Rev 2.0 | SC5309A & SC5310A
Hardware Manual
SignalCore, Inc.
18
SC5309A & SC5310A Hardware Manual
Converted Signals
Selecting the narrower filters in the IF2 stage improves the leakage spurs, and allows for lower
frequencies at both IF and RF. For example, IF3 could be set to lower for a narrow BW IF2 filter
without the risk of possible increase of in-band spurs arising from LO2-LO3 intermodulation or
having significant LO leakages even when the device is tuned for RF as low as 5 MHz.
The final IF3 LPF is only a 5
th
order Chebyshev filter so the rejection slope is not particularly
aggressive. This means the harmonic rejection is not as effective for some applications, especially
when IF3 is low. It is recommended that an external filter be used to further suppress these
harmonics.
Dynamic Range Setting
There are 2 sets of digital step attenuators to control the conversion gain of the downconverter.
The first set consists of RF_ATTEN1 and RF_ATTEN2 and the second set consists of IF3_ATTEN1
and IF3_ATTEN2. All attenuators, except for IF3_ATTEN2, have 30 dB range and 1 dB step.
Attenuator IF3_ATTEN2 has 30 dB range and 0.25 dB step.
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 IF3 attenuator is then used to adjust the final IF output level.
The drawback is that the signal level starts off higher as it enters the first mixer as well as the
subsequent components. As a result, the apparent linearity of the device is lower.
To set the device for better linearity, the gain should be shifted to the output IF3 path (after the
third mixer) and reduced in the RF path. The signal power level at the first mixer should be lower
than -20 dB for improved linearity. Since the input signal is low, the relative SNR will be lower.
But, as the first mixer and subsequent components experience lower power levels, the apparent
linearity of the device is improved.
When the device gain is balanced well, the device can achieve SNR better than 130 dBc/Hz while
maintaining IMD3 levels close to 80 dBc. These numbers are representative of converters used
in large box high end spectrum analyzers. When the device is optimized for best SNR, typical
values better than -150 dBc/Hz can be achieved, and when the device is optimized for sensitivity,
the input spectral noise floor is typically lower than -160 dBm/Hz. The flexible use of these
attenuators allows the downconverter to achieve better than 190 dB of measurement dynamic
range.