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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 2 - INSTALLATION, SETTING UP AND FIELD VALIDATION 2-19
CROSS-CHANNEL EQUALIZING VIA INTERNAL LOOPBACK
(EXCLUDING ANTENNAS AND CABLES)
2:64. Identical response of all receive channels of the Digisonde-4D to the input signal is required to ensure
correct directional analysis of four-channel data. Frequency-dependent mismatches in phase and amplitude
response characteristics are introduced primarily by the narrowband analog circuitry that is used in the
Tracking Filter cards and analog amplifiers of the Receiver card.
2:65. Periodically running a Cross-Channel Equalizing program removes the cross-channel, frequency-
dependent phase and amplitude variation in the Digisonde-4D sounder. The results of the cross-channel
equalization are saved in the latest.ceq file in /digisonde/dispatch folder. This file automatically corrects
phase and amplitude differences in the four receiver channels for all subsequent Ionogram and Drift meas-
urements.
2:66. A Cross-Channel Equalizing program is configured by setting Operation mode to “Channel Equalizing”
and Operation Option to “Internal Loopback” (see Figure 2-8 and Section 3). An appropriate Cross-
Channel Equalizing program is pre-configured prior to the sounder shipment, but it is important that its fre-
quency coverage is appropriately adjusted with time to match the range of operating frequencies. In the
Cross-channel equalizing mode with internal loopback, the sounder:
i. a. Switches the XMT card output through an attenuator (on the XMT card) and injects it directly
into the antenna switch assembly (via the coaxial cable labeled “Cal In”).
j. b. Disables the RF power amplifier by gating off the XMTR pulse signal (accessible on the front
panel) which normally activates the bias voltage of the RF power FET.
k. c. Switches the antenna switch inputs from the four external antennas to the directly injected
transmitter signal.
Figure 2-8: Program for Cross-Channel Equalization
2:67. Cross-Channel Equalization program shall be run prior to checking cable lengths and the phase re-
sponse of the loop antenna pre-amplifiers.
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