Electronics > RF, Microwave, Ham Radio

Cheap stable VFO design

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uncle_bob:

--- Quote from: GK on June 03, 2016, 12:15:42 pm ---
--- Quote from: uncle_bob on June 03, 2016, 12:11:15 pm ---
--- Quote from: GK on June 03, 2016, 12:08:13 pm ---
--- Quote from: uncle_bob on June 03, 2016, 11:44:18 am ---Hi

As soon as you forward bias the diode, that curve changes a lot. Since an LCR meter (or an oscillator) could be putting several volts p-p on the diode, you may start to "see" forward bias at the low reverse bias levels.

One way to look at it:

By the time the diode gets to 0.7V forward bias, it's basically a short RF wise. As it approaches the "zero ohm" point, it's capacitance goes up quite a bit.

Bob

--- End quote ---


The junction capacitance does typically rise at an accelerated rate as 0V DC bias is approached, but the slope in this case would have to be ridiculously steep to effect the capacitance variation required to tune this oscillator with the limited range of variable DC bias that the OP must be getting with the diode shorted by the DCR of the inductor.

--- End quote ---

Hi

.... which is why several of us have been pointing out the short circuit issue. Of course, there are many ways to get a circuit to change frequency.

Bob

--- End quote ---


You are talking about something completely different now. That there are numerous things wrong and sub-optimal in this design is beyond question. Precisely how the described circuit can function exactly as claimed is what I am trying to figure out.
   

--- End quote ---

Hi

Well, that's not exactly the point of the DC short either.

How can it work:

1) The schematic and the circuit are not the same

2) The parts on the breadboard are not what they are "supposed to be".

3) The measurements are in error.

That's a short list.

Bob

GK:

--- Quote from: uncle_bob on June 03, 2016, 12:18:19 pm ---Hi

Well, that's not exactly the point of the DC short either.

How can it work:

1) The schematic and the circuit are not the same

2) The parts on the breadboard are not what they are "supposed to be".

3) The measurements are in error.

That's a short list.

Bob

--- End quote ---


:palm:

Sherlock Holmes I presume?

StillTrying:
A few random notes from last nights simulations on the off-chance someone finds them interesting, or even useful. - I simulated(on and off) for over 2 hours so you don't have to. LOL

Despite it only going down to 0.1V, GK's 1N4002 junction capacitance/reverse voltage graph above is very close to what I 'measured' in LT. A 1Amp diode starts off at 35pf at 0V, and applying a real 1V to 8.5V tuning voltage gives only a 2:1 change in capacitance.
LT's variactor starts at ~52pF, - still with a 2:1 range.

I 'measured' the tuning diodes capacitances by probing the 5.4Mhz current through it and it's parallel 30pF, and compared the 2 pk-pk currents ratio.

In the simulation if I fix 'the short' so that the tuning voltage of 1V to 8.5V gets across the tuning diode it makes very little difference to the frequency or its full range - it's still about 5.4Mhz with about a 250Khz change.

You'll like this next one.

Now knowing the tuning diodes capacitances, if I swap out LT's variactor diode for a fixed 50pF with ESR 10R does the frequency still change with the 1 to 8.5V? - Yes! But in the opposite direction - at 8.5V it drops about 1Mhz.

Just for some colour, using LT's variactor, no short, green =output, red =tuning voltage, blue = emitter-collector.

G0HZU:
Obviously the impedance at the emitter of the 'tuning' transistor/buffer will change with changes in bias. At very low bias voltages the impedance at the emitter will be much higher and the (potentially severe) damping impact of the 1K resistor will be less because there is a high impedance at its cold (emitter) end. So there should be more RF voltage developed across the T50-6 inductor at low tuning voltages because of the reduced damping. But I think the diode/transistor may interact as a form of crude RF clamp here with the side effect of changing the RF frequency downwards as this ALC/clamping happens. So maybe you don't 'see' this increased RF voltage because the diode is doing something in the form of ALC/limiting. Maybe the side effect of this ALC process is how it appears to tune with changes in tuning voltage (at the emitter) despite the diode being shorted at DC by the inductor? The diode will be a different animal if this clamping/limiting is happening and the limiting probably only happens strongly/significantly at low tuning voltages?

Ideally, the emitter node should be bypassed at RF so maybe some of the tuning strangeness will go away when there is a decent RF decoupling cap across the 10k resistor in the emitter? This would clamp this node at RF and this would define the damping effect of the 1k resistor across all tuning voltages and frequencies. A 1k resistor across the inductor is going to damp the loaded Q of the system quite a bit and this will degrade the phase noise and drift/stability.

The circuit just looks dodgy to me... I'd bin it and start again with a JFET and connect the tuning diode(s) differently to prevent clamping and also aim for a higher loaded Q (I.e. less damping of the main resonator!).

uncle_bob:

--- Quote from: StillTrying on June 03, 2016, 05:12:40 pm ---A few random notes from last nights simulations on the off-chance someone finds them interesting, or even useful. - I simulated(on and off) for over 2 hours so you don't have to. LOL

Despite it only going down to 0.1V, GK's 1N4002 junction capacitance/reverse voltage graph above is very close to what I 'measured' in LT. A 1Amp diode starts off at 35pf at 0V, and applying a real 1V to 8.5V tuning voltage gives only a 2:1 change in capacitance.
LT's variactor starts at ~52pF, - still with a 2:1 range.

I 'measured' the tuning diodes capacitances by probing the 5.4Mhz current through it and it's parallel 30pF, and compared the 2 pk-pk currents ratio.

In the simulation if I fix 'the short' so that the tuning voltage of 1V to 8.5V gets across the tuning diode it makes very little difference to the frequency or its full range - it's still about 5.4Mhz with about a 250Khz change.

You'll like this next one.

Now knowing the tuning diodes capacitances, if I swap out LT's variactor diode for a fixed 50pF with ESR 10R does the frequency still change with the 1 to 8.5V? - Yes! But in the opposite direction - at 8.5V it drops about 1Mhz.

Just for some colour, using LT's variactor, no short, green =output, red =tuning voltage, blue = emitter-collector.

--- End quote ---

Hi

So what does the schematic look like now?

Bob

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