Varicap use - Is this correct?

[atferrari]

Isn't a direct path for DC through the inductor?

 

[Nigel Goodwin]

That's incorrect, you need a DC blocking capacitor between the varicap and the other capacitor.

 

[Sangoma]

That's incorrect, you need a DC blocking capacitor between the varicap and the other capacitor.

I agree, incorrect, at the moment you have a DC path from the top of the varicap to ground through the coil, so it will never vary from maximum capacitance, and you may well kill the oscillation as well.

Depending on the circuit, 1n or 10n will be OK. you dont want it too small, otherwise the effective varicap capacitance will be affected by the series capacitance of the blocking capacitor.

 

[dr pepper]

Yes your right L1 will short your dc to ground and the 47k will load the osc and possibly stop it, all previously posted.
Its possible that the inductor could be of suffciently high dc resistance not to affect the tuning voltage (saturation might then be problem though), but very unlikely to be high enough not to be affected by the 47k resistor, and even then the oscillator would be uneccesarily loaded

 

[Nigel Goodwin]

Its possible that the inductor could be of suffciently high dc resistance not to affect the tuning voltage

Can you really imagine a coil high enough inductance and thin enough wire to do that

Particularly bearing in mind the small capacitance change of a varicap, which means you're usually talking VHF or above.

 

[fernando_g]

Where did you find that schematic?
The way it is drawn, it appears to have come from a data sheet or app note.

 

[dr pepper]

Nige: yep it'd be like winding a coil with a spiders web, a subminiature spider, one turn around the albert hall with 40swg wouldnt even do it.

 

[Sangoma]

Can you really imagine a coil high enough inductance and thin enough wire to do that

Particularly bearing in mind the small capacitance change of a varicap, which means you're usually talking VHF or above.

The last month or so I have been doing quite a bit with varicaps that go to 500p

I replaced a the crappy polyvaricon in a simple "antenna analyser" with two back to back, and still had enough to reduce the bands from 4 to 3 and with a 10 turn pot, very nice to use. Made a sweep generator that will give me 1024 steps between two preset frequencies using a pic and a resistor da converter to give me a rough idea of filter bandpass, made an effective SW receiver, again ten turn pot and switched presets to create "bands", and probably the most useful one was a variable bandwidth IF filter.

All below 30megs, so they can be very useful

 

[dr pepper]

I've a frequency standard that tunes to radio4, it uses a 1n4007 as a varicap, that particular diode works well as a varicap, the capacitance of the diode is adjusted using dc to 'pull' the frequency of a crystal osc to sync up with the radio 4 signal using a very simple pll.

 

[Nigel Goodwin]

I've a frequency standard that tunes to radio4, it uses a 1n4007 as a varicap, that particular diode works well as a varicap, the capacitance of the diode is adjusted using dc to 'pull' the frequency of a crystal osc to sync up with the radio 4 signal using a very simple pll.

Rectifier diodes make pretty decent varicaps, their large chip size gives them decent capacitance.

 

[Sangoma]

Rectifier diodes make pretty decent varicaps, their large chip size gives them decent capacitance.

Red LEDs are good to. Light has been shown to have minimal effect, most are enclosed anyway, and a tin of black spray paint would eliminate any undesirable effect.

 

[dr pepper]

I didnt know that, what sort of capacitance and control voltage are we talking?

 

[Sangoma]

about 25p min to 50p max 0-12V

 

[Nigel Goodwin]

about 25p min to 50p max 0-12V

Rectifier diodes are better than that

And cheaper!

 

[Sangoma]

I think the curve on the LED is more usable in some cases. The rectifier diode has most of its range in the first couple of volts, the LED has the same range over the first 6 volts. Neither are anywhere near linear, so I won't use that as a description.