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Frequency Measurement and Signal Isolation

Question asked by SOLT_guy on Dec 30, 2012
Latest reply on Dec 31, 2012 by SOLT_guy
Dear Sir:

     I own an Agilent spectrum analyzer with a comb generator.

     I also own an Agilent frequency counter.

    I was measuring frequencies generated by my comb generator with a spectrum analyzer and frequency counter at very high frequencies - tens of GHz..  

    The comb signal was connected to a simple T connector which went out to my spectrum analyzer and frequency counter.   

    To my surprise when I changed the measurement settings on my frequency counter using the front panel buttons, I was led to believe that my comb signals changed frequency on my spectrum analyzer due to the instantaneous disappearance of some signals and the new appearance of other frequency signals.  

     It is my belief that by changing my frequency counter settings, the interaction between my frequency counter and signal generator caused a reflection, or permitted a stray signal to flow, that would be displayed on my spectrum analyzer.   I do not know the origin of these new frequencies, all I know is that they exist, and it is possible that these new signals will not appear if I can implement better isolation between my signal source, frequency counter, and spectrum analyzer (minimize the undesirable effects).

     I was hoping that someone at Agilent might tell me what type of device I should use to replace the T connector with - a device that will provide better measurement isolation and prevent this problem from occurring?

     Also, I do believe that since I am manually setting my frequency counter and all input signals must fall within 40 MHz of the frequency counter's center frequency setting, it is possible that those frequencies that fall outside this range might be affected by my frequency counter when I change the center frequency setting.   This may explain why I see a signal on my spectrum analyzer disappear (note that my spectrum analyzer span setting is far greater than the 40 MHz) after I manually change the counter's center frequency setting.

       Can anyone tell me how signals, which lie outside of the frequency counter's manual setting's range, are "physically" affected when one changes the center frequency such that the incoming signal is now outside of the counter's frequency counter's measurement range?

Edited by: SOLT_guy on Dec 30, 2012 9:46 PM

Edited by: SOLT_guy on Dec 30, 2012 9:53 PM

Edited by: SOLT_guy on Dec 31, 2012 6:36 PM  

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