**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).

![](media/image1.wmf){width="6.315972222222222in" height="4.14375in"}

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.

![](media/image2.wmf){width="5.976388888888889in"
height="4.101388888888889in"}

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.

![](media/image3.wmf){width="5.786111111111111in"
height="3.970833333333333in"}

Figure 3. Calibration curve for IDS miniature Search Coil Magnetometer

![](media/image4.wmf){width="5.995833333333334in" height="4.1in"}

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.

![](media/image5.wmf){width="4.761805555555555in"
height="3.2493055555555554in"}

Figure 5. Response function for a typical IDS E-field spectrometer
channel

![](media/image6.wmf){width="5.999305555555556in" height="4.11875in"}

Figure 6. Filter response for IDS PWS E-field low-frequency channels.

Table 1. IDS PWS E-Field (PWA) Low-Frequency Channel Characteristics

+----------+--------------+----------------+----------+--------------+
| #        | **Center     | **Bandwidth**  | **Q**    | # Gain       |
|  Channel | Frequency**  |                |          |              |
|          |              | **(Hz)**       |          | ## **Co      |
|          | **(Hz)**     |                |          | mpensation** |
+----------+--------------+----------------+----------+--------------+
| 0        | 15           | 10             | 1.5      | 34.5         |
+----------+--------------+----------------+----------+--------------+
| 1        | 28           | 16             | 1.8      | 8.4          |
+----------+--------------+----------------+----------+--------------+
| 2        | 45           | 26             | 1.8      | 4.4          |
+----------+--------------+----------------+----------+--------------+
| 3        | 76           | 23             | 3.3      | 1            |
+----------+--------------+----------------+----------+--------------+
| 4        | 137          | 76             | 1.8      | 1.25         |
+----------+--------------+----------------+----------+--------------+
| 5        | 235          | 131            | 1.8      | 1            |
+----------+--------------+----------------+----------+--------------+
| 6        | 412          | 238            | 1.7      | 1.1          |
+----------+--------------+----------------+----------+--------------+
| 7        | 784          | 450            | 1.7      | 1            |
+----------+--------------+----------------+----------+--------------+
| 8        | 1001         | 572            | 1.8      | 0.99         |
+----------+--------------+----------------+----------+--------------+
| 9        | 1812         | 1018           | 1.8      | 0.99         |
+----------+--------------+----------------+----------+--------------+
| 10       | 3281         | 1854           | 1.8      | 0.99         |
+----------+--------------+----------------+----------+--------------+
| 11       | 6510         | 3657           | 1.8      | 0.945        |
+----------+--------------+----------------+----------+--------------+
| 12       | 11960        | 6757           | 1.8      | 1            |
+----------+--------------+----------------+----------+--------------+
| 13       | 23200        | 13260          | 1.8      | 1            |
+----------+--------------+----------------+----------+--------------+
| 14       | 42160        | 24800          | 1.7      | 1            |
+----------+--------------+----------------+----------+--------------+
| 15       | 82400        | 49000          | 1.7      | 1            |
+----------+--------------+----------------+----------+--------------+

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.

![](media/image7.wmf){width="6.498611111111111in"
height="4.461805555555555in"}

Figure 7. Filter response for IDS PWS E-field high-frequency channels.

Table 2. IDS PWS E-Field (PWA) High-Frequency Channel Characteristics

+-----------+--------------+----------------+----------+-------------+
| # Channel | **Center     | **Bandwidth**  | **Q**    | # Gain      |
|           | Frequency**  |                |          |             |
|           |              | **(Hz)**       |          | ## **Com    |
|           | **(Hz)**     |                |          | pensation** |
+-----------+--------------+----------------+----------+-------------+
| 0         | 170k         | 170k           | 1        | 0.9         |
+-----------+--------------+----------------+----------+-------------+
| 3         | 360k         | 327k           | 1.1      | 0.55        |
+-----------+--------------+----------------+----------+-------------+
| 2         | 730k         | 561K           | 1.3      | 0.58        |
+-----------+--------------+----------------+----------+-------------+
| 4         | 1.6M         | 1.8M           | 0.9      | 3.2         |
+-----------+--------------+----------------+----------+-------------+
| 7         | 2.6M         | 3.3M           | 0.8      | 2           |
+-----------+--------------+----------------+----------+-------------+
| 6         | 13.5M\*      | 32M            | \*       | 1.75        |
+-----------+--------------+----------------+----------+-------------+
| 5         | 13M          | 18.6M          | 0.7      | 6.2         |
+-----------+--------------+----------------+----------+-------------+
| 1         | 34M          | 15.5M          | 2.2      | 0.8         |
+-----------+--------------+----------------+----------+-------------+

\* Channel six is best-fit with two second order band path filters with
their two center frequencies at 9MHz and 18MHz

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

+--------+-------------+-----------+-------+-------+-----------------+
| # C    | Center      | **Ba      | **Q** | #     | # Pre-a         |
| hannel | Frequency   | ndwidth** |       |  Gain | mp Scale Factor |
|        |             |           |       |       |                 |
|        | **(Hz)**    | **(Hz)**  |       | **    | **pT/V**        |
|        |             |           |       | Adj** |                 |
+--------+-------------+-----------+-------+-------+-----------------+
| 0      | 14          | 8         | 1.6   | 17.5  | 9.713E8         |
+--------+-------------+-----------+-------+-------+-----------------+
| 1      | 28          | 15        | 1.8   | 5.8   | 2.888E8         |
+--------+-------------+-----------+-------+-------+-----------------+
| 2      | 44          | 24        | 1.8   | 3.15  | 1.314E8         |
+--------+-------------+-----------+-------+-------+-----------------+
| 3      | 77          | 26        | 3     | 1     | 4.080E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 4      | 131         | 65        | 2     | 1.16  | 4.652E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 5      | 223         | 106       | 2.1   | 0.95  | 3.620E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 6      | 408         | 227       | 1.8   | 1     | 3.630E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 7      | 767         | 426       | 1.8   | 1     | 3.260E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 8      | 1000        | 556       | 1.8   | 1\.   | 3.110E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 9      | 1800        | 1000      | 1.8   | 1     | 2.785E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 10     | 3300        | 1833      | 1.8   | 1     | 2.700E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 11     | 6505        | 3614      | 1.8   | 1     | 2.530E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 12     | 11900       | 6611      | 1.8   | 1     | 2.595E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 13     | 23000       | 12778     | 1.8   | 1     | 3.336E7         |
+--------+-------------+-----------+-------+-------+-----------------+
| 14     | 43200       | 24000     | 1.8   | 1     | 1.483E8         |
+--------+-------------+-----------+-------+-------+-----------------+
| 15     | 81800       | 45444     | 1.8   | 0.99  | 4.069E8         |
+--------+-------------+-----------+-------+-------+-----------------+

### 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

![](media/image8.wmf){width="5.5680555555555555in"
height="3.4180555555555556in"}

Figure 8. Amplitude conversion for IDS low-frequency E-field
spectrometer.

![](media/image9.wmf){width="5.995833333333334in" height="4.1in"}

Figure 9. Amplitude conversion for IDS high-frequency E-field
spectrometer.

![](media/image10.wmf){width="5.691666666666666in" height="3.9in"}

Figure 10. Amplitude conversion for IDS low-frequency B-field
spectrometer.