Re: LTSpice simulation of uBitx Power Amp

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On Feb 5, 2018, at 12:47 PM, Diver Martin <diver.martin@...> wrote:

> Let me know if anybody figures out why my model of the WA2EBY amp shows the output to be so much lower at 30mhz

To quote George E.P. Box, "All models are wrong, but some are useful."

Spice models are just that, models.  They're rarely right.  But they can tell you useful things about some of the mechanics of what's going on inside the system.

On Mon, Feb 5, 2018 at 10:40 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
Have now uploaded version two of the simulation:
    https://groups.io/g/BITX20/files/KE7ER/WA2EBYandUBITX_2.asc
    https://groups.io/g/BITX20/files/KE7ER/WA2EBYandUBITX_2.pdf

Has the following changes:
Quiescent drain current reduced to 10ma on both.
Added a switching mechanism to show response at both 7 and 30 mhz with a single run.

Observations:

Power out on WA2EBY is still much lower at 30mhz than he reported in his QST article.
Input impedance at 30mhz falls to 12 ohms, which is the primary factor here.
If Vin source impedance is reduced from 50 ohms to 0 ohms, then 30mhz output exceeds 7mhz output.
 
Removing C3 increases output a bit at both 7 and 30mhz 

L3 helps make the exciter see a resistive load, but doesn't help much.
 
I'm moving on to other things.
Let me know if anybody figures out why my model of the WA2EBY amp shows the output to be so much lower at 30mhz
Here's my notes on how I got the WA2EBY inductor values, includes a python script for single layer coils:

# http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html
#   Single layer coils, wheeler formula:  Henrys = 0.001 * N**2* R**2 / (228R + 254L)
#   where N is number of turns, R is radius in meters, L is length in meters (>0.8r)
def h(n, d, w): # turns, form diameter in inches, wire diameter in inches
   length = w*n/39.37 # length of coil in meters, assuming close wound
   radius = (d+w)/2/39.37 # radius of coil from middle of wire in meters
   henries = 0.001 * n**2* radius**2 / (228*radius + 254*length)
   return(henries)

L1, L2   9-1/2 turns #24 enameled wire, closely wound 0.25-in. ID       >>> h(9.5, 0.25,  0.02):  5.285e-07 = 0.5285uH
L3       3-1/2 turns #24 enameled wire, closely wound 0.190-in. ID      >>> h(3.5, 0.190, 0.02):  8.222e-08 = 0.0822uH
T1       10 bifilar turns #24 enameled wire on an FT-50-43 core  uH=(AL*Turns**2)/1000  AL=440     (440*100)/1000 = 44uH/winding
T2       10 bifilar turns #22 enameled wire on two stacked FT-50-43 cores. AL doubles (880*100)/1000 = 88uH/winding
T3       Pri 2 turns, sec 3 turns #20 Tefloncovered wire on BN-43-3312 balun core. AL=7000    (7000*4)/1000=28uH, (7000*9)/1000=63uH

Jerry,  KE7ER



On Sun, Feb 4, 2018 at 10:21 pm, Jerry Gaffke wrote:

I've now uploaded an LTSpice simulation schematic file that allows easy comparison of the uBitx and WA2EBY finals,
also a pdf version for those who have not installed LTSpice:
    https://groups.io/g/BITX20/files/KE7ER/WA2EBYandUBITX.asc
    https://groups.io/g/BITX20/files/KE7ER/WA2EBYandUBITX.pdf

I haven't played with it too much, haven't taken any notes.
But here's what I think I'm seeing:

The WA2EBY does perform better. 
Power in is about the same between the two, power out from the WA2EBY is generally a factor of 3 greater.
The gates on the WA2EBY have nice sine waves, the uBitx is quite distorted there by capacitive loading.
Input impedance on both is about 50 ohms at 7mhz, both go down to about 12 ohms at 30mhz,
as determined by monitoring the voltage after the 50 ohm source impedance resistor.
Both show a 4x power increase when switching from 30mhz to 7mhz, more than I would
have expected on the WA2EBY, perhaps I have some wrong values on the inductors?
The IRF510 model assumes worst case for Rds(on) and Qgate, though I doubt that's just too far off.

Jerry, KE7ER




--
Martin Held - AE7EU
http://ae7eu.com/
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