential scheme [3–5], shown in figure 1. Signals
from the sky and from a 4 K reference load are combined using a (180◦ ) hybrid. Both signals
–1–
2009 JINST 4 T12003
1
Figure 1. Planck Mission radiometer scheme.
2
Design of the Back-End Modules
In this section, we will describe all the fundamentals of each RF component designed and their
individual performance before integration.
2.1
Low Noise Amplifiers
Each Low Noise Amplifier (LNA) consists of two cascaded amplifiers in order to have enough
gain and to provide a signal in the square law region of the diode detector. Monolithic Microwave Integrated Circuits (MMIC) on GaAs technology have been chosen at 30 and 44 GHz.
The MMIC at Ka-band are commercial circuits, model HMC263 from Hittite. They have four
stages of Pseudomorphic-High Electron Mobility Transistors (PHEMT), with an operating bandwidth from 24 GHz to 36 GHz, and offer 23 dB of gain and 3 dB of noise figure from a self-biasing
supply of +3 Volt, 52 mA. A picture of the MMIC at Ka-band, its noise figure, gain and input and
output return loss are shown in figure 2.
–2–
2009 JINST 4 T12003
are amplified by Low Noise Amplifiers (LNA) in the Front-End Module. Two identical phase
switches introduce a differential (180◦ ) phase shift in one branch with relation to the other. The
second (180◦ ) hybrid delivers a signal proportional to sky temperature in one output, and a signal
proportional to the reference load temperature in the other.
The balanced structure of the FEM permits to obtain two signals, sky and reference, contaminated by the same amount of noise because both have passed through the same LNA and phase
switches. By differencing signals across the two outputs using post-processing, the noise can be
cancelled. Phase switching at 4 kHz is done also to remove the 1/f noise of the BEM, because it
does not have a balanced structure like the FEM. This BEM noise is noticeable at low frequencies,
at some hundreds of Hz. Noise cancellation can only be perfect if the two FEM branches are identical and the balance is perfect as well. In practice a little unbalance is tolerated and it was tested
as a leakage from one input to the theoretically isolated output.
Next sections describe in detail the 30 and 44 GHz BEM units of Planck-LFI, from their design
principles and individual subsystems performance, to the final Flight Model units which are integrated in the Planck satellite. Prototype and Elegant Breadboard units are presented and described
showing their main characteristics. Electrical and environmental tests have been performed in all
the Qualification and Flight Model units delivered to the Planck satellite Project System Team.
Figure 3. Q-band LNAs, (a) Normally ON; (b) Normally OFF.
At Q-band two custom designed MMICs have been assembled [6]. They have been manufactured with the process ED02AH from OMMIC, which employs a 0.2 µ m gate length PHEMT on
GaAs. The first MMIC has four stages of depletion mode transistors (Normally ON: N-ON) with
gate widths of 4x15 µ m and 6x15 µ m, shown in figure 3(a). The second MMIC, in figure 3(b),
has four-stages of enhancement mode transistors (Normally-OFF: N-OFF) with gate widths of
6x15 µ m. The N-ON LNA preceding the N-OFF, was found to be the best in terms of input and
output matching and noise performance. The main requirements of the two LNAs are to provide
low noise and low power consumption with enough gain. Using as first amplifier an LNA, based
on N-ON PHEMT transistors, a good noise performance was obtained with a power consumption of only 90 mW. The second LNA, with N-OFF PHEMT transistors, was added to increase the
gain with a very low impact in the noise figure and a very low power consumption of 32 mW. All
the circuits were measured on wafer using a coplanar probe station. Noise figure, return loss and
associated gain for the Q-band depletion and enhancement transistor LNAs are plotted in figure 4.
2.2
Band Pass Filters
A band pass filter was used to define an effective bandwidth of 20% and to reject undesired signals
out of the band of interest. Low bandpass losses, more than 10 dB out of band losses and small size
were considered the main objectives to fulfil. The filter was based on microstrip coupled line structure that was chosen because it provides inherently good band pass characteristics. The design is a
three-order Chebyshev resonator filter. Electrical models of coupled lines and an electromagnetic
–3–
2009 JINST 4 T12003
Figure 2. (a) Ka-band LNA MMIC (b) MMIC performance.
Figure 5. Microstrip bandpass filter.
simulator was used in the design phase. The design method was based on the classic prototype
filter tables provided by [7], but a design methodology has been developed to achieve a predictable
frequency response in microstrip filters using commercial CAD software. After a careful evaluation of the validity of the CAD models, comparing simulated and accurately measured results, the
design was restricted to microstrip elements than could be well characterized [8]. The selection of
the substrate became critical due to the gaps and widths of the microstrip lines because it sets the
line-etching precision required and the minimum losses achievable. Microstrip lines were made
on Duroid 6002 substrate with 10 mils thickness and dielectric constant of 2.92. Several units of
microstrip band pass filters were fabricated and tested. Typical test results for the 30 GHz filter are
insertion losses lower than 0.84 dB in the band, and for the 44 GHz filter, insertion losses better
than 1.5 dB and return losses better than 10 dB across the operating bandwidth. A photograph of a
30 GHz filter is shown in figure 5. The response of the filters when they have been measured with
coplanar to microstrip transitions using a coplanar probe station is depicted in figure 6.
2.3
Diode Detectors
The diode for the detector was a GaAs planar doped barrier Schottky diode. The specific model
chosen was a zero-bias beam-lead diode HSCH-9161 from Agilent Technologies. This diode has
suitable characteristics to be used at microwave frequencies. Among the main specifications in
the detector design are the input matching, the sensitivity and the tangential sensitivity. The diode
–4–
2009 JINST 4 T12003
Figure 4. Q-band LNAs on wafer performance, (a) Normally ON; (b) Normally OFF.
Figure 7. Diode detectors, (a) 30 GHz; (b) 44 GHz.
Figure 8. (a) Ka-band detector sensitivity at 30 GHz; (b) Q-band detector rectification efficiency at 44 GHz
for -30 dBm of input power.
equivalent circuit is not a good match to 50 Ohm, so it was necessary to synthesize a network
that would transform it to something close with an input matching network. Thus the detector is
composed of a hybrid reactive/passive matching network, and the Schottky diode. Both detectors,
for 30 GHz and 44 GHz, were mounted with a coplanar-to-microstrip transition to make on-wafer
tests as a previous step to the BEM integration. The practical implementation is performed with
transmission lines printed on a standard dielectric substrate, Alumina with 10 mils thickness and
dielectric constant of 9.9. A view of the detectors is shown in figure 7. The output voltage sensitivity has been measured at 30 GHz for the Ka band detector and it is shown in figure 8(a). Figure 8(b)
depicts the rectification efficiency of the Q-band detector at the frequency of 44 GHz for −30 dBm
of input power.
–5–
2009 JINST 4 T12003
Figure 6. Band-pass filter performance, (a) 30 GHz; (b) 44 GHz.
2.4
DC Amplifier
The detector diode output is connected to a low noise DC-amplifier, with an adequate voltage gain
to have the required detected signal level for the data acquisition electronic module. A schematic
of the DC-amplifier is shown in figure 9. The first stage has an OP27 precision operational amplifier that combines low offset and drift characteristics and low noise, making it ideal for precision
instrumentation applications and accurate amplification of a low-level signal. A second balanced
stage, implemented with an OP200, provides a balanced and bipolar output. DC amplifier total
power consumption with a high impedance load is 37 mW.
The amplifier gain is given by the ratio between the differential output voltage Vout , and Vin
(input voltage from the detector), where Vout is given by:
R4
R2
· 1+
·Vin
Vout = Vout+ −Vout− = 2 ·
R1 + R2
R3
(2.1)
The detector resistive load is R1 + R2 . In order not to affect the detector RF response, the
condition R1 + R2 ≥ 50 kOhm was fulfilled.
Since the phase switch frequency rate in the receiver is 4096 Hz, the output signal has to
provide a video bandwidth of at least 50 kHz, which means that the output signal contains more than
ten harmonics in order not to degrade the information. The DC amplifier measured gain-bandwidth
product was 5.7 MHz. This operational amplifier gain bandwidth product has been taken into
account, and the maximum achievable balanced gain without loosing output bandwidth was 100.
A voltage gain of 50 is due to the OP27 and a further of 2 due to unbalanced to balanced conversion
of the OP200 with unit individual gain. Another constraint of this DC amplifier is to provide an
output voltage in a window between 0.2 Volt and 0.8 Volt, where the data acquisition electronics
(DAE) works properly. The designed DC amplifier was adjusted for each channel, taking into
account small RF gain differences, in order to have the output DC voltage inside the window and to
achieve the output bandwidth requirement. The resulting bandwidth was 163 kHz. Figure 10 shows
the DC amplifier measured low-frequency noise referred to the amplifier input. The white noise
√
√
voltage level is 8 nV / Hz (= −162 dBV / Hz), and the flicker noise knee-frequency, defined as
√
the point where the noise voltage is 2 times the white noise voltage, is 2.8 Hz.
–6–
2009 JINST 4 T12003
Figure 9. DC-amplifier schematic.
Figure 11. General view of the EBB 30 GHz BEM branch with LNA, band pass filter and detector.
3
3.1
Manufacturing
Elegant breadboard prototype
The objective of the Elegant Breadboard (EBB) prototypes was to experimentally demonstrate the
radiometer concept, by integrating a fully representative unit in the laboratory. An EBB demonstrator must have the same electrical functionality as the final flight unit. The internal architecture,
electrical scheme, and the electronic components were the same as for flight. The electronics components do not need to be space qualified. The size and weight of the EBB demonstrator can
be different from the flight unit. To make easier the integration with the EBB of the Front End
Module, the EBB version of the 30 and 44 GHz BEM included only one branch. In fact the EBB
version is a quarter of one full BEM. Figure 11 shows a photograph of one EBB at 30 GHz. This
EBB branch contains a Low Noise Amplifier (LNA), a Band Pass Filter (BPF), a Schottky diode
detector (DET) and a low frequency amplifier (DC Amp). The central frequency is 30 GHz and
the nominal fractional bandwidth is 20%. Figure 12 shows two EBB BEM units connected to the
Prototype Demonstrator (PD) FEM in the laboratory at Jodrell Bank Observatory.
The first functional element of each BEM is a waveguide-to-microstrip transition that was
designed using a ridge waveguide [9]. The input waveguide flange is WR-28 for the 30 GHz BEM
and WR-22 for the 44 GHz BEM. The transition is a four sections stepped ridge waveguide to
microstrip line transition which provides a broadband performance. This transition has been chosen
for its broad bandwidth, low insertion loss, and repeatable performance. Figure 13 shows the
experimental results of back to back rectangular waveguide to microstrip transitions using ridge
waveguide transformer. Microstrip line losses are included in the result. The length of microstrip
50 Ohm line, on Alumina substrate 10 mils thickness, was 10 mm in both units (Ka band and Q
band). The estimated insertion loss of one single ridge waveguide transition is lower than 0.2 dB.
–7–
2009 JINST 4 T12003
Figure 10. DC amplifier low frequency noise, referred to the amplifier input.
Figure 13. Insertion loss and return loss of back to back rectangular waveguide to microstrip transitions
through stepped ridge sections. (a) Ka band unit, (b) Q band unit.
3.2
Qualification and Flight Model units
The final BEM mechanical configuration for the Qualification Model and Flight Model has four RF
branches, providing signal amplification and detection for two complete radiometers. Mechanical
design had been carried out by Mier Comunicaciones S.A. within an allowed envelope of 70 x 60
x 39 mm3 including all the RF and DC circuitry. Mass is 305 g for 30 GHz BEM and 278 g for the
44 GHz BEM. Figure 14 shows the external view of the BEM. Internally there are 5 different levels
of PCB circuits, from top to bottom, as follows:
–8–
2009 JINST 4 T12003
Figure 12. The EBB 30 GHz BEM branches connected to the Prototype Demonstrator FEM in Jodrell Bank
Observatory.
Figure 15. (a) DC Amp PCB contains operational amplifiers; (b) DC PCB contains voltage regulators.
1. DC PCB: It contains voltage regulators for a half BEM (one receiver, two branches)
2. DC amplifiers PCB: It contains the DC operational amplifiers for a half BEM (two signal
detected outputs)
3. RF part: It contains two RF branches (one receiver)
4. DC amplifiers PCB: It contains the DC operational amplifiers for the other half BEM (two
signal detected outputs)
5. DC PCB: It contains voltage regulators for the other half BEM (one receiver, two branches)
Pictures of the DC and RF circuits are in figure 15 and figure 16.
Qualification Model (QM) units have been manufactured using identical electrical and mechanical components as for the Flight Model (FM) units. The QM and FM components have
identical quality level and have been space qualified following the same procedures. The only
difference between QM and FM units is the different environmental tests done on each case. In
–9–
2009 JINST 4 T12003
Figure 14. Back End module external view.
Figure 17. (a) FM units of 30 GHz BEM; (b) QM unit of 44 GHz BEM.
particular vibration levels and thermal cycling tests for QM units have been more demanding than
for FM units. Summarising: QM units are identical to the FM units, but they are not intended to be
installed in the satellite. In order to be ready to deal with limited unexpected component failure in
the FM units, before launching the satellite, spare FM units of the 30 and 44 GHz BEM have been
also manufactured and tested. Pictures of FM or QM units of 30 and 44 GHz BEM are showed in
figure 17.
4
Electrical characterization tests
A set of basic tests were performed at three different temperatures in the range of possible operating
temperature; Tlow (−25◦C), Tnom (26◦C) and Thigh (48◦C), to verify proper operation of the FM BackEnd Modules and fulfillment of the specified electrical requirements. They were assembled and
tested in a clean room equipped with a vacuum chamber which enables us to achieve low pressure
levels and to vary the base plate and shroud temperature within the acceptance ranges. The principal
tests performed were frequency response, equivalent noise temperature, stability and linearity. Test
– 10 –
2009 JINST 4 T12003
Figure 16. RF parts (two branches) in: (a) 30 GHz BEM, (b) 44 GHz BEM.
equipment available included vector network analyzers, swept frequency sources, noise sources
and low frequency spectrum analyzers.
4.1
Frequency response and effective bandwidth
The frequency response (RF to DC) measurements were done injecting a CW small signal into
the waveguide input, for a constant power level. The level of the signal was set to yield a readily
measurable response, but not so large as to cause non-linear effects. This test was needed in order to know the bandpass response. It was measured stepping the synthesizer source through 201
frequencies and recording the output voltage for each frequency, with the RF output of the synthesized enabled and disabled. Figure 18 shows the RF to DC response for the flight model BEMs at
30 GHz and 44 GHz.
The measured results obtained for the RF to DC response are used to calculate the effective
bandwidth. The effective bandwidth is defined according to next expresion.
| G( f )d f |2
R
BWeff = R
(4.1)
|G( f )|2 d f
G( f ) is the RF power gain of the BEM, including the RF detector response. This G( f ) gain
is obtained by RF to DC response test. This test is performed with a microwave sweep generator
providing a constant input power versus frequency, so the effective bandwidth can be calculated
as (4.2), using only the output voltage values taken at discrete frequencies.
N
BWeff = ∆ f ·
N +1
∑Ni=1 Vout (i) −Voutoff
2
2
∑Ni=1 (Vout (i) −Voutoff )
(4.2)
where N is the number of frequency points, ∆ f is the frequency step, Vout (i) the DC output voltage
at each frequency and Voutoff is the DC output voltage when the sweep generator is off. Voutoff is
typically about 25 mV for the 30 GHz BEM and lower values for the 44 GHz BEM.
Table 1 shows the values of the effective bandwidth for each flight model BEM at three different temperatures in the range of possible operating temperature.
– 11 –
2009 JINST 4 T12003
Figure 18. RF to DC response at three temperatures for a CW power input of - 60 dBm, (a) BEM 30 GHz
Flight Model; (b) BEM 44 GHz Flight Model.
Table 1. Effective bandwidth of the BEM Flight models in GHz. (One unit of each band is a Flight Spare).
Estimated error = ± 1%.
4.2
Channel B
Tlow Tnom Thigh
9.17 9.14 9.02
9.35 9.29 9.32
9.04 9.09 9.02
7.70 7.50 7.29
7.87 7.94 7.92
7.43 7.38 7.27
8.35 8.30 8.04
Channel C
Tlow Tnom Thigh
9.34 9.36 9.27
9.16 9.10 9.06
9.03 9.00 8.88
8.36 8.23 8.06
8.76 8.74 8.58
8.08 8.02 7.83
7.72 7.74 7.60
Channel D
Tlow Tnom Thigh
9.54 9.45 9.28
9.84 9.77 9.72
8.88 8.88 8.72
8.39 8.32 8.04
8.55 8.52 8.35
8.33 8.31 8.16
8.51 8.46 8.22
Equivalent noise temperature
A method has been developed to achieve an accurate and unique equivalent noise temperature of
the whole receiver. This method takes into account commercial noise sources, which have not a
flat Excess Noise Ratio (ENR) versus frequency in millimetre-wave range, and RF to DC receiver
performance along the band. Because the hot temperature of the used noise source and the BEM
RF gain show variations across the operating bandwidth, the next expression was used to obtain
the global equivalent temperature (Trec ) [10]:
f
Trec =
f
∑ f21 Th ( f )Vdet ( f ) −Y Tc ∑ f21 Vdet ( f )
(Y − 1) ∑ ff21 Vdet ( f )
.
(4.3)
Where Th and Tc are the hot and cold temperature of the noise source, Y is the noise Y − f actor, and
Vdet is the detected voltage at each frequency when a CW signal, with a constant power sufficiently
above white noise level, is applied at the BEM input. The Y − f actor is given by:
Y=
Vdet |h
.
Vdet |c
(4.4)
Where Vdet |h and Vdet |c are the receiver output voltages when two known source temperatures, hot
and cold loads, are connected at the BEM input.
The Y − f actor was tested with a cold load and a hot load using a commercial noise source
Q347B from Agilent. The equivalent noise temperature as a total power radiometer was slightly
worse than on wafer measured noise figure of a naked MMIC due mainly to losses in the waveguide to microstrip transition and to the readjustment of the bias point to decrease the ripple in the
operating band, trading off noise temperature and effective bandwidth. This noise temperature has
minimum impact on the global radiometer performance due to the high gain of the FEM.
Table 2 shows the values of the equivalent noise temperature for each flight model BEM at
three different temperatures in the range of possible operating temperature. The large variability
of the equivalent noise temperature of 44 GHz BEM units was due to their large dependence on
the input matching network result, which was observed to be a very critical parameter, not easy to
control during the assembly process of MMIC.
– 12 –
2009 JINST 4 T12003
BEM
30 GHz FM1
30 GHz FM2
30 GHz FM3
44 GHz FM1
44 GHz FM2
44 GHz FM3
44 GHz FM4
Channel A
Tlow Tnom Thigh
9.11 9.16 9.07
9.23 9.26 9.29
9.34 9.26 9.13
7.85 7.60 7.51
8.57 8.55 8.44
8.02 7.93 7.77
7.87 7.86 7.75
Table 2. Equivalent noise temperature of the BEM Flight models in Kelvin. (One unit of each band is a
Flight Spare). Estimated error: ± 20 K.
Channel B
Tlow Tnom Thigh
159 294 349
179 299 332
129 272 323
734 676 745
513 674 587
520 595 755
464 459 609
Channel C
Tlow Tnom Thigh
231 349 413
202 316 357
257 342 410
798 662 744
349 426 386
385 437 591
271 380 609
Channel D
Tlow Tnom Thigh
150 292 346
167 307 350
185 301 364
546 643 881
299 405 509
392 437 484
342 510 598
Table 3. 1/f knee frequency (Hz) of 30 GHz FM2 BEM unit.
Channel
A
B
C
D
4.3
Tlow
75
75
74
87
Tnom
90
100
100
100
Thigh
103
94
117
100
Stability: 1/f noise
The raw measurements of the output spectrum are used for the determination of the 1/f knee frequency. First, the output spectrum data are handled in a logarithmic scale in frequency. This way,
it is possible to plot the spectral noise density composed by a white noise constant value, at high
frequencies, plus the contribution of the 1/f noise at low frequencies. According to the definition,
the 1/f noise knee frequency is the frequency at which the noise voltage spectrum density is at a
√
level of 2 of the white noise voltage spectrum density level, for instance in V rms/sqrt(Hz). Using power density instead of voltage density then the level is 3 dB above the white noise level (for
instance in a scale of dBm/Hz).
The BEM low frequency power spectrum was characterized with a Hewlett Packard Vector
Signal Analyzer HP81490A when a wave-guide matched load is connected to the input. The test
was done at three temperatures: nominal (299 K), low (273 K) and high (326 K). The 1/f knee
frequency was below 400 Hz in all BEM units, much lower than the phase switching of the FEM
(4096 Hz), so gain fluctuations of the back-end module did not impact on the global performance
of the radiometer. The dominant 1/f noise source is attributed to the Schottky diode detector, since
it refers directly to the diode current. The 1/f noise spectrum of each LNA alone was tested and the
knee-frequency was about 13 Hz for the N-ON LNA and about 15 Hz for the N-OFF LNA. These
results make evident that diode detector is mainly responsible for the knee-frequency of the BEM.
The results for the four channels of a 30 GHz BEM FM unit are given in table 3. Figure 19 shows
a typical noise spectrum of a flight model BEM at 30 GHz.
– 13 –
2009 JINST 4 T12003
BEM
30 GHz FM1
30 GHz FM2
30 GHz FM3
44 GHz FM1
44 GHz FM2
44 GHz FM3
44 GHz FM4
Channel A
Tlow Tnom Thigh
196 317 365
176 288 324
164 286 347
923 856 1006
346 494 397
419 467 525
342 561 939
Figure 20. 44 GHz QM BEM dynamic range.
4.4
Linearity
The detected voltage versus input power was measured in order to get the BEM dynamic range. A
HP83650B generator was used as CW source. Results for the 44 GHz QM representative BEM are
depicted in figure 20 (solid line) for a 40.3 GHz CW signal input. In order to use a more realistic
input signal, a wide band noise stimulus was used to measure the BEM sensitivity. This noise
was obtained from a broadband white noise source, Q347B from Agilent, accordingly filtered and
amplified by another 44 GHz BEM branch with only the radiofrequency chain (detector and DC
amplifier not included). Results are also plotted in figure 20 (circles). Taking into account the FEM
gain and noise temperature, the BEM is always working under compression regime. The BEM
non-linearity is a combination of the second MMIC LNA gain compression and the non-linearity
of the diode detector. Given the dynamic range and the compression response of the BEM, it is not
possible to identify the 1 dB compression point. Due to this compression effect, measured signal
must be converted using the calibration curves, according to the procedures described in detail in
in [11].
Results of compression for channel A of a FM unit at 44 GHz are shown in figure 21. The test
was done at three temperatures: nominal, high and low.
– 14 –
2009 JINST 4 T12003
Figure 19. Typical low frequency noise power spectrum at the BEM output. Tested in a 50 Ohm spectrum
analyser.
Figure 22. BEM planes.
5
5.1
Verification tests
Vibration and thermal vacuum
Comprehensive vibration tests were performed, at different planes and frequency profiles, from
5 Hz to 2000 Hz. The sequence applied in the QM BEM unit test was the following one for each
axis: Low sine vibration, sine vibration, random vibration and low sine vibration surveys. The
low level sinusoidal vibration surveys were conducted per each axis, prior and after performing
sinusoidal and random vibration, at a sweep of 2 oct/min and an acceleration of 0.5 g, in the range
5-2000 Hz, only one sweep per run. The sinusoidal vibration test consisted of a single sweep per
axis, at a rate of 2 oct/min. In all vibration tests the BEM units were not operating.
The units were tested in 3 mutually perpendicular axes, 2 of them parallel to the base plate and
the third one perpendicular to it. Figure 22 shows the axes orientation with relation to the BEM
unit.
– 15 –
2009 JINST 4 T12003
Figure 21. Compression at 44 GHz (FM unit).
Table 4. Acceptance sinusoidal vibration test levels.
Axis
Out-of-plane (OOP)
In-plane (IP − NFP, IP − PFP)
Frequency range (Hz)
5 - 21.25
21.25 - 100
5 - 21.25
21.25 - 100
Level
+/-8 mm
20 g
+/-8 mm
16 g
As the specified vibration levels were defined with relation to the spacecraft axis system, a
stiff fixture providing the right inclination for the BEM units was envisaged in order to match to
the shaker axis. The BEM units were hard mounted on the fixture. This fixture guaranteed that the
major modes of the BEM were not modified, in the sense that frequency shifts were below 5% for
lower frequency modes. The units were tested at the environmental temperature expected during
satellite launch. A view of one BEM unit mounted on the shaker is in figure 23.
For the FM units the acceptance sinusoidal vibration test consisted of a single sweep per axis, at
a rate of 4 oct/min. The acceptance levels applied in this test are indicated in table 4. The acceptance
random vibration test was done according to the levels and durations presented in table 5.
The proper BEM performance, after the vibration of each unit, was checked by post-dynamic
performance verification tests. These checks were performed by measuring a small number of
indicative requirements of the BEM, like power consumption, in order to be sure that the unit
was still alive and performing well. The electrical behaviour, in terms of noise and RF to DC
conversion, did not change after vibration and thermal vacuum tests. The tested deviations were
within the range of the test equipment measurement accuracy.
Thermal vacuum tests for BEM flight units were done basically by six thermal cycles, with a
total duration of about 25 hours, between −35◦ C and +55◦ C. The thermal profile is depicted in
figure 24. During thermal cycling, a set of Reduced Performance Tests (RPT) have been performed.
As in the vibration tests, after thermal vacuum test the survival condition and functionality of each
unit was checked.
Each unit in turn was screwed to the base plate of the vacuum chamber and low thermal
resistance ensured by thermal silcone compound. Temperature cycling in the acceptance range was
thus achieved by changing the base plate temperature. The DC feedthroughs of the chamber were
connected to the BEM, in order to have available the DC power lines outside the chamber. Output
– 16 –
2009 JINST 4 T12003
Figure 23. 30 GHz QM BEM attached to the shaker.
Table 5. Acceptance random vibration test levels.
Frequency (Hz) Power Spectral Density (g2 /Hz) Grms Time per axis (s)
20
0.0482
80
0.192
Out − o f − plane
220
6.912
(OOP)
240
6.912
25.05
60
300
0.832
500
0.832
2000
0.00822
20
0.0241
80
0.096
In − plane
150
3.456
(IP − NFP, IP − PFP)
170
3.456
20.01
60
300
0.704
500
0.704
2000
0.00411
Axis
channel signals and DC consumption were continuously monitored by the Agilent 34970A Data
Logger with the 34901A switching unit.
As it is shown in figure 25, the BEM waveguide inputs were left open. In order to avoid the
detection of any noise signal, an aluminium wall, covered with a layer of microwave absorber, has
been located at a distance of 2.5 cm from the BEM.
5.2
Electromagnetic compatibility
Very strict EMC tests were performed on BEM units, according to specifications. Both conducted
and radiated emission and susceptibility tests were made. Interfering signals covering the range
from 30 Hz to 18 GHz depending on the individual test were used. Radiated tests included electric
and magnetic fields emission and susceptibility. In the case of conducted EMC tests the emission
and the susceptibility were tested through the power supply lines ±5 Volt. The most difficult test
to fulfil was the conducted susceptibility, because the BEM units do not have DC to DC converters
– 17 –
2009 JINST 4 T12003
Figure 24. Thermal profile for a FM unit test (RPT instants highlighted with squares).
inside, and power supply fluctuations appeared at the BEM detected output. Special filtering on the
power supply input lines was used to avoid susceptibility at low frequencies.
6
Conclusions
Back End Modules at 30 and 44 GHz for Planck Low Frequency Instrument have been designed,
manufactured and tested. They have successfully fulfilled the electrical, mechanical and environmental requirements of Planck satellite mission. Qualification model units and Flight Model units
have demonstrated the compliance with the required performances.
Acknowledgments
This work has been supported by the Spanish ”Plan Nacional de I+D+i”, Programa Nacional de
Espacio, grants ESP2002-04141-C03-01/02/03 and ESP2004-07067-C03-02. The authors would
like to thank Eva Cuerno and Alexandrina Pana for the assembly of the BEM prototypes. Planck is
a project of the European Space Agency with instruments funded by ESA member states, and with
special contributions from Denmark and NASA (USA). The Planck-LFI project is developed by an
International Consortium lead by Italy and involving Canada, Finland, Germany, Norway, Spain,
Switzerland, UK, USA.
References
[1] N. Mandolesi et al., Planck pre-launch status: the Planck-LFI program, accepted by Astron. Astrophys.
(2009).
[2] R.J. Davis et al., Design, development and verification of the 30 and 44 GHz front-end modules for the
Planck Low Frequency Instrument, 2009 JINST 4 T12002.
[3] E.J. Blum, Sensibilité des Radiotélescopes et Récepteurs à Correĺation, Ann. Astrophys. 22 (1959) 140.
[4] M. Bersanelli, N. Mandolesi and J. Marti-Canales, Multi-band radiometer for measuring the cosmic
microwave background, proceedings of 32nd European Microwave Conference, Milan, Italy,
Sept. 2002, Eur. Microw. Conf. 32 (2002) 547.
– 18 –
2009 JINST 4 T12003
Figure 25. BEM fixation to base plate of thermal vacuum chamber.
[5] M. Bersanelli et al., Planck pre-launch status: Design and description of the Low Frequency
Instrument, accepted by Astron. Astrophys. (2009).
[6] B. Aja et al., Q-Band monolithic GaAs PHEMT low noise amplifiers: comparative study of depletion
and enhancement mode transistors, proceedings of GAAS 2002 Symposium, Milan, Italy, Sept. 2002,
pp. 53–56.
[7] G.L. Matthaei, L. Young and E.M. Jones, Microwave Filters Impedance-Matching Networks and
Coupling Structures, Mc. Graw-Hill (1964).
[9] W.J.R. Hoefer and M. Burton, Closed-Form Expressions for the Parameters of Finned and Ridged
Waveguides, IEEE MTT 30 (1982) 2190.
[10] B. Aja et al., A new method to obtain total power receiver equivalent noise temperature, proceedings
of the 33rd European Microwave Conference, Munich, Germany, Oct. 2003,
Eur. Microw. Conf. 33 (2003) 355.
[11] A. Mennella et al., The linearity response of the Planck-LFI flight model receivers,
2009 JINST 4 T12011.
– 19 –
2009 JINST 4 T12003
[8] M. Detratti et al., Millimetre Wave Broadband Bandpass Microstrip Filters. Design and Test
proceedings of 32nd European Microwave Conference, Milan, Italy, Sept. 2002,
Eur. Microw. Conf. 32 (2002) 573.