RF Oscillator & Buffer Amp

Discussion in 'Homebrew and Kit Projects' started by KB1LQC, Jul 30, 2008.

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  1. AC0FP

    AC0FP Ham Member QRZ Page

    Don't make this harder than it really is. If a crystal is specified for a 32 pf load capacitance this just means the frequency of oscillation is the series resonate frequency of the crystal with a 32 pF capacitor connected in seres with the crystal.

    I agree with the statement about the book, I have read it quite a few years ago and its quite good.

    73,

    Frank:)
     
    Last edited: Aug 2, 2008
  2. NA5Z

    NA5Z Ham Member QRZ Page

    It's Microsim... what we have now at work is a blend with Orcad, version 10 I think it was. At home I still use 8.

    Production and eval engines are the same though. The eval engine just limits the nodes and active devices. The schematic capture is more limited, too, but for 99% of what I used it for, it doesn't matter. The device/node limit is normally only a problem when you try do op-amp and gate circuits.
     
  3. WY6K

    WY6K Ham Member QRZ Page

    Bryce,

    Please keep us up to date on your oscillator and amp! Do you have it working OK yet?

    Mike
     
  4. WY6K

    WY6K Ham Member QRZ Page

    The eval version of Orcad seems to not allow me to add spice models. Or am I missing something?

    Thanks,
    Mike
     
  5. NA5Z

    NA5Z Ham Member QRZ Page

    I think after all the taking over and merging that happened with Msim, Orcad, and Cadence that it might have been lost in the shuffle. What we have at work actually has a Orcad/Pspice bridge app that runs Orcad's schematic window with the Msim PSpice engine we already had. Why, I don't know. They looked identical to Msim's capture to me.

    Anyway....

    I'd try backing up a version in your search for an eval.


    I did a quick search for PSpice Eval and found a link to some older versions that looks valid:

    http://www.engr.uky.edu/~cathey/pspice061301.html

    8.0 is what I use at home. I downloaded 9.1 just now to make sure it's real and it looks ok (didn't seem to scare Trend Micro's virus scan either).

    The catch with eval copies:
    Node limited
    Active device limited
    Can't use the device modeling module (used to be the case - don't know if that's still true)
    Schematic library's are very limited.

    But... it doesn't mind if you make your own library. So if you have an existing model for a new transistor, for example, copy an existing schematic symbol to a new library, tag the model to it you want to use, make sure all the pins are hooked up and you should be good to go. It is a little painful the first time you make a new part, I'm not gonna lie. LOL

    Then you also have to remember to configure it to actually use the new libraries.

    If there's an interest, I can zip up the libraries I have on hand for 8.0 and make them available. It looks like about a 56M file, though, so be forewarned.



    Another thing occurred to me this week as I was sim'ing something for work _ inverted my impedance equations on those graphs, so I need to run that over again. I dyslexia hate. :-/
     
  6. WY6K

    WY6K Ham Member QRZ Page


    I want to thank you for getting me onto using spice! I found a free (eval) version of 5Spice and I've been having a ball with it. It's almost as much fun as actually building things!

    It is limited in the size of sheet of paper and some other things related to complexity. But it's enough for small amps and things.

    Back when I was actually doing design engineering we didn't have spice or any other modeling software. I wrote my own in Fortran, which really just did a lot of calcs and sensitivity analysis to figure out how to use up the error budget - ran on an IBM 360. I did that while I was designing the analog portion of an attitude control system for scientific rockets for NSF. Nothing graphical about it - huge printouts on green and white paper with holes on each side. By the time simulation arrived on the scene it required big computers and we just started using it as an aid in chip design when I transitioned into management and pretty much quit doing actual engineering.

    Anyway, I used 5Spice to design a one stage circuit that meets the needs of the fellow who started this post: 50 mv pk to pk in and 5v pk to pk out to drive a cmos digital chip. He has probably moved on by now, but I took it as a bit of a challenge and enjoyed it. I had to use a PN5179 - couldn't get there with the 2n2222, even using a couple of them. It wasn't as easy as I thought it would be. :)

    Thanks again for doing that sim and stimulating me to simulate!


    Mike

    PS I'm going to keep trying to make Orcad eval versions (or find them) that will let me do useful things. I may take you up on your offer for the models. If they are generic spice models I would love to have them. My yahoo email will not accept big attachments. But I have a couple of others that might. So I think the important question is whether they are generic spice models or Msim specific?
     
    Last edited: Aug 10, 2008
  7. NA5Z

    NA5Z Ham Member QRZ Page

    That's what we're here for :)

    There's a lot to be said for doing it on pencil and paper or writing your own code. Pspice is so quick and so handy that I hate to say, it's made me lazy when it comes to remembering how to do it by hand... I can't imagine being on an interview and getting asked a design question, then whipping out a laptop with Pspice on it LOL


    Amyway, I went ahead and zipped it and it compressed all the way down to 6meg. I have some webspace, so I just tossed it up there.

    The SLB, PLB, and FLB files I would believe are all Msim specific, but since Orcad absorbed them, MAYBE they still would work? Those are all symbol and part libraries. The LIB files are the device models and those should be generic to spice. So if the SLB won't work, just make your own symbol and tag it with the device model in that you want.

    Library link (6meg):
    http://br549.thetumbleweeds.net/lib.zip


    Glad I could be useful for something LOL
     
  8. KB1LQC

    KB1LQC Guest

    Hey guys, good to see the conversation still going on. I am still working on the oscillator and amp. I've been on vacation for that past two weekends with almost no internet access and busy working/getting ready for school during the week so right now I had a chance to visit. Will be reading whats happened so far to catch up!
     
  9. WY6K

    WY6K Ham Member QRZ Page

    I have to say that I think you should just use a comparator or cmos inverters to make the oscillator if the purpose of the oscillator is just to clock digital circuits. However, I got interested in the buffer amp and have done some things.

    I built a version using a single 2N2222 but was not at all satisfied with it. So I added an emitter follower between the source and the input of the amp. This yielded a circuit that produced about 2.5 pk to pk output at 10 Mhz but when to heck at 28 Mhz. The 2N2222 just runs out of gain. I may have been able to push it a bit farther but decided it was a lost cause.

    Then I obtained 5Spice and spent quite a bit of time simulating various approached. I was able to solve the problem two ways using a PN5179 (with the simulator). I have not built either circuit yet. But I also simulated the circuit I built previously using the 2N2222 and was very impressed with the results - it matched almost exactly what I saw with the actual circuit.

    I will try to take some screen shots of the circuit and the simulation results later today. I'll post them here.

    I really recommend 5Spice. The free trial version is capable of handling this circuit and your oscillator. Not only is it useful, but I've been having a ball with it. :)

    Mike
     
  10. KB1LQC

    KB1LQC Guest

    OK, I will definitly take a look a 5Spice. I may have to run it under WINE because my main OS is Ubuntu Linux, but I dual boot vista so worse comes to worse Ill see how that partician i holding up :) with little use hihi.

    I have the oscillator working with a PN918 and am trying to get a PN918 buffer amp to work too. However, I am noticing a loading problem. Using either a 33 pf (closest to 22pf as suggested earlier that I can find) or a .1 uf capacitor to DC block the oscillator into the buffer amp (biased at 1/2 VCC or 6V) the load seems to stop the ocillation. I am extracting the signal from the emitter rather than the collector as suggested (its a lot cleaner signal too!). I tried the inverter gate method for oscillator with a 74HC104 but havent been able to get that to work...
     
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