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Automatic Gain Control ( AGC )
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This project is a digital Automatic Gain Control (AGC”) system using a PIC16F876 MCU. The ability to set the gain level in a circuit and have it control itself is a very useful function. This circuit is a building block of another project I am working on. A 30W power amp for either the PCS, iDEN or CDMA frequency bands. I will settle on one of those frequencies sometime soon. I needed to control the gain so the signals for a digital downconverter (DDC) and digital upconverter (DUC) would be in a specific range so the A/D that processes the IF of the mixers does not get over-driven by the amplitude of the input signals. Using a digital attenuator, logarithmic detector / controller, and a MCU with an A/D converter I was able to accomplish this task fairly easy with minimal components...
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This Design Idea shows how you can use a frequency-to-voltage converter and a DDS (direct-digital-synthesizer) chip for precise digital-to-analog conversion. The DDS chip generates a precision frequency proportional to its digital input. This frequency serves as the input to a voltage-to-frequency converter, thereby generating an 18-bit analog voltage proportional to the original digital input. Figure 1 shows how the AD650 is configured for frequency-to-voltage conversion...
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The high-accuracy frequency-to-voltage converter (FVC) in Fig 1 demonstrates how a synchronous, charge-balance, voltage-to-frequency converter (VFC) can function as a single-supply FVC given proper biasing and level shifting. This FVC can maintain a monotonic 0.01% linearity error over a 60-dB range of 9.7 kHz to 9.7 MHz; it operates from 12 to 36V power supplies. You can modify the circuits prescaler to adapt the circuit to considerably higher frequency ranges...
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This PLL can operate over a wide frequency range, not just
1 or 2 octaves but over 1 or 2 or 3 decades. It naturally
provides a voltage output which responds quickly to frequency
changes, yet does not have any inherent ripple.
Thus, it can be used as a frequency-to-voltage (F-to-V)
converter..
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This circuit is designed to detect the narrow 1uS pulses produced by the above amplifier circuit. The clean logic type pulses produced by the discriminator are then sent to a frequency to voltage converter. The circuit is designed to process a pulse frequency of 10KHz that is frequency modulated by voice audio signals...
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The circuit in Figure 1 converts pulse information to a clean dc voltage by the end of a single incoming pulse. In another technique, an RC filter can convert a PWM signal to an averaged dc voltage, but this method is slow in responding. Converting low-duty-cycle pulse information is slower yet. The circuit in Figure 1 uses two low-input-bias-current LT1880 op amps, IC2 and IC3, and an LTC202 quad analog switch,..
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The improved circuit in Figure 2 makes an end-run around
these compromises. A low-cost sample-and-hold circuit
such as LF398 can sample the F-to-V`s output at the peak
of its ripple, and hold it until the next cycle. The LF398 has
fairly low output ripple (rms) but it does have some short
duration noise spikes and glitches which can be removed
easily with a simple output filter...
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Frequency to voltage converters are available in a number
of forms from a number of sources, but invariably require
significant additional components before they can be put to
use in a given situation. The LM2907, LM2917 series of
devices was developed to overcome these objections. Both
input and output interface circuitry is included on chip so
that a minimum number of additional components is required
to complete the function...
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Simplify your F/V converter designs with versatile V/F ICs.
Starting with a basic converter circuit, you can modify it to
meet almost any application requirement. You can spare
yourself some hard labor when designing frequency-tovoltage
(F/V) converters by using a voltage-to-frequency IC
in your designs. These ICs form the basis of a series of
accurate, yet economical, F/V converters suiting a variety of
applications...
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The AD650 is a versatile monolithic voltage-tofrequency
converter (VFC) that utilizes a chargebalanced
architecture to obtain high performance in
many applications. Like other charge-balanced VFCs,
the AD650 can be used in a reverse mode as a
frequency-to-voltage (F/V) converter. This application
note discusses the F/V architecture and operation,
component selection, a design example, and the
fundamental trade-off between output ripple and circuit
response time...
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The circuit in Figure 1 is a simple, low-cost voltage-to-time converter using the ubiquitous 555 timer chip. You can use the IC`s monostable multivibrator as a voltage-to-time converter by connecting the analog-voltage input to the charging resistor, R, instead of connecting R to VCC. With this modification, the timer chip`s output-timing cycle, tP, is proportional to the input voltage, VIN...
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The circuit in Figure 1 stems from a radio-controlled modeling application, which requires a voltage proportional to the width of the incoming servo pulses. The circuit is optimized for a positive-going pulse width of 1 to 2 msec, repeating at intervals of approximately 17 msec. The output produces a voltage of 0.95V for a 1-msec pulse to 2.25V for a 2-msec pulse. The circuit operates similarly to a PLL, but it locks onto the pulse width, rather than to the frequency, of the incoming signal...
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