How to connect OLD IF transformer to Siglent spectrum analyser with tracking gen - Page 1
Forum section: electronics / rf-microwave

author: AlexResch
author_rank: Regular Contributor
timestamp: August 09, 2018, 08:50:47 pm
content: Hi everybody,I am a High School student getting into electronics. I would like to see the response of some old IF transformers ( 30s Pilot radio ) on a Siglent spectrum analyser with tracking gen. But I know those IF transformers are not 50 Ohms in/out impdenace. Then how should I connect them to the tracking gen/spectrum analyser input ?Thank you for your help and hope this is the right place in the Forum.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2018, 10:40:57 pm
content: Probably just as well to do it grid-dip style. That is, put a wire loop on a BNC-to-binding-posts adapter, and use that to couple into the coils. Connect TG to loop, and either SA to another probe loop (and probe the coils with both -- see how the response changes with position and orientation), or connect all three together with a tee.The response will be a notch for the one-loop case, or a peak for the two-loop case (assuming the loops are placed at opposite ends of the coil).Alternately, or if you can't remove the coils from their shielding cans (go carefully, if you do, by the way), then use two BNC adapters, connect the grounds together, and one side of each coil to ground (there's probably a bypass cap to ground, in the circuit, showing which side is naturally at RF ground; if these are still in circuit, then just tie off to that ground, no need to unhook or short out the circuit's bypass caps!). Use a large value resistor, a few kohms, to connect the TG to one coil, and another resistor to connect the other coil to the SA. The impedances must be high because tube impedances are high -- you won't see the right response otherwise. The insertion loss will be very high, because of the resistors; you probably won't get a good read on stop-band behavior (because it will be down in the noise floor).You can get unloaded Fo and Q this way. You can't get Zo with the loop probe method. You might get k this way.Mind that vacuum tubes have impedances as well: resistance and capacitance, typically. They load the coil, detuning it. Expect to see unloaded (or "cold", if in-circuit but unpowered) values being a bit different from the intended value.I don't recommend trying to do alignment this way!Tim

author: SpencerTC
author_rank: Contributor
timestamp: August 10, 2018, 12:19:10 pm
content: I'm not super experienced myself, so take what I say with a grain of salt.I would do my research and find what impedance the device you want to measure is (or what it most likely is) and make an impedance matching pad to match 50Ω to whatever you are trying to measure. I work in a company that makes equalizers, filters, and attenuators for the cable TV industry and that's how I measure the 75Ω devices that I am working on with 50Ω equipment.Just search for a impedance matching attenuator calculator and use that to get the resistance values correct easily.You might want to either put an attenuator in between the matching attenuator you make and your testing equipment in case the signal is stronger than you expect (or if you know how much attenuation you need, you could build that into your matching attenuator and achieve both with the same device)

author: AlexResch
author_rank: Regular Contributor
timestamp: August 10, 2018, 11:21:00 pm
content: I will try the resistors approach because it seems the easiest. I managed to remove the coils from the can and was easy but the wires were very stiff after almost 80 years. But I made a mistake - I neglected to note which coil goes where. Both coils look the exact same and are wound on a simple card board core at some distance between. No ferrite, just two "compression trimmers". I'll report what I saw.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 11, 2018, 01:03:42 am
content: Yeah, using large (~kohm) series resistors is a minimal loss pad. Same thing, just said differently.Downside is, even at minimum loss, it's a lot (30dB+?).Tim