Voltage to Frequency

Simple DC to 20 MHz Driver

The output cable from my 20 MHz function/sweep generator dangled over the side of the workbench, the alligator clip hovering over the floor. Deeply engrossed in a project, I moved the power strip on the floor a little closer so I could plug in the circuit upon which I was working. That's when the alligator clip contacted the 240 VAC. The function/sweep generator had been gravely injured. Poking around with a scope probe, the signal output was down in the noise, but it was there, and the other outputs worked. The expensive, rare, and discontinued MAX038 function generator chip was still working. Upon opening the case, I found that the Ztex ZXFY202N8 wide band opamp had been destroyed and the output termination resistor burnt and broken. ..

This circuit was designed to frequency modulate a 40KHz carrier, using human voice frequencies. A common flip/flop is used at the core of the circuit...

The basic VFC (voltage-to-frequency converter) in Figure 1 comprises an integrator (IC1) and a Schmitt-trigger circuit (IC2). The integrator converts the dc input voltage, VIN, to a linear voltage ramp, and the Schmitt trigger sets the limits of the integrator`s output voltage. Feedback around both circuits provides the condition for oscillation. The DPP (digitally programmable potentiometer) in Figure 2 adds programmable limits to the Schmitt trigger and adds two powerful features to the VFC...

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The circuit`s simplicity, unconditional stability, and consistent high-drive capability over an octave make the oscillator ideal for many applications, such as synthesized sources, local oscillators, and transmitters. The AHC logic family (Texas Instruments, www.ti.com) makes the circuit`s performance possible. AHC is a relatively new line of CMOS logic whose high speeds and good noise performance allow oscillator operation into regions in which bipolar-junction-transistor and FET designs prevail...

When you develop the prototype of a circuit using capacitors or when you replace an old, defective capacitor with a new one, you often need to know the value of the capacitor you use or replace. At times, the values printed on the capacitors are no longer readable. Also, the wide tolerance band of the capacitors can leave you making a wild guess about the capacitor value...

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Many portable RF products use VCOs to generate the RF carrier frequency. These applications often require low-noise VCO power-supply voltages that are higher than the primary battery voltage. Many designs use a dc/dc converter powering a low-noise linear regulator. This approach has several inherent disadvantages. The dc/dc converter produces noise that the regulator may not reject, resulting in regulator-output noise far greater than the thermal-noise levels...

The main purpose for building the circuit in Figure 1 is to study the idiosyncrasies of X5R, Z5U, and Y5V multilayer ceramic capacitors. The circuit is also an inexpensive VCO (voltage-controlled oscillator) with only five components. Many types of ceramic capacitors for surface-mount placement are on the market...

This is a 0 - 6 MHz DDS VFO controlled by a PIC16F84 (or `C84). The VFO is separated into two modules, the DDS module and the controller module. The PCB layout (double sided) for the DDS module is included as two Postscript files (scale 1:1). The .PS files can be copied to a Postscript capable printer such as HP LaserJet 4MP. Please note that the layout is mirrored to get the toner side of the film toward the PCB surface...

A VCO (voltage-controlled oscillator) is an analog circuit, so you cannot find it in the libraries for the design of digital programmable chips. When you need such a circuit for synchronization or clock multiplication, you need to find a circuit that works with the standard digital functions, such as AND and NAND. Several possibilities exist for building variable-frequency oscillators. For example, you can change the frequency using a varactor diode. Unfortunately, these diodes have a small change of frequency per volt...

This circuit is used to convert a mono audio signal into a stereo signal that can be panned between the left and right channel by a 0-10V control signal, it is intended for analog synthesizer systems. The circuit is mainly here for historical reasons, a higher quality panner may be built with the National Semiconductor LM13700 dual transconductance amplifier set up as a pair of voltage controlled amplifiers...

The circuit in Fig 1 generates a 50-Hz to 1-kHz sinusoidal waveform that exhibits lower than 60-dB THD. The input voltage controls the output frequency with the relationship 1 kHz/V. The output amplitude is invariant with frequency over the entire operating range. IC1 is a V/F converter with an input control range of 0 to 1V. You adjust R5 so that the output frequency equals 1 kHz with a 1V input...

The circuit in Figure 1 is a quadrature-output VCO that provides both positive and negative output frequencies, depending on the polarity of the control-voltage input. The circuit provides a function that designers traditionally implement in analog music-effects units, such as Bode/Moog frequency shifters. Bode/Moog shifters use fixed-beat-frequency oscillators at 20 kHz and variable sine oscillators that go higher and lower than 20 kHz. Both oscillators feed into mixers...

The circuit in Figure 1 illustrates a narrowband VCO, which has a digitally programmable center-frequency ratio of greater than 2-to-1. With a VCO having center-frequency ratio (F HIGH/FLOW) of greater than 2-to-1, you can derive any output frequency by simple binary division. The VCO has extremely high spectral purity, and typically operates at frequencies greater than 230 MHz by using high-performance ECL. Also, you can enable and disable the VCO almost instantaneously...