# How to make a variable frequency circuit

Anybody can explain how do I make my circuit for variable frequency output with variable resistor not trimmer/variable capacitor my circuit shown in fig

• Your question makes no sense - there is no variable frequency being generated. This is not an oscillator. Your choice of labels - there are no mosfets or ICs or oscillators here. Jun 27, 2017 at 17:01

If you replace C4 with back to back varicap diodes you will get some control of the filter frequency. A BB171 has about a 20:1 control range from about 5 pF to about 80 pF but don't quote me on this. The centre point of the two varicap diodes is used for dc control and can be connected to a pot via a 100 kohm resistor. There are plenty of google pictures of examples.

A 20:1 range in capacitance produces a bit more than a 4:1 range in resonant frequency tuning. It's a square root thing. One thing to watch out for of course is the change in tank Q is capacitance changes across the range. This may or may not be a problem to you.

The output of your circuit is also very susceptible to loading impedance so watch out for this two and maybe consider using an emitter follower stage after the tank.

• When i connect 10nF to transistor (Tip 31) the output not showing even Transistor stopped working but when i disconnect the signal working why ? Jun 29, 2017 at 11:40
• I have no idea what you mean - there isn't a TIP31 in your posted circuit and what connection did you make anyway with the 10 nF capacitor? Jun 29, 2017 at 12:29
• I wnat to drive a coil for RF wave induction @ higher Frequencies above 10Mhz so i m trying to drive NPN transistor " i want to make this type of circuit : youtu.be/Ddd_GFf8yzc " Jun 29, 2017 at 12:36
• In this circuit capacitor C1 one pin is connected to Inductor and another is connected to TIP31 " Not shown in diagram " Jun 29, 2017 at 12:38
• A TIP31 will never work at 10 MHz - read the data sheet - current gain falls to is unity at 3 MHz and above that you have no gain at all. I recommend you open a new question for this and post a proper circuit and not a video (that didn't work anyway!). Jun 29, 2017 at 13:16

This is called a VCO or resistor tuned oscillator. The easiest are Realaxation OScillators with CMOS Schmitt Trigger, feedback pot (1M) and input shunt cap. For sine waves, A VCO is used with linear output or a BPF to make it a sine wave.

Define inputs and outputs for f , harmonics and voltage first. then look for hundreds of suitable solutions that exist for VCO, RF Tx etc. Varicaps were once common for FM and used in VCXO's as well VC tuneable oscillators. Define your sensitivity in Hz/V or KHz/V or MHz/V or kHz/Ohm in a relaxation oscillator with a fixed R and variable R as a % of total R or Voltage controlled Varicap for fine tuning.

Basically you must DEFINE SPECS before starting ANY design.

But VCO's come in a wide range of solutions from very unstable but sensitive to wide tuining range, or very stable and precice control for small FM or PM modulation using VCXO.

• I had use CD4047 as an oscillator at 1MHZ and there is no feedback pinout Jun 27, 2017 at 17:15
• CD4047 is a Schmitt trigger with R external feedback used as an astable Osc. just like a simple 4069 Schmiitt inverter but different. The R voltage pull up can be used to adjust f or one shot time on the 4047 as it uses comparators with hysteresis, whereas the CD4069 or equiv uses feedback R with hysteresis at 1/3 of Vdd for high impedance CMOS thresholds. same results but 4047 is voltage tuneable with V(R) pullup over a limitd range ?? 2.5 to 5V? I forget Best bet is a CD 4046 PLL with a VCO built in. logic out. Jun 27, 2017 at 17:21
• until your define specs that is !!!!! Otherwise answers go in circles. Now get 'er dun Jun 27, 2017 at 17:27