Evan,
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I had read the LT-Spice docs and used some of the advanced features a few years ago.
Comming at it fresh late yesterday, it was all I could do to edit a schematic and get a transient analysis going.
I had enough head scratching to do without dealing with advanced features.
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You may have noticed that for the 1:1 version of T501 that I was using, I assumed the inductance
of the two primary windings added together should equal the secondary inductance.
Unfortunately, the two primary windings interact on the same core to give a larger total inductance for the primary.
Instead it is the total turns count of the primary windings that is equal to the secondary winding turns count for a 1:1 transformer.
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Here's how I now think of it:
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Inductance on a given core is proportional to turns squared.
Lets assume an AL of 1uH per turn for the core, a 16uH secondary, and a 3:2 turns ratio.
Then the number of turns for a 16uH secondary winding is sqrt(16) = 4 turns
? (working back we have: ? 1uH/turn * 4turns**2 = 16uH)
The total number of turns on the primary is 4 * 3/2 = 6 turns
Each half of the primary has 3 turns.The inductance of each half of the primary is: ?1uH/turn * 3**2 = 9uH
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In general the formula for the inductance of one winding of a center tapped primary given a turns ratio of N:1and a secondary inductance of Ls is:
L_one-primary-winding = ( sqrt(Ls) * N / 2) **2?
For the above example:? ? ( sqrt(16) * (3/2) / 2 ) **2 = 9
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I've used the above to adjust the transformers in the new simulation schematic QMX_sim04.asc
In addition, I changed the turns ratio of T501 to 1:1 for the "Stock with L502 removed" case.
This increases the output to be slightly more than the stock case, but Vds-max across the BS170's is far less than the stock build.
There may be reasons not to do this, but it is worth investigating since the change in build proceedure would be trivial.
If we use this scheme, then everybody builds the QMX with the simpler 9v version of T501, but 12v builds short across L502.
There should also be a 0.1uF cap to ground at the T501 center tap if L502 gets shorted.
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Here's some figures for peak voltages using QMX_sim04.asc:
Vds? ? ? Output
38.3? ?21.4 ? ? ? ? ? ?Stock QMX, has 3:2 turns ratio on T501
23.8 ? 17.5 ? ? ? ? ? ?With L502 removed, still has 3:2 turns ratio on T501
23.0? ? ?21.6 ? ? ? ? ? ?With L502 removed, now has 1:1 turns ratio on T501
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Note that I am showing 38.3 for Vds-max across the BS170's when simulating a stock QMX at 3.57 Mhz.
Chris reported in a forum post that he measured well over 60 volts using a scope on a stock QMX.
(Unfortunately his graph has been removed.)
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With the 1:1 turns ratio at T501 and with L502 removed, I see a Vds-max of only 23v
and 21.6v across the load. The power into the 50 ohm resistor is? ?(21.6*0.707)**2/50 = 4.66W
My simulation for a stock QMX 12v builds shows slightly less, but real world tests can give over 5W.
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That's all just in simulation, should be interesting to try it on some hardware.
And try it with a whole range of complex impedances at the antenna port, not just a 50 ohm dummy load.
Once we figure out operation at 3.57mhz we might find 4.0mhz significantly different.
And then we only have another dozen bands to figure out.
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It's complicated!
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Jerry, KE7ER
