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Thread: VCO stability

  1. #1

    Default VCO stability

    I'm new RF design, so please bear with me. I have a situation where I want to do FM on one frequency only. I've read a lot about Phase-Locked Loops, but it seems to me that it's pretty difficult to tune their charge pump filters to get a lock yet not fight with the FM.

    My question is if it is feasible to use some sort of a precision voltage reference, divide it to get the voltage for the desired carrier frequency, and sum that voltage with the information signal to feed into the VCO? This is assuming a commercial VCO, not a homemade Colpitts oscillator. Almost all of the circuits I see out there are either unstable (subject to frequency drift), or use a PLL.

    If PLL is the only way to go, can someone recommend a chip that is easy to interface with, and will be good for the 2-meter band?
    Jay Kickliter

    www.chasingtrons.com (projects and tutorials)

  2. #2

    Default Re: VCO stability

    Jay,

    All PLLs rely on low-pass filtering in the feedback loop for stability. If the audio modulating frequency summed into the VCO's control input is above the filter's cut-off frequency, you should be able to produce FM without the PLL fighting to regain control. It will simply filter out the modulation and use the average frequency to maintain a lock. Of course, if the filter's time constant is too long, it will take longer to QSY and lock onto a new operating frequency, so there is a small tradeoff.

    I'm not sure what to recommend for a VHF VCO. Most of my experimenting has been in HF bands:
    One thing I cannot recommend is using a Propeller counter in PLL mode. Although it will get you to the 2-meter band, and you can do FM, there's too much phase jitter, which causes spurious sidebands throughout the entire band (and outside) that cannot be filtered.

    You might be able to generate your FM signal at a lower frequency using the PLL/FLL method noted in my thread, then run it through a mixer to reach 144+ MHz.

    -Phil

  3. #3

    Default Re: VCO stability

    If you really want to get into RF, you might want to ponder the Universal Software Radio Peripheral. This device plugs into a desktop/laptop computer via USB and is supported by software in Windows/OSX/Linux OSes.

    Ettus Research at www.ettus.com is a really interesting supplier.

    These days, it is very challenging to build RF items from scratch and there is a heck of a lot that Ettus does to eliminate the drudgery of involved. That leave more time and energy to explore what is one the airwaves, how a better antenna might help, and so on.

    The key word here is 'low budget'.

  4. #4

    Default Re: VCO stability

    Mini-Circuits makes nice VCOs:

    http://www.minicircuits.com/products/vco_sm_5v.shtml

    Use one of those with a PLL chip:

    http://www.fujitsu.com/global/servic...duct/assp/pll/

    You will need an amateur radio license to transmit on the 2m band, of course.
    Leon Heller
    G1HSM

  5. #5

    Default Re: VCO stability

    Jay, here's another possible option using a mixer/oscillator (e.g. SA602A): Do the Propeller FLL/PLL on the built-in LO (via a varactor) at, say 20MHz or so, to establish the operating frequency. On the input connect a VCXO (around 125MHz) that receives the modulation signal. Mix it up to 144+ MHz and filter out the difference frequency (~105 MHz) via an LC network.

    -Phil

  6. #6

    Default Re: VCO stability

    Jay:
    You can use a fixed frequency and phase modulate it with a network of variable capacitance diodes and inductors of the appropriate values for the frequency you are using. This will be demodulated by an fm rx as if it was fm, If you are transmitting low speed data, this will not work as it does not have dc capability.

  7. #7

    Default Re: VCO stability

    Hey Phil. Thanks. Do you know a rule of thumb for that time constant? Say if my lowest freq is 1200 Hz (using your Bell 202 code). Or what if I wanted to do bandpassed voice with -3 dB corners at 300 and 3000 Hz?

    Actually, I already have a VCO on order I'm going to evaluate. It's the one I linked to in my first post. It's pretty close to the 2 meter band.

    For now I want to use traditional hardware, and maybe try using the Prop for greater control later. I'm not building anything specific; just learning. However, I do like your idea of generating the signal on the Prop and mixing it up.


    Quote Originally Posted by Phil Pilgrim (PhiPi) View Post
    Jay,

    All PLLs rely on low-pass filtering in the feedback loop for stability. If the audio modulating frequency summed into the VCO's control input is above the filter's cut-off frequency, you should be able to produce FM without the PLL fighting to regain control. It will simply filter out the modulation and use the average frequency to maintain a lock. Of course, if the filter's time constant is too long, it will take longer to QSY and lock onto a new operating frequency, so there is a small tradeoff.

    I'm not sure what to recommend for a VHF VCO. Most of my experimenting has been in HF bands:
    One thing I cannot recommend is using a Propeller counter in PLL mode. Although it will get you to the 2-meter band, and you can do FM, there's too much phase jitter, which causes spurious sidebands throughout the entire band (and outside) that cannot be filtered.

    You might be able to generate your FM signal at a lower frequency using the PLL/FLL method noted in my thread, then run it through a mixer to reach 144+ MHz.

    -Phil
    Jay Kickliter

    www.chasingtrons.com (projects and tutorials)

  8. #8

    Default Re: VCO stability

    Loopy, maybe I'm looking at the wrong product, but those SDR's are well over $1k. Are you referring to something else?

    Quote Originally Posted by Loopy Byteloose View Post
    If you really want to get into RF, you might want to ponder the Universal Software Radio Peripheral. This device plugs into a desktop/laptop computer via USB and is supported by software in Windows/OSX/Linux OSes.

    Ettus Research at www.ettus.com is a really interesting supplier.

    These days, it is very challenging to build RF items from scratch and there is a heck of a lot that Ettus does to eliminate the drudgery of involved. That leave more time and energy to explore what is one the airwaves, how a better antenna might help, and so on.

    The key word here is 'low budget'.
    Jay Kickliter

    www.chasingtrons.com (projects and tutorials)

  9. #9

    Default Re: VCO stability

    Hey Leon. I already ordered some VCO's to try, but I do like those PLL's you linked. I'll give them a try. On a whim the other day I ordered a bunch of LMX2322 PLL's off eBay for real since they're out of production. But they seem to be popular with folks on the internet. However, when I got them I realized I ordered QFP chips. I have no Problem soldering QFP's usually, but they are a pain in the ***. I'll try those Fujitsu's.

    Oh, just yesterday I saw that my FCC Technician call sign was posted. I took the test about a week ago.

    Quote Originally Posted by Leon View Post
    Mini-Circuits makes nice VCOs:

    http://www.minicircuits.com/products/vco_sm_5v.shtml

    Use one of those with a PLL chip:

    http://www.fujitsu.com/global/servic...duct/assp/pll/

    You will need an amateur radio license to transmit on the 2m band, of course.
    Jay Kickliter

    www.chasingtrons.com (projects and tutorials)

  10. #10

    Default Re: VCO stability

    Quote Originally Posted by Jay Kicklitr
    Do you know a rule of thumb for that time constant? Say if my lowest freq is 1200 Hz (using your Bell 202 code). Or what if I wanted to do bandpassed voice with -3 dB corners at 300 and 3000 Hz?
    No, but that won't stop me from making a guess. For a simple, first-order RC low-pass filter, I'd try picking R and C such that fc, given by
    is one-half or less of your lowest modulating frequency.

    -Phil

  11. #11

    Default Re: VCO stability

    I got my first RF circuit working. I didn't think it was working, since when I probe the oscillator's output, I see nothing. Yet the signal is making it into the antenna. The weird thing: I do see the signal when I probe the + or - rails, even with the probe clip on the - rail. It works pretty well, but if I even look at it wrong it gets out of tune.

    Jay Kickliter

    www.chasingtrons.com (projects and tutorials)

  12. #12

    Default Re: VCO stability

    Man, that's good fidelity for just a two-transistor circuit! (And since when does Milwaukee make radios? Or does it transform into a circular saw somehow? )

    Can you post a schematic for the transmitter?

    -Phil

  13. #13

    Default Re: VCO stability

    Phil, I didn't design it. Here's the schematic I used: http://www.pyroelectro.com/projects/...schematic.html

    It's a little different than most of the FM circuits I've found on the internet. Most seem to have the top cap in parallel with the transistor and and the bottom cap from the emitter to ground.

    Here's the most comprehensive document I've found reguarding PLL's: http://web.itu.edu.tr/~pazarci/pll/N...sBook_4_01.pdf A little too comprehensive, it's a bit much to digest.
    Jay Kickliter

    www.chasingtrons.com (projects and tutorials)

  14. #14

    Default Re: VCO stability

    Ano way to use a PLL to generate FM is to use a Xtal oscillator at several MHz, with the FM applied directly to the Xtal oscillator. A PLL then locks a higher frequency VCO to a multiple of the Xtal frequency. The PLL needs a low-pass filter with a roll-off _higher_ than the modulation frequencies. The deviation is multiplied up by the PLL so quite low deviations are sufficient at the Xtal oscillator.

    Alternatively a series of harmonic frequency multiplier stages can be used, but thats more complex than a PLL divider chip these days.

    This won't work with broadcast FM though as its very wideband and crystals can only be pulled a small amount. For narrowband FM at VHF frequencies its pretty good (except that you need special crystals with a simple PLL). Using a sigma-delta PLL with fractional multiplication would fix that, but is more complicated (and may generate more spurious outputs).

  15. #15

    Default Re: VCO stability

    Phil, do you think it would be practical to use your PLL/VCO method with the prop, and just use a simple prescaler chip to divide the input instead of down-mixing like you suggest? Or if I'm going to have a prescaler, should I just not bother using the prop for PLL and just use a dedicated chip? I mainly ask since there's a few through-hole prescalers available out there these days, and I'm really getting tired of prototyping with surface mount parts. Maybe if I had UV light box I'd feel different, but I haven't been getting great results with PNP.
    Jay Kickliter

    www.chasingtrons.com (projects and tutorials)

  16. #16

    Default Re: VCO stability

    Jay,

    If I read you correctly, what you propose is to have a VCO running at the final VHF frequency, and divide that down to a feedback frequency that the Prop can handle for doing the VCO control. Right? That should work. Just keep in mind that any frequency or phase errors will be multiplied by the divider ratio. 'Still worth an experiment, though!

    -Phil

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