Update on on new PCBs, and request for input

[Carl]

OK, my plan to get new Hammerhead PCBs got delayed a little. I had a new PCB layout ready to go, back in January, and home-etched one to test out... good thing, too, found a layout error. OK, fixed that.

But, I never liked the VCO audio I used, not enough frequency range, which is a fundamental limitation of using the 555. So, I decided to rip it out and do something different. I had hoped to use the VCO I designed for the fluxgate magnetometer, which worked quite well, and was roughly the same parts count.

But getting it to work with the PI second stage, is giving me fits. For one, probing the LM393 comparator doesn't give the signals I expect, at all. The inputs are clearly crossing, but the output doesn't budge. Yup, pullup installed, tried several chips, and all that. Also, I've picked up some new source of noise that's driving the PI crazy. Which makes working on it at the bench, a real pain.

Anyway, I'm open to suggestions on a good VCO audio stage. Would really like it if it fits in the current board space, and needs to use fairly standard chips. If I can't figure it out by the weekend, then the 555 goes back in.

- Carl

[bbsailor]

Re: Update on on new PCBs, and request for input

Please post the layout and schematic of the new board so we can see what space is available to work with.

The VCO audio range should be a function of the voltage level. What about using an adjustable gain log amplifier before the VCO?

bbsailor

[Beachcomber]

Re: Update on on new PCBs, and request for input

Hi Carl,

Don't what kind of voltage swing that you are working with but I found this interesting VCO Circuit on the Internet. It has a fairly wide frequency range given enough swing in voltage.

http://www.discovercircuits.com/PDF-FILES/4584vco.pdf



VCO Oscillator Schematic

[Gary]

Re: Update on on new PCBs, and request for input

Hi Carl, Have you ever played with an XR4151?

Its a really good "Volt to Freq." or "Freq to Volt" converter. Its also capable of going to Zero Hz. at zero volts. I've used it may times in a lot of circuits.

Sorry I don't have a internet link to data on it but I have the Exar data book.

Gary

[Randy Seden]

Re: Update on on new PCBs, and request for input

Carl,

I would like to see you add stacking to your PI which will greatly increase the depth many fold. What I had in mind was using the Panasonic Bucket Brigade Delay lines or similar as they are cheap and the newer line has a low noise floor and would be easy to run the received signal thru it and feed the output to a Dual Gate MOSFET as the summer.
Plus the fact it's adjustable and would also sharpen up the ability to discriminate between gold and iron I would suspect.

Just a thought.

Randy Seden

[Carl-NC]

Re: Update on on new PCBs, and request for input

Except for the XR2206 signal generator that I used many years ago, I am completely clueless as to what Exar offers. In fact, I thought they had been bought out years ago, but I guess not. I'll take a look.

- Carl

[Carl-NC]

Re: Update on on new PCBs, and request for input

Well, for this rev I want to minimize design changes. This will be the final version of this design; the next design is PIC-controlled, but still with all parameters user-variable. Once I get that done, I'll look at performance enhancements.

Interesting thought on the BBD, I still have some of the old Signetic chips from my audio enthusiast days.

- Carl

[Gary]

Re: Update on on new PCBs, and request for input

Hi Carl, I'm sure Exar still makes it and it was also made by Fairchild, A uA4151, but not sure about them now. There was also a uA7151, with an additional op-amp on the chip, but in a 16 pin dip.

The Exar is a 8 pin dip

[Carl-NC]

Re: Update on on new PCBs, and request for input

Thanks, but only a 20:1 or so range is not enough. Looking for DC to a few kHz.

- Carl

[Dave Emery]

Re: Update on on new PCBs, and request for input

Carl, Check out the CD4046 or the newer 74HC4046
This is a PLL chip with one wild VCO. The VCO is almost dead linear and can tune over any desired range from a few Hz to RF frequencies. The frequency range is simply set by two resistors. The VCO is separate from the phase detector which you can leave disconnected. The chip comes in both DIP and SO16. Dave. * * *



74HC4046 Data Sheet

[Carl]

Re: Update on on new PCBs, and request for input

From the data sheet, it looks like the VCO has at best about a 10:1 tuning range, which is typical in PLLs, else you'll end up fighting phase noise. I really need about 1000:1 range, or better. See my other post, where the LM393 may have solved it.

- Carl