Hey Gabriel and hey everyone,
First: Thanks to all of you and of course Gabriel in particular for this really nice and worthwhile discussion! I hope that we can gather some more experiences and ideas. ?
Concerning the caps: compared to 100n or 470nF caps those 4u7 have a lower resonance point and hence are able to filter lower frequencies too. So in total, they are really broadband!
Frankly I never had such radiation problems concerning the ¡®normal¡¯ SMA connector type. That¡¯s why I¡¯m a bit astonished that you had one at your design as 4cm distance between 2 SMA connectors (mid to mid) gave me one roughly 90dB isolation. Maybe your connectors are a bit closer¡
Funny that we nearly had the same ideas! You surely did a good and detailed prototyping, wow! ?
But concerning the caps between mixer and BB amp: maybe a smaller cap may do the trick? A smaller cap is faster at its charging and discharging but still blocks DC.
I¡¯m sorry that those TC-1-13Ms do not solve your problems. If your looking for a transformer having a compensation for its asymmetry within the component may you can have a look at those:
MABA 011082 or TCM1-63ax. Unfortunately, the S-Parameters are just starting from 5MHz and 1MHz. But within the balun simulation they still had a nice performance at those low frequencies. Of course, they might be a bit more expansive but as they are built up symmetrically you may not need to build in two but just one inside your directive coupler¡
May I ask why the OIP3 is important to you? In my opinion to ensure a good performance the system needs to run at the linear region of the amplifier so values like P1dB might be more important than OIP3¡?
And Wow to your plans building up a ¡®proper¡¯ VNA! That¡¯s amazing! I wish you all the best for it ? But programming (modelling) a FPGA is totally different then a microcontroller. I thought the new SMT32 generation have some specials like higher clock and special filter setup internally etc so maybe those might be already fast enough to shift the IF frequency into a proper value.
As you¡¯ve asked at which project I needed to work on an directional coupler: I did my master thesis at Rohde & Schwarz at their VNA department and developed a directional module from 9kHz to 20GHz (actually it had been more than 30GHz at the end) using PCB material. ?
At the moment I¡¯m just a normal RF-Engineer with several different projects from schematic to layout design and from simulations to antenna setups. But I¡¯m far from being able to lead a whole ne VNA development :¡¯)
Although, maybe I might build up a small directive coupler if I have some spare time just to do some measurements on my one !
And ones more: Thank you very much for sharing your knowledge with us! ?
Mel