THE
HIGH FREQUENCY RECEIVER (RPWS-HFR)
The HFR consists of 2 receivers, fed by 2 inputs
selected among the 3 RPWS electric antennas (monopoles, effective length ~7
meters): +Ex, -Ex, and Ez. +Ex and
-Ex can be used together as a dipole. The two inputs E1
and E2 can thus be:
For E1 : +Ex, -Ex,
Dipole, Off.
For E2 : Ez, Off.
Each receiver contains three filters (A,B,C) 2.2 octaves each, with fixed
frequency ranges, covering together the band 3.5-319 kHz without redundancy
and one tunable 25 kHz band (H) frequency-shifted with an
oscillator. The total HFR range is 3.5
kHz - 16.125 MHz.
The digital filters have an out-of-band rejection
>45 dB. They are preceded by an analog front end providing anti-aliasing
filtering and automatic gain control (AGC, with ~1 msec time constant). The AGC
normalizes to an optimum value (0.5 Veff) the input to the digital
receivers (A/D 8-bit conversion of the range 0-5 V). The dynamic range (or
allowable gain variation) is about 90 dB
for the AGC (on 8 bits therefore »0.4 dB quantization step) + ~30 dB through the in-band digital analysis.
There are solid-state 26 dB attenuators on all bands. They are put back to the default
(Off) at each Sleep mode (~once/day). A real-time command is required to put
them On altogether (not separately selectable), or a new mode will be loaded
(after launch) to put them back On after Sleep modes. They are expected to be
necessary only during high-latitude passes within the LF SKR sources, late in
the tour.
Each of the 3 fixed bands ABC can be analyzed in up to 32 channels (log-spaced), and the
tunable one H in up to 8 channels
(with linear spacing). The channel bandwidth at -3 dB is respectively 5/10/20%
of the center frequency in bands ABC with 32/16/8 channels per band, and
25/n_channels kHz in the H band. The noise equivalent bandwidth is 1.1 times
the bandwidth at -3 dB.
Auto-correlations of the two electric sensor input
voltages |E1|2 and |E2|2 and
cross-correlation products Re(E1.E2*) and Im(E1.E2*)
are computed in the HFR's ADSP for each frequency channel. Several in-band
spectra are averaged to reduce the noise level to the quantization step (1 bit
= 0.375 dB or 8 levels/octave; more integration may cause the loss of sporadic
events). The exact number of integrated spectra depends on frequency and
integration time. The errors on auto- and cross-correlation products due to
quantization is ~5% (no additional error is brought by the calculations in the
ADSP, performed with 32 bits precision).
For the tunable band, the IF filtering and
amplification is made near 21 MHz, and is followed, after a second frequency
conversion to the range 50-75 kHz, by linearly-spaced frequency analysis by the
digital analyzer. This band is tunable to any frequency between 150 kHz and
16.125 MHz, with 25 kHz steps up to 4.125 MHz and 50 kHz steps above. The 25
kHz width and 25/50 kHz stepping were chosen to avoid interference lines from
S/C converters, at n x 50 kHz (the PRA receiver onboard Voyager suffered
from a ~20 dB sensitivity loss in its HF band due to S/C generated
interference). Converter lines may however pollute the fixed frequency bands
below 319 kHz. The HFR sensitivity is 5-7
nV.Hz-1/2.