Using an E4446A spectrum analyzer I was seeing spurious images caused by an large input signal.
I'm not using exact numbers intentionally, but they are close.
Input was +14 dBm with 0 dB atten internal.
Fin was 2.5 GHz
Span was 300MHz to 1 GHz and I believe the RWB was at 1MHz.
The observed spurious result was significantly above the noise floor at aprox 0.5 Ghz.
I know the mixer level at Fin was higher than it should be but (s/b -10dBm max) but I thought the preselector would prevent spurs when moved off the high level input.. in this case quite alot.
I later injected my own signal with a calibrated sig gen and obtained the same result. I know it was a spurious artifact because adding 10 dB atten caused it to vanish completely..by much more than 10 dB.
So.. am I doing something wrong? Should I have to manually adjust the YTF or does it auto adjust at each sweep?
Edited by: Timbo on May 16, 2012 2:10 PM Correction.. intially had 0 dB attenuation
Edited by: Timbo on May 16, 2012 2:11 PM
I'm not using exact numbers intentionally, but they are close.
Input was +14 dBm with 0 dB atten internal.
Fin was 2.5 GHz
Span was 300MHz to 1 GHz and I believe the RWB was at 1MHz.
The observed spurious result was significantly above the noise floor at aprox 0.5 Ghz.
I know the mixer level at Fin was higher than it should be but (s/b -10dBm max) but I thought the preselector would prevent spurs when moved off the high level input.. in this case quite alot.
I later injected my own signal with a calibrated sig gen and obtained the same result. I know it was a spurious artifact because adding 10 dB atten caused it to vanish completely..by much more than 10 dB.
So.. am I doing something wrong? Should I have to manually adjust the YTF or does it auto adjust at each sweep?
Edited by: Timbo on May 16, 2012 2:10 PM Correction.. intially had 0 dB attenuation
Edited by: Timbo on May 16, 2012 2:11 PM
Spectrum analyzers use mixers to heterodyne the signal up and down to make measurements. Mixers are non-linear devices. Therefore, a strong signal at 2.5 GHz can also produce a harmonic at twice the frequency (5 GHz). This harmonic can mix with the LO and cause an image. Nevertheless, the solution is to simply add in more attenuation, so as not to drive the mixer so hard. This will increase the noise floor, and in order to reduce the noise floor to compensate you will need to reduce your resolution bandwidth. The spurious responses specification for PSA is for an input mixer level of -40 dBm and not +14 dBm. A +14 dBm mixer level should cause an IF overload message on screen. All spectrum analyzers will have images, especially when you drive their input mixer so hard. The advantage the PSA has is that it has RBWs in 10% steps to allow for fine tuning of the RBW to choose the right RBW to meet your sweep speed and selectivity and sensitivity needs.
I sure hope this helped -