12.1 WBR Subsystem Description

This section of the RPWS Calibration Document describes the Wideband Receiver (WBR), extending the description contained in the Section 3 instrument overview. The WBR subsystem provides very high frequency/time resolution electric and magnetic field measurements for a selection of frequency passbands, and is similar to the wideband receivers previously designed for the Voyager, Galileo, Polar and Cluster missions.

12.1.1 General WBR Characteristics

The WBR processes signals from one of the electric or magnetic sensors, or from the Langmuir Probe, as selected by instrument command A block diagram of the WBR subsystem is shown in Figure 12.1.1.1.

Input signals for the WBR are chosen via an analog antenna selection switch located at the receiver input. The output of this switch goes to a 50 Hz to 80 kHz bandpass filter which sets the bandwidth of the input waveform. To provide the capability for obtaining waveforms at frequencies above 80 kHz, the HFR is utilized to down-convert higher frequency signals into a 50 kHz to 75 kHz bandpass (HF input in Figure 12.1.1.1).

To allow the WBR to accommodate the large (>100 dB) dynamic range of input signals, a set of gain amplifiers are used to regulate signal amplitudes in the receiver in steps of 10 dB over a range of 0 dB to 70 dB. The output from the gain select amplifiers then goes to a 50 Hz to 10 kHz and a 1 kHz to 80 kHz bandpass filter. A switch determines which filter output is sent to the HRP for 8-bit sampling, and also to a peak detector. The peak detector creates a voltage that is used by an automatic gain control (AGC) implemented in software.

12.1.2 Receiver Inputs

The WBR subsystem has the capability of processing signals from one of the electric or magnetic field sensors, or from the Langmuir Probe. The WBR is provided with the following signal inputs:

Sensor Designation	Description

ExLo	10 meter electric antennas. The HFR can provide for the ExLo input either the dipole configuration (default); or either the Ex+ or Ex- as a monopole

EzLo	10 meter electric antenna (monopole)

HF	High frequency electric field signals are down-converted by the HFR into a 50 kHz to 75 kHz band. This mode requires that the WBR 1 kHz to 80 kHz output passband be selected. The possible frequency bands for conversion are any between 150 kHz and 4.125 MHz, and any band except those centered at multiples of 50 kHz between 4.125 and 16 MHZ. It should be noted that HF input signals have been through the HFR AGC, and thus are kept at a fairly constant amplitude. Also, proper use of the HF input mode restricts the WBR band select to the 1 kHz to 80 kHz passband.

Bx	Magnetic search coil antenna with axis aligned with spacecraft x-
axis

LP	Langmuir Probe

12.1.3 Output Passbands

The bandwidth of the WBR output waveform is determined by one of two bandpass filters selected by the HRP. The bandwidths of the WBR output waveform and associated sampling rates are:

Filter	Sampling Rate

50 Hz - 10 kHz	36 s

1 kHz - 80 kHz	4.5 s

12.1.4 Gain Select

The gain select stage of the WBR employs three switchable amplifiers (0/10 dB, 0/20 dB, and 0/40 dB) which may be commanded via the HRP to provide gain combinations of 0 dB to 70 dB in 10 dB increments.

Additionally, the WBR has the capability of auto-ranging through the gain steps via an automatic gain control (AGC) loop. The automatic gain select function is controlled by the HRP and allows the WBR to respond to large changes in signal intensity. In this operational mode, the signal from the output bandpass filter (50 Hz to 10 kHz or 1 kHz to 80 kHz) goes through a peak detector stage as shown in the block diagram. The output from the peak detect, after sampling, is then compared in software to a pair of reference amplitudes, and the gain state is either increased by one step (10 dB) or decreased by one step accordingly for the next set of contiguous samples. In order to avoid excessive toggling between gain steps, a threshold must be exceeded in either direction, thereby introducing hysteresis in the AGC loop. The gain update rate is adjustable, with dwell times of 0.25 seconds or greater possible, depending on output data mode. The autoranging gain capability allows the WBR to respond to large changes in input signal levels so as to maximize the signal-to-noise ratio and to avoid saturation of the receiver.

12.1.5 Receiver Output

The output signal from the WBR is routed to the HRP to be sampled (8 bits), and

packed into WBD mini-packets before being transmitted to the ground. To increase the amount of wideband waveform data which can be transmitted with the available telemetry, the output of the WBR is usually compressed.

12.1.6 Control

WBR control functions which determine instrument modes are managed by the HRP. The WBR receives the following control inputs:

Function

Antenna Select

Band Select

Gain Select

Gain Time Constant

12.1.7 Power

The WBR is provided with four power supply lines. The regulated supply voltages and measured average current loads are as follows:

+12 V	5.5 mA

−12 V	6.9 mA

+ 6 V	39.0 mA

− 6 V	39.0 mA

The total power requirement for the WBR is 313 mW.

12.2 Converting WBR Telemetry Values to Physical Units

In this section, the procedure for obtaining calibrated data values for the Wideband Receiver is described.

1.	First a snapshot consisting of 2 N raw WBR data samples is obtained. These data samples are 8-bit unsigned values (0-255).

2.	The DC component is obtained by averaging the 2 N samples. The DC component is subtracted from the 2 N samples, because the receiver is an AC-coupled system and the DC-component is not related to any sensor measurement. Furthermore, removal of the DC offset is needed for the next step.

3.	The proper counts-to-voltsrms factor is now applied to the data. This is the amplitude in counts of a sine wave in the middle of the pass-band which would be measured by the WBR if a 1 Voltrms signal were injected into the Eu differential amplifier and the WBR had no gain amplifier turned on. The 2 N samples are all divided by this factor. Throughout the calibrations, another way this is expressed is the decibels below a maximum amplitude sine wave; for 8-bit data, which can range 0 through 255, this is dB below a sine wave of amplitude 127.5 counts (or 255 counts peak-to-peak) and it is referred to as dBmax. This factor depends upon the filter mode:

Translation Mode	Filter	counts/Vrms	dBmax

Baseband	10 kHz	264.25	+6.33

Baseband	80 kHz	267.31	+6.43

4.	Next the WBR gain amplifier value must be divided out. The gain can vary from 0 dB through 70 dB in steps of 10 dB. If the gain is G, then the 2 N samples should all be divided by 10G/20.

5.	Next a Hanning window can be applied to the 2 N samples. For a Hanning window the coherent gain is 0.5 (see "Digital Filter Design Handbook" by Fred J. Taylor). Therefore the 2 N samples must then be multiplied by 2. The equation for a Hanning window is Hi=0.5*(1-cos(2*P*i/(2 N-1))) for i=0, 2 N-1.

6.	An FFT should be performed on the 2 N samples. The FFT output must be normalized using whatever normalization factors are necessary to give the amplitude of a sine wave when real and imaginary parts are squared, summed and the square root taken. The normalization factors may be different for different FFT implementations. The phase information from the FFT is not useful, so the magnitudes can now be calculated by squaring the real and imaginary parts, summing them and taking the square root. This yields voltsrms in each FFT bin. This is the voltage difference Vdiff between the Ex+ and Ex- antennas at the frequency corresponding to that FFT bin.

7.	Once the voltage Vdiff is found, the sensor-dependent conversion factor must be used to adjust the voltage. The voltage difference between the Ex+ antenna and the Ex- antenna is given the symbol VDEx. Since the voltage difference Vdiff calculated in steps 1 through 6 is for the voltage difference between Ex+ and Ex-, no conversion is necessary for VDEx. VDEx is equal to Vdiff. The voltage between the Ex+ antenna and spacecraft ground is given the symbol VEx+. The voltage between the Ex- antenna and spacecraft ground is given the symbol VEx-. The voltage between the Ez antenna and spacecraft ground is given the symbol VEz. The voltage out of the Bx magnetic preamp is given the symbol Vbxpa. Figure 12.2.1 shows the location of these voltages. The conversion factors and their symbols are listed below. The conversion factors convert the Vdiff values to the voltage at the various sensors.

Symbol	Factor

CF+ex	1.0	Conversion factor for a monopole using the the Ex+ antenna

CF-ex	1.0	Conversion factor for a monopole using the the Ex- antenna

CFez	1.0	Conversion factor for the Ez antenna

CFbx	24.0	Conversion factor for the Bx Search Coil

The formulas for converting the Vdiff value to the sensor input voltage are:

VDEx	=	Vdiff	(units are Vrms)

VEx+	=	(CF+ex)*(Vdiff)

VEx-	=	(CF-ex)*(Vdiff)

VEz	=	(Cfez)*(Vdiff)

Vbxpa	=	(Cfbx)*(Vdiff)

8.	Now the sensor-dependent calibrations must be applied. For the Bx Search Coil, the voltage at the output of the Search Coil preamplifier must be converted to nanotesla. See Table 7.1.1 in the section about the Magnetic Search Coils for the frequency-dependent conversion factors. For a particular frequency let this conversion factor be given the symbol CBxnt; for example, Table 7.1.1 shows that at 1000 Hz the Bx Search Coil's response is 147.8 mV/nT, so CBxnt is 0.1478. To convert from voltage at the output of the Bx Search Coil preamplifier, VBxpa must be divided by CBxnt. This is shown in the following equation:

NTBx	= VBxpa / CBxnt	(units are nTrms)

NTBx is the field detected by the Bx Search Coil. At this point we have meaningful physical units. For the electric antennas we have voltage measured at the antenna elements. For the magnetic sensor we have the magnetic field in nanotesla at the sensor. Next the magnitude of the electric field for the electric antennas can be obtained. The voltages at the antennas are divided by the effective antenna length, producing units of volts per meter. Here the effective antenna lengths are defined as the physical distances between the geometric centers of the antennas for the Ex dipole, and the geometric center for the Ez antenna and also for the individual monopole configurations of Ex+ and Ex-. The effective length for the Ex+ to Ex- dipole antenna configuration is given the symbol Lexdelta. The Ex+ monopole effective length is given the symbol L+Ex. The Ex- monopole effective length is given the symbol L-Ex. The Ez monopole effective length is given the symbol LEz. For a more detailed discussion of the effective lengths of the electric antennas, see Section 5.

Effective Antenna Lengths in Meters for various mode configurations

Antenna	Effective Length (meters)	Configuration

LExdelta	8.66	dipole

L+Ex	5.00	monopole

L-Ex	5.00	monopole

LEz	5.00	monopole

The electric field on the Ex dipole is represented by VMExdelta, the electric field on the Ex+ monopole by VM+Ex, the electric field on the Ex- monopole by VM-Ex, and the electric field on the Ez monopole by VMEz. The following equations show the method for calculating the electric field on the antenna in volts per meter. These equations do not include effects due to stray capacitive divider effects between the antennas and the spacecraft.

VMExdelta	=	(VDEx) / (LExdelta)

VM+Ex	=	(VEx+) / (L+Ex)

VM-Ex	=	(VEx-) / (L-Ex)

VMEz	=	(VEz) / (LEz)

9.	Finally, to obtain power spectral density, one must square the value and divide by the bandwidth, which depends upon the type of window function used before performing the FFT. If no window function is used, then the equivalent noise bandwidth is the FFT bin width, which is the sample frequency divided by the number of samples (fS/2 N). If the Hanning window suggested above is used, then the equivalent noise bandwidth is 1.5 times the FFT bin width (again see "Digital Filter Design Handbook" by Fred J. Taylor).

As an illustration of how to obtain calibrated values from WBR telemetry, let us look at some real data:

Example 1: SCET 1997-October-25-UT06:34:25 (SCLK 4AE3F190.1)

After electric antenna deployment, a broad-band emission is examined. The WBR was in 10- kHz mode with 60 dB of gain and connected to the Ex dipole. A waveform of length 2048 was captured. Following the recipe for obtaining calibrated values, the DC component is computed and subtracted from all 2048 samples. Since the filter mode is 10-kHz, all 2048 samples are then divided by 264.25 counts per Vrms, and also divided by the gain amplifier factor, 1060/20 (or 1000). A Hanning window is applied to the data and then all windowed values are multiplied by 2 to offset the Hanning window's coherent gain. An FFT is performed on the data, and after normalizing, the magnitudes of the resulting 1024 components are computed by squaring the real and imaginary parts of the complex outputs, summing them, and taking the square root. The broad-band emission under examination extends from about 1 kHz to 3 KHz and is about 10 to 20 dB above the Ex dipole's noise floor. The peak amplitude occurs at frequency 2835 Hz, and this peak amplitude is Vdiff at 2.835 kHz:

Vdiff	=	1.33x10-5 Vrms.

Since the sensor is the Ex dipole, we know that the voltage VDEx is the same as Vdiff. So the electric field strength can be calculated directly:

VMExdelta	=	(VDEx) / (LExdelta)

=	1.33x10-5 / 8.66

=	1.54x10-6 Vrms/meter.

Finally, to determine the electric field power spectral density, we must square this value and divide by the equivalent noise bandwidth. Since we have 10-kHz mode, the sample frequency is given by

Fs	=	1. / 36.x10-6

=	27778 Hz

and since the sample set size is 2048, the FFT bin size is given by

Df	=	Fs / 2048

=	27778 / 2048

=	13.563 Hz.

But since a Hanning window was applied to the data, the equivalent noise bandwidth is given by

ENBW	=	1.5 * Df

=	20.345 Hz.

So the power spectral density at 2.8 kHz is given by

PSD	=	(VMExdelta)2 / ENBW

=	(1.54x10-6) 2 / 20.345

=	1.17x10-13 Vrms2 / meter2 Hz.

Example 2.	SCET 1996-July-31-UT18:52:32 (SCLK 31FFAB70.1)

During unit level testing the Bx Search Coil was stimulated with a 1 kHz tone at 0.5 Vrms, which should have produced a magnetic field of 0.5 nTrms. The WBR was in 80-kHz mode with 30 dB of gain. A waveform of length 2048 was captured. The raw data shows a sine wave with maximum value of 152 and a minimum value of 109, giving an initial estimate of 43 counts peak-to-peak for the sine wave. Following the recipe for obtaining calibrated values, the DC component is computed and found to be 130.35 counts. This value is subtracted from all 2048 samples. A Hanning window is applied to the data and then all windowed values are multiplied by 2 to offset the Hanning window's coherent gain. An FFT is performed on the data, and after normalizing, the magnitudes of the resulting 1024 components are computed by squaring the real and imaginary parts of the complex outputs, summing them, and taking the square root. The peak amplitude occurs in the 9th FFT bin, corresponding to frequency 977 Hz, and this peak amplitude is 19.46 counts (note that this is close to our initial estimate of 21.5 counts for the amplitude). Dividing this by the calibration factor of 267.31 counts/Vrms and also by the gain factor of 10G/20, we obtain Vdiff at 1 kHz:

Vdiff	=	19.46 / (267.31*1030/20)

=	19.46 / (267.31*31.62)

=	2.30x10-3 Vrms

Since the sensor is the Bx Search Coil, we now want to know what the voltage VBxpa from the Search Coil preamplifier is. It is:

Vbxpa	=	(CFbx)*(Vdiff)

=	24*2.30x10-3	Vrms

=	55.25 mVrms

Now referring to the Search Coil calibration in Table 7.1.1, we find that at 1000 Hz the Bx Search Coil's response CBxnt is 147.8 mV/nT. So we find that the magnetic field strength NTBx is

NTBx	=	VBxpa / CBxnt	(units are nTrms)

=	55.25 / 147.8 nTrms

=	0.374 nTrms

This is probably more correct than the 0.5 nTrms presumed value, since the MFR also gives a similar answer. In this case, it makes little sense to calculate spectral density since the signal is a tone.

12.3 WBR-Specific Calibration Tests and Results

12.3.1 WBR Frequency Responses

In this section, frequency response calibration data for the wideband receiver are presented. Transfer functions for each of the filters and translation modes is shown. Figure 12.3.1.1 illustrates the stimulus configuration used for the WBR frequency responses. The oscillator output is attenuated by a fixed amount and a balancing transformer is used to drive the Ex+/- inputs differentially.

During the calibrations, the HP workstation commands the oscillator to step in frequency. For each frequency step, a waveform is captured and telemetered to the spacecraft stimulator. An FFT is performed on the data after removing the DC component. The peak amplitude is chosen from the frequency-domain result, and the peak frequency (which should be very close to the stimulus frequency) is chosen. The amplitude is calculated by a simple time domain rms average and then is adjusted by 21/2/2 to give the amplitude of the sine wave in WBR counts. This rms calculation yields a value very close to the FFT result at the peak frequency, but does not suffer from the "scallop loss" effect of the FFT results, which are sensitive to the proximity of the frequency to the center of an FFT bin. For further discussion on this effect, see Section 12.3.2, where the FFT amplitudes of the peak frequencies are presented for the gain calibrations. Finally, the amplitude is expressed in dBmax, i.e., 0 dB corresponds to a sine wave of amplitude 127.5 counts.

Although the frequency response could be used to adjust the calibrations of the WBR data, it is recommended that this not be done, since the noise floor becomes accentuated outside the pass- band. Instead, one should simply note from the frequency response tests that the filters are flat to within a dB or less in the pass-band. And data should be ignored if it is outside the 3-dB points. From the frequency response curves, those 3-dB points are approximately:

Translation Mode	Filter	3-dB points

Baseband	80 kHz	0.80 - 75 kHz

Baseband	10 kHz	0.06 - 10.65 kHz

Table 12.3.1.1 WBR 10 kHz Mode Frequency Response

Frequency

(Hz)	Amplitude

(dBmax)	Frequency

(Hz)	Amplitude

(dBmax)
27.13	-40.33	1152.89	-13.74
40.69	-20.79	1220.70	-13.77
54.25	-18.04	1315.65	-13.81
67.82	-14.91	1383.46	-13.84
81.38	-14.15	1478.41	-13.90
94.94	-13.82	1627.60	-13.99
108.51	-13.74	1722.55	-14.03
122.07	-13.48	1831.05	-14.07
149.20	-13.45	1966.69	-14.12
162.76	-13.41	2088.76	-14.16
203.45	-13.46	2224.39	-14.21
217.01	-13.41	2360.03	-14.25
230.58	-13.43	2522.79	-14.30
244.14	-13.37	2699.11	-14.35
271.27	-13.41	2861.87	-14.38
284.83	-13.40	3051.76	-14.40
298.39	-13.41	3255.21	-14.42
311.96	-13.36	3472.22	-14.44
339.08	-13.39	3702.80	-14.45
366.21	-13.37	3960.50	-14.45
393.34	-13.41	4218.21	-14.44
406.90	-13.41	4503.04	-14.44
434.03	-13.43	4801.43	-14.43
461.15	-13.42	5126.95	-14.43
501.84	-13.42	5466.04	-14.42
528.97	-13.43	5832.25	-14.41
569.66	-13.46	6212.02	-14.40
610.35	-13.49	6618.92	-14.40
651.04	-13.50	7066.51	-14.37
691.73	-13.52	7541.23	-14.34
732.42	-13.53	8043.08	-14.33
773.11	-13.55	8558.49	-14.38
827.37	-13.58	9168.84	-14.56
881.62	-13.61	9752.06	-14.81
935.87	-13.64	10430.23	-15.17
1003.69	-13.67	11121.96	-19.87
1071.51	-13.71	11840.82	-30.33

Table 12.3.1.2 WBR 80 kHz Mode Frequency Response

Frequency

(Hz)	Amplitude

(dBmax)	Frequency

(Hz)	Amplitude

(dBmax)
976.56	-23.99	50998.26	-24.27
1953.12	-24.00	51974.83	-24.32
3038.19	-24.16	52951.39	-24.34
4014.76	-24.15	54036.46	-24.41
4991.32	-24.03	55013.02	-24.49
5967.88	-23.93	55989.58	-24.54
7052.95	-23.83	56966.15	-24.59
8029.51	-23.76	58051.22	-24.66
9006.08	-23.64	59027.78	-24.75
9982.64	-23.57	60004.34	-24.81
10959.20	-23.51	60980.90	-24.93
12044.27	-23.47	61957.47	-24.99
13020.83	-23.41	63042.53	-25.08
13997.40	-23.39	64019.10	-25.18
14973.96	-23.39	64995.66	-25.31
15950.52	-23.38	65972.22	-25.40
17035.59	-23.35	66948.78	-25.54
18012.15	-23.37	68033.85	-25.73
18988.72	-23.34	69010.42	-25.88
19965.28	-23.31	69986.98	-26.05
21050.35	-23.38	70963.54	-26.21
22026.91	-23.35	72048.61	-26.29
23003.47	-23.35	73025.17	-26.39
23980.03	-23.38	74001.74	-26.51
24956.60	-23.38	74978.30	-26.58
26041.67	-23.42	75954.86	-26.64
27018.23	-23.45	77039.93	-26.72
27994.79	-23.44	78016.49	-26.77
28971.35	-23.47	78993.06	-26.89
29947.92	-23.50	79969.62	-27.19
31032.99	-23.55	81054.69	-27.62
32009.55	-23.58	82031.25	-28.39
32986.11	-23.63	83007.81	-29.60
33962.67	-23.63	83984.38	-31.14
35047.74	-23.67	84960.94	-32.95
36024.31	-23.69	86046.01	-34.92
37000.87	-23.71	87022.57	-36.65
37977.43	-23.77	87999.13	-38.69
38953.99	-23.79	88975.69	-40.29
40039.06	-23.84	89952.26	-41.40
41015.62	-23.87	91037.33	-42.34
41992.19	-23.90	92013.89	-43.25
42968.75	-23.94	92990.45	-44.47
44053.82	-23.99	93967.01	-45.37
45030.38	-24.06	94943.58	-46.05
46006.94	-24.06	95052.08	-46.15
46983.51	-24.10	96028.65	-46.53
47960.07	-24.10	97005.21	-47.18
49045.14	-24.17	97981.77	-47.69
50021.70	-24.22	98958.33	-47.37

12.3.2	WBR Gain Calibration

In this section, the amplitude calibrations for the wideband receiver are presented. Data at selected stimulus frequencies are shown for all filters and translation modes. The stimulus configuration used for the WBR gain calibration is similar to that used for the WBR frequency response tests; see Figure 12.3.2.1. The GSE workstation is used to command the oscillator and programmable attenuators. The attenuator is converted to a differential signal using a balancing transformer. The Ex+ and Ex- inputs are driven differentially. The WBR amplitude response to any of the other sensors may be determined by applying the appropriate sensor and differential amplifier curves to the data here.

The procedure used for the performance of the calibrations is as follows. A command is sent to the oscillator and attenuator to set the desired setting. A waveform is captured by the WBR and telemetered to the spacecraft simulator. An FFT is performed on the WBR capture and both the time and frequency domains are plotted. The amplitude of the the signal at the stimulus frequency is shown on each plot, where the amplitude is given in dB below maximum signal detectable. Throughout the amplitude calibrations, this is expressed is the decibels below a maximum amplitude sine wave; for 8-bit data, which can range 0 through 255, this is dB below a sine wave of amplitude 127.5 counts and it referred to as dBmax. This can be converted to the amplitude in counts of a sine wave in the middle of the pass-band which would be measured by the WBR if a 1 Voltrms signal were injected into the Eu differential amplifier and the WBR had no gain amplifier turned on. The results for the 10-kHz WBR mode are presented in Table 12.3.2.1 and for the 80-kHz WBR mode in Table 12.3.2.2. These results are also presented in graphical form in Figures 12.3.2.1 and 12.3.2.2.

The amplitude calibrations show that the gain amplifiers are very accurate. At any fixed frequency, the variations in the gain are on the order of 1 dB. Furthermore, the variations across the frequency pass-band are mostly due to the "scallop loss" for a Hanning window. The scallop loss is defined as the ratio of coherent gain for a tone located half a bin from an FFT sample point to the coherent gain for a tone at the FFT sample point (see "Digital Filter Design Handbook" by Fred J. Taylor). For a Hanning window the scallop loss is 1.42 dB. White noise does not exhibit the scallop loss evident when using tones; the response is flatter like that shown in Section 12.3.1. Because of these considerations, a single conversion factor can be used for each filter mode. The calibration factors will be given for a frequency in the middle of the pass-band. For the 10 kHz mode this is 1 kHz; for the 80 kHz mode this is 10 kHz. If frequency-dependent variations are to be removed, then the calibration factors can be adjusted across the pass-band using the frequency response data of Section 12.3.1. The conversion factors which should be used are:

Translation Mode	Filter	counts/Vrms	dBmax

Baseband	10 kHz	264.25	+6.33

Baseband	80 kHz	267.31	+6.43

Table 12.3.2.1: WBR Low-Band Gain Calibration vs. Frequency

(Entries are in units of dBmax).

Freq. (kHz)	Gain 0dB	Gain 10dB	Gain 20dB	Gain 30dB	Gain 40dB	Gain 50dB	Gain 60dB	Gain 70dB
1	-4.60	-4.28	-4.26	-4.23	-3.97	-3.68	-3.67	-3.57
2	-6.11	-5.79	-5.78	-5.75	-5.49	-5.20	-5.18	-5.10
3	-5.47	-5.16	-5.15	-5.11	-4.86	-4.56	-4.55	-4.45
4	-5.15	-4.84	-4.83	-4.80	-4.54	-4.24	-4.23	-4.14
5	-5.59	-5.29	-5.27	-5.25	-4.98	-4.69	-4.67	-4.59
6	-6.21	-5.90	-5.89	-5.87	-5.60	-5.31	-5.29	-5.19
7	-5.24	-4.93	-4.92	-4.89	-4.62	-4.33	-4.30	-4.21
8	-5.09	-4.79	-4.78	-4.75	-4.48	-4.19	-4.17	-4.05
9	-6.01	-5.70	-5.69	-5.67	-5.39	-5.11	-5.09	-4.96
10	-6.29	-6.00	-5.99	-5.97	-5.69	-5.41	-5.38	-5.24
11	-9.14	-8.86	-8.85	-8.83	-8.54	-8.26	-8.23	-8.09
12	-23.93	-23.51	-23.50	-23.53	-23.20	-22.96	-22.92	-22.79
13	-41.67	-40.71	-40.27	-39.92	-39.73	-39.20	-39.23	**
**No data was acquired at this setting

Table 12.3.2.2: WBR High-Band Gain Calibration vs. Frequency

(Entries are in units of dBmax)

Freq. (kHz)	Gain 0dB	Gain 10dB	Gain 20dB	Gain 30dB	Gain 40dB	Gain 50dB	Gain 60dB	Gain 70dB
10	-4.40	-4.09	-4.08	-4.06	-3.77	-3.48	-3.47	-3.35
20	-4.50	-4.24	-4.24	-4.27	-3.90	-3.65	-3.58	-3.31
30	-5.25	-5.05	-5.06	-5.18	-4.66	-4.48	-4.34	-3.82
40	-4.80	-4.58	-4.61	-4.86	-4.14	-4.04	-3.81	-3.01
50	-4.59	-4.31	-4.37	-4.76	-3.80	-3.81	-3.49	-2.34
60	-4.85	-4.50	-4.58	-5.16	-3.91	-4.07	-3.67	-2.47
70	-6.00	-5.58	-5.71	-6.48	-4.91	-5.25	-4.83	-3.90
80	-7.31	-6.82	-6.97	-7.99	-6.07	-6.62	-6.22	-6.12
90	-23.82	-23.23	-23.39	-24.76	-22.46	-23.23	-22.94	-23.82
100	-41.92	-41.86	-42.02	-43.55	-41.85	-42.93	**	**
** No data was acquired at this setting

12.3.3 WBR AGC Characteristics

Tests were conducted to characterize the response of the Wideband Receiver Automatic Gain Control (AGC) hardware. This corresponds to the "Peak Detect" section of Figure 12.1.1.1. The WBR software reads an 8-bit value provided by this hardware, and can make automatic gain decisions based upon the value it reads. As stated in Section 12.1.4, if the software AGC function has been enabled, then the output from the peak detect is compared to a pair of reference amplitudes, and the gain state is either increased by 1 step (10 dB) or decreased by one step accordingly for the next set of contiguous samples. The pair of reference amplitudes can be reprogrammed, so it is necessary to understand what the response of the peak detect hardware is. For the first three tests, see the test configuration block diagram of Figure 12.3.3.1.

The first test was performed during Spacecraft Thermal Vacuum testing, on February 3, 1997. The Spacecraft was at room temperature. The WBR was configured in the 10-kHz mode, the input sensor was the Ex dipole, and the WBR gain was fixed at 0 dB gain (i.e., the AGC software was disabled and the WBR was in "manual" gain mode). The input frequency was 526.5 Hz (this is the "drive" frequency for MFR step 16), and the input voltage was varied in steps of 2 dB. The results are plotted in Figure 12.3.3.2 and given as AGC data number versus dBmax (the decibels below a maximum amplitude sine wave measured by the WBR) in Table 12.3.3.1.

Table 12.3.3.1 WBR 10 kHz mode: AGC vs. dBmax

dBmax	AGC	dBmax	AGC	dBmax	AGC	dBmax	AGC
-0.6	209	-8.8	86	-20.6	32	-32.4	24
-0.8	201	-10.6	71	-22.6	29	-35.2	24
-1.6	186	-12.7	59	-24.6	27	-36.3	24
-2.8	163	-14.7	49	-26.6	26	-41.0	23
-4.7	132	-16.7	42	-28.5	25
-6.7	106	-18.6	37	-30.4	24

Clipping in the WBR data and harmonic distortion occurred at dBmax values greater than -2.8, so the useful range of the AGC extends from data numbers 163 to 24. This corresponds to about 30 dB of signal range.

The second test was performed during Unit Level Thermal Vacuum testing, on August 19, 1996. The unit was at +75 degrees C. The WBR was configured in the 10-kHz mode, the input sensor was the Ex dipole, and the WBR gain was fixed at 40 dB gain (i.e., the AGC software was disabled and the WBR was in "manual" gain mode). The input frequency was 5 kHz, and the input voltage was attenuated at various levels. The results are plotted in Figure 12.3.3.3 and given as AGC data number versus dBmax, the decibels below a maximum amplitude sine wave measured by the WBR, in Table 12.3.3.2.

Table 12.3.3.2 WBR 10 kHz mode: AGC vs. dBmax

dBmax	AGC	dBmax	AGC	dBmax	AGC	dBmax	AGC
-0.1	218	-13.0	59	-24.9	30	-36.2	27
-3.0	159	-14.9	50	-27.0	28	-39.7	26
-5.0	129	-17.1	42	-29.9	27	-42.2	26
-7.2	103	-20.0	35	-32.6	27
-10.1	77	-22.9	31	-34.9	27

Again, clipping in the WBR data and harmonic distortion occurred at dBmax value -0.1, so the useful range of the AGC extends from data numbers 159 to 26. Notice the similarity to the first test, where a different gain setting and different signal amplitudes and frequency were used. The likely reason for the higher AGC values at the low end of the curve here is the temperature: at +75 degrees C, the bottom value has raised slightly. For gain-setting decisions, the room temperature test is the better calibration.

The third test was also performed during Spacecraft Thermal Vacuum testing, on February 3, 1997. The Spacecraft was at room temperature. The WBR was configured in the 80-kHz mode, the input sensor was the Ex dipole, and the WBR gain was set to several gains, including 30, 40, and 50 dB gain. The input frequency was 20 kHz, and the input voltage was attenuated to various levels. The results are plotted in Figure 12.3.3.4 and given as AGC data number versus dBmax, the decibels below a maximum amplitude sine wave measured by the WBR, in Table 12.3.3.3.

Table 12.3.3.3 WBR 80 kHz mode: AGC vs. dBmax

dBmax	AGC	dBmax	AGC	dBmax	AGC	dBmax	AGC
-11.7	64	-14.0	53	-16.0	44	-22.1	30
-12.0	63	-14.0	52	-18.0	38	-23.1	29
-12.9	58	-15.0	48	-20.0	33	-24.0	28
-13.0	57	-15.7	45	-21.0	32	-25.0	27

Although a variety of gain states are combined in this data set, notice that the dBmax relative scale is insensitive to this. And notice that this AGC response is very close to the response of the WBR 10-kHz mode to a 526 Hz tone. This indicates that the same gain control window points can probably be used for both the low-band and high-band modes.

The fourth test was performed on August 7, 1996 during unit level testing at room temperature. During a white noise response test for the MFR Band 3, data was also acquired by the WBR. The WBR was configured in the 10-kHz mode, the input sensor was the Ex dipole, and the WBR gain was set to 70 dB gain. For the stimulus configuration see Figure 10.3.3.2.3 in the MFR section. With white noise as the input, the signal voltage was stepped in 2-dB intervals. The results are plotted in Figure 12.3.3.5 and given as AGC data number versus dBmax in Table 12.3.3.4.

Table 12.3.3.4 WBR 10 kHz mode: AGC vs. dBmax

dBmax	AGC	dBmax	AGC	dBmax	AGC	dBmax	AGC
-1	238	-7	99	-17	37	-26	26
-2	179	-8	83	-19	34	-28	25
-3	152	-10	69	-20	33	-30	24
-4	124	-12	58	-21	30
-5	122	-14	49	-23	28
-6	102	-16	42	-24	27

To compare to the sine wave data of the previous tests, the random noise signal was processed as follows: an RMS average was calculated for the time series and the result was multiplied by 2. This result can then be converted to the dBmax value, which is rounded to the nearest dB. The reason that the higher values of dBmax do not step by 2 dB is because there was clipping of the input signal at 70 dB gain. Notice that this AGC response is very close to the response of the WBR 10-kHz mode to tones.

The final conclusions for the WBR AGC response are as follows. One commonly used rule of thumb in setting the gain is that 10 dB of "headroom" is desired for random noise spikes. This determines the upper set point, and corresponds to about 70 on the AGC response curve. The other rule of thumb is that the window formed by the upper and lower set points must be wider than one gain step, which is 10 dB. The wider the window is, the larger the hysteresis produced. To get a window which is 14 dB wide with the upper end of the window at 10 dB headroom, the lower threshold would be set at AGC value 27. These values can be changed a little, but there is not much adjustment possible because of the limited range of the AGC peak detector.

12.3.4 WBR Random Noise Response

Tests were conducted to characterize the response of the Wideband Receiver to random noise. These bench tests were performed on July 28-29, 1996, at +40 degrees C. Two test configurations were used, one for the 10 kHz WBR mode and one for the 80 kHz WBR mode. These configurations are shown in Figures 12.3.4.1 and 12.3.4.2. The GSE was connected to the instrument Ex+ and Ex- inputs. The noise spectral density was measured using the HP8535 Spectrum Analyzer, and graphs showing the frequency spectrum for the two GSE configurations are shown in Figures 12.3.4.3 and 12.3.4.4. Also a measure of the total power was made using a true RMS meter, and an estimate of the noise spectral density was obtained by using that and the signal bandwidth. For each of the WBR modes, 100 spectra were averaged and the average spectra are shown in Figures 12.3.4.5 through 12.3.4.8. Using an estimate of the average level for each plot, an estimate of the noise spectral density was done using the calibration procedure described in Section 12.2. Note that the FFT size for all of these tests is 2048 samples, and a Hanning window was applied to the data. The results are summarized for the 10-kHz mode in Table 12.3.4.1.

Table 12.3.4.1 WBR 10 kHz mode: White Noise Response

RMS voltage (Vrms)	1.00	1.00	1.00	1.00
Bandwidth (kHz)	20	20	20	20
Attenuation (dB)	30	20	10	00
Est. PSD (dBV/Hz1/2)	-73	-63	-53	-43
HP8535 SD (dBV/Hz1/2)	-74	-64	-54	-44
Avg. WBR PSD (dBV/Hz1/2)	-75	-65	-56	-52

Notice the large discrepancy for the test at 0 dB attenuation: the HP8535 spectrum analyzer measured -44 dBV/Hz1/2, while the WBR measured -52 dBV/Hz1/2. The reason for this discrepancy is that the data was clipped, so a low estimate of the power results. This overloading is apparently just starting to occur at the 10 dB attenuation setting, where the WBR estimate is 1 dB lower relative to the tests at higher attenuations. Those tests at 20 and 30 dB attenuation are both within 1 dB of the HP8535 estimate. The scatter in the data seems to indicate that these are reasonable answers.

The results for the 80-kHz mode are summarized in Table 12.3.4.2.

Table 12.3.4.2 WBR 10 kHz mode: White Noise Response

RMS voltage (Vrms)	1.00
Bandwidth (MHz)	2
Attenuation (dB)	10
Est. PSD (dBV/Hz1/2)	-73
HP8535 SD (dBV/Hz1/2)	-71
Ave. WBR PSD (dBV/Hz1/2)	-72

Notice that for the 80 kHz test, there was an interference tone present. It is thought that the 32 kHz interference originates from the Noise Generator. Nevertheless, it does not interfere with the test. The HP8535 measured the spectral density at 50 kHz, avoiding the noise line, and the WBR PSD estimate is within one dB of that.

12.4 WBR Noise Levels

In this section, the in-flight receiver noise floors are detailed for the commonly used modes. These modes will include:

Translation Mode	Filter	Antenna

Baseband	80 kHz	Ex dipole

Baseband	80 kHz	Ez monopole

Baseband	10 kHz	Ex dipole

When flight data is acquired for other sensors, those sensors' noise levels will be added to this document.

For the 80 kHz, Ex dipole configuration, see Figure 12.4.1. The data plotted in red (the lower noise level) was acquired just prior to antenna deployment, from the following data times, which are referred to as Spacecraft Event Times (SCET):

Start SCET	Stop SCET	# Spectra Averaged

1997-Oct-25-00:20:09	1997-Oct-25-00:30:01	75

One can see the low-end roll-off, with the cutoff frequency of about 800 Hz. There is no apparent 1/f noise, so the WBR rather than the pre-amps appears to set the noise floor. There are many noise lines, some caused by the MFR and some by the HFR.

In Figure 12.4.1, the data plotted in green was taken after the electric antennas were deployed, from the following data times:

Start SCET	Stop SCET	# Spectra Averaged

1997-Oct-25-04:15:05	1997-Oct-25-04:21:37	50

In this data, one sees the shot noise at about 10 dB higher than the pre-deployment noise level for frequencies up to about 10 kHz. But above that, the internal noise lines are comparable to or even greater than the antenna shot noise. Some of the noise lines have now disappeared below the shot noise floor.

For the 80 kHz, Ez dipole configuration, see Figure 12.4.2. The data plotted in red (the lower noise level) was acquired just prior to antenna deployment, from the following data times:

Start SCET	Stop SCET	# Spectra Averaged

1997-Oct-25-00:15:05	1997-Oct-25-00:19:53	37

There are many noise lines, and these are exacerbated by the lack of common mode rejection.

In Figure 12.4.2, the data plotted in green was taken after the electric antennas were deployed, from the following data times:

Start SCET	Stop SCET	# Spectra Averaged

1997-Oct-25-04:10:09	1997-Oct-25-04:14:57	37

In this data, one sees the shot noise a few dB higher than the pre-deployment noise level for frequencies up to about 10 kHz. But above that, the internal noise lines are comparable to or even greater than the antenna shot noise. Some of the noise lines have now disappeared below the shot noise floor, but some, like that around 1200 Hz, are no better.

For the 10 kHz, Ex dipole configuration, see Figure 12.4.3. The data plotted in red (the lower noise level) was acquired just prior to antenna deployment, from the following data times:

Start SCET	Stop SCET	# Spectra Averaged

1997-Oct-25-00:30:09	1997-Oct-25-00:40:01	71

The noise level here is set primarily by the Ex preamps. One can see the low-end roll-off, with the cutoff frequency of about 60 Hz, followed by 1/f noise for most of the spectrum. At the high end the WBR itself appears to set the noise level, and there are many noise lines, some caused by the MFR and some by the HFR.

For the 10 kHz, Ex dipole configuration, see Figure 12.4.3. The data plotted in red (the lower noise level) was acquired just prior to antenna deployment, from the following data times:

Start SCET	Stop SCET	# Spectra Averaged

1997-Oct-25-00:30:09	1997-Oct-25-00:40:01	71

The noise level here is set primarily by the Ex preamps. One can see the low-end roll-off, with the cutoff frequency of about 60 Hz, followed by 1/f noise for most of the spectrum. At the high end the WBR itself appears to set the noise level, and there are many noise lines, some caused by the MFR and some by the HFR.

In Figure 12.4.3, the data plotted in green was taken after the electric antennas were deployed, from the following data times:

Start SCET	Stop SCET	# Spectra Averaged

1997-Oct-25-06:08:01	1997-Oct-25-06:17:45	74

In this data, one sees the shot noise at about 20 dB higher than the pre-deployment noise level. Also some of the noise lines have now disappeared because of the higher noise floor.