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An help with a sawtooth generator for varactor diodes

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MisterBeppe

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Hi,

I wish some help with the following circuit:

**broken link removed**

The purpose of the following circuit is to generate a sawtooth waveform, to continuously change the value in pF of these varactor diodes (these diodes replace a manual variable capacitor in a tank circuit, to feed a sweep oscillator).

The point is the following: currently, using this circuit, the sawtooth waveform change the value from about 0 V to about 8 V; but these varactor diodes need a max value of 28 V, to fully cover their pF range (at about 28 V these varactor diodes achieve their minimum pF value).

I was thinking to add a transistor or an opamp to the sawtooth output (the R4 resistor, however, must stay present before the diodes), to increase the max volt value (so from 0 to 28/29 V): this option would be a viable way? And if so, can I have some suggestions about how to implement this solution?
I can also consider to adopt another circuit, considering that I prefer if the power supply of the circuit itself would not exceed the value of 12 volt.

Many thanks.
 

Yes, you can use a circuit supplied at higher voltage to increase the amplitude of the sawtooth waveform.

 

Thank you for the reply and for the schematic, vfone!

Well, from what I can see I need a supplementary power source of 30 V, Am i right? The fact is that this circuit with the oscillator should be portable, and for this purpose I'm using a lead battery which provides 12V DC. So: there is some way to obtain - using some similar circuit - a max voltage of 30V using the current power supply of 12V?

Thank you.
 

you realize your varactor diodes are drawn backward, and will need a DC return to ground too.

the only way to do what you want with a 12v supply is to find hyperabrupt diodes that change their capacitance over the full range at a much lower voltage.

Otherwise your circuit will need a switching regulator to boost the voltage
 

you realize your varactor diodes are drawn backward, and will need a DC return to ground too.

So my schematic is wrong? Sorry, but I don't correctly understand (please, also consider the fact that English is not my main language: I'm very sorry).

the only way to do what you want with a 12v supply is to find hyperabrupt diodes that change their capacitance over the full range at a much lower voltage.

Yes: in facts I was looking for some hyperabrupt varactor which could be driven with a low voltage: I was only able to find BB208 (in SMD format), which, connected in a "back to back" series, would give me a range of 2 - 12 pF, with a sawtooth signal of 0 - 8 V. But 2 - 12 pF isn't enough for me. Also consider the fact that I'm not able to solder SMD components, so the BB229, which is in a PTH format (and with a better capacitance range) would be the better choice: but choosing the BB229 I need to increment the sawtooth signal.

Otherwise your circuit will need a switching regulator to boost the voltage

From what I know, this way is a kind of waste for the battery, am I wrong?
But I can give it a try: do you have a schematic which I can connect to my circuit?

Thank you very much.
 

the only way to do what you want with a 12v supply is to find hyperabrupt diodes that change their capacitance over the full range at a much lower voltage.

However (since is the first time that I'm dealing with varactor diodes), can you suggest me a varactor that:

1: has a PTH format (so not SMD)
2: Can achieve a low value in pF (about 1 - 2 pF, and I can also consider to connect two varactor in series to get an half value in pF) using a max value of 8 volt
3: at 0 volt can achieve a value that at least is equal or higher than 15 pF.

The fact is that in my town I can only find varactors that work with a voltage higher than 20 volt, but I've seen that on Ebay there is a good choice for these diodes.

Anyway let me know if the solution of using a switching regulator to boost the voltage, would be viable; I can try.

Thank you very much for your gentle attention.
 

Try to build the additional circuit. It will take you about 5 minutes.
Using high voltage (30V) from a linear supply or from a chopper, in both cases you still need an additional circuit to increase the slope amplitude, if use standard varicap diodes which needs high bias voltage.
 

Try to build the additional circuit. It will take you about 5 minutes.
Using high voltage (30V) from a linear supply or from a chopper, in both cases you still need an additional circuit to increase the slope amplitude, if use standard varicap diodes which needs high bias voltage.

Unfortunately I don't have a linear supply which can provide me a voltage of 30V, also considering the fact that the circuit should remain portable.
However I've also find some other varactors (FC54) which can operate to a max voltage of 15V, so (maybe) I just need a voltage doubler circuit (adjustable to reach a max Vout of 15V).

Do you have a schematic for a voltage doubler which could be OK for my circuit?

I thank you very much for your attention.
 

the way your schematic is drawn, you need 0 to minus 8 volts on the varactor diodes.
 

Here is a voltage doubler. A full H-bridge chops 12VDC.
This makes AC which is applied to two stacked capacitors.



The clock pulses can come from a 555 timer IC. Send clock 'A' through an inverter gate to get clock 'B'.

I made the load 1k as a 'guesstimate'.

It is possible to adjust the output voltage somewhere between 12 and 24 V, by changing bias on the transistors, capacitor values, frequency, etc.
 

First of all: many thank to everyone for replies and suggestions!

Well, in the meantime I have done some attempts to simulate some circuit using LTSpice.
The following image is the current waveform of my circuit, without any "booster":

broken link removed

Then I have done a simulation of a voltage booster based on NE555, but now the output waveform is wrong: **broken link removed**

So I've tried to simulate just a transistor on the output, to increase the output gain: (**broken link removed** [1]) but again the output waveform is wrong:

**broken link removed**

The latest circuit (with one NPN transistor: [1]) theoretically, should just increase the gain, leaving unalterated the output waveform. There something wrong with LTSpice or these theories are just wrong?
 
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the way your schematic is drawn, you need 0 to minus 8 volts on the varactor diodes.

af Biff44 say ... your variocap is turning wrong.
a variocap is like other díode it lead current only in one direction. but if you want to use it like a variocap, you need to make it reverse to "pull" the P layer away from the p to n layer so it will give a smaller capasitor in the diode layer. don't look at wikipedia they are wrong in this case but try read here https://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/circuits.php

- - - Updated - - -

what is the purposes to build this ugly circuit ? what frequency range do you want to make ? sin? squire ? sawtooth?
 

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