Tuesday, September 26, 2017

Playing with the RF Amp in spice

The sim will run faster if you set the display time to a smaller value. I made that change and will point it out when I post the picture. That run took about 5 minutes to load before I changed the setting. Ok back to the DBM LO requirement.
The level 7 will be .7 volts peak. Now let's look  at the oscillator.
I have built the one on top of the page. I plan to try the other when I get some shop time. The one I made produced about 250 mv so it will need a little boost. I posted the  model for a RF amp in my last post. The last line in the post. The questions are will it produce the output level we need and at what frequencies will it function. If it works as desired we can build the oscillator for the frequency range we want to receive.
Notice I set the input at 50 mv and 10mhz. So at 10 mhz we should see greater than .7 volt peak.
At 20 mhz it has dropped off a bit but still good.
At 30 mhz dropped some more but still good. Note the orange circle. It took 5 minutes to run this sim and I lowered the time for the viewing window. With the new times it loads in less than a minute. Shortwave is generally 3 - 30 mhz so it should be good for our set but I had to do one more test to push it limit.
At 100 mhz it is still providing some gain.
The transformer I'm using in the sim is a tri-coil supplied by BG Micro. The little coil is 25 cents each and works well. You can wind your own but will have to adjust the values to fit. I wound a Fair-Rite T50-43 with 11 (trifiliar) turns and it measures 60uh per winding. Substituting the value for the transformer will tell the story about the transformer requirements. That will be a project for later.
NOTE: I made a DBM with the Fair-Rite cores and 1SS98 diodes and it did function when driven with my signal generator.

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