I am preparing to do some RF mixer experiments with the SA612 Gilbert cell mixer. I started out with the impedance matching as I understand that the input and output need to be matched. From what I read, the impedance should match 1500 ohm single or 3000 ohm in a balanced configuration. So I need to match 50 to 3000 when using a balanced configuration.
I decided to try winding a toroid. I used a T 80-2 (red, which should be good below 30MHz) with 23:3 turns:
But before I move on, I would like to measure it. I don't have any experience with this, so I would like to get a feeling if and how well it performs.
But how? I have some basic equipment (scope, signal gen, nanoVNA, etc,) but I don't know how to approach this.
At first I was thinking of doing some tests with the nanoVNA (I'm no expert in using it). I figured an SWR value would give me a good indication. But I wasn't able to get a good reading (I got extremely high or even negative SWR values). Maybe because the setup I used (I terminated the high-Z side with 3000 ohms as if it was connected to the SA612). I also wondered how this would work, I mean, doesn't the nanoVNA just see an inductor (the primary side of the transformer) and doesn't this not just act like one (with somewhere a dip where it's resonant)?
Then I figured I could do some measurements with my signal generator and an oscilloscope. I connected the signal generator to the low-Z side and tried to measure the high-Z side which was again terminated by a 3000 ohm resistor (gnd and probe on each side of the resistor). I could not get any reading (even at the lowest setting in the mV range and with 10 Vpp). I inserted a 50 ohm resistor in series with the signal gen to see if the signal is visible at the input side and it was as expected.
So, now I have built myself this mysterious device that should work, but which I can't measure...
My question: Can anyone explain how to measure an impedance transformer? And please elaborate and be explicit, as I am quite new to this particular area.
Test setup for measuring impedance:
Result after calibration (calibrated at the end of the cable used):
I was inspired by this video from Ham Radio operator W2AEW. His toroids "look" the same, and he didn't use enameled wire. So I was hoping I would come close to his results this way.
The datasheet I used for the toroid values as stated for the current test: datasheet
The sweep on the nanoVNA was set to 1 MHz - 30 MHz. I'm not sure what signal level the nanoVNA is using (I would have to investigate that) but what I am planning for my SA612 experiment will be:
LO -50dBm @ 17.7 MHz, RFin -50dBm @ 28.4 MHz
expecting a response at 10.7 MHz
I will move on by finding a (ferrite) toroid with better properties for this. As I have no experience with this, I probably have to order and test multiple candidates and see if they match (calculated) expectations.
I found this document to be useful: Inductor Construction
I calculated that for Z=50 (which would be 0.00053 mH inductive) a ferrite core with 3 turns would need an Al of ~60 mH/1000. This would lead to a toroid FT-114-61 (which is 79.3) as closest. So that I am going to try.
The mentioned document immediately makes clear that for wideband use the iron core toroids are not useful. So things fall into place...