Varicap step-up supplies

[Albert H]

This should be separated into another thread, to make it easy to find.

Varicaps are (relatively) expensive components, so it's nice to minimise the number of them you need to cover the whole of Band II. You can buy more sensitive ones, or you can put several in parallel, but there are two reasons that make this a poor practice - varicaps are noisy (particularly if they're high sensitivity types) and a big frequency swing per volt makes the swing required for 75kHz deviation very small, making the modulator susceptible to hum and noise from other things in the rig.

I never agreed with Roger at BW - his modulator and VCO control varicap (both were the same component) would tune the whole band with a swing of around 2 - 12 Volts. That made the voltage swing for 75kHz deviation around 35mV. He used a potential divider on the audio path to drop the level, but that also introduces noise as well. It's a poor design (but cheap). His board is really susceptible to noise in the box, and is difficult to stabilise when there's a lot of RF about! Also - particularly when tuned to the lower end of the band - the varicap is nowhere near its linear region, so will introduce its own distortion of the modulation.

My solution to this conundrum was to increase the Voltage across the varicap so that the sensitivity can be much lower, and the varicap can operate in its linear region. It's easy enough to develop 30V on the board:

Varicap Supply.png

This is a simple oscillator and charge pump that I've used to feed to the Vcc3 pin of an SAA1057. This reduces the problems of phase noise in the VCO, and allows a sensible modulation sensitivity - scaled by use of a series capacitor - but with 20V bias to the mod varicap derived by potential divider from the 30V rail giving a sensitivity of about 2V p-p for 75kHz deviation.

Another trick I've used is to drive a charge pump from a 4049 CMOS oscillator or from a 4049 used as a buffer clocked by a spare output from the reference divider IC in a CMOS synthesiser. The 4049 is a great choice, because it's really cheap and has plenty of power output to drive the diode and capacitor voltage multiplier.

[Analyser]

Am I correct in thinking you also posted a circuit a while back which was a boost converter instead of the Cockcroft-Walton multiplier?

Or perhaps I'm going mad and it was only just the one above?

[Banus_radio]

Thanks Albert, appreciated
Couple of questions though.

is that 82uH and 180uH inductors? what purpose do these serve, is it just a dc filter? If I used a dedicated 9v positive regulator feeding this circuit would I still need the inductors?

Also what do you mean by linear range of a varicap?. I normally tune my osc so my pll is at around 4volts (my pll can swing 2-9v) on a BB809 and I use a separate varicap diode with approx 2v applied which I modulate audio onto

[sinus trouble]

Mr Banus the inductors are crucial to the circuit, especially the 82uH! it provides feedback to allow oscillation!
the circuit Mr Albert has shown is a charge pump which basically boosts the 12v supply to around 30v
When using a synth IC like the SAA1057 you need to choose the supply voltage range that will go to your varicap (usually the same as the main supply voltage)
For most oscillators 1v to 13.8v will work great to tune the whole band but not all will!

[Albert H]

The kHz / Volt of a varicap in a VCO circuit should exhibit a straight-line characteristic, especially when it comes to modulation. If you're using the varicap outside its linear range, you'll get audio distortion.

[Banus_radio]

Thankyou

[Albert H]

Here's the 4049BE inverter I sometimes use:

Varicap Supply Inverter.png

If you have a suitable high frequency source from (for example) a 4060 in a PLL reference circuit, you can use that instead of the oscillator (pins 9, 10, 7 and 6).

[Banus_radio]

Here's the 4049BE inverter I sometimes use:

Varicap Supply Inverter.png

If you have a suitable high frequency source from (for example) a 4060 in a PLL reference circuit, you can use that instead of the oscillator (pins 9, 10, 7 and 6).

I am going to use this 4049BE circuit, I only need an 18v output (or more) from a 12v input..The 12v input wont be from a regulator as it will be the 12v input to my board. So would it be good practice to use an 18v regualtor after the 4049 ? or would you suggest I use a 9v regulator before the 4049 to double to around 18v ? im guessing the 18v after the 4049 with a nice smoothing cap would be cleaner?

Hope I explained that clear enoguh

[Albert H]

Use an 18V zener to set the output voltage, and increase the 1k to (perhaps) 3k3, so the zener diode doesn't sink too much current. There won't be enough current for an ordinary 18V regulator IC to work very well.

[Banus_radio]

Albert I've spider wired that circuit using cd4049UBE.
Without the 30v diode I'm only getting 19volts out. It stays stable at 19v out with input from Only 7 or so volts right upto 15volts.
I've gone over it and it's defo exactly same as your schematic,
Only thing I've done that maybe affect it is I've used plastic caps for the 47n as I don't have any discs. can you please check it my ol'mate. Cheers

[Banus_radio]

I found some 47n ceramic and still no joy.
I've also just built that other circuit I uploaded and I still only get 19volts. Using another ic (still 4049) so it's not the ic at fault. I've tried 3 different multimeters incase they were dragging it down. I really wanted to see at least 24v or thereabouts

[Analyser]

Ok, one of the limitations of that circuit is the current it can provide.

Are you loading it while measuring the voltage output or just using a multimeter with no load connected?

If you want to increase the current capability you can increase the value of the capacitors in the charge pump, in this case the 47nF caps. Try paralleling some to make 100nF or 150nF and see if that helps things.

[Banus_radio]

It's unloaded, only multimeter connected.
I've tried 100nf discs and makes no difference.

I've also tried using different Electrolytic caps at the output incase they had some kind of loading effect

[Analyser]

Are you still using a resistor and zener at the output, or is that all gone?

[Analyser]

You could also try reducing the value of the 1nF capacitor in the oscillator.

[Analyser]

Apart from that I'm at a loss, the circuit looks like it should work.

[Albert H]

That's weird!

If you've got another IC, try making an extra stage with two paralleled gates, 47n cap and diode, and see what voltage you get. There may be something strange going on with your 4049s. I had one that just wouldn't work properly at all - it was "SEC"-branded (I've never heard of them either!).

[Banus_radio]

I will try that later, i do have another ic so ill see what happens.
I will try and buildthe version above with the bc547, only i dont hav inductors of that value, i might stick a few inductors in series to get close to them values.