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Type 2 PHSNA Transformer T1 replacement

Steven Dick
 

Hi all. I decided to replace the handwound transformer T1 on my Type II PHSNA with a ready made transformer from Coilcraft, part number WB1-6TSLB.  They are spec'd .05 MHz to 200 MHz with 0.2dB loss max. They cost $2.61 in quantity 1 though shipping is about $8.00. You can request a sample if you have a company name.  I purchased a bunch and they are now my go-to inexpensive 1:1 wideband transformer with a center tap on one side.  I added a photo in the K1RF folder in the files section.  Position it so the transformer center tap is right on the primary ground pin of the PCB. I held it to the board with a small dab of quick set epoxy.  Be sure you are not to0 close to a resistor pin on the output side. You can clip off the lead a bit if you wish.

This eliminates the hand-winding of T1 and probably provides better performance.  I didn't compare to the handwound one in efficiency but I am getting close to 0dbm output of the PHSNA at the default 7.04 MHz as measured on a scope.

See http://www.coilcraft.com/pdfs/wb.pdf for technical data

Regards,
"Digital Steve", K1RF


Alfredo Mendiola Loyola
 

Steve:

What is the output impedance from 1 to 30
mhz of your signal generator using your transformer?

Alfredo
OA4AJP

Steven Dick
 

Alfredo, I’m not understanding your question.  I’m using the transformer on a Type II PHSNA board which uses the true and complementary outputs of an AD9850 or AD9851. The PHSNA board is meant to drive a 50 ohm impedance and its output amplifier uses an era3+ MMIC IC.  The transformer is driven by the AD9850 or AD9851 and is a 1 to 1 turns ratio center tapped transformer.  The AD9851 has two current source complementary outputs.  The secondary of the transformer drives a 50 ohm impedance from a 6db pad. See the AD9851 data sheet for discussion of push pull center tapped transformer configuration.  1 to 30MHz is way in the middle of the sweet spot of the transformer
 
“Digital Steve”, K1RF
 

Sent: Saturday, January 30, 2016 9:38 AM
Subject: [PHSNA] Re: Type 2 PHSNA Transformer T1 replacement
 
 

Steve:

What is the output impedance from 1 to 30
mhz of your signal generator using your transformer?

Alfredo
OA4AJP

This email has been sent from a virus-free computer protected by Avast.
www.avast.com

Alfredo Mendiola Loyola
 

I would like to know if the output impedance on the secondary is 50 ohm from 1 to 30 Mhz.

Is there any impedance mismatch frpm the tranformer output?

Regards
Alfredo

Curt
 

Alfredo

A transformer does not have an impedance until its other port is loaded.  You are correct to be interested in its frequency response, which tells the story when used in the amplifier.  So when the other builders report its frequency response in the circuit,  that is sufficient.  

Curt

Steven Dick
 

To test the transformer by itself, I suggest the following:

A simple experiment can be done if one has an antenna analyzer.
1. Put a precision 50 ohm (or 49.9 ohm) resistor on the output
2. Drive the full primary with the antenna analyzer and sweep from your lowest desired frequency to highest.
3. Plot SWR across the band of interest.  If it deviates much from 1:1 across your band of interest then you will see a problem.

If your antenna analyzer can determine complex impedance, you can measure the impedance seen by the primary with a 50 ohm load at the secondary for any frequency you might be interested in.

Out of curiosity, I will try this in the next few days and do the same test with a coilcfraft WB1-6TSLB up to 50 MHz, the limits of my antenna analyzer.  I'll post the plots when I have them.

Alternately, you can use a PHSNA with return loss bridge to test a separate transformer over the frequency band of interest. 

Regards,
"Digital Steve", K1RF

Alfredo Mendiola Loyola
 

Thank you for the information.

I asked this because I'm using an opamp THS3201 to amplify my AD9851 signal (256mv peak to peak).
The only thing I don't like about this high speed opamp is the second harmonic level at 30 Mhz, about -60dBc. I don't want to put two low pass filter, one at the ouput of the AD9851 and another at the output of the opamp.

Regards.
Alfredo Mendiola Loyola
Lima, Peru
OA4AJP

Steven Dick
 

I put a file called "Type II PHSNA transformer comparison" in the Files section in the K1RF folder.  I tested various transformers with a Rig Expert antenna analyzer. In summary, the Coilcraft WB1-6TSLB worked the best followed by a bifilar-wound FT37-43 toroid, while the standard wound FT37-43 toroid was substantially worse than the other two.  If you wind your own on an FT37-43 toroid, I would recommend winding it with bifilar-wound primary and secondary (I.E. 12 turn twisted pair with a center tap on one of the wires).  If you are using a AD9851 for high frequency operation, I would strongly recommend using a commercial wideband center tapped transformer such as the Mini-circuits T1-1T or a Coilcraft WB1-6TSB.

Regards,
"Digital Steve", K1RF

Alfredo Mendiola Loyola
 

Dear Steve:

Could you show what is the Impedance at 10Mhz and 29.5Mhz for your transformer WB1-6TSB using your Rig Expert antenna analyzer?

Regards
Alfredo Mendiola Loyola
Lima, Peru
 

Steven Dick
 

Sorry Alfredo - can not do further testing at this time. Borrowed antenna analyzer has been returned to owner.  Why is it so critical to you to get exact numbers at exact frequencies?  The data I included in the previously reference file shows that you will get good performance with the  WB1-6TSB transformer.  Any small mismatch will result in very small reductions in output amplitude of the DDS which are not all that significant and any small impedance mismatches will have little effect on following filter as the impedance match is still fairly close.

Regards,
"Digital Steve", K1RF

Alfredo Mendiola Loyola
 

What is the output impedance seen by a low pass filter a the output of your transformer connected to your 50 ohms load?

DDS + TRANSFORMER + 50 ohm load = 50 ohm of output impedance?

I think that the transformer impedance (50 ohm) in parallel with the 50 ohm load equals to 25 ohm.

Regards
Alfredo

Nick Kennedy
 

In general, you wouldn't look at the transformer as a separate element having an impedance.  You'll be reasonably close considering it to be an ideal transformer with impedance transformation properties in accordance with its ratio.

If that's not accurate enough, you'd factor in its magnetizing impedance, which is generally reactive and in shunt.  If it's 10 times the connected impedance, the effect is minimal but you could figure it in if you are looking for high accuracy, or pushing the transformer to the limits of its frequency response, on either the high or low end.

Of course, you can make it even more complicated, depending  on how far you wish to go.

73-

Nick, WA5BDU

On Tue, Feb 2, 2016 at 8:47 PM, mendiola_loyola@... [PHSNA] <PHSNA@...> wrote:
 

What is the output impedance seen by a low pass filter a the output of your transformer connected to your 50 ohms load?

DDS + TRANSFORMER + 50 ohm load = 50 ohm of output impedance?

I think that the transformer impedance (50 ohm) in parallel with the 50 ohm load equals to 25 ohm.

Regards
Alfredo


Steven Dick
 

A similar experiment was done by another ham, Kerry VK2TIL. He posted his data in the EMRFD Yahoo group on Jan 31, 2015. I am copying his post here, as I think the info might be interesting to this group:

"RF transformers and the ferrite inside them are very interesting devices.

I think that the best reference to their design is in the Philips application notes ECO6907 & ECO7213 which cover both conventional and transmission-line types; Google will find these app notes.

The references tell us that the tx-line types are better but I like to see things for myself.

I try to emulate the scientist who was travelling through the countryside in a train; his companion looked out the window and remarked "Those sheep have just been shorn" to which the scientist replied "Yes, on this side anyway".

On this quiet Sunday morning I built a transformer on an FT37-43 core; it had two windings, one of 20 turns and one of 10 turns.

I placed the windings on opposite sides of the core; I have previously found that winding the shorter winding over the longer gives passable results but I wanted to see the effect of separating the windings.  Here it is, in a test fixture and with a 200-ohm resistor across the larger winding;

http://s18.postimg.org/6wxwzo2l5/003.jpg

 

Here is a sweep of Real Z, Imag Z and VSWR;

http://s30.postimg.org/ptns7u14x/trfr.png

It's very poor; indeed, I didn't expect it to be as bad as this so I made another one and the test results were almost identical.


I then made a tx-line version, using the same FT37-43 for consistency.


The new transformer used 10 turns bifilar connected as a 4:1 device; I didn't measure the line Zo but I wound it a little less tightly than the twist that I have found to give 50 ohms Zo so it's probably somewhere near the desirable 100 ohms.  With a pin-connector and a 200-ohm resistor installed as per the previous model, it looked like this;


http://s2.postimg.org/xjnbq5vs9/011.jpg


Here are the test results;


http://s8.postimg.org/suo0laaph/bifilar_trfmr.png


This is a very good transformer; it is wideband well beyond the 50 MHz limit of my tests.

That answers Vince's question 2; the designer, a very smart bloke, knew what he was doing!  :)

Re question 1; these devices can be tested with an antenna analyser in the same setup that I used.

Terminate the "non-50-ohm" winding with the appropriate resistance and apply the analyser to the 50-ohm winding to see how good the VSWR is.

Keep connections as short as possible; you can see that I use SIL pin-connectors which are very convenient for this kind of work.  In this case, a short SIL strip is soldered to a BNC(f); one SIL pin goes to centre, the next pin is removed and the outer two are soldered to the body.  Silver-plated BNCs ease soldering.

If, as occasionally occurs, neither winding is 50-ohms, apply the analyser to that which is closest to 50.

Kerry VK2TIL."