Richard,
Essentially, you take whatever is the latest & fastest version of a '74HCU04. (The "U" is very important...it MUST be unbuffered). You capacitively couple a varactor to the input and another to the output and drive them from the same tuning voltage. An inductor from output to input completes the circuit, where L resonates the two varactors. Using appropriate varactors, I have made HF VCO's that tune almost a 3:1 range. I have operated this circuit at over 150MHz. I don't have numbers for phase noise but it was good....not as good as a crystal of course, but pretty good. This particular VCO was used as the first LO in an HF (60MHz) spectrum analyzer. It was driven from a phase detector whose other input was a DDS. The PLL/DDS combo allowed fine tuning resolution along with wide loop bandwidth, resulting in a very good, fast tuning LO.


Joe
W3JDR

"Richard Hosking" wrote in message . au...
Joe
I agree the LO is the hardest part of a radio, especially if you want fine resolution and digital programmability.
I would be interested in your VCO circuits using inverters if you are willing to share these. What sort of phase noise performance have you got with these circuits?
I wonder if you could use a coax transmission line as the tunable element, and ground different sections of it using diodes for different bands? I would be aiming to use the LO at 4X operating freq, so it can be divided for a quadrature generating circuit. I plan to use the Tayloe circuit as a bilateral SSB RX/TX, using a quad bus switch (say the FST3253)

Richard
W3JDR wrote in message ...
Richard,

Now you're talking my language...almost!

I also want to built an ultra-portable rig, except I want a PSK31-only rig that I can pack with my laptop for my travels. I know Small Wonder Labs already makes this, but this is a homebrew forum, and that's what I want to do. When building any HF rig, the frequency control systems are usually the most complex part. I'm hoping that by developing this little configurable & programmable module, I'll have a building block I can press into service for VFO's BFO's, and other heterodyne oscillator applications.

The Atmel chips are certainly good candidates for the application...low power, fast, powerful, & cheap. I know that with a PIC (and possibly the Atmel) chip, you can make a 50mhz frequency counter with as much resolution as you care to program into it, commensurate with the counting speed tradeoff. Thus, 10hz measurement resolution could be achieved with a 10 frequency corrections per second. As you pointed out, the counter could be programmed for a lower resolution, faster update mode that would be used to acquire, and then slip into the slower, higher resolution mode to maintain. A simple 8 bit R-2R D/A converter could be used for coarse tuning so that the loop doesn't have to work so hard. In a VFO with a 500khz bandspread, that would initially place the frequency within a few khz. I've built VCO's using fast CMOS unbuffered inverters that work to over 150Mhz, so that part isn't difficult.

Overall, I think all the technology is proven...someone just has to do it. My PICLite Starter Kit should arrive any day (only $36!) and I hope to get started.

Joe
W3JDR




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