The RI7710G, RI7725B and RI7725C sources are high performance synthesizers with excellent power and phase noise. However, like all microwave sources, these sources contain low-level spurious signals, resulting from the frequency generation circuitry.
When the source is used as a local oscillator (LO) for Roos receivers, the spurs might need to be considered. When measuring very low powers, at specific frequencies, the spurs can cause interference in the measurement results. In the receiver, the spur on the LO can mix with the actual LO and produce an IF, limiting the minimum power that can be measured.
Mixers need a high power LO, so a spur that is very far down in dBc can still result in a significant increase in the minimum detectable signal. For example, if a +15 dBm LO has a -80 dBc spur, the resulting IF can appear to be -65 dBm.
These sources use a fundamental oscillator from 2 GHz to 20 GHz, using multiple phase lock loops referenced to an internal 100 MHz clock. A down-converter achieves frequencies below 2 GHz.
The chart below shows the minimum detectable signal of a typical Cassini tester. The receiver's settings for this graph were 56 dB IF gain, 1000 averages, the default 200 kHz IF bandwidth, and 'measure voltage'.
The most significant source of spurs is the source's down-converter. It contains an internal oscillator at 6.5 GHz that leaks into the source output at about -80 dBc. As can be seen, the down-converter spurs occur at precisely 6.5 GHz and its harmonics, 13 GHz and 19.5 GHz, with the 6.5 GHz spur being the most significant.
A second, much less significant source of spurs is the internal 100 MHz crystal oscillator that the source uses for a reference. You can see that, below 2 GHz, there are multiple very small spurs at precise multiples of 100 MHz.
Note that TIMs with multiplying LO's will see the spurs the at the multiplied frequency. For example, the RI8563C, 40 GHz TIM uses a doubling mixer, so the down-converter spurs will occur at 13 GHz, 26 GHz and 39 GHz.
There are multiple ways to avoid the spurs, all involve moving away from the spurious frequency. The easiest way is simply to offset the frequency by a small amount.
The following graph is the same measurement as the previous graph, except the sweep is offset by 1 MHz (101 MHz to 20001 MHz).
Note that we used the 'Voltage' measurement. This measurement uses vector averaging, and will average away anything that is not precisely the frequency measured. As such, we could offset a small amount, even less than 100 kHz, and average away the spur. If 'power' or 'RMS power' is measured, then the offset needs to be outside the tester's actual hardware bandwidth, as specified by the receiver's IF Bandwidth button..
Another avoidance technique is the use of IF offsets and spectrum power measurements. That implementation is beyond the scope of this document and is described in the user training.
In summary, the RI7710G, RI7725B and RI7725C sources, like all microwave sources, have inevitable low-level non-harmonically related spurs. The largest spurs are at 6.5 GHz and 13 GHz. These spurs can limit the minimum measured power at the precise spur frequency and can easily be avoided by offsetting the frequency a small amount.