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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
After the explosion of my simple-antenna-feedback SSTC, I thought I could try the PLL version insted as it should not abuse the FETs as much.
I can't access Steve Conner's website where is the original PLL SSTC schematic so I'll talk from what I remember.
I remember there are some (two) resistors you must set which will adjust minimum and maximum frequency of the PLL. What is the importance of this? The simple-feedback 74HC14 version has no min and max freq. settings and it locks to resonance every time, is there a reason why the PLL can not do this? I think this is the part that discourages me from building a PLL coil as I have no way to measure the Fres, I don't know what would happen if I set the resistors wrong, etc...
Registered Member #180
Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187
When I was using the PLL (Steve's schematic too) I was curious about this too. If you look at the datasheet there is a graph of frequency vs resistance and another for the capacitor... I couldn't really figure it out, it was hard to read. Instead of knowing exactly what value you need, you could just use a couple pots instead, say around 50kR and maybe a 10k in series with that, thats what I did anyway. Run the coil at low power (really low) and twiddle the knobs till something happens, then increase the power and make sure everything runs good.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I couldn't find an exact formula for the frequency either, but I've never really missed it. Just use two ~33k resistors for timing and find a capacitor which puts the frequency approximatly in range. Then set the resistors so they have a 1.5 ratio, like the 33 to 46k Steve used. When you turn the phase potentiometer with no feedback the VCO frequency will shift from the timing cap + small resistor to timing cap + large resistor, so basically from min to max frequency. I like to have my secondary fres right in the center frequency of the VCO. Once you set up feedback adjust the phase for perfection/breakout. It's that easy!
I've made a simple fres tuner if you're interested, it can use the parts form your old SSTC. Steve's PLL schematic is also on my site, just slightly modified to suit my SSTC. PLL isn't hard to set up. With Steve's circuit it's just template work.
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PLL SSTC question
