©2018
Rev 2.0
15
Functional Description
The Input Path Switch
Immediately following the capacitor is an RF switch to selectively direct the RF signal down two
paths:
1.
The default is the conversion path where the RF signal is converted to a final IF.
2.
The alternative path is directly to the IF port, bypassing all means of frequency conversion. For
lower RF frequency that can be digitized directly by a digitizer at the IF port, this path provides
the convenience. Another reason for choosing this path for lower frequencies is that converting
low RF to an IF may result in more unwanted spurs at the IF output.
When the conversion path is bypassed, the conversion path and the internal LO are disabled by
powering down.
The RF Path
The first device in the RF conversion path is a step attenuator with 1 dB step resolution. This
attenuator (RF_ATTEN) is used to control the signal level at the mixer or at the RF preamplifier. If
the RF level at the input of these devices is relatively large with respect to their input compression
points, the signal would experience greater nonlinear effects, distorting its waveform, and
producing large spurious 3
rd
intermodulation distortion effects.
Following RF_ATTEN is a switchable RF amplifier that can be switched into the signal path to
improve the device sensitivity, effectively lowering the device noise figure. In other words, the
effective input noise level of the device is lowered when this amplifier is enabled. Turning on this
amplifier is recommended to improve the signal-to-noise dynamic range (DR
SNR
) when expected
signal levels are lower than -30 dBm. The typical maximum gain of the downconverter without the
amplifier enabled is +30 dB. When the amplifier is enabled, its addi15 dB will boost the
typical maximum gain to +45 dB.
The attenuator (RF_ATTEN2) following the amplifier switch is used together with RF_ATTEN1 to
control the level at the input of the mixer. Unless the input level is very high, and the attenuation
range of RF_ATTEN1 is exhausted, RF_ATTEN2 is not commonly used except when the RF amplifier
is enabled. RF_ATTEN1 and RF_ATTEN2 provide 60 dB of attenuation range, which is enough to
keep the device in the linear mode of operation even for signal levels near the maximum
recommended value of +27 dBm.
Between RF_ATTEN2 and the mixer is the RF elliptic low pass filter. This filter cuts off just above
2.7 GHz and has better than 75 dB of rejection for frequencies above 3.6 GHz. This suppresses
back-travel LO leakages to better than -100 dBm (typically -120 dBm) at the input RF port. Another
function of this filter is to provide superior suppression of RF image frequencies. The lack of
suppression of these RF images could result in mixing with LO1 and appear as spurious components
in the IF.
The First Mixer and IF1 Path
The first mixer (IF1 mixer) of the downconverter is very critical as it sets the dynamic ranges of the
device for both the signal-to-noise and third order IMD (DR
IMD
) dynamic ranges. The DR
SNR
and
DR
IMD
are directly related to the mixer input compression point (IP1dB) and to the input third-order