Aerospace and Electronic Systems Magazine December 2017 - 52

Sensitivity and Linearity Requirements of an OTH Radar Receiving System

Figure 11.

Analysis output for 24 hours showing channel corruption effects as a function of frequency and time of day.

Figure 12.

Analysis output for 24 hours showing the size in dB of channel corruption effects as a
function of frequency and time of day.

IEEE A&E SYSTEMS MAGAZINE

ideal, and therefore those frequencies would be flagged
as internally noise limited. In this case this occurs at
frequencies above 15 MHz. Similar arguments can be
applied to the ADC spurs, which are dominant in this
particular case. This is a situation where WOFE is failing at the higher end of the band due to system imposed
limitations within the receiver.
The dark blue spectrum represents the second order
distortion terms and the black represents the third order products. As with the noise, if these spurious products come within 6 dB of the ideal signal they will be
flagged as causing degradation.
Since the noise and distortion is propagated independently, attribution to a particular form of corruption
can be made.
With this methodology in mind, a time series of effects can be established and visualized. Figure 11 shows
the 24 hour period of data with each channel assessed for
dominant impacts of the receiver chain. The red channels,
generally forming horizontal stripes on the plot, are channels determined to be utilized (above the noise floor) at
the measurement point, generally these should be third
party users of the spectrum or local radio frequency interference that has been detected through the antenna.
The yellow represents channels that were determined to be internally noise limited at the measurement
point. This situation can happen at HF when the antenna provides insufficient power at its output to overcome
internal noise in the receiver chain to the measurement
tap point. In practice, that can generally be overcome by
matching the antenna better to the band of interest and
ensuring sufficient radiative efficiency, and designing
the receiver chain with knowledge of the power levels
coming from the antenna. The wider the band required
the more challenging this can become.
The light blue color represents channels that were
externally noise limited at the measurement point but
subsequently became internally noise limited at the
output of the receiver. This occurs if the receiver noise
figure is excessive by way of comparison with external
noise levels presented at its input. The darker blue signals (generally appearing as horizontal stripes) are second order distortion products. The band limited nature
of these products occurs because they are formed by
nonlinear interactions of the broadcast band signals (the
red stripes at lower frequencies, that are also band limited). This indicates the IP2s on the filter blocks were
insufficient to deal with the spectral environment as it
presented from the receive chain. No third order products appear to be affecting channels in this case indicating sufficient IP3 in the filter and gain blocks. ADC
effects also appear to be minimal in this case.
The impact of any of these effects can be quantified
in dB by how much the ideal noise level would is raised
by their inclusion. The corresponding data is illustrated
in Figure 12.
DECEMBER 2017

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine December 2017