10.58 to 10.74 MHz VFO Oscillator Circuit

Figure 1, below, shows the 10.58 to 10.74 MHz VFO oscillator circuit. Yes, this is the same circuit that was presented last time, but I’ve re-drawn it to show the Colpitts oscillator more clearly. “L” is approximately 1.5 uH; 19 turns in a T50-6 core (yellow). The value of C6 is found experimentally. I used 69 pF (a 47 pF and a 22 pF in parallel). More about this during check-out.

10.58 to 10.74 MHz VFO Oscillator Circuit

“L” and the V V C along with C6 make up the parallel-tuned circuit that determines the frequency of the VFO. The V V C is an MV2104. Capacitors marked with an asterisk must be silvered mica, COG, or NPO.

You might wonder how the component values for the resonant circuit are determined. Well, when Jupiter is aligned with Mars, and Mercury is on the cusp of a new moon, if the inductive reactance equals the capacitive reactance, then we have a resonant circuit. For any given frequency there are endless combinations of L and C that will produce a resonant circuit. Experience shows that variable capacitors and V V C’s in the 5 pF to 300 pF range are altogether practical. Combine this with the fact that a 1.5 uH coil and a 150 pF capacitor resonate near the frequency of interest, and you have the component values for this VFO’s resonant circuit. You can find formulas for figuring all this out in your Handbook, if you like to do arithmetic. The component values actually used in the circuit take into account “stray” inductance and capacitance that occurs in the real world. More about this during check-out.

RESONANT CIRCUITS are fascinating critters! While I’m NOT going to get into a theoretical discussion, there are some fundamental characteristics of resonant circuits that you (and I) should keep in mind when building VFO’s, filters, RF amplifiers, etc.

Two types of resonant circuits are commonly used: series resonant circuits and parallel resonant circuits, as shown below. Here are two important things to keep in mind: PARALLEL resonant circuits attenuate the flow of AC current at the resonant frequency (and pass all other frequencies); SERIES resonant circuits pass AC current at the resonant frequency (and attenuate all other frequencies).

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Flashy Christmas Lights

This simple and inexpensive circuit built around a popular CMOS hex inverter IC CD4069UB offers four sequential switching outputs that may be used to control 200 LEDs (50 LEDs per channel), driven directly from mains supply. Input supply of 230V AC is rectified by the bridge rectifiers D1 to D4. After fullwave rectification, the average output voltage of about 6 volts is obtained across the filter comprising capacitor C1 and resistor R5. This supply energises IC CD4069UB.

All gates (N1-N6) of the inverter have been utilised here. Gates N1 to N4 have been used to control four high voltage transistors T1 to T4 (2N3440 or 2N3439) which in turn drive four channels of 50 LEDs each through current limiting resistors of 10-kilo-o Base drive of transistors can be adjusted with the help of 10-kilo-ohm pots provided in their paths. Remaining two gates (N5 and N6) form a low frequency oscillator. The frequency of this oscillator can be changed through pot VR1. When pot VR1 is adjusted To get the best results, a low leakage, good quality capacitor must be used for the timing capacitor C2

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Sawtooth Wave generator Circuit Diagram

Sawtooth wave generators using opamp are very common. But the disadvantage is that it requires a bipolar power supply.

A sawtooth wave generator can be built using a simple 555 timer IC and a transistor as shown in the circuit diagram.

The working of the circuit can be explained as follows:
The part of the circuit consisting of the capacitor C, transistor,zener diode and the resistors form a constant current source to charge the capacitor. Initially assume the capacitor is fully discharged. The voltage across it is zero and hence the internal comparators inside the 555 connected to pin 2 causes the 555's output to go high and the internal transistor of 555 shorting the capacitor C to ground opens and the capacitor starts charging to the supply voltage. As it charges, when its voltage increases above 2/3rd the supply voltage, the 555's output goes low, and shorts the C to ground, thus discharging it. Again the 555's output goes high when the voltage across C decreases below 1/3rd supply. Hence the capacitor charges and discharges between 2/3rd and 1/3rd supply.

Note that the output is taken across the capacitor. The 1N4001 diode makes the voltage across the capacitor go to ground level (almost).

The frequency of the circuit is given by:

f = (Vcc-2.7)/(R*C*Vpp)

where:

Vcc= Supply voltage.
Vpp= Peak to peak voltage of the output required.

Choose proper R,C,Vpp and Vcc values to get the required 'f' value.

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Simple Variable Frequency Oscillator Circuit Diagram

This is a very simple circuit utilising a 555 timer IC to generate square wave of frequency that can be adjusted by a potentiometer.

With values given the frequency can be adjusted from a few Hz to several Khz.
To get very low frequencies replace the 0.01uF capacitor with a higher value.

The formula to calculate the frequency is given by:

1/f = 0.69 * C * ( R1 + 2*R2)

The duty cycle is given by:

% duty cycle = 100*(R1+R2)/(R1+ 2*R2)

In order to ensure a 50% (approx.) duty ratio, R1 should be very small when compared to R2. But R1 should be no smaller than 1K.
A good choice would be, R1 in kilohms and R2 in megaohms. You can then select C to fix the range of frequencies.

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