Rev 2.0 | SC5309A & SC5310A
Hardware Manual
SignalCore, Inc.
20
SC5309A & SC5310A Hardware Manual
is selected, it is important to have it remain powered on to maintain its temperature stability. Like
the VCXO, it is also phase locked to the external source.
The reference output is derived from the buffered VCXO. When its output is enabled, its frequency
can be selected for 10 MHz or 100 MHz. Typically, output power is +3 dBm as its accuracy and
stability is that of either the internal VCTCXO or external reference source.
The First Local Oscillator (LO1)
The first local oscillator (LO1) is a YIG based synthesizer tunable between at least 3.3 GHz and more
than 6.2 GHz typically. Its multiple hybrid PLL approach gives it the advantages of having lower
phase noise, very low spurs, and a small frequency step of 1 Hz. This signal drives the first mixer of
the signal chain.
The frequency of LO1 is set indirectly by setting the frequency plan for IF1, IF2, and the RF signal
frequency.
The Second Local Oscillator (LO2)
The second LO (LO2) is normally fixed at 3.32 GHz, by it may be set via the frequency plan to tune
between 3.1 GHz and 3.45 GHz in 5 MHz frequency steps. LO2 drives the second mixer of the signal
chain. Its frequency is indirectly set using frequency plan IF1 and IF2 values using the following
relationship:
𝐿𝑂2 = 𝑠𝑡𝑒𝑝 ∗ 𝑓𝑙𝑜𝑜𝑟 (
𝐼𝐹1 − 𝐼𝐹2
𝑠𝑡𝑒𝑝
)
The ability to tune LO2 allows for the optimization of IF combinations to move higher order
intermodulation products that may form inside the passband to outside the passband.
The Third Local Oscillator (LO3)
The third LO (LO3) frequency is set indirectly with IF2 and IF3 frequency values, and the polarity of
the IF3 spectrum is set with respect to the RF. IF3 can be set ideally from DC to 100 MHz. When
the IF3 spectrum is set to be non-inverted, LO3 tunes from IF2 up to IF2 + 100 MHz, otherwise it
tunes from IF2 down to IF2
–
190 MHz. Caution needs to be taken when tuning to the limits because
of larger LO leakages and intermodulation products that could form in-band. For example, when
the LO3 is set to IF2 - 100 MHz in the inverted case, the IF3 low-pass filters may not sufficiently
provide the suppression needed to keep the leakage to an acceptable level at the IF output port.
In this case, an external low-pass filter may be needed.