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Posted by: WB8RCR, John Posted On: 04/01/03 Subject: Re: 14 to 42 Mhz VCO Message Posted: In Reply to: Re: 14 to 42 Mhz VCO posted by Mikrovolt on 03/21/03 That is an awful range for a VCO, and you are going to have a horrible time getting the sort of stability you need with a varicap tuned oscillator at 42 MHz. At lower frequencies it's not so bad. However, the 1.4 to 4.2 isn't going to help much, because when you multiply it up, you are also going to multiply the instability. Let me make a (pricey) suggestion. The 9850 based DDS from Far Circuits tunes 1 Hz to 30 MHz in 1 Hz steps. Several folks have moved it up to 50 MHz by increasing the reference oscillator to 120 MHz, or you could tune some lower range and mix it up, although the post-mixer filtering is going to be a bit of a dice with that wide a range. Now, you simply use a nice shaft encoder (this is where the bucks come from). Good optical encoders that don't click are availble from a number of sources (DigiKey for one), that feel GREAT. The only problem is, they are about $50 just for the stupid encoder. There are a few mechanical encoders available without detents that are only a few dollars, but they typically only have 12 or 16 steps per revolution ... at 1 Hz per step, that's a bunch of turning. Of course, you can fix the step size in the firmware, but that defeats the nice, smooth, tuning you are looking for. However, if you are looking only for phone, 100 Hz steps probably wouldn't be too bad. The other alternative is to do some sort of bandswitching to keep the range of your VCO manageable. This isn't as nice as continuous tuning, though, and you would be playing the game of trading off VCO frequency for images and the number of switched. I can tell you from experience that making a stable VCO much above 8 MHz is quite a challenge. |
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