I purchased one of these items..note attached PDF wit all details. Proceeded to break the PCB due to a yanked cable and was in the process of recalculating the 'impedance' of the microstrip for 1.6mm FR4.
I realized that it adds up to several hundred ohms.
My question is this...why would a 50 ohm RF signal require a several hundred ohm microstrip (to couple some signal to a detector) and then connect to another 50 ohm output? See Note 4 on page 11.
Those microstrip impedance values shown in Note 4 are transmission line impedances.
They don't add together.
Ideally they should be all the same but there is some mismatch between them (apparently not enough to cause a problem).
Do you understand what the characteristic impedance of a transmission line is?
I was treating them as lumped element, but that's a wrong approach.
I have to do some more learning in this regard.
I think the stepped impedance.is deliberate to create some kind of Low pass arrangement, otherwise just make a single 50 Ohm microstrip for lowest VSWR and be done.
I am working on figuring it out. I am going to do some empirical VNA tests as well, I have a new PCB from the PDF to work with . Sometimes learning works better by doing.
I think that this may be a case of "over thinking" the problem.
The highest frequency involved here is about 1000MHz, wavelength is 30cm.
What are the line lengths involved here, 1cm maximum?
I think that this is going to have little effect on the signal.
Did Tektronix even design these as microstrip or are they just traces on a piece of exotic circuit material?
It is matched to replace a squiggle on the original , both stepped impedance and phase delay. It's length is adjusted for the different propagation/permittivity of FR4 vs Alumina. The designer of the substitute said he just duplicated the functionality without finding out why it's there. As I am only 3 months into RF (IoT motivated) I have a lot to do in the fundamentals and so here I am.
I gather the following:
Since the peak Freq is 1 Ghz the microstrip is well below 1/4 wavelength resonance and can be considered an inductor and not a tx line. VSWR then is not as significant.
A series inductor is a choke/LP pass filter, presumably to knock down harmonic spurs.
What makes this complicated for me is figuring out what is the inductance of this design.
Can you help with that?
Edit:
using coil32 freeware I calc'd the individual inductor steps and came up with 7nH total for the original TEk strip and 9nH for the substitute. Thus it seems from a lumped element perspective the substitute is somewhat incorrect. From a tx line perspective (which appears invalid at the Frq range) the impedances and phase are correct.