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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:



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.
 

vfone

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

 

MisterBeppe

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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.
 

biff44

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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
 

MisterBeppe

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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.
 

MisterBeppe

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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.
 

vfone

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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.
 

MisterBeppe

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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.
 

biff44

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

BradtheRad

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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.
 

MisterBeppe

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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":



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


So I've tried to simulate just a transistor on the output, to increase the output gain: (http://s24.postimg.org/726qc2khx/Circuit_Transistor_Boost.png [1]) but again the output waveform is wrong:



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?
 

Ivan-Holm

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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 http://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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