AnsweredAssumed Answered

PSA E4446A spurious artifacts

Question asked by timbo on May 16, 2012
Latest reply on May 31, 2012 by xmo

Like • Show 0 Likes0
Comment • 6

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

No one else has this question

Outcomes

tabbott

May 16, 2012 1:40 PM

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 -

Actions

timbo @ tabbott on May 16, 2012 2:26 PM

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

Actions

tabbott

@ timbo on May 17, 2012 11:36 AM

to be migrated, sourceId: 101781

Actions

timbo @ tabbott on May 18, 2012 6:36 AM

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

Actions

tabbott

@ timbo on May 31, 2012 7:48 AM

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 -

Actions

xmo May 31, 2012 11:38 AM

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."

Actions

6 Replies

tabbott May 16, 2012 1:40 PM
Mark Correct
Correct Answer
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 -
Like • Show 0 Likes0
Actions
- timbo @ tabbott on May 16, 2012 2:26 PM
  Mark Correct
  Correct Answer
  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
  Like • Show 0 Likes0
  Actions
  - tabbott @ timbo on May 17, 2012 11:36 AM
    Mark Correct
    Correct Answer
    to be migrated, sourceId: 101781
    Like • Show 0 Likes0
    Actions
    - timbo @ tabbott on May 18, 2012 6:36 AM
      Mark Correct
      Correct Answer
      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
      Like • Show 0 Likes0
      Actions
      - tabbott @ timbo on May 31, 2012 7:48 AM
        Mark Correct
        Correct Answer
        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 -
        Like • Show 0 Likes0
        Actions
xmo May 31, 2012 11:38 AM
Mark Correct
Correct Answer
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."
Like • Show 0 Likes0
Actions

PSA E4446A spurious artifacts

Outcomes

Related Content

Recommended Content