Rev 1.1 | SC5317A & SC5318A
Hardware Manual
SignalCore, Inc.
12
SC5317A & SC5318A Hardware Manual
The First Mixer and IF Path
The mixer (RF mixer) of the downconverter is very critical as it sets the dynamic ranges of the device,
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 input third-order intercept point (IIP3). The IIP3
of the mixer is typically better than +18 dBm, and its P1dB is better than 5 dBm. In applications that
require better signal linearity, it is recommended that the level at the mixer input is kept at -20 dBm or
less. Typically, the IMD is better than -75 dBc for two -20 dBm tones at the mixer. Increasing the level at
the mixer improves SNR of the IF output signal at the expense of higher nonlinear byproducts.
Another important characteristic of the mixer is the LO-IF isolation and LO-RF isolation. The higher the
isolation, the lower the leakage LO signal is at the RF and IF ports. The RF mixer is driven with a ~+17 dBm
LO signal and the LO-IF isolation is about 45 dB, resulting in a -28 dBm LO leakage in the IF path. The low-
pass IF filters suppress this leakage to levels well below -60 dBm. The LO-RF isolation is also about 45 dB
so the LO leakage at the RF port of the mixer is on the order of -28 dBm. The high reverse isolation of the
RF linear power amplifier suppresses this LO leakage to < 60 dBm. When the low noise pre-amplifier is
enabled, it will provide another 30 dB of reverse isolation, lowering the LO leakage at the RF input port to
< 90 dBm.
The frequency relationship between the three ports of the mixer is given as:
𝐼𝐹 = {
𝐿𝑂 − 𝑅𝐹
𝑅𝐹 − 𝐿𝑂
From the above equation, the IF output spectrum is inverted with respect to the RF in the first
relationship, whereas in the second it is noninverted. When the LO > RF, the IF signal is inverted and it is
not inverted when LO < RF.
Signal Chain Configurations
Recommended signal chain configurations are provided below for various application scenarios. These
are strictly recommendations, they are not an indicative of the downconverter function limitations; users
will need to adapt the device to their applications at hand.
Signals Bypassing the Conversion Stage
For frequency less than 6 GHz, the RF input signal can be directed to the RF output port, bypassing the
conversion process. This RF output signal may be used by another downconverter to bring it down to
baseband IF, for example, the SC5308A 6 GHz downconverter mates well with this device to form an
integrated converter with continuous frequency coverage from 100kHz to 26.5 GHz.
Dynamic Range Setting
There are 2 digital step attenuators to control the conversion gain of the downconverter, the RF_ATTEN
in the RF stage and IF_ATTEN in the IF stage. Both attenuators have 30 dB of settable range, however the
IF attenuator has fine resolution of 0.25 dB while the RF attenuator has resolution of 1 dB.
To set the downconverter for better sensitivity or better SNR, the gain should be shifted to the RF input
path of the device, before the mixer. The RF pre-amplifier should be enabled if necessary and/or RF
attenuation reduced. The IF attenuator is then used to adjust the final IF output level. The drawback is