Design Logamp Pulse Monolithic logarithmic Detector amplifiers offer s
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DESIGN FEATURE Pulse Detector
Design Logamp Pulse Monolithic logarithmic Detector amplifiers offer sensitivity, dynamic range, speed needed
detect bursts.
Eamon Nash
Analog Devices, Inc., Woburn St., MS-124, Wilmington, 01887; (781) 937-1239, FAX: (781) 937-1024, e-mail: eamon.nash@analog.com
OGARITHMIC amplifiers (commonly known logamps) useful burst detection measurement their ability detect signals that vary over very large dynamic range. Monolithic logamps capable detecting bursts short frequencies GHz, they detect amplitude variations large dB.1 Logamp burst detectors commonly used applications such radar demodulation amplitude-shift-keying (ASK) signals. This article describes issues that designers must consider when applying logamps these tasks, discusses techniques pitfalls associated with measuring logamp's pulse response time.
understand logamp detects bursts, first necessary understand basics logamp operation. Figure shows simplified block diagram typical logamp. core device cascaded chain linear amplifiers, each with gain typically between simplicity, this example shows chain five amplifier stages, each with gain 10X. small continuous sine wave into first amplifier chain progresses through chain. some stage, becomes that begins clip. this example, clipping limiting) level been +1-VDC peak, occurs output third stage. clipped signal continues through signal chain, maintaining +1-VDC peak ampli-
Logamps successive detection calculate envelope signal. full-wave rectified outputs from detectors summed must filtered before output. corner frequency lowpass filter determines response time logamp change input.
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DESIGN FEATURE Pulse Detector
typically ranges from tude goes. approximately signal output GHz, whereas video each amplifier into bandwidth output filfull-wave rectifier detecter typically ranges from tor, outputs these MHz. table lists rectifiers summed maximum input frequentogether. summed outcies video bandwidths then applied lowfor number Analog pass filter, which removes Devices' logarithmic amplithe ripple summed, fiers. Note that AD640 full-wave-rectified signal. AD641 have (Some logamps have built-in on-chip, lowpass filter, lowpass output filters, require external filtering. whereas others require advantage this external filters.) arrangement that corsummed current increases frequency linearly exponential arbitrarily high frequenincrease input signal, output signal propor- Applying signal logamp whose input frequency This yield rise times tional enve- equal lower than video bandwidth will result When selecting logamp lope input signal. excessive ripple output. This ripple easily response time, When input signal con- eliminated with additional external lowpass filtering. designer must consider tinuous (not pulsed), linear tail output signal caused non-ideal primary application. Figure logamp responds putting (exponential) decay input signal. shows logamp used steady-state voltage. more-detailed explanation designers customarily mini- circuit detect simple signal. operation logamp con- input frequency value this example, presence somewhere between five absence burst conveys tained reference consider what happens times video bandwidth. Howev- digital information. input signal continuous, logamp used detect also used radar applications pulses instead. lower-frequency inputs without where arrival time burst logamp's response time-the time penalty long sufficient external critical parameter takes output change lowpass filtering used. some measured. Although signal detected response change input-is cases, this trivial adding logamp vary over large dominated time constant load capacitor output.3 lowpass output filter. logamp's video bandwidth dynamic range, logamp's output bandwidth this filter commonly should confused with input- amplitude interest. What matreferred video bandwidth. signal bandwidth. input signal ters that detects presence Setting video bandwidth very bandwidth monolithic logamp absence burst. Indeed, high obviously will produce residual output ripple lowLogamps with high video bandwidths frequency input signals. Figrespond quickly bursts example, shows response AD8313 monoMaximun Rise time input Video Dynamic Limiter lithic logamp 10-kHz Device bandwidth bandwidth percent range conformance output input burst. AD8313 work frequencies AD640 (see text) ±1.0 achieves 65-dB dynamic range. Since onAD641 (see text) ±2.0 chip video bandwidth AD8313 approxiAD8306 ±0.4 mately MHz, there excessive output ripple AD8307 ±1.0 response this low-frequency input. This scenario AD8309 ±1.0 demonstrates fact that corner frequency AD8310 ±1.0 lowpass output filter determines logamp's minimum AD8313 2500 ±1.0 input frequency. Logamp
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DESIGN FEATURE Pulse Detector
application shown, logamp's output comparator. comparator's threshold voltage that corresponds logamp input level that slightly above bottom dynamic range. applications such this, standard practice specify response time 10-to90-percent rise time-that time takes signal from percent final value. While this standard does indicate long takes before logamp provides precise reading input amplitude, does give good indication narrow pulse logamp detect. applications where measurement size input signal critical, more appropriate define response time time between onset burst point where logamp's output reaches certain portion final value (0.5 final value commonly used). Figures show results pulse-response measurements AD8314, which optimized detection control transmitted time-division-multiple-access
4(a)
4(b)
Excessive fall time characteristic many logamp response-time measurements. closer examination, however, input burst, which first glance seems have clean decay (a), actually decays quite slowly from logarithmic perspective (b). logamp faithfully measures this signal, which still relatively large from logarithmic perspective.
(TDMA) bursts mobile handsets. logamp operates from dynamic range Figure shows output responses input levels +10, -10, (the +10-dBm input signal shown). immediately apparent that there problem with falling edge logamp's output signal. falling edge this plot long tail that very slow
simple amplitude-shift-keying (ASK) system, logamp converts bursts that vary over large dynamic range into pulses that vary amplitude over very narrow range. comparator used provide constant amplitude output input levels.
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settle, compared with rising edges. However, closer examination, concluded that logamp doing exactly what should doing-detecting signals that vary over very large dynamic range. Looking closely decay input signal Figure seen that does immediately completely turn 300-ns burst. This signal decays level that barely visible linear scale such that oscilloscope. However, domain, signal remains relatively large after burst. course, logamp detects this relatively large signal. Figure shows +10-dBm input signal magnified larger scale. Here, clear that burst persists lower level additional takes some additional time after that settle. result this 100-ns burst extension clearly visible output logamp. Note that this problem visible onset burst. When rising, input signal quickly ramps from zero value that close final value. settling portion rise time entails moving through voltage range that, decibel terms, quite small. example, log(20) log(10) (100) log(90). Clearly, precise control turnoff burst critical measuring fall time logamp's output-more critical than linear amplifiers. Figure shows circuit
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DESIGN FEATURE Pulse Detector
measure response time fast rise-time logamps, pulse generator with picosecond resolution required precisely control turn-on turn-off burst. probes used throughout avoid adding excessive load capacitance, which affect quality rising falling edges.
accurately measuring logamp rise fall times. pulse generator used turn source off. receive sharp falling-edge response from logamp, necessary adjust pulse width pulse generator 100-ps increments. This allows RF-generator control tweaked until turn-off pulse disables burst just makes zero crossing. ability generator react burst-enable signal also critical. Generally, this characteristic generator unknown, determining requires some experimentation. Throughout these signal measurements, field-effect-transistor (FET) probes used because they contribute very little load capacitance. This especially true measurement logamp's output, because even small amounts load capacitance form lowpass filter with logamp's output impedance slow down output edges. now, discussion focused detecting bursts (also called pulses)-AC signals that turn stay short period time, then turn off. Logamps also detect baseband pulses, they optimized this function.
baseband pulse defined signal that changes from level (usually, always, VDC) another level short period, then returns original value. good example might signal from photodiode. general, coupling this pulse required. Since most logamps internally DC-coupled, this fundamentally feasible. How-
ever, there practical constraint when using single-supply logamps with differential inputs. input signal must positioned volts above ground potential proper biasing first stage. Furthermore, source usually singleended, ground-referenced signal, will also necessary provide single-ended-to-differential conversion order correctly drive logamp's differential inputs. Figure shows signal level shifted converted differential form using AD8138 differential amplifier. AD8138's differential outputs then drive AD8310 logamp, which input impedance approximately four 499- resistors differential amplifier's gain unity. output common-mode bias) voltage +2.5 achieved applying +2.5 (from supply-referenced resistive divider) AD8138's VOCM pin. this application, necessary trim circuit's offset voltage. Remember that offset only millivolts dramatically reduce dynamic range domain. Under normal (AC-coupled) operation, AD8310 compensates internal offset voltages. (This another reason coupling normally recommended.) When inputs
Logamps detect DC-coupled baseband pulses, some signal conditioning necessary. pulse must converted differential signal with bias level that around mid-supply. Baseband pulses from detected. pulse width narrow
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DESIGN FEATURE Pulse Detector
coupled, this offset-compensation circuitry must disabled. This performed applying nominal voltage approximately +1.9 AD8310's OFLT pin. Note that this does trim logamp's offset voltage-it merely holds fixed level prevents logamp's offset-compensation circuitry from misinterpreting DC-input signals offsets. AD8138's trim, therefore, compensates both devices' offsets. trim occurs grounding circuit's input slightly varying gain resistor AD8138's inverting input potentiometer used this example) until voltage AD8310's output reaches minimum. After trimming, lower dynamic range limited broadband noise output AD8138, which approximately peakto-peak. Figure shows this circuit responds series 100- pulses having amplitudes VDC. circuit detect pulses narrow excessive noise output signal before after pulse signal-generator noise.
References
THROUGHOUT THESE SIGNAL MEASUREMENTS, FIELD-EFFECTTRANSISTOR (FET) PROBES USED BECAUSE THEY CONTRIBUTE VERY LITTLE LOAD CAPACITANCE. THIS ESPECIALLY TRUE MEASUREMENT LOGAMP'S OUTPUT, BECAUSE EVEN SMALL AMOUNTS LOAD CAPACITANCE FORM LOWPASS FILTER.
Measurement System with Dynamic Range. AD8307 datasheet, Available http://www. analog.com Eamon Nash, "Ask Applications Engineer Logarithmic Amplifiers Explained," Analog Dialogue, Volume Number March 1999. Available http:// www.analog.com/logamps/. AD8307, Operation Frequencies. AD8307 Datasheet, Available http://www.analog.com/ logamps.
DC-coupled logamp circuit Fig. provides constant output step sizes constant ratio changes input. output signals represent response 100- input pulses amplitude VDC.
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