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Spurious Emissions Mode: detecting the carrier as a spur?

Question asked by DanHernandez on Mar 18, 2019

The spurious emissions mode (SEM) is picking up spurs exactly 100 KHz away from the carrier frequency, when in fact there are no spurs to be found.

I'm using the N9030b PXA to measure spurious emissions.

Let me set up an example to describe my problem.

Let's say the carrier frequency is 15GHz at 0dBm and my SEM has the following 4 ranges greater than the carrier.

Range    Start                        Stop                        Span            RBW

1            15,000,000,050      15,000,000,100        50 Hz             1Hz

2            15,000,000,100      15,000,001,000        990 Hz           10Hz

3            15,000,001,000      15,000,100,000        990 KHz        100Hz

4            15,000,100,000      15,001,000,000        9.9 MHz         1KHz

For some unknown reason the SEM is finding a spur in range 4, at exactly 15.0001GHz (+100KHz from the carrier) with a power level that will be ~ -1dBm. When going into spectrum analyzer mode and checking that frequency there is no spur.

The strange thing is, if there was a spur at +100KHz then range 3 should have also caught the spur. Correct? Also, the power level on the "spur" is so high it seems like the analyzer is mistaking the carrier for the "spur".

I have tried reducing both the RBW and VBW to as low as 50 Hz with no luck.

Has anyone seen this before? Does anyone know what I'm seeing? I'm not very familiar with this mode and I assume I'm just missing some setting that needs to be turned on or off.


Thanks in advance.