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    Frankling VFO converted to VCO - problems

    I am trying to convert my Franklin-based VFO http://qrp.gr/supervfo/ into a VCO.
    I have done it as in the attachment, which works (it oscillates) but the tone in a near by receiver is bad. In AM mode in the receiver it sounds like hum.

    I notice that when I touch my hand onto the bare PCB copper the tone gets somehow better (half the hum).
    Any ideas?
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    Re: Frankling VFO converted to VCO - problems

    The main rule in any VCO design is to keep very low RF voltage across the varicaps. Rule that your oscillator doesn't follow, due to the location of your varicaps.
    When use variable capacitors should be fine, but using varicaps with high RF voltage across, and also placed in the feedback path of the oscillator, you get behavior that you mentioned.
    Franklin oscillator has only advantage that is relative wideband, otherwise has poor frequency stability and poor phase noise.
    Make a simple Colpitts (or Clapp) and you get rid of all these problems.


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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by neazoi View Post
    I notice that when I touch my hand onto the bare PCB copper the tone gets somehow better (half the hum).
    Touching our hand to an amplifier often changes the sound. It might be worse buzzing, it might be less buzzing. It's hard to be sure what your project needs, whether more shielding of signal-carrying wires. Or an enclosing metal box. Or a different grounding arrangement, or star-ground. Etc.

    I suspect that high-impedance components isolate a section of the circuit, making it prone to pick up ambient hum. Example, your schematic has a 1M resistor. This is not necessarily an error, although it may pay to try lower ohm values.


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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by vfone View Post
    The main rule in any VCO design is to keep very low RF voltage across the varicaps. Rule that your oscillator doesn't follow, due to the location of your varicaps.
    When use variable capacitors should be fine, but using varicaps with high RF voltage across, and also placed in the feedback path of the oscillator, you get behavior that you mentioned.
    Franklin oscillator has only advantage that is relative wideband, otherwise has poor frequency stability and poor phase noise.
    Make a simple Colpitts (or Clapp) and you get rid of all these problems.
    It is super-stable and it works excellent with a capacitor. On HF I do not matter too much about phase noise although it does play a role, but not in this hobbyist circuit.
    Your mentioned high voltage on varicaps was something that I suspected, some designs just don't allow varicaps due to that reason.
    I have seen a few UHF franklin VCOs though ans I wonder if they are all suffer from the same problem.
    https://sites.google.com/site/linuxd...80mhz-franklin
    https://www.qsl.net/va3iul/High_Frequency_VCO_Design_and_Schematics/High_Frequency_VCO_Design_and_Schematics.htm

    The use of the vco will solve me the problem of band-switching in this configuration where a stereo potentiometer is used to tune the vco in the band of interest (it covers most of the HF with the same inductor), then the fine tune will allow for real tuning within this band. It avoids any switches. I could probably do it with capacitors as well though.

    - - - Updated - - -

    Quote Originally Posted by BradtheRad View Post
    Touching our hand to an amplifier often changes the sound. It might be worse buzzing, it might be less buzzing. It's hard to be sure what your project needs, whether more shielding of signal-carrying wires. Or an enclosing metal box. Or a different grounding arrangement, or star-ground. Etc.

    I suspect that high-impedance components isolate a section of the circuit, making it prone to pick up ambient hum. Example, your schematic has a 1M resistor. This is not necessarily an error, although it may pay to try lower ohm values.
    I will try a smaller value varicap resistor like 100k or even 10k to see how it goes, although I doubt if this will help.

    Recently in another oscillator I had the same problem but much less occurring and when I enclosed it in a metal box and switched all the devices in the room, it seemed it cured it. There was a digital clock that caused interference. These things are hair-pulling!
    Last edited by neazoi; 3rd October 2018 at 08:14.
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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by BradtheRad View Post
    I suspect that high-impedance components isolate a section of the circuit, making it prone to pick up ambient hum. Example, your schematic has a 1M resistor. This is not necessarily an error, although it may pay to try lower ohm values.
    You got that, thanks so much!
    I paralleled 1K to the 1M feeding the varicaps and the problem cured to 80%. I touched mu finger on the copper board too and it was cured to 95%.
    But this is an unshielded board so when shielding it might go away.

    Any other suspicions for other high impedance points that might pick up noise?
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    Re: Frankling VFO converted to VCO - problems

    The varicap is high impedance, so feeding with 1MOhm is sensitive to external coupled voltages. I would prefer an inductor instead, so that you can have low impedance for the control voltage (DC) without loading the oscillator tank circuit (RF).

    Another disadvantage of the 1MOhm is that thermal noise of the resistor will make your oscillator noisy (resistor noise voltage tunes the varicaps).


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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by volker@muehlhaus View Post
    The varicap is high impedance, so feeding with 1MOhm is sensitive to external coupled voltages. I would prefer an inductor instead, so that you can have low impedance for the control voltage (DC) without loading the oscillator tank circuit (RF).

    Another disadvantage of the 1MOhm is that thermal noise of the resistor will make your oscillator noisy (resistor noise voltage tunes the varicaps).
    What value of inductor should I use for 3-30MHz? Maybe 100uH?
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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by volker@muehlhaus View Post
    The varicap is high impedance, so feeding with 1MOhm is sensitive to external coupled voltages. I would prefer an inductor instead, so that you can have low impedance for the control voltage (DC) without loading the oscillator tank circuit (RF).

    Another disadvantage of the 1MOhm is that thermal noise of the resistor will make your oscillator noisy (resistor noise voltage tunes the varicaps).
    Here is the complete VCO schematic that includes yous suggestion for lowering the varicap feeding resistor. I have tried it with inductors as well and the results are the same.

    FvM said that the LC voltage is too high and I suspect this causes this problem, although this does not explain why when I touch my finger to the PCB the frequency (I think) modulated noise is reduced.

    But anyway I want to try to fix this high energy on LC. The LC Sine level has great variations alone. But I have this ALC in the amplifier feeding the amplifier input so that it reduces the amplification at high LC voltages.
    Now my thought is to use this ALC mechanism, but vary the oscillator gain when the LC sine is high, so that to keep the LC sine level low at all times.
    How can I do this?
    Could this be the solution for the high LC level?
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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by neazoi View Post
    Now my thought is to use this ALC mechanism
    Your schematic appears to have two potentiometers at left. Did you install them? Do they tune frequency?
    Or could they be for adjusting gain?

    Or else you could experiment with a component elsewhere on the board. For instance the amount of bias going to a transistor. Or a resistor value in the LC loop.


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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by BradtheRad View Post
    Your schematic appears to have two potentiometers at left. Did you install them? Do they tune frequency?
    Or could they be for adjusting gain?

    Or else you could experiment with a component elsewhere on the board. For instance the amount of bias going to a transistor. Or a resistor value in the LC loop.
    I used them and they do change the frequency. The big one (stereo) changes the "range" the "band", it is the coarse tuning of you like. The small one is the fine tuning. I have thought of this arrangement so that the fine tuning always stays fine. If two mono pots were used, connected in series (the wiper of the first to one end of the second and the other end of the second to the ground), the fine tuning range of the second one would depend on the setting of the first. For example when the first one would set all the way to VCC, the second one would have no fine tune action.

    I have tried to create this Reristor-BJT attenuator in the feedback path (next to the 470pF capacitor) but it did not work and I am not sure why. Maybe I shall try at the collector side near the 3.9pF capacitors?
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    Re: Frankling VFO converted to VCO - problems

    Accepting the previous comments about noise and stability, I would expect a degree of frequency pulling by the ALC circuit. I'm not sure a BC549 would make a good shunt across the signal at high frequency either.

    A better method would be to follow the oscillator with an emitter follower so it presented a high impedance to the oscillator output and maybe apply the ALC voltage to a subsequent stage. I've never tried it but I don't see any reason why a fixed cap & varicap can't be used to bypass the emitter resistor in a conventional common emitter stage to control negative feedback and hence gain.

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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by betwixt View Post
    Accepting the previous comments about noise and stability, I would expect a degree of frequency pulling by the ALC circuit. I'm not sure a BC549 would make a good shunt across the signal at high frequency either.

    A better method would be to follow the oscillator with an emitter follower so it presented a high impedance to the oscillator output and maybe apply the ALC voltage to a subsequent stage. I've never tried it but I don't see any reason why a fixed cap & varicap can't be used to bypass the emitter resistor in a conventional common emitter stage to control negative feedback and hence gain.

    Brian.
    Oh you mean to replace the bjt with a varicap (placed in shunt to the GND with the 470pF feedback capacitor and control it from the VCC of the ALC loop?

    - - - Updated - - -

    Quote Originally Posted by neazoi View Post
    Oh you mean to replace the bjt with a varicap (placed in shunt to the GND with the 470pF feedback capacitor and control it from the VCC of the ALC loop?
    Oh no, you are talking about controlling the oscillator emitter. Which one of the two bjts emitters should I control with the varicap?

    The problem I see with the varicap approach, is that the voltage out of the germanium diodes may be too small to drive the varicap effectively.
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    Re: Frankling VFO converted to VCO - problems

    Controlling the oscillator emitters would be disastrous and the ALC loop may even cause a secondary oscillation. What I meant was to add a buffer stage before the 2n2222 and shunt it's emitter resistor to AC with a varicap. I agree the voltage recovered from the rectifiers will probably be too low (~2V at a guess) but you can use that voltage amplified to drive a varicap. The essential problem is that the way the oscillator works is rather like a multivibrator with gain too low for it to reliably run so it relies on the external LC to give it enough gain at the resonant frequency. It makes it very susceptible to changes around the tuned circuit which is why I am worried about 'shorting' it out with the ALC shunt. A buffer stage would help by isolating the tuning from the effects of ALC voltage.

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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by vfone View Post
    ...otherwise has poor frequency stability...
    Not true! The frequency stability when using an air-spaced capacitor instead of a varactor is excellent and crystal-like in lower bands. Tested, believe me.
    As far as I read, a major characteristic of the Franklin topology IS the frequency stability that is achieved mainly due to the very low coupling (low loading) of the resonating elements.

    - - - Updated - - -

    Quote Originally Posted by betwixt View Post
    Controlling the oscillator emitters would be disastrous and the ALC loop may even cause a secondary oscillation. What I meant was to add a buffer stage before the 2n2222 and shunt it's emitter resistor to AC with a varicap. I agree the voltage recovered from the rectifiers will probably be too low (~2V at a guess) but you can use that voltage amplified to drive a varicap. The essential problem is that the way the oscillator works is rather like a multivibrator with gain too low for it to reliably run so it relies on the external LC to give it enough gain at the resonant frequency. It makes it very susceptible to changes around the tuned circuit which is why I am worried about 'shorting' it out with the ALC shunt. A buffer stage would help by isolating the tuning from the effects of ALC voltage.
    Brian.
    I have started to abandon the idea of using varactors, no matter how convenient it is. I cannot make it produce a good tone and the frequency stability is not too good even at lower bands. Using varactors seems to have a big effect in the so precious characteristics the Franklin has.
    I was trying to find a miniature air-spaced capacitor that would accommodate a knob and I have found none. Eventually I have found these beehive types, I remember I had some in the lab. See here at about the middle of my page http://qrp.gr/components/ A knob can be soldered onto them and there you have it, a small air-spaced variable capacitor with reduction drive and no backlash at all, at a very low cost!
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    Re: Frankling VFO converted to VCO - problems

    Varactors really benefit when the frequency is controlled in a loop, for example a PLL where mechanical tuning is impossible.
    Beehive trimmers are good but I would have reservations about longevity. The screw thread is intended for 'once off' adjustment and may not last long in continuous use particularly with the extra 'wobble' of an extended shaft.

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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by betwixt View Post
    Varactors really benefit when the frequency is controlled in a loop, for example a PLL where mechanical tuning is impossible.
    Beehive trimmers are good but I would have reservations about longevity. The screw thread is intended for 'once off' adjustment and may not last long in continuous use particularly with the extra 'wobble' of an extended shaft.
    Brian.
    Variable capacitors were once controlled in a loop using motors! I know at least one receiver that did that the Micro-Tell 1200. It was fun to see the frequency know moving around by itself.

    I also concern the longevity of the screw on beehives. Time will show how it goes. They are quite tight anyway so I put a bit of oil at the part where it is needed to reduce friction and wearing. I use the thick-screw types because I thing the plates are kept more steady in place. I guess with time they will get more loose, but I dare to say that wearing to the point that they "wobble" so much to be unusable, will take a long time before it happens. I am really curious though.
    Last edited by neazoi; 12th October 2018 at 14:08.
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    Re: Frankling VFO converted to VCO - problems

    Variable capacitors were once controlled in a loop using motors!
    My HF transceiver still uses motors to tune the PA output!

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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by betwixt View Post
    My HF transceiver still uses motors to tune the PA output!

    Brian.
    I am curious which one is it?

    If you are antenna-ready we could arrange a QSO. I (sv3ora) have talked to mobile stations in Whales successfully, when prop conditions are forgiving.
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    Re: Frankling VFO converted to VCO - problems

    Sorry for the delayed reply. We have been hit by storms over the past few days and had no electricity most of the time!
    My HF transceiver is an Icom IC-756ProII so it is all 'new' technology but still uses servo motors to tune the PA stage because of the high power (up to 100W) involved.

    My antenna is not good I'm afraid. I live in a valley and inside a national park that does not allow masts to be erected. It is an end fed horizontal wire about 25m long with an automatic ATU. Unfortunately only about 70m above sea level and the antenna only about 3m above ground level and surrounded by tall trees. We can try if you like but don't expect good results.

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    Re: Frankling VFO converted to VCO - problems

    Quote Originally Posted by betwixt View Post
    Sorry for the delayed reply. We have been hit by storms over the past few days and had no electricity most of the time!
    My HF transceiver is an Icom IC-756ProII so it is all 'new' technology but still uses servo motors to tune the PA stage because of the high power (up to 100W) involved.

    My antenna is not good I'm afraid. I live in a valley and inside a national park that does not allow masts to be erected. It is an end fed horizontal wire about 25m long with an automatic ATU. Unfortunately only about 70m above sea level and the antenna only about 3m above ground level and surrounded by tall trees. We can try if you like but don't expect good results.

    Brian.
    It should be nice to try at least and see how it goes. My machine is 30 years old but it works ok.
    I will email you to arrange the QSO.
    I did not know that these new machines have motors in them other than the fan!
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