SC5305A Operating & Programming Manual
Rev 2.1.0
14
For applications where the SNR must be maximized, such as examining the close-in characteristics of a
sine tone, the input mixer should be set to accept 0 dBm power and the IF set at 0 dBm or higher. This is
a likely setting for making phase noise measurements of an RF signal (assuming the specified phase
noise of the SC5305A is low enough for measuring that particular signal). It is important to first set the
necessary attenuators before injecting a 0 dBm level signal to the mixer, otherwise heavy saturation of
the mixer or the output amplifiers may cause degradation or even possible failure of the receiver over
time.
The SC5305A is designed for a nominal output IF level of 0 dBm, ensuring the IF signal is about 3-4 dB
below the full-scale value of many 50
analog-to-digital data converters (ADCs). Depending on the
application, the user will need to set the appropriate gain of the device (via attenuation), and hence the
output level, to suit the particular application. For broadband signals, it is recommended that the IF
output level be about 7 dB below the full-scale value of a digitizer because of possible high crest factors
that may saturate the digitizer. For sine-tone or narrowband application, the output IF level should be
about 3 dB below full-scale of the digitizer to maximize its signal-to-noise dynamic range.
SignalCore provides a simulation tool that mimics the behavior of the SC5305A. The user may run the
simulator to get an understanding of what the parameters need to be set on the downconverter to
achieve certain performance. Additionally, the function
sc5305a_CalcAutoAttenuation
helps the user
obtain the necessary attenuator parameters to setup the device for the best compromise of linearity
and noise performance for a given set of input and output parameters.
There is a programmable attenuator in the first IF section, IF1_Atten, that can be used to improve
linearity in general. The primary use of this attenuator is to suppress the LO1 leakage in the IF band
when the downconverter is tuned below the bandwidth frequency. This in-band leakage affects the
linearity of the device as it may inter-modulate with the IF signal to produce third order spurious
products. The level of the leakage is equivalent to a typical -25 dBm RF signal at the mixer. The user
should set 5 dB to 10 dB OF attenuation when operating at these low frequencies.
Operating the SC5305A Outside Normal Range
The SC5305A is capable of tuning below 1 MHz and above 3.9 GHz. These frequencies lie outside of the
specification range and performance will be degraded if operated in these outer margins. However, for
some applications, the reduced dynamic range or elevated spurious levels in these ranges may not pose
an application concern. The lowest tunable frequency is 0 MHz (DC). However, for input frequencies
below 1 MHz, the AC coupling capacitors in the circuit limit/attenuate the signal significantly. On the
upper end of the spectrum, the input low-pass filter will attenuate the signal rapidly as the frequency
increases above 3.9 GHz. Calibration stored on the device EEPROM does not account for these out of
range frequencies, so applying any correction using the stored calibration is not valid.