Description and Calibration Approach for DS1
IPS Diagnostics Sensors:
Plasma Wave Spectrometer Channels
David E. Brinza and Michael D. Henry
Jet Propulsion Laboratory, Pasadena, California
(PWS_describe4_1_calib.doc)
Plasma Wave Spectrometer Description
====================================
The Deep Space 1 (DS1) Ion Propulsion Subsystem (IPS)
Diagnostic Sensor (IDS) includes a Plasma Wave
Spectrometer (PWS) to characterize the AC electric and
magnetic field environment. A block diagram of the IDS
PWS is provided in Figure 1. The PWS includes a 2-meter
tip-to-tip dipole Plasma Wave Antenna (PWA, E-field)
and a miniature Search Coil Magnetometer (SCM, B-field)
sensor. A second higher sensitivity SCM was integrated
in the IDS, but had failed on the launch pad when
subjected to high-level 60 Hz AC fields. The PWA has a
pre-amplifier with voltage-controlled gain at the
antenna. The SCM pre-amplifier is integral with the
PWS circuit board. The PWS provides 24 spectrometer
band-pass (16 ELFagc and 8 EHFagc) channels for the
PWA measurement over the range of 15 Hz to 34 MHz and
16 (BLFagc) channels for the SCM (B-field) measurement
over the range of 14 Hz to 82 kHz. The PWS also outputs
waveform signals (ERAWout, BRAWout) through a 10 kHz
low-pass filter to the IDS Fields Measurement Processor
(FMP).
Figure 1. IDS Plasma Wave Spectrometer (PWS) block diagram
PWA and SCM Calibration
=======================
The PWA pre-amplifier was separately characterized with
the frequency response determined to be flat between the
20 Hz and 30 MHz 3dB points. The voltage-controlled gain
was determined using an input signal at 1 kHz and varying
the gain voltage over the operating range. The pre-amp
gain response is shown in Figure 2. The "end-to-end" gain
response (upper) line reflects the internal 20dB amplifier
in the low-pass filter for the waveform output channel
(ERAWout) in the PWS. The upper curve in Figure 2 is used
to generate engineering unit conversion for time-domain
plasma wave signals.
Figure 2. Response of the PWA voltage-controlled gain
pre-amplifier
The SCM sensor was separately calibrated in a magnetometer
calibration facility (mu-metal shielded housing and known
solenoid magnetic field source) to provide Volts/Tesla
response over the nominal frequency range (7 Hz to 25.6
kHz) as shown in Figure 3. For B-field time-domain
measurements, the engineering unit conversion factor was
taken to be 40,000 V/T.
Variable-gain for the SCM sensors was achieved via a
voltage-controlled amplifier within the PWS circuit board.
The SCM signal was amplified and then routed to both the
spectrometer and the time-domain signal chains. The
time-domain signal chain (BRAWout) includes a low-pass (10
kHz) filter with an additional 20 dB gain. The
voltage-controlled amplifier response function for the SCM
is shown in Figure 4.
Figure 3. Calibration curve for IDS miniature Search Coil
Magnetometer
Figure 4. Response of the SCM voltage-controlled gain
pre-amplifier.
Plasma Wave Spectrometer Calibration
====================================
Calibration of the PWA (E-field) and SCM (B-field) sensor
spectrometer channels for the IPS Diagnostics Sensors
(IDS) were performed with the flight hardware during
final thermal testing. The low-frequency E-field and
B-field spectrometer channels are derived from
voltage-controlled fourth-order band-pass filters with
constant Q. Calibration signals were injected directly
into the PWA dipole antenna leads at the pre-amp and the
external SCM channel inputs shown in Figure 1. The
calibration activity described herein was intended to
characterize the center frequency, bandwidth and
sensitivity of the IDS spectrometer channels.
The IDS spectrometer channel calibration was performed
over the operating temperature range (-25C to +50C). At
elevated temperatures (>40C), internal oscillation was
observed in the 400 Hz to 700Hz frequency in the
spectrometer board. Fortunately, the IDS spectrometer
board remained below +25C throughout most of the DS1
mission, and was approximately 15C during the Comet
Borrelly encounter. Very little sensitivity to
temperature was observed during calibration over the
temperature range of -25C to +25C, thus only the +25C
calibration data set was analyzed to produce the
engineering unit conversions for the spectrometer
channels.
Characterization of the spectrometer channels was
performed by injecting sinusoidal signals of known
frequency and amplitude into the spectrometer inputs.
For frequency response characterization, the input
amplitude was held constant (100 mV peak-to-peak) and
the injected signal frequency was varied stepwise
from 1 Hz to 50 MHz in logarithmic fashion. A few
channels were characterized with higher frequency
resolution to verify the response of tunable band-pass
circuit. The frequency characterization data for
Channel 7 (E-field, 780 Hz) is accurately fit by a
fourth-order band-pass filter response function as
shown in Figure 5. The band-pass filter response
functions for the 16 low-frequency E-field channels are
show in Figure 6 with the best-fit characteristic
parameters listed in Table 1.
Figure 5. Response function for a typical IDS E-field
spectrometer channel
Figure 6. Filter response for IDS PWS E-field
low-frequency channels.
Table 1. IDS PWS E-Field (PWA) Low-Frequency Channel
Characteristics
The modeled response functions for the 8 high-frequency
E-field channels are plotted in Figure 7 with the
best-fit parameters listed in Table 2. Two of the
high-frequency channels exhibit anomalous behavior as
will be described presently. When tested as a
stand-alone board, PWS high-frequency channels 5 and 6
had center frequencies near 15 MHz and 7.1 MHz
respectively. Subsequent to assembly and environmental
test, some form of coupling and large baseline offsets
developed for these channels (now the apparent center
frequencies are near 13 MHz and channel 6 is best fit by
two second-order filters). Also in reviewing the
calibration data we found that approximately 40% of the
amplitude of signals in the 34MHz channel are coupled
into the two ~13MHz channels. Due to the late discovery
of these issues and a tight delivery schedule for the
hardware, the source of these anomalys could not be
investigated. The high-frequency channels above 1 MHz are
of little scientific interest for the comet encounter,
but were valuable for measurement of plasma noise
produced by the DS1 ion engine.
Figure 7. Filter response for IDS PWS E-field high-frequency
channels.
Table 2. IDS PWS E-Field (PWA) High-Frequency Channel
Characteristics
The spectrometer channels for the search coil (B-field)
sensor were also characterized during IDS calibration.
Response functions for the 16 low-frequency channels
were determined by injecting signals from a function
generator directly into the search coil voltage inputs
of the PWS board. The conversion factors from voltage to
B-field (Figure 3) for each channel were take at the
center frequency and are listed with the other B-field
spectrometer channel characteristics in Table 3.
Table 3. IDS PWS B-field (Search Coil) Low-Frequency Channel
Characteristics
Amplitude Response Function
===========================
Amplitude response characterization was performed at
constant frequency (2 kHz) while increasing the input
amplitude from 1 mV to 1 V (peak-to-peak). The AGC
detector circuit has an active "log-linear" region and
highly non-linear response at amplitude extremes (<30 mV
or > 3V peak-to-peak). Since transient events can easily
saturate the detection circuit, a substantial amount of
spectrometer data has been recorded using low gain at the
pre-amp stage. A significant effort was taken to
characterize the low-end response of the IDS PWS. The
amplitude response for the low-frequency PWS (E-field) is
shown in Figure 8. Similar amplitude sweeps where
performed for the high-frequency PWS(E-field) as shown in
Figure 9 and the search coil (B-field) spectrometer as
shown in Figure 10
Figure 8. Amplitude conversion for IDS low-frequency
E-field spectrometer.
Figure 9. Amplitude conversion for IDS high-frequency
E-field spectrometer.
Figure 10. Amplitude conversion for IDS low-frequency
B-field spectrometer.