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2nd November 2017, 02:04 PM  #1 
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Join Date: Apr 2009

Lundahl LL1545A LTSpice Simulation Issues
Transformer LTSpice simulation issue.
I tried to LTSpice simulate a Lundahl LL1545A transformer to the AK4490 differential dac output, but I have FR simulation issue. I have done an internet search about how is the right transformer LTSpice setup but the information are less. For example, see for the Lundahl LL1545A the right LTSpice setup will be like 2 primaries coils and 4 secondaries coils with K L1 L2 L3 L4 L5 L6 0.999 spice directive. I have done the following schematics. shematics.jpg If you done a real FR measurement with 20K load on the above setup by Spectrum Analyser or oscilloscope or rms multimeter, you will see that FR output is flat btw 2015KHz, with a small increase voltage btw 17K50K and then there is significant decrease until cutoff. The best FR output that I have with LTSpice is the following, that is wrong. FR.jpg The LTSpice have a dialogue window for Coil setup like this. LTSpice Coil Setup.jpg But for the example, if you measure each primary coil by DER EE DE5000 you have the following table. LL1545A primaries.jpg How is the right setup on LTSpice, can someone help? I have included and the LTSpice .asc file 
2nd November 2017, 03:00 PM  #2 
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A couple of issues you have  firstly the series resistance of the windings needs to come from Lundahl's own data (305R for primaries, 122R and 182R for secondaries). Sorting this out fixes up the frequency response very nicely (I zeroed your shunt resistance and capacitance though to get clear what's happening initially). With the correct series R I get 1dB at almost 400kHz.
Second the inductances of a transformer depend on the frequency (and to some extent the amplitude used to measure with). The turns ratio comes out of the ratio of inductances so its best to tweak the secondary inductance to be in line with what the turns ratio is.
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2nd November 2017, 03:30 PM  #3 
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2nd November 2017, 04:36 PM  #4 
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6k7 resistor and 470 pF capacitor should be in series; sim is wrong.

2nd November 2017, 04:39 PM  #5 
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@ abraxalito = From Lundahl's datasheet the 305R for primaries, 122R and 182R for secondaries, are described like static resistance and not series resistance.
Probably it is the same, but I confused with the Rs (Series Resistance) and Rp (Parallele Resistance) of Deee measurings. @ pieter t = like 5H on primaries inductance and 20H at the secondaries (in series)? Abraxalito, can you attach me the file that you described like 1dB at 400K? Here a new .asc file with the Lundhal's datasheet values. Thanks for the help guys! 
2nd November 2017, 04:43 PM  #6 
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2nd November 2017, 04:45 PM  #7 
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When I am right you use the transformer with 1:1 ratio (2+2:1+1+1+1).
Then you must have equal inductance for the primary and secondary. 
2nd November 2017, 04:47 PM  #8 
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2nd November 2017, 05:27 PM  #9 
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Oops, it is clear to me now!!
pieter t you had right, my sim is wrong, I have the R//C at the output. Now, it is very close to the real measurement. I have used the Series Inductance that I have measured with Deee at 1KHz, 8.3H in primaries and 2H at primaries. With the inductance ratio 1:4 (Prim. to Sec. ratio 1:2) which is the right? 
2nd November 2017, 06:02 PM  #10 
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Wrong typo...I meant "I have used the Series Inductance that I have measured with Deee at 1KHz, 8.3H in each primary and 2H at each secondary.

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