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Brookdeal 128A Amplifier
Manufacturer:
Model:
128A
Date:
1983
Class:
Type:
SubType:
Lock-In Amplifier

Information

INTRODUCTION The Model 128A Lock-In Amplifier enables the accurate measurement of signals contaminated by broad band noise, power line pickup, frequency drift, or other sources of interference. It does this by means of an extremely narrow band detector which has the center of its passband locked to the frequency of the signal to be measured. Because of the frequency lock and narrow bandwidth, large improvements in signal-to-noise ratio can be achieved, allowing the signal of interest to be accurately measured, even in situations where it is completely masked by noise. Signals applied to the input (single-ended or differential) i routed through a series of amplifiers which allow .ull-scale sensitivity ranges down to one microvolt. Switch selectable low-pass and high-pass filters allow considerable noise reduction ahead of the phase-sensitive detector. This pre-detector noise reduction can be further enhanced by making use of the optional Plug-In (internal) selective amplifier. At the phase sensitive detector, the signal is compared with the reference signal derived from the experiment. Only those signal components which are synchronous with the reference yield a net dc detector output. Noise and other non-synchronous signals do not contribute a net dc output, but only ac fluctuations which can be reduced to any arbitrary value according to the amount of filtering selected with the Time Constant switch. This switch allows time constants as large as 100 seconds to be selected, with provision for achieving larger externally determined time constants if necessary. Post-detector dc amplifiers drive the panel meter and signal output connectors. Other features include provision for calibrated zero suppression of up to 10 x full scale, a two-position dc prefilter, and the capability of driving the reference input the detector at double the frequency of the signal applied to the Reference Input connector to facilitate second harmonic measurements. An optional Plug-In Oscillator (internal) is available for use in applications where the experiment does not produce a reference signal itself, but is capable of being driven by a signal furnished by the Lock-In Amplifier. With its wide range of capabilities and ease of operation, the Model 128A Lock-In Amplifier should find extensive application in situations where the accurate measurement of signals is complicated by the presence of noise and interference. 2.2 SPECIFICATIONS SIGNAL CHANNEL (1) INPUT TYPE Single-ended or differential as selected by front-panel switch. (2) INPUT IMPEDANCE 100 Mi2 shunted by no more than 20 pF. (3) SENSITIVITY 12 full-scale ranges in 1-2.5-10 sequence from 1 jjV to 250 mV. (4) FREQUENCY RANGE 0.5 Hz to 100 kHz. (5) COMMON MODE REJECTION At least 100 dB at 1 kHz. (6) MAXIMUM COMMON MODE VOLTAGE 3 V pk-pk to 20 kHz; then —6 dB/octave above 20 kHz. (7) DETECTOR BIAS Internal network allows dc bias current of either polarity to be provided at the "A" Input to facilitate operation with diode detectors which require biasing. (See page VII-3 and Parts Location Diagram on page VII-2.) (8) NOISE At 1 kHz the signal channel noise will not exceed 10 nV/Hz1/a. (9) LOW PASS Filter Switch selectable 6 dB/octave low-pass Filter which can be set to 3 dB down frequencies of 100 Hz, 10 kHz, or MAX (greater than 100 kHz). (10) HIGH PASS Filter Switch selectable 6 dB/octave high-pass Filter which can be set to 3 dB down frequencies of 50 Hz, 5 Hz, or MIN (below 0.5 Hz). (11) OVERLOAD DETECT Front-panel indicator lights if applied signal plus noise is large enough to cause overload at any of several critical overload monitor points. (12) GAIN STABILITY 0.1%/°C. (13) GAIN LINEARITY 0.05%. (14) OVERALL GAIN ACCURACY ±2%. REFERENCE CHANNEL The Model 128A reference channel automatically locks onto and tracks an applied reference signal over the entire operating frequency range of the instrument. As a result, the instrument is immune to frequency and phase shifts as long as the reference and signal to be recovered change together. (1) TRACKING RANGE 5 Hz to 100 kHz (FAST) or 0.5 Hz to 100 kHz (SLOW) as determined by the setting of two internal switches. Faster lock-on time and slewing rate obtained with switches set to FAST make this range preferable except when operating below 5 Hz. (2) MODES Either of two modes, f and 2f, can be selected by means of a front-panel switch. In the "f" position, the phase-sensitive detector is driven at the same frequency as the applied reference signal. In the "2f" position, the phase-sensitive detector is driven at twice the frequency of the applied reference signal to facilitate second harmonic measurements. (3) INPUT IMPEDANCE 10 Mf2 shunted by no more than 20 pF. (4) MINIMUM REFERENCE SIGNAL REQUIREMENT 100 mV pk-pk, any waveshape crossing its mean only twice each cycle. Minimum time required on either side of the mean is 100 ns. Amplitude excursions must be at least 50 mV on each side of the mean. Maximum input signal is 5 V (pk-to-mean). Best phase accuracy is obtained with a 1 V rms sinewave. (5) LOCK-ON TIME A function of internal switch setting as follows. Selected Range Lock-On Time SLOW (0.5 Hz to 100 kHz)............. 20 sec. per octave FAST (5 Hz to 100 kHz)...................2 sec. per octave (6) PHASE Calibrated Phase controls allow the phase of the reference drive to the Phase-Sensitive Detector to be set at any angle relative to the input signal. The controls consist of a Phase Dial with a range of 100° and a Phase Quadrant switch which provides incremental phase shifts of 90°. The phase shift accuracy of the dial is better than 0.2° over the entire frequency range. The resolution of the dial is better than 0.1°. The incremental phase shifts provided by the Quadrant switch are accurate to 0.2°. The overall phase accuracy of the instrument, including shifts in both the reference and signal channels, is typically better than 5°. (7) DETECTOR BIAS Internal network allows dc bias current of either polarity to be provided at the REF. IN connector to facilitate operation in situations where the reference signal is taken from diodes requiring biasing. (See page VII-6 and Parts Location Diagram on page VII-5.) PHASE SENSITIVE DETECTOR, DC Amplifier (1) OUTPUT DRIFT 0.1%/ °C. (2) OVERLOAD CAPABILITY 1000 times full scale up to a maximum at the input of 650 mV rms. Overload capability is defined as the ratio, at the input of the Model 128A of the maximum pk-pk non coherent signal which can be applied without overloading the Model 128A to the pk-pk coherent signal required to yield full scale Model 128A output. Note that, expressed as the ratio of the pk-pk non-coherent signal to the rms value of the coherent signal required for full-scale output, this number can be as great as 2800. Maximum acceptable signal is a 650 mV rms sinewave. (3) NON COHERENT REJECTION 50 ppm maximum. Non-coherent rejection is defined as that offset which results from applying a non-coherent signal having a pk-pk amplitude 1000 times the pk-pk amplitude of the coherent signal required to obtain full-scale output. Example: With a non coherent signal applied having a pk-pk amplitude 1000 times the pk-pk coherent signal required to obtain full-scale output, there will occur an offset at the output caused by the non coherent input signal. The amplitude of this offset will be no greater than: 50 x 10“6 x 1000 = 50 x 10~3 of f.s. output = 50 mV (f.s. = 1 V) (4) TIME CONSTANT Front-panel switch allows selection of 6 dB/octave Filter time constants of 1 ms, 10 ms, and .1 s through 100 s in 1-3-10 sequence. Also MIN (time constant =0.7 ms) and EXT, which allows time constants longer than 100 s to be achieved by means of external

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Service and User Manual
Manual type:
Service and User Manual
Pages:
60
Size:
7.22 Mbytes (7574415 Bytes)
Language:
english
Revision:
Manual-ID:
Date:
1983 01 01
Quality:
Scanned document, all readable.
Upload date:
2017 05 26
MD5:
94fcf4d892471ab71cdb66f058c9c225
Downloads:
570

Information

CONDENSED OPERATING INSTRUCTIONS ... 1-1 II CHARACTERISTICS ... 11-1 2.1 Introduction ... 11-1 2.2 Specifications ... 11-1 III INITIAL CHECKS ... 111-1 3.1 Introduction ... 111-1 3.2 Equipment Needed ... 111-1 3.3 Procedure ... 111-1 IV OPERATING INSTRUCTIONS ... IV 1 4.1 Introduction ... IV-1 4.2 Preliminary Considerations ... IV-1 4.2A Power Requirements ... IV-1 4.2B Fusing ... IV-1 4.2C Warm-Up Period ... IV-1 4.2D Operating Frequency ... IV-1 4.2E Grounding ... IV-1 4.2F Noise ... IV-3 4.3 Operating the Model 128A ... IV-5 4.3A Introduction ... IV-5 4.3B Reference Channel ... IV-5 4.3C Signal Channel ... IV-6 4.3D Output Channel Controls ... IV-8 4.4 Mixer Function and Harmonic Sensitivity ... IV-9 4.5 Interface Connector ... IV-10 4.6 Battery Operation ... IV-10 4.7 Operation with the Internal Reference Oscillator ... IV-10 4.7A Introduction ... IV-10 4.7B Operation ... IV-10 4.7C Installation ... IV-11 4.8 Operation with the Internal Tuned Amplifier ... IV-12 4.8A Introduction ... IV-12 4.8B Operation ... IV-13 4.8C Installation ... IV-15 4.9 More Reference Channel Operating Hints ... IV-15 4.9A Reference Channel Slewing Rate ... IV-15 4.9B Phase Errors with Small Reference Signals ... IV-15 V ALIGNMENT ... V-1 5.1 Introduction ... V-1 5.2 Required Equipment ... V-1 5.3 Preliminary Steps ... V-1 5.4 Procedure ... V-1 5.4A +15 V Adjust (R310),-15 V Check, and+5 V Check ... V-1 5.4B Reference Board Adjustments ... V-1 5.4C Signal Board Adjustment ... V-3 5.4D Mixer Adjustments ... V-3 5.4 E Other Adjustments ... V-4 VI TROUBLESHOOTING ... VI-1 6.1 Introduction ... VI-1 6.2 Equipment Required ... VI-1 6.3 Initial Steps ... VI-1 6.4 Power Supply Checks ... VI-1 6.5 Reference Checks ... VI-1 6.6 Signal Channel Amplifiers ... VI-2 6.6A Preamplifier ... VI-2 6.6B Intermediate AC Amplifiers ... VI-2 6.6C Final AC Amplifier ... VI-2 6.7 Mixer ... VI-2 6.8 DC Amplifiers ... VI-2 VII SCHEMATICS, Table of ... VII-1

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