Inductors, Transformers and Baluns

[Steve Haynal]

Hi Group,

Upon request, Jim WA2EUJ shared the BOM and layout for the new opensource RS-HFIQ. I learned several interesting things. First, he is using standard Bourns and Murata inductors like the filter people are already experimenting with, but also many from Abracon:

IND 15UH 400MA 560 MOHM	MURATA	82153C
IND 390NH 210MA 2.2 OHM	VISHAY	IMC0805ERR39J01
IND 1UH 370MA 1.8 OHM SMD	ABRACON	AISC-1008-1R0G-T
IND 680NH 400MA 1.47 OHM	ABRACON	AISC-1008-R68G-T
IND 270NH 190MA 2 OHM SMD	MURATA	LQW2BHNR27J03L
IND 180NH 250MA 1 OHM SMD	MURATA	LQW2BHNR18J03L
IND 120NH 320MA 400 MOHM	MURATA	LQW2BHNR12J03L
IND FIXED 1.8UH 1.7A 100 MOHM	WURTH	7440450018
IND 6.8UH 200MA 7.7 OHM	ABRACON	AISC-1008-6R8J-T
IND 3.3UH 450MA 1.7 OHM	ABRACON	AISC-1008F-3R3J-T
IND 1.8UH 600MA 840 MOHM	ABRACON	AISC-1008F-1R8J-T
IND 1.2UH 650MA 800 MOHM	ABRACON	AISC-1008F-1R2G-T
IND 560NH 460MA 1.1 OHM	ABRACON	AISC-1210-R56J-T
FERRITE BEAD 1K OHM 500MA 0805	TDK	MMZ2012R102A
IND 100NH 300MA 900 MOHM	BOURNS	CE201210-R10J
IND 270NH 630MA 400 MOHM	ABRACON	AISC-1210-R27J-T
IND 180NH 700MA 300 MOHM	ABRACON	AISC-1210-R18J-T
IND 470NH 490MA 800 MOHM	ABRACON	AISC-1210-R47J-T

Second, for the 4:1 (I believe) unbalanced-unbalanced transformer on the output of the PA, he is using 

IND ARRAY 2 COIL 4.7UH SMD

BOURNS

SRF1260A-4R7Y

This is a $1.25 coupled inductor from Mouser that is wired as an unun. I am interested in alternate parts for those who don't want to wind transformers, and this is an interesting way to go for single transistor PAs. I wonder if there are any drawbacks. 3 of these could be used (2 ununs, 1 balun) to support the push-pull amplifier, but at that point a $0.50 bn43-202 is more attractive in my mind.

We've never found an off the shelf 1:1 transformer that could handle the power for use in instrument mode with Jim's opamp design. These coupled inductors may fit the bill. See this and this. I am not an expert on this topic, but would someone with more expertise please comment?

73,

Steve

KF7O

[Steve Haynal]

Hi Group,

The SRF0703-100MCT-ND would match the inductance seen by the 4-turns on a BN-43-2402 that we currently use. The footprint is less than ideal but workable. The price is nice at $0.80. I'm not sure how to judge these into the MHz range without trying one. It is reassuring that the BN-43-2402 lists power transformers as an application.

73,

Steve

KF7O

[Glenn P]

Interesting use of what would appear to be aimed at power supplies http://www.bourns.com/docs/Product-Datasheets/SRF0703.pdf
The one you mention saturates at 1.24A so even from that point of view i don't think its suitable. However others appear to be doing this, perhaps in some clever way.

glenn

[Glenn P]

I assume you mean for a single ended amp?

[Steve Haynal]

Hi Glenn,

Sorry for not being clear. In the last post, I was considering this as the 1:1 balun for the instrument TX output path as an off-the-shelf alternate to the current BN43-2402 for those who don't want to wind a transformer. Since the output here is under 25 dBm, the 1.24A should be more than adequate. We've had a hard time finding an off-the-shelf part that we are sure can handle the output power of the op amp.

There is another thought of using an optional power inductor for a single transistor impedance matching like seen in the RS-HFIQ. But for that, I'd stick with the SRF1260A that is already proven in the RS-HFIQ and handles >4A. 

73,

Steve

KF7O

[James Ahlstrom]

Hello Steve,

On Saturday, July 16, 2016 at 1:19:49 PM UTC-4, Steve Haynal wrote:

This is a $1.25 coupled inductor from Mouser that is wired as an unun. I am interested in alternate parts for those who don't want to wind transformers, and this is an interesting way to go for single transistor PAs.

These parts are sold as inductors for switching power supplies, so I would be worried about using them above 2 MHz.  I don't see any frequency or ferite specs.  We could try them, I guess.

 

We've never found an off the shelf 1:1 transformer that could handle the power for use in instrument mode with Jim's opamp design. 

We need to look at this again.  The commonly available transformers from Minicircuits have a max power of 250 mW.   The op amp puts out 280 mW nominal into 100 ohms, but half of that is dissipated in the 24 ohm back termination, and a max of 145 mW goes through the transformer to the 50 ohm load.  There is almost no DC current, so that is not a problem.  So if we retain the two 24 ohm resistors at the output of the op amp, we can use common transformers.

It is true that when the back termination is reduced, the power through the transformer increases to 500 mW.  It may be true that a load lower than 50 ohms (like a short) will stress the transformer due to the higher current, even though the power through the transformer is lower.  But our design still has the two 24 ohm (or equivalent) resistors in place, outputs 100 mW (20 dBm) nominal, and that is less than the max 250 mW.

The minicircuits T1-1T and T1-6T are nice parts, but not readily available overseas.  Digikey has the M/A Com  MABAES0060 1:1 transformer that is rated for 500 mW and 500 ma DC.  So it looks like we use that part, and notify the builder that if they reduce the back termination too much, they need to replace the transformer.

Jim

N2ADR

[Steve Haynal]

Hi Jim,

Jim WA2EUJ is using these parts successfully up to 30 MHz in the hobbypcb RS-HFIQ. Jim has won the ARRL homebrew challenge I think more than once. His earlier PA used the BN 43-202 and I asked him how an autotransformer with these coupled inductors compares to a BN 43-202.. He said the BN 43-202 is more flexible regarding what turns ratio you can use, but the coupled inductors lend themselves to SMT pick and place assembly. He didn't mention any difference in performance. I asked him whether he thought these would work well as 1:1 transformers, and he thought so. He independently came up with the same idea as I did for using 2 of these as 4:1Z autotransformers and then 1 as a 1:1 balun. He claims he can get 15W out of a push-pull design with the RD16HFF1 and this configuration.

I want to try this one for the 1:1 Balun on the instrument TX path in the first HL2. The footprint will also support the standard wind your own BN43-2402 as a fallback. The minicircuits parts are too costly and hard to obtain. 

73,

Steve

KF7O