Installation Instructions

Hardware Installation
RVP8 User’s Manual
September 2005
2–9
2.2.7 IF Bandwidth and Dynamic Range
The RVP8 performs best with a wide bandwidth IF input signal. This is because a wideband
signal can be made free of phase distortions within the (relatively narrow) matched passband of
the received signal. The RVP8 uses an external analog anti-aliasing filter at each of its IF and
Burst inputs. The purpose of these filters is to block frequencies that would otherwise alias into
the matched filter passband. The anti-alias filters have a nominal passband width of 14 MHz
centered at 30MHz, i.e. from 23MHz to 37MHz. This is the recommended operating bandwidth
for the IF signal, although the RVP8 will still work successfully with lesser IF bandwidth.
At the 36MHz sampling rate the quantization noise introduced by LSB uncertainties is spread
over an 18MHz bandwidth. For an ideal 14-bit A/D converter that saturates at +6dBm the
effective quantization noise level would be:
)6dBm * 20log
10
(2
14
) * 10log
10
(
18MHz
1MHz
) +*90dBm (at 1MHz BW)
If samples from this ideal converter were processed with a digital filter having a bandwidth of
1MHz, then an input signal at –90dBm would have a signal-to-noise ratio of 0dB. A narrower
FIR passband (corresponding to a longer transmitted pulse) would decrease the quantization
noise even further, so that 0dB SNR would be achieved at even lower input power.
In practice, the 14-bit A/D converter used inside the IFD does not behave quite this well. The
Analog Devices AD6644 chip has been measured to have a wideband SNR of 76dB, i.e., 8dB
less than the 84dB range expected for an ideal converter. The above calculation for noise
density thus becomes:
)6dBm * 76dB * 10log
10
(
18MHz
1MHz
) +*82dBm (at 1MHz BW)
Indeed, the RVP8’s receiver power monitor described in Section 4.5 will show a filtered power
level of approximately –82dBm when the FIR bandwidth is 1MHz and the IFD inputs are
terminated in 50–Ohms.
The inverse correspondence between filter bandwidth and the 0dB SNR signal level leads to an
interesting and useful property of wideband digital receivers: they can operate over a dynamic
range that is much greater than the inherent SNR of their A/D converter would imply. If this
particular A/D chip were performing direct conversion at “base band” it would have a dynamic
range of only 76dB. However, by utilizing the extra bandwidth of the converter, the RVP8 is
able to extend the dynamic range to approximately 100dB.
To understand this, begin with the 88dB interval between the converter’s +6dBm saturation level
and the –82dBm 0dB SNR level at 1MHz bandwidth. Add to this:
S 6dB for the statistical linearization that is performed on signals that exceed the saturation
level. The RVP8 can recover signal power accurately even when the A/D converter is
driven beyond saturation. Velocity data will also be valid, but spectral width may be
overestimated.
S 4dB for usable dynamic range below the 0dB SNR level. In practice, a coherent signal at
–4dB SNR can easily be measured when 25 or more pulses are used.