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VNA Fixtures

 

Hi all,
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I am sorting through my capacious parts bin and came across hundreds of old IF transformers of the kind you see in the transistor radios of the 60s and 70s (see attached pictures). Anyway, they're all color coded to indicate their intended use. Online sources differed in their interpretation of what the colors meant, so I decided to measure them and find out. I have about a hundred of each color and there are about 7 different colors altogether in my collection.
The simplest approach seemed to be to create a fixture so the transformers could be swapped in and out of a socket and readings taken of each in turn. I freely admit I struggled with this part. It ain't pretty, but it's the best I can manage with my shaky hands and poor eyesight. I've hooked this up to my HP VNA, but then it occurred to me I would also need to create some cal standards for this fixture (does that make sense?) Anyway, if you take a look at the pictures you will see what I've at least *attempted* to do here! I know there are terrible strays and parasitics from this lash-up, but the IFs will be relatively low, so I figure I can get away with it.
My question is: given that I'm going to be performing an S21 forward transmission test sweeping from 300khz through 50Mhz, which type of calibration should I choose from the analyzer's calibration menu? Or can I get away with just a normalization? I've only ever done S11 measurements in the past and calibrated with short, open and load. But this is different.....
Suggestions, please.
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Jinxie
 

thru calibration should be enough in S21 case plus we are talking about pretty low frequencies.
 

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What is the impedance of these transformers? ?If significantly different than 50 ohms you won’t get good results.?

Peter

On Feb 17, 2025, at 11:20?AM, Maciej Kawalkowski via groups.io <kawalkowski.m@...> wrote:

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thru calibration should be enough in S21 case plus we are talking about pretty low frequencies.
 

The impedance of the transformers obviously changes with frequency. They are, in effect, band-pass filters. As for the 50 ohms, that's the 'system impedance' and there's nothing I can do to change that!
I did remember *after* I'd gone to all this trouble that I do actually have a vintage 'wobbulator' in good working order which would have been a much more sensible approach than using a VNA!
 

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You can make wideband transformers with small toroids and a bit of wire.?

I did that to test some crystal filters that were closer to 500 ohms. You can also make resistive pads.?

At 50 ohms the response looked horrible and I couldn’t even identify the BW but with the transformers the response looked quite proper.?

Peter

On Feb 17, 2025, at 3:15?PM, Jinxie via groups.io <paul666@...> wrote:

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The impedance of the transformers obviously changes with frequency. They are, in effect, band-pass filters. As for the 50 ohms, that's the 'system impedance' and there's nothing I can do to change that!
I did remember *after* I'd gone to all this trouble that I do actually have a vintage 'wobbulator' in good working order which would have been a much more sensible approach than using a VNA!
 

Perhaps it's because it's very late here in my timezone, but I'm not really seeing why the 50 ohm system impedance would be an issue in this regard?
I'll have to read any answers to that tomorrow as it's past time for me to hit the hay.
If there are any other issues, please summarize them so I can address them at the same time as the above.
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thanks,
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J.
 

The 3577A has both 50-ohm and 1-Moh’s natively available. 100MHz upper limit.

Pete

_._,_._,_
 

You can rest assured that none of those are for 50 ohms or even close, but you can still fool around with them on a VNA and get some ideas of what they are.
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Those color codes can identify them somewhat since the functions of the ones used in transistor radios were somewhat standardized long ago and assigned to color codes, but different manufacturers may have their own or modified codes. But, it's a good place to start. A good info source is the fun with tubes group here:
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https://funwithtubes.groups.io/g/main/messages
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I know I have seen and been involved in some pertinent IF transformer discussions there, and have seen references to some sites with info on the color codes and such, within the past couple years or so. It's not a high traffic group, so it should be fairly easy to search for the right stuff there. It's easy to join if necessary to ask for advice and look through their archives.
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I have similar but much larger troubles with IF cans of all sorts, with small numbers of very many kinds that I have pulled and saved over decades - lots of brands, sizes, styles, color codes, and so on. I started sorting them somewhat a couple years ago, but have a very long way to go, and most will never be properly ID'd. I have maybe a thousand pieces. The ones you have probably were high volume broadcast radio units with standard functions in the signal chains, seven different ones may have been all that were needed for a wide range of AM/FM transistor sets.?
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The easiest to spot are the IFTs, which often have a tiny dogbone ceramic capacitor mounted in a cavity on the bottom. These tune for the nominal IF 455 kHz AM or 10.7 MHz FM and possibly 19/38 kHz pilots for stereo decoding. Sometimes AM LO and FM discriminator transformers have the built in cap too, which can confuse things a bit. I think FM LOs usually used open coils that are deformed for tuning so not in cans. The cans without the caps are more general purpose. It's possible to have TV parts too, including video IF around 43 MHz VSB, and audio IF and traps around 4.5 MHz. My collection is mostly in these types but includes lots of others like VCRs, cordless and cell phones, ISM bands, CB radios, satellite, cable, and industrial stuff. I also have a lot of just plain variable inductors, which are very handy, and of course, using only one winding of an appropriate transformer can serve the purpose. Note also that many types can be opened up and modified.?
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Another big clue is the style of the magnetic tuning slug. If it's fat (and will be ferrite) and goes around the outside of the coil assembly like an inverted cup, it's likely for very low frequencies below 10 MHz. That yellow one in your? picture might be that kind. Try to screw it all the way down and see if the coil form is visible under the can lip. If not, it's a low frequency one. Midband types will have more of a screw-shaped slug of ferrite (lower end) or powdered iron (upper end) that threads into the coil form. If the slug is aluminum, that's for VHF like satellite 480 or 600 MHz IF and helical filters.
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Anyway, the nature of the part will tend to show on a VNA even without matching - it's a sloppy measurement, but probably enough to ID the basic function types. The best thing to start with is a 50 ohm series damping R in each line to/from the VNA which will reduce any crazy resonances so things are a little easier to decipher. No fancy fittings are needed - just tack in whatever parts help get a cleaner picture. It's actually a helluva lot of fun to experiment with and re-task various IF can type parts into new uses. I have done it a lot especially for custom LC filters, where adjustable tuning is essential. I like to use fixed padded mica caps with variable slug-tuned chokes in cans for most filters below a hundred MHz.
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Ed
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That's an interesting setup. I would start with a two-port S21 Response calibration.
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Not sure what VNA you're using. My 8510+8515A won't go that low of frequency but if yours are similar to 8510,
it's?
1) press CAL
2) Select CAL kit(I think any calkit will do for your setup)
3) Press S21,
4) Select "Calibrate Response"
5) Connect thru
6) Press "Thru" standard key.
7) Done and save CAL Set#
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Then measure S21.?
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Good luck!
Calvin
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Thanks, guys; some very useful info overnight! I should have mentioned the analyzer I'm using is the good old venerable 8753C. Though its screen is growing somewhat dim, it still does the job. I just have to close the curtains if it's too bright outside. :-)?

 

Ed, many thanks for that comprehensive answer. All bar one of these devices have that capacitor you describe tucked away in the base. Can't be more than a few tens of pF at most I'd have thought. The sole exception is one with 6 pins instead of 5 for some reason. The 5 pin ones have 3 on one side and 2 on the other and I *believe* the 3 side is input and the 2 pin side is output. Normally only two of the three input pins are used. I think there's an extra inductor in there if you want to use it in which case you connect to the two outer pins. Seems there's a bit more to these trannies than first appears!
 

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If you have a lot you can carefully disassemble one of each to get further knowledge.?

Peter

On Feb 18, 2025, at 8:34?AM, Jinxie via groups.io <paul666@...> wrote:

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Ed, many thanks for that comprehensive answer. All bar one of these devices have that capacitor you describe tucked away in the base. Can't be more than a few tens of pF at most I'd have thought. The sole exception is one with 6 pins instead of 5 for some reason. The 5 pin ones have 3 on one side and 2 on the other and I *believe* the 3 side is input and the 2 pin side is output. Normally only two of the three input pins are used. I think there's an extra inductor in there if you want to use it in which case you connect to the two outer pins. Seems there's a bit more to these trannies than first appears!
 

That may well be necessary if I encounter one of those 'problem child' devices that Ed described!
 

On Mon, Feb 17, 2025 at 03:15 PM, Jinxie wrote:


As for the 50 ohms, that's the 'system impedance' and there's nothing I can do
to change that!
Sure there is. Download the reference impedance renormalization utility near the top of the following page. Download instructions are at the bottom of the page.



Try various renormalization impedances until you get a reasonable response. Google AI says the impedance is around 1.5k ohms for typical 455 kHz IF filters. I'm not sure I'd trust that number, but I think you'll know when you've chosen something close to the correct value.

Brian
 

Yes it's well worth taking some apart to see the innards, and how to put them back together. Those cans seem to be a decent size (1/2" and 3/8" square ones are very common and workable) for surgical mods for other uses. You can delete the built-in caps, and change the windings within reason. The ones smaller than that can be quite difficult to work on, especially due to the very fine wire needed to fit.
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Ed
 

One more thing - you can get some idea of the transformer pinouts and circuit topologies in old transistor radio schematics, from say the 1960s - 1980s.
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Ed
 

Ed, yes I can see how it? would be possible to rewind one of these things in practice, but for me I think it would be a task too far. I just don't have the dexterity to do something like that. I did take one completely apart and the actual core was miniscule; I'd estimate the diameter as less than 2mm. I'd really struggle with that!
 

Brian, I'm sorry. What does this program actually do? I should also say I only have Linux computers, so that adds another barrier to your suggestion, I'm afraid.
 

You'd have to run a Windows emulator.

The program changes the 50 ohm reference impedance to the impedance you specify. For example, if you set the renormalization impedance to 1.5k ohms, the renormalized |S21| response of a filter will be as if had you driven and loaded it with 1.5k. If you measure a device using an impedance it's not designed for, you won't get the response it would exhibit in a circuit that uses the proper impedances. The program lets you see the correct response without actually driving/loading the device with the proper impedance.

The attached plot shows the response of a Murata 10.7 MHz ceramic filter. The blue curve is |S21| for the 50 ohm VNA measurement. The red curve is after the data was renormalized to 330 ohms, which is the source/load impedance the filter was designed for. The renormalized response is as expected.

Brian

1.png

 

Okay, thanks, Brian. I may be able to run this Windows program under WINE, I'll have to investigate further on the appropriate discussion forums.......
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J.

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