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Thread: PSA E4446A spurious artifacts


Permlink Replies: 6 - Pages: 1 - Last Post: May 31, 2012 12:38 PM Last Post By: xmo Threads: [ Previous | Next ]
Timbo

Posts: 5
Registered: 05/16/12
PSA E4446A spurious artifacts
Posted: May 16, 2012 11:14 AM
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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
tabbott


Posts: 1,483
Registered: 07/24/09
Re: PSA E4446A spurious artifacts
Posted: May 16, 2012 2:40 PM   in response to: Timbo in response to: Timbo
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Hi -

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 -
Timbo

Posts: 5
Registered: 05/16/12
Re: PSA E4446A spurious artifacts
Posted: May 16, 2012 3:24 PM   in response to: tabbott in response to: tabbott
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That helps me verify what I though to be true at least.
But I also I thought that the TYF preselector would significantly attenuate the "strong signal" when I was way out of its band pass and therefore eliminate overload. I am trying to run a MIL-STD conducted emission test on radio transmitter. It's transmit level is about 16 dBm (only 40 mW).
My measurement requires I be able to measure 80 dB down from the transmitter level. I'm not sure I have the head room left with the attenuation added so I was hoping the preselector would be my answer.

Edited by: Timbo on May 16, 2012 3:25 PM.. oops
tabbott


Posts: 1,483
Registered: 07/24/09
Re: PSA E4446A spurious artifacts
Posted: May 17, 2012 12:36 PM   in response to: Timbo in response to: Timbo
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Hi -

The built-in preselector in PSA is for frequencies above 3 GHz. When making EMC measurements it is necessary to use an RF preselector. There was an RF preselector available for PSA that is now discontinued (N9039A). The replacement product is the N9038A MXE EMI Receiver.

Regards -
Timbo

Posts: 5
Registered: 05/16/12
Re: PSA E4446A spurious artifacts
Posted: May 18, 2012 7:27 AM   in response to: tabbott in response to: tabbott
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Is there a spec or discussion about the internal preselector? I'm having problems finding much information.
Basic questions such as
Is the preselector automatically on? Does it center on each freq sample? What is the BW of it? What is the typical out of band attenuation.

Edited by: Timbo on May 18, 2012 7:33 AM
tabbott


Posts: 1,483
Registered: 07/24/09
Re: PSA E4446A spurious artifacts
Posted: May 31, 2012 8:48 AM   in response to: Timbo in response to: Timbo
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Hi -

There is a good application note that describes the preselector. Please refer to pages 9 and 10 of Application Note 1443: http://cp.literature.agilent.com/litweb/pdf/5988-7814EN.pdf

The preselector is automatically turned on when tuned to frequencies above 3.05 GHz. Its bandwidth is nominally 30 MHz at 3 GHz and increases to about 60 MHz at 26.5 GHz. The preselector sweeps with the Yig Tuned Oscillator unless you are in a narrow span (approx. 10 MHz or less). We don’t have a specification on the rejection for our preselector YTF, but we do have a specification for multiples and out of band images. That specification is 80 dBc down. So, the YTF would have a rejection for multiples of more than 80 dB.

Hope this helps -
xmo

Posts: 50
Registered: 03/05/07
Re: PSA E4446A spurious artifacts
Posted: May 31, 2012 12:38 PM   in response to: Timbo in response to: Timbo
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Another relevant document is Product Note 5980-3079EN

"Agilent PSA Performance Spectrum Analyzer Series
Optimizing Dynamic Range for Distortion Measurements"

On page 32 it states:

"One should also be aware of the potential of overloading the front end when the lowband path is selected."

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