Right from the start (after I "improved" my SSA that is) I wasn't too happy with the performance of the tracking generator. Once the machine has warmend up, the level drops below -7dB close to 3GHz. That is the case even though I modified the level calibration of the TG, but the calibration range just isn't wide enough. While the SSA is still cold, the situation is somewhat better, it drops by round about 5dB in this case.
Of course, normalization takes care of this during measurements that permit to normalize the instrument without having to change the settings afterwards. Always being nosey, I decided to take a look if there are any hardware tweaks possible that may ameliorate the situation. I decided to take a closer look at the TG module, and there, only behind the TG mixer so the repercussion on the first local oscillator signal in the main RF module would be neglible / non-existant. So I first started to take a close look at the quite complex distributed element filter directly after the mixer:
(Photo probably borrowed from Dave's "teardown collection")
I vectorized the graphics in my CAD system and used the edges to produce a "proper" drawing of it. Using the mounting flange of the N connector as a reference (it's specified to be 1" square), I scaled the drawing to the proper size. The size of the stripline to the N connector, which should have an impedance of 50 Ohms and is approx. 1mm wide, together with the assumption that the PCB material is some Rogers R4003 hybrid multilayer laminate (since for economical reasons, I don't expect Siglent used PTFE based material here), a copper thickness of 35µm and a dielectric thickness of 0.406mm and an ε
r of 3.38 appear correct (those parameters are both stock values from Rogers).
Finally, I calculated the inductances and capacitances of the elements (there are nice online tools available for this; google is your friend) and entered the schematic of the distributed element filter in LTSpice. Actually, I considered my approach to be not much more than a "wild guess", the more I was surprised of the result, see here:
At 4GHz, the attenuation of the signal is 50dB+ !
So even though this approach didn't take me closer to a solution of my TG improvement attempt, I'm more than impressed how accurate characterizations of such delicate circuitry are possible with "household" methods. I tweaked some of the component values of the filter in the simulation to find its performance only gets worse...
Time to think about different solutions to get the TG straight
. An interesting approach it was anyway, I think
.
Cheers,
Thomas