MC145170

[XXL]

I’m trying to increase the lock range of my pll using the mc145170 as its so dreadful.
My thinking is if I increase the voltage to the varicaps, it will extend.
I only seem to be getting around 1v to the varicap on lock. Is this right ? It seems very low for a vhf varicap. Shouldn’t I be getting 5v on lock ? I’m not sure.

[nrgkits.nz]

There’s is a charge pump circuit somewhere on here that Albert posted - it will generate 30v from 12v which you can then use to power the opamp used with the 145170. There’s some schematics floating around online (use google) of the older BW v1 exciters and it has all the schematics for the 145170 setup.

[Shedbuilt]

The “in lock” varicap voltage, depends on frequency, tank circuit inductance, coupling capacitance, and other tank circuit capacitances. If you keep the frequency the same, and increase the inductance (or other capacitances), the PLL voltage has to increase (to cause the varicap capacitance to reduce). Once the PLL voltage loses the ability to rise, the PLL will go out of lock. Increasing the available PLL voltage range, will therefore increase the lock range.

[sinus trouble]

Yes the PLL decides and applies the relevant voltage to the varicap it needs to correct the phase error.

If your oscillator is tunable? Any adjustments you make? The PLL/Varicap voltage will follow and track it.

If your oscillator is No Tune? Then the PLL/Varicap voltage ideally would increase in steps as you increase your lock frequency!

As a very rough example -
1V=88Mhz 1.5V=89Mhz 2V=90Mhz 2.5V=91Mhz And so on.......

[sinus trouble]

Oh, Just to clear up any confusion! I may have mentioned in the past having 4V lock voltage on my varicap?

The reason i did this was i had a single varicap performing both audio and PLL functions.

Most other exciters use two independent varicaps in which the audio one needs to be DC biased, Otherwise the audio will sound shite!

[Albert H]

If you want to have consistent mod sensitivity across the band, you have to use (at least) two varicaps - one for the control and the other for the modulation. You have to take a portion of the loop control voltage to adjust the bias on the mod diode if you want to get the same mod sensitivity right across the band.

FYI - I've only ever seen three exciters that have an exact modulation voltage vs. deviation that's exactly the same at all frequencies - one made by Bext, one that was made by Rhode & Schwartz, and one that I designed. There are a few others that come close, but most don't even bother to try!

[OldskoolPirate]

Yes I’m using 2 varicaps. But sound quality isn’t the issue. It’s the super low voltage I’m getting to the varicap on the oscillator. 1v just isn’t enough. There’s hardly any swing on the oscillator. Heat could probably take it off frequency, it’s that bad. Anyway I’m looking into it using some comments made above.

[Albert H]

You should (perhaps) vary the oscillator coil, or change the capacitors in the oscillator to raise the required varicap voltage. I had a batch of NPO capacitors that turned out to be wrongly marked. The oscillator started off at the wrong end of the band, and no matter what voltage I applied to the varicaps, I couldn't get it right! The capacitors came from a reputable source as well - they weren't nasty Chinese "seconds"!

[sinus trouble]

As mentioned several times, The PLL phase correction voltage will depend on the state of your oscillator?

The symptoms do suggest that your oscillator operating frequency is too high! The PLL has reached its limit (1V) and is struggling to pull the frequency lower?

Either that or you have some other serious problems?

[Shedbuilt]

As mentioned several times, The PLL phase correction voltage will depend on the state of your oscillator?

The symptoms do suggest that your oscillator operating frequency is too high! The PLL has reached its limit (1V) and is struggling to pull the frequency lower?

Either that or you have some other serious problems?

Absolutely Sinus. Another thing you (XXL) could try. Assuming you have variable components in the VCO, tune it in both directions, to find the position where it drops out of lock, note the end points of lock, then tune to the halfway point. Once you've done that, re-check the voltage. If it will only lock right at one end of the adjustment (which in your case, since you have such a low PLL correction voltage, may be at the lowest end of the VCO frequency adjustment), you will need to make some changes or adjustments to the VCO (ie add more inductance or capacitance). If the inductor is air cored, you may be able to just compress it to achieve more inductance (if needed - ie if the frequency adjustment is right at the low end).