Rf Oscillator and Its Neeeds

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RF OSCILLATOR AND ITS NEEEDS:

Any communication system that transmits and receives data on a high frequency carrier needs to be able

to generate the carrier, usually using an oscillator. We need to be able to design an oscillator that can

produce a high-quality signal at a pre-determined frequency with little noise and low harmonics. In real

receivers, the oscillator is phase and frequency locked to the incoming carrier using a carrier recoverycircuit

OSCILLATOR CHARACTERISTICS:

An oscillator is a device that generates a high frequency signal with only a DC signal input. It begins

oscillating due to amplification of noise or a transient signal. Oscillators consist of two major blocks:

An active, nonlinear element that provides gain

A frequency selective circuit (e.g. LC tank, crystal oscillator, dielectric resonator)

We design oscillators with criteria such as:

frequency stability over bias and temperature variations

spectral purity (low power in harmonics, and low phase noise)

power consumption

area consumption

Phase noise and its effect in a communication systemOscillators produce generally produce fairly sinusoidal signals, but noise inside the oscillator (1/f noise,

thermal noise, shot noise, etc.) leads to variations in the generated signal. In the time domain this leads to

"jitter" at the fundamental frequency, i.e. the period of the signal differs very slightly from one time to

another. In the frequency domain, this is called phase noise, and leads to a rapidly decreasing power

around the desired frequency. ELEC-853 (Silicon Integrated Circuits) has more information

aboutoscillator phase noise.

This phase noise has a significant effect on the feedthrough of interfering signals. If an undesired signal is

closely spaced in frequency to the desired signal, both signals will be mixed by the oscillator signal. Since

the oscillator has a finite power at small frequency offsets from the central frequency, it will result in a

"spreading" of both the desired signal and the interfering signal. This will lead to the interfering signal

overlapping the desired signal. If the interfering signal is larger than the desired, this can cause significant

problems. For this reason, oscillators with low phase noise are required for communication systems.

Feedback OscillatorsThe simplest way to create an oscillator is to use an explicit frequency-selective feedback network with a

gain element (transistor). This is often an LC tank, as we will examine here, butdielectric resonatorsor

transmission lines can also be used. These last two offer lower noise but are physically larger. Feedback

oscillators are becoming more popular at very high frequencies ( > 50 GHz).

A amplifier with appropriate feedback can be designed as an oscillator. For the system on the right the

output voltage is
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Vo() =AVi() +H()AVo

or rearranging to solve for Vo

We will have a non-zero output (i.e. oscillaton) with a zero input when 1 AH() = 0

PREAMPLIFIER

A preamplifier (preamp) is anelectronic amplifierthat prepares a small electricalsignalfor further

amplification or processing. A preamplifier is often placed close to thesensorto reduce the effects

ofnoiseandinterference. It is used to boost the signal strength to drive the cable to the main instrument

without significantly degrading thesignal-to-noise ratio(SNR). The noise performance of a preamplifier is

critical; according toFriis's formula, when the gain of the preamplifier is high, the SNR of the final signal is

determined by the SNR of the input signal and thenoise figureof the preamplifier.

In a homeaudio system, the term 'preamplifier' may sometimes be used to describe equipment which

merelyswitchesbetween differentline levelsources and applies avolume control, so that no actual

amplification may be involved. In an audio system, the second amplifier is typically apower

amplifier(power amp). The preamplifier providesvoltagegain (e.g. from 10 millivolts to 1 volt) but no

significantcurrentgain. The power amplifier provides the higher current necessary to driveloudspeakers.

Preamplifiers may be:

incorporated into the housing or chassis of the amplifier they feed

in a separate housing mounted within or near the signal source, such as a turntable, microphone ormusical instrument.

Examples

The integrated preamplifier in a foilelectretmicrophone.

The first stages of aninstrument amplifier.

A stand-alone unit for use inlive musicandrecording studioapplications.

As part of a stand-alonechannel stripor channel strip built into an audiomixing desk.

Amastheadamplifier used withtelevisionreceiverantennaor asatellitereceiverdish. The circuit inside of a hard drive connected to the magnetic heads or the circuit inside of CD/DVD

drive which connects to thephotodiodes.

Aswitched capacitorcircuit used to null the effects of mismatch offset in mostCMOScomparator-

based flashanalog-to-digital converters

Matching the Preamplifier to the Detector and the Application
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The primary function of a preamplifier is to extract the signal from the detector withoutsignificantly degrading the intrinsic signal-tonoiseratio. Therefore, the preamplifier is located as close as possible to the detector, and the inputcircuits are designed to match thecharacteristics of the detector. Different pulse processing techniques are typically employed,depending on whether the arrival time or

the amplitude (energy) of the detected event must be measured. Pulse shaping for eitherapplication is normally implemented in asubsequent module. This module can be located at some distance from the preamplifier,provided that the signal fidelity is notdegraded due to the length of the interconnecting coaxial cable.

Preamplifier Types

Three basic types of preamplifiers are available: the current-sensitive preamplifier, the parasitic-capacitance preamplifier, and thecharge-sensitive preamplifier. The following paragraphsdescribe their functions and primary performance characteristics.

Current-Sensitive Preamplifiers

Several detector types, such as photomultiplier tubes and microchannel plates, generate amoderately large and fast-rising output signal through a high output impedance. Pulseprocessing for timing or counting with these detectors can be rather simple. Aproperlyterminated

50-coaxial cable is attached to the detector output, so that the current pulse from the detector

develops the desired voltage pulse across the 50-load presented by the cable. For scintillatorsmounted on 14-stage photomultiplier tubes, this voltage signal is usually large enough to drivethe input of a fast discriminator without further amplification. For single-photon counting, 10-stage photomultiplier tubes, or microchannel plate PMTs, additional amplification is neededbetween the detector and the discriminator, and this is the function of the current-sensitive

preamplifier.

Charge-Sensitive PreamplifiersThese preamplifiers are preferred for most energy spectroscopy applications. The signal from asemiconductor detector or ion chamber is a quantity of charge delivered as a current pulselasting from 109 to 105 s, depending on the type of detector and its size. For mostapplications the parameters of interest are the quantity of charge and/or the time of occurrenceof an event. A chargesensitive preamplifier (Fig. 3) can deliver either or both. Because itintegrates the charge on the feedback capacitor, its gain is not sensitive to a change in detectorcapacitance, and in the ideal case, the rise time of the output pulse is equal to the detectorcurrent pulse width.

Parasitic-Capacitance PreamplifiersPhotomultiplier tubes, electron multipliers, microchannel plates, and microchannel plate PMTs produce moderatelylarge output signals with very fast rise times. Parasitic-capacitance preamplifiers have a high input impedance

(~5 M). Hence, the current pulse generated by the detector is integrated on the combined parasitic capacitancepresent at the detector output and the preamplifier input. This combined capacitance is typically 10 to 50 pF. Theresulting signal is a voltage pulse having an amplitude proportional to the total charge in the detector pulse, and a risetime equal to the duration of the detector current pulse. A resistor connected in parallel with the input capacitancecauses an exponential decay of the pulse with a time constant ~50 s. An amplifier having a high input impedance

and unity gain is included as a buffer to drive the low impedance of a coaxial cable at the output. The 93-resistor inseries with the output absorbs reflected pulses in long cables by terminating the cable in its characteristic impedance.


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Rf Oscillator and Its Neeeds