4:1 Transfomers 50:12.5Ω - Success

I hadn't expected this to work out so easily. I already had a selection of bifilar wound transformers wound on these cores so it seemed like a good place to start. Terminated in 12.5Ω I looked at the 50Ω connection with the nanoVNA:

8t bifilar on a single core

Another single core with 12turns bifilar was tested and it was too many turns. Good for 160m to 40m though. 

Shifting to two cores side by side like a binocular core with 7 turns proved useful for all of the HF bands. Five turns could be a suitable starting point for 6m.

7t bifilar on two cores binocular style

To check the insertion loss I wired two of these transformers back to back. (50:12.5 + 12.5:50) 

The loss was negligible.

Loss across two transformers back to back

With 1W applied to this arrangement for 15 minutes the cores showed no sign of heating, perhaps 1degree above ambient.  

The goal was to find a suitable transformer for matching 50Ω to input of a 10W amplifier. If a 4:1 transformer is the solution then this has been achieved. But a 9:1 transformer may be required so another round of testing is needed.

73's

Richard

4:1 Transfomers 50:12.5Ω - Preliminary

Having resolved which ferrites to use for 200:50 ohm transformers it is time to see if any of the cores I have are suitable for matching from 50Ω to a lower impedance using a bifilar winding. The application in mind is for the input stage matching on a single ended mono band HF amplifier delivering 10W or more. 

While I have built push pull HF amplifiers it always struck me as ironic that semiconductor manufacturers often specify the IMD properties of RF transistors in a single ended circuit. However, that just gets ignored because push-pull configurations are claimed to deliver lower distortion and harmonic cancellation. Maybe that's possible, but I know single ended amplifiers can deliver very good results. And I struggle with the concept that a transistor in the off state can cancel harmonic energy generated by a transistor in the on state. Perhaps the overall result is less about the topology and more about the implementation?

My approach will be to determine which core can offer a good transformation from 50 to 12.5 ohms, then use two back to back to see what the loss is and how well the core copes with 1W of RF. 

73's

Richard

4:1 (200:50 ohm) Transformers - Yet Another Suggested Core

So this core is surprising but bang for buck is pretty amazing. It's quite small but 8 bifilar turns spaced around the core is useful from 160m (just) to 6m.

8 bifilar turns

 Perhaps you have the patience to use more turns of still finer wire?

 

12 bifilar turns

But glue, or hold while you start winding, two side by side and wind 7 bifilar turns using fine wire also results in a good transformer:

Given the extra time it takes to work with these small cores it might be false economy. But it does remind me how useful the nanoVNA is for making such measurements of unknown components. And these ferrites are really low cost.

Details of power handling and loss soon.

73's

Richard

4:1 (200:50 ohm) Transformers - Another Suggested Core

I had some more cores arrive, one of which appears to be as useful as those mentioned in this blog here.  This one is a binocular core from LCSC, part C498903. It is larger than the binocular core in the earlier blog.

Instead of showing you a Smith Chat of the swept response I present a plot of S11 versus frequency. This should make it easier to discern how good the transformer is. Six turns is good for HF, while 7 or 8 turns improves things at the bottom end of HF. I haven't measured a 5 turn transformer but it might be better for 6m.

6t bifilar

7t Bifilar

8t Bifilar

 

73's

Richard

4:1 (200:50 ohm) Transformers - Two Suggested Cores

 Still more testing but I have narrowed it down to a choice of two:

Small Binocular Core ~A$0.30 each

My initial test with this binocular core was only 3 bifilar turns due to the size wire being used. I  suggested if 4 turns didn't perform better it should be put to one side. I twisted up some fine wire and had little difficulty passing 7 turns through the core. It appears useful across HF:

 

 

 Small Toroid Core ~A$0.20

Now this core was a real winner. It's like two FT-24-43 cores stacked. I had tried some smaller cores which appeared to be too small for HF. I wound 10 turns and measured in my fixture and thought this could work. I rewound with some twisted wire and increased the winding to 13 turns. It just felt right looking at the curve for 10t.

The result was outstanding and far exceeded my expectation being useful beyond 150MHz as the sweep below from 1MHz to 150MHz shows. 

I am very happy that a small amount of time with the nanaVNA, some systematic investigation and thought have resulted in a 4:1 transformer I can use from 160m to 2m.

73's

Richard

4:1 Transmission Line transformers - Which core?

 Right. So I know have a rule of thumb and a growing pile of test cores and results. The rule of thumb I found, see earlier posts on this subject, is very helpful when dealing with unknown cores. But I went searching and I was surprised at the reasonable cost of buying brand new cores from Mouser. 

For 200:50 ohm bifilar wound transformers I am narrowing down on a "standard" part. I am confident that small cores will cope with the maximum power a 12V class A amplifier can deliver across 200ohms (12^2/400 ~ 0.4Watts) so size is unlikely to be a selection criteria.  Taking into account price and performance across HF I present some results for your consideration:

Small Binocular Core ~A$0.30 each

I could only get 3 bifilar turns through this tiny toroid with the relatively large wire I was using so those bifilar windings were jammed on top of each other. Despite this the results were surprising:

3 turns 200 ohm termination and Open circuit termination

With finer wire and 4t I expect this core will result in a useful transformer. Confirmation in due course.

The next physically larger size core is substantially larger than the toroids below with a similar cost. So if 4 turns doesn't cut it then I'd put these to one side for now in this application.

EPCOS / TDK Toroid ~A$0.60 each

After 7 bifilar turns suggested more turns were needed I guessed 11 turns. Very good transformer at 30MHz but still needs a few more turns. I'll try 13 turns next.

11 turns 200 ohm termination and Open circuit termination

 

Fair-rite Toroid 43 Mix ~A$0.75 each

Results for 7 bifilar turns were pleasing. 8 turns might be better. I only had one of these cores and have put it to one side for now.

7 turns 200 ohm termination and Open circuit termination

Next steps:

The Epcos/TDK toroid is, on results to date, the leading contender. I still have a few smaller (and cheaper) ferrite toroids to test and I am interested to see how they perform. At some point I will have picked my standard for 200:50 ohm transformers and I will write it up.  Then it will be time to get back to my amplifier testing which is how I ended up down this rabbit hole.

 

73's

4:1 Transmission Line transformers - Rule of Thumb

Having unwound some of the cores I recently measured and finding some were not bifilar wound at all I started again. 

I took another, different small core, and wound a few turns on it. The exact number is unimportant at this point. It's the red curve below. I then wound a bifilar winding on another identical core with the same number of turns, that's the yellow line when the second winding is open circuit. What I noticed was the bifilar wound toroid appeared to be "stretched". When terminated in 200 ohms the blue curve eventuated.

Where either the red or yellow curve crossed the R axis, that frequency was a reasonable approximation of the frequency where the 200 ohm load was transformed to 50 ohms.

 

 

I picked up three cores, the same as each other but different from previous tests, and wound 9,10 and 11 bifilar turns on each one.

9 turns

10 turns

11 turns

Again, this relationship of the frequency where the R axis is crossed when the load is open is close to the best frequency for a 200:50 ohm transformation. 

 

I  believe I'm onto something now. Let's repeat this with 3 larger, EMI suppression cores taken from a VGA cable. Not the huge ones, but about 15mm long.

 

 

 

 

11 turns 

  

  

8 turns

7 turns

With 7 tuns I had a transformer suitable for 2 MHz (marker 1) to over 10 MHz (marker 2). I will repeat this for 6 turns and if suitable use that transformer in the test boards for general purpose transistors as RF amplifiers I am working on.

Conclusion:

You can quickly determine the frequency a toroid can be used to make a 200:50 ohm transmission line transformer with a bifilar winding. 

• If the toroid has a single winding, or adding some turns if it has no windings, adjust the turns until the R axis is crossed at the lower end of the frequency range you are interested in. Then replace the winding with a bifilar wound transmission line.
  
  
• If the toroid already has a bifilar winding you can treat it like a core with a single winding by leaving the hot end open circuit.

 

73's

Richard VK6TT

4:1 Transmission Line Transformers - Experimental results

Progress on my testing of general purpose transistors for RF amplifiers had stalled because I ran out of 22uH chokes when populating the test boards. Using a 4:1 transformer on the collector of the first stage would work but I wanted to understand, if I could not eliminate, variation between the test boards.

I had 5 different 4:1 transformers already wound in the parts bin. The pic shows these and acts as a legend for the charts to follow. There is also a 1206 sized part for scale.

 

Now to determine just how well these transformers worked. I swept each transformer over the range 1 - 100 MHZ with the nanoVNA. Firstly with the transformer un-terminated, then terminated in 200 ohms. A perfect transformer would have transformed the 200 ohm load to 50 ohms across all frequencies, showing as a dot in the center of the Smith Chart.

Core 1, the red lines below, appeared to be a ferrite core and I had expected it to work well as the frequency increased. It failed. While it presented roughly a 45ohm resistive load to the VNA across these frequencies, the reactance was significant until the frequency approached 100MHz. Expanding the swept range showed 100MHz was about as good as it got. Given this came from commercial equipment, perhaps it works better in a 75:300 ohm application.

Core 2, the yellow lines,  was a tiny twin hole balun core. I know this came from a commercial TV distribution amplifier. The open circuit trace suggested it might work up towards 100MHz, but when terminated in 200 ohms it also presented 45 ohm resistive at the top end of the swept frequencies. At 2MHz it was 34+j17 ohms.

Core 3, the green lines, was something I had wound on a ferrite bead, say 15mm x 10mm,  removed from a power cord for EMI suppression. I had no expectations. The open circuit trace showed it might have enough reactance at the low end, but it quickly presented a capacitive reactance to the VNA above marker 4, 10MHz. While the 200ohm load transformed well in the 2-4MHz range, but above 10MHz it was unsuitable.

Core 4, the light blue lines, appeared to have potential above 30MHz when inspected in the open circuit configuration. Terminated in 200 ohms it was a pass above 30Mhz. I suspect a few more turns would probably make this a suitable toroid for a HF transformer.

Core 5, the purple lines, appeared unsuitable from the open circuit trace. This proved to be the case when terminated.

Tentative Findings:

There is still more investigation to take place but some themes were noted from the work to date.

If you have a core with unknown characteristics choose one that tracks the outside of the Smith Chart when a representative number of turns is swept with a VNA. There are frequencies for which the blue trace tracks the outside of the Smith Chart. More turns would rotate the curve clockwise. Perhaps core 3 would benefit from less turns.

The useful frequency of the core will be in the range of frequencies where the open circuit response of the bifilar winding crosses the axis eg 2-4Mhz for the green trace. Getting the right number of turns could be the key to having a useful transformer for your applications. 

Being on the edge of the Smith Chart means the core losses are low and do not impress themself onto the response as a series resistance. The blue line above (LHS) is an example of this though it starts to spiral inwards above 30MHz which suggests it is not ideal above 30MHz.

The EMI suppression core would work well at 80m and perhaps 40m with further testing of turns. Above that the core losses, which a suppression core would be expected to have, appear limit it's usefulness.