This might interest some of you who tried or looked into the Huff and Puff VFO (H&P). I think that this circuit is cool, I hope it can help some of you sometime. I have been wanting to get the 74HC74 spice model running and thanks to David here at the group I got it up on line here.
File: Huff_And_Puff.zip http://groups.yahoo.com/group/soft_radio/files/SDR%20Spice%20Analysis%20Files/Huff_And_Puff.zip This circuit simulation demo is a model of the 74HC74 comparator section. Through the lock loop to that Integrator (charge pump) section. With a lock loop filter and a final RF filter for the Varactor diode. The circuit ends, looking into the VFO resonance tank circuit. You can look at the digital waveforms and at the noise in this model. The noise is low and so are the harmonics. The integrator time constant is 20 seconds. LTspice revealed that the 74HC74 prefers that both its outputs like to look into a load rather than have one output unloaded as some folk do with their circuits. The circuit then works better if everything is loaded right. The current in the chip is balanced between the Q and _Q sections and this works better. The circuit uses a 80 MHz clock frequency with a divided down sample of a VFO in the 10 kHz region (for speeding up the simulation). Signal generators serve for the clock and divided sample inputs. The complete Transient Analysis time is 20 seconds if you want to run it that far to see what happens after the full charge time constant. You might use this file to explore things you could not do with limited test equipment. And to get ideas from. A few notes included and the complete circuit is in a gif file. P.S. You can make crystal oscillators and RF mixers with the 74HC74, as well as modulate it. And I have been looking into using it for a QSD. I have had a few problems with that idea when I compare the I and Q outputs though. I think I have to explore all of the possible connection methods to the pins. But I can get it to modulate as a single Flip Flop. If I can be successful at making a QSD out of this one chip I will post that. Dan ka9rza
