I've always been fascinated by PLL etc, and when Huff and Puff first came along in the early 70's I decided to have a go. However due to other attractions (mainly beer and girls) I didn't tackle it until the mid 90's. The circuit I copied was a fairly complex one, but it operated in a manner that I could understand so I was able to debug it when it didn't work. After a bit of fiddling it worked well, with free 'knob' tuning and lock points every 64 Hz. Then I lost interest...
Then I stumbled across Han's Summer's site and started reading the Huff & Puff reference library. Oh dear. Out came the HC CMOS box and a simple two-chip stabiliser was born. It uses a 4060 osc/divider to provide the reference signal and a74HC74 dual D-type for the detector/latch and the VFO. It refused to give a lock, and after a lot of hair-pulling I made one small change to the circuit which resulted in lovely clean locks every 1KHz. One thing that surprised me was the amount of pull the 'varicap' LED gives!
Update: I got 50 Hz steps out of this simple circuit but it was a bit iffy, so I've moved on to a different design, also from Hans site. This is the 'Huff & Puff in practice' circuit by G3DXZ, and can be found at:
The finished product. This one fits just nicely underneath the VFO in my Topband rig. It works very well and I could sit and listen to its crystal-like note all day. Step size (at 2.4MHz VFO frequency) is 52 Hz - a 28.4 MHz ref osc would give 50Hz, but I didn't have one to hand. In operation it is almost invisible. When I sit down at the rig I press the 'setup' button, which sets the varicap voltage to half supply. After that it's just like normal - twiz the tuning knob to the required frequency and as soon as you stop twizzing the circuit pulls the VFO to the nearest 50Hz lock point, where it stays.
May 2009: More action on the Huff & Puff front. I built the unit below as part of an all-singing hybrid PLL synthesizer, but once I'd got it working I took it apart again! This is what happens when you don't drink enough. It's essentially the same as the one in my Topband rig, but with an extra divider to give small steps at higher frequencies. It tunes 12 to 13 MHz in 30Hz steps and is a joy to twiddle.
This is a new photo of the 12 to 13 MHz osc in its MK2 form. It now tunes 15 to 20 MHz. The VFO is in the small box behind the tuning cap, and the bigger box contains the huff & puff gubbins. The bit of circuitry at the back is just voltage reguation.
A new plot to go with the rebuild. 200KHz span and 200Hz bandwidth shows a nice clean output, although it gets a little noisy close in. Still a good signal.