And now for something completely different: Fun with TEA
Phase noise - this is just another rabbit hole to explore
These signal generators are said to be very good in terms of phase noise.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319420;image)
How does one measure this? First attempt, use the spectrum analyzer (100MHz set to 0dB) - this one doesn't have a special phase noise measurement mode.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319426;image)
Hmm, the datasheet says -119dBc/Hz at 1kHz offset and 100MHz carrier, so there's about 20dB missing here.
So lets do it the other way, mixing the signal down to DC and measuring the demodulated phase noise. One needs a mixer for this, by some luck I've got this one around, it's a mixer and a low noise preamplifier / comparator that I built while experimenting with the DMTD clock stability measurements.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319438;image)
First dial in some beat note (100Hz) at the signal generators and adjust the low frequency spectrum analyzer to 0dB.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319444;image)
Next, set them to the same frequency (of course they have to be synchronized, I simply connected one reference input to the reference output of the other one, while the other one uses its internal reference). By offsetting one frequency and watching the DC output of the mixer, one has to set the signal generators phase to 90° (minimum DC output). This way, phase noise gets demodulated.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319450;image)
This is what it looks like on a scope:
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319456;image)
Now, set the imput sensitivity of the 3580 to -50dB and wait for the sweep to complete:
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319462;image)
This is 100Hz per division at 3Hz RBW, the center line equals -100dBc. So at the right hand side of the sweep, at 1kHz offset, one can see it's about -120dBc phase noise.
For reference, this is the noise floor of the mixer setup (about -130dB at 1kHz)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319468;image)
The data sheet shows this plot, also having a slight "peak" at about 300Hz, similar to what one can see in my measurement.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1319474;image)