Dear Sir:
I was reading Agilent application note 150, entitled, "Spectrum Analysis."
I have uploaded a copy of this application note for your convenience.
Page 8 of the application note states:
"When the local oscillator equals the IF, the local oscillator signal itself is processed by the system and appears as a response in the display...L.O. feedthrough actually can be used as a 0 Hz marker."
I have viewed Agilent tutorials with regard to this condition and a question always arises whenever I see this teaching model (the model seeks the viewer to observe the difference frequency). We see a constant LO frequency but, I am led to believe that the LO signal is a swept signal. I would surmise that, in reality, the LO signal would be sweeping so fast that we would not be able to see the frequency sweep.
Why was the signal data processed in this manner?
Edited by: SOLT_guy on Oct 5, 2012 8:38 AM
I was reading Agilent application note 150, entitled, "Spectrum Analysis."
I have uploaded a copy of this application note for your convenience.
Page 8 of the application note states:
"When the local oscillator equals the IF, the local oscillator signal itself is processed by the system and appears as a response in the display...L.O. feedthrough actually can be used as a 0 Hz marker."
I have viewed Agilent tutorials with regard to this condition and a question always arises whenever I see this teaching model (the model seeks the viewer to observe the difference frequency). We see a constant LO frequency but, I am led to believe that the LO signal is a swept signal. I would surmise that, in reality, the LO signal would be sweeping so fast that we would not be able to see the frequency sweep.
Why was the signal data processed in this manner?
Edited by: SOLT_guy on Oct 5, 2012 8:38 AM
yes normally the LO is swept and the sweep speed can be changed depending on what type of signals you want to measure.
Take a look at the picture on page 8. The first LO, (3.62 to 6.52GHz), is normally swept and the sweeping range will give the frequency range that can be seen on the screen. If you want to look at the lowest frequency, "0Hz" then the first LO will be 3.6214GHz, if it's 100MHz then the first LO will be 3.7214GHz. If you want to look at 0 to 100MHz then the first LO will sweep between 3.6214 to 3.7214GHz. The other LO's are fixed, and always fixed and so are the filters.
The first filter, at the input, will not let any signals at this higher IF frequency to get through, only "0Hz" to 3GHz will get through and the first IF is 3.6214GHz.
I think, (but this is my way of reading the text so there might be other interpretations), that the writer of the article actually means that you'll see "0Hz" when the first LO is fixed at 3.6214GHz. This LO frequency will get to the IF port of the first mixer, (since 0+3.6214=3.6214), and get through the first IF filter, (3.6214GHz). A signal at this frequency will then be mixed in the LO chain and result in 0Hz at the detector.
Does this clearify your question?
Regards
Jens