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Originally Posted by Radio Wrangler
When having to do fairly extreme TOI measurements in the past, i've sometimes been forced into using older valve-based sig gens with tuned power amp output stages to get broadband noise down. I was suffering from noise*noise convolution with noise and phase noise in the DUT (a receiver) The DUT was giving a floor below -140dBm with 22Hz FDM pilot filter selected.
David
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You are saying that valve signal generator is having lower thermal noise than solid state counterparts. The phase noise is proportional to frequency of a negative power nth except the PLL and broadband noise components are independent of frequency (attachment). If OcXO or TCXO are used, the main contributor of phase noise near the carrier, the reference frequency would be reduced.
It is not easy to build my own frequency crystal oscillator source. It must have very low phase noise and 50 ohms output. I have no idea at the moment.
I attached the measurement of phase noise of a crystal oscillator. It seems insanely difficult to get reliable results. You multiply a square wave with an anti-phase sine wave from the crystal oscillator to eliminate the carrier, obviously. You make a rabbit disappearing in your hat. The product of the two frequency sources is the difference, giving the noises of the sig gen only after some filtering. So far so good until you know the mixer is the weakest link that it is not really a simple Math multiplication operation of a square wave and a sine wave. What about the noises of the square source? So the measurement is plagued with flaws.
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he tool of choice at work for the last 20 years has been the Agilent E4440A PSA analyser and in this frequency range it has 104dB SFDR with 30Hz RBW. I'm lucky in that I've also got one of these analysers here at home.
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I have been reading PSA application notes since last weekend and where i got the equations from in my calculation. Thats when things start to make more sense when they elaborate their points in more details. The optimum TOI mixer level is about -38dbm for the E440A at band 0.