If there's one thing that I was blissfully ignorant of before trying to build a spectrum analyzer, and is now an annoyance and borderline obsession, it's phase noise. Phase noise is exactly the same as any other kind of noise, but unlike noise on a DC signal, phase noise is more like tiny variations in the frequency of the signal.
To get a little more specific, think about phase as a quantity that describe how far along the sine wave we are at any given instant. It can be described in degrees, radians, or even if you're a little radical, tau radians. The idea is the same. Phase increases until it "rolls over" at the end of the cycle. It looks a bit like a sawtooth wave, assuming that we're discussing a constant frequency, unmodulated wave. Hopefully you believe me that phase noise looks just like noise superimposed on the sawtooth-shaped phase ramp.
I think the graphs above helps to illustrates the relationship between phase(above) and a sine wave (below). I've re-scaled the phase so that it goes from zero to one over the range of one whole cycle.
In the frequency domain, phase noise is a little easier to understand, and see. The graph at the head of this article is an extreme example of phase noise. Really, there are two kinds of phase noise here, and one of them we can do something about.
I tried really hard to find some nice graphics to use to describe, simply, the basic operation of a PLL-based oscillator. The best I could come up with is the above diagram. This was lifted from the Linear Technology LMX2326 PLL IC's datasheet. This is the same (or damn near) as the PLL chip that's used in the analyzer. The bottom left corner is the oscillator. All it does it generate a single frequency set by the voltage coming into pin 6. On the PLL chip, pin 2 is the output from the "charge pump," which is how the PLL sets the tuning voltage.
Unfortunately, the PLL in the spectrum analyzer isn't this simple (if you can call a normal PLL simple!). Notice the Op-Amp, in the center and near the top. The high-side supply to this Op-amp is +20 volts, to supply the VCO (voltage controlled oscillator) with the range of tuning voltages it needs. It isn't possible for the PLL to produce voltages like this, so we need this extra supply.
Now, the question is: "What happens when there's noise on the +20 voltage supply" The waveform on the oscilloscope above shows about 20mV of noise on the 20 volt supply. The frequency of this noise is about 20kHz. It's no coincidence that the spacing between the peaks in the comb-like plot is about 20kHz. What's happening is that the noise on the 20 volt supply is literally modulating the output. Incidentally, that's exactly what you do if you want to frequency modulate something.
Now that we know what the cause is, what can we do about it? If we eliminate that noise, we can fix the problem. I had made a second 20 volt supply, and used cheap capacitors. Apparently, when using high voltages (relatively speaking) the amount of capacitance decreases in cheap, small, ceramic capacitors. I went back to the first supply I made, and added even more capacitance.
The lower trace is the new +20 volt supply, and it's peak-to-peak noise voltage is about 3mV. But the proof of the pudding is in the eating, so how does it affect the phase noise?
It squashes it like a bug! The above plot is almost textbook for phase noise.The large peak in the center is representing the width of the final filter(I'll get to that in a later post) and the skirt is caused by traditional phase noise. If I zoom in to the center of that plot it's easier to see:
Here, I've highlighted another common cause of phase noise: PLL loop bandwidth. This is the bandwidth of the filter that smooths out the pulses that come out of the PLL chip. This is a compromise between low phase noise, and how quickly the PLL is able to tune between different frequencies.
That's all I have for now… I've tried to make this topic, which is very technical and dry, interesting and accessible to those that haven't spent the last 5 years trying to build a spectrum analyzer. I hope you've enjoyed it.
If you want a much more in-depth and technical analysis, see Scotty's website.