. Missing a 3rd dds to make for the cuadrature reference.
what I haven't seen is the mixer, as said the only mixy thing there are the switches but filtering that is kind of a pain, as switching mixer generates lot of side bands, should do the math for that.
Filtering may not be needed, it depends on the architecture. VNAs are less demanding to filtering. Some, like the VNWA, dont have filtering at all. Moreover, it makes use of the aliases as the operating principle per se and to extend the bandwidth.
Although it doesn't answer your question on what the wideband transformer winding style is, this page does explain why it's done that way.
http://www.richieburnett.co.uk/temp/gdt/gdt1.html
(unfinished page)
I think the real trick is in the brown ferrite core itself, the colours denote frequency/saturation properties, I have heaps of rings but not any brown ones which I assume are RF ones.
Oh s***t, it's blue! http://allegro.pl/rdzen-magnetec-m-083-rtn-40x25x15-nanoperm-i7430998721.html I don't know, I can't find the actual one, this looks like the closest, but still could work. I couldn't find a way to source the cores, always hard here in Argentina, if someone can source them would be cool to see some tests.
Hf response would only depend on the parasitic inductance and capacitance, to keep inductance low they have twisted the wires, to get low capacitance low turns count and thick insulators, the tricky part is the low frequency. I do have some audio transformers I could use, they go up to tens of kHz but LF response is better than seen in the first video, having a few Henry inductance, after that a different transformer could jump in and make up for the rest of the way.
We have all we need to reverse engineer that transformer! Construction method, number of turns, impedance analysis, freq response... It's just matter of finding the right core material, with the impedance and number of turns permeability could be estimated... Would someone sell cores out of that material?
Although it doesn't answer your question on what the wideband transformer winding style is, this page does explain why it's done that way.
http://www.richieburnett.co.uk/temp/gdt/gdt1.html
(unfinished page)
I thought bifilar implies same coil opposite direction and (usually?) non twisted?
This one is opposite pair and twisted, totally different thing to a bifilar coil winding IMO
Bifilar just the wires are wound next to each other, regardless of the direction.
That's as I understand it too.
Twisted pair is different.
What intrigues me the most: When i spoke to the guys at Omicron they've told me that one challenge for the transformer was that it had to be low in capacitance. But the coupling capacitance in a bifilar winding is usually higher than a sectional wound inductor.
. Missing a 3rd dds to make for the cuadrature reference.
You can generate quadrature LO sequentially with a single DDS as it is done in the N2PK VNA.
We have all we need to reverse engineer that transformer! Construction method, number of turns, impedance analysis, freq response... It's just matter of finding the right core material, with the impedance and number of turns permeability could be estimated... Would someone sell cores out of that material?
If you could sell a $100 unit with the same performance, you'd sell a truck load of them.
Would be interesting to try and duplicate their design and what performance it has with just some random core.
That's not ferrite and it's definitely not powdered iron, that's got to be nanocrystalline and nothing else. Most likely Vacuumschmeltze, probably
...
HF response is easily calculated from the transmission line length. It's a transmission line transformer, simple as that. The twisted pair will have Zo ~ 100 ohms, so that for a step input, each port of the transmission line looks like 100 ohms. That is, the equivalent circuit for short transients is:
pri start -- 100 ohms -- sec start
| |
~open circuit ~open circuit
| |
pri end -- 100 ohms -- sec start
The open circuit is because the core gives the transmission line a very large common mode impedance, i.e., the two ports act as ideal ports, with no common mode connection (again, for short transients, but as it turns out, also for rather low frequencies, down to ~Hz).
After one transmission line delay, the start and end waves interfere with each other, and normal transformer action is had.
Note that this does NOT magically have extreme CMRR -- there's as little as 100 ohms, directly from primary to secondary (again, for short transients). At frequencies well below the electrical length, it approximates as a capacitance from each end of primary, to the respective ends of the secondary. (The exact capacitance can be calculated from line length and impedance.)
Likewise, leakage inductance is the LF equivalent of transmission line inductance, and can be calculated from length and impedance.
If the line is, say, 5m long (to take a ballpark guess), and vf ~ 0.8, then Cp = 208pF (total, so, say, 104pF where the 100 ohmses are indicated above), and LL = 1.67uH.
Tim
Bifilar just means the wires are wound next to each other, regardless of the direction. The opposite would be sectional winding where the two windings are on opposite sides of the core, like seen in chokes for common mode line filters.
What intrigues me the most: When i spoke to the guys at Omicron they've told me that one challenge for the transformer was that it had to be low in capacitance. But the coupling capacitance in a bifilar winding is usually higher than a sectional wound inductor.
(EDIT: T3sl4co1l beat me to it. Bifilar is not a brand thing)
. Missing a 3rd dds to make for the cuadrature reference.
You can generate quadrature LO sequentially with a single DDS as it is done in the N2PK VNA.
There is no need for a quadrature LO if the ADC is not seeing zero IF, but more like an IF in the audio band. The 2 nd DDS would than be the LO, slightly offset from the output frequency. The quadrature part is than at the second virtual IF done in software. So to a good part this is very similar to the simple SDR receivers. It is kind of doing the quadrature at a different time though more continuous phase shift instead of switching between 2 cases.
As the LO is used to drive CMOS switches as mixers, there would be digital LO signal and thus no problem generating quadrature signals with 2 D flip-flops and running the DDS at twice the LO. Still the ADC seems to be single channel only - so this way is likely not used, though it might offer slightly lower noise.
I would guess the FPGA might have enough processing power to do the IF processing and thus much less data transferred via USB. Using an FTDI USB chip also points to a rather low data rate.
It is not a transmission line transformer. You cant excite opposite ends of a transmission line and call it TLT just because you used a transmission line for it.