672 Spectroscopy Amplifier

The Model 672 input accepts either positive or negative polarity signals from a radiation detector preamplifier and provides a positive 0 to 10-V output signal suitable for use with single or multichannel pulse-height analyzers. Its gain is continuously variable from 2.5 to 1500.

Automation of all the critical adjustments makes the Model 672 easy to set up with any radiation detector, and provides a performance that is nearly independent of operator expertise.

The Automatic Pole-Zero Adjustment feature significantly simplifies the tuning of the amplifier to compensate for the decay time of the preamplifier pulse. This minimizes the operator skill and effort needed to achieve good energy resolution and peak position stability at moderate to high counting rates. When changing time constants or detectors, an accurate pole-zero (PZ) adjustment is achieved by simply pushing the AUTO PZ button and waiting a few seconds for the AUTO PZ BUSY LED to turn off. No oscilloscope is required for this procedure. The AUTO PZ memory is protected against power failures. 

In extreme situations, where the preamplifier pulse shape is deformed from the normal exponential decay, complete PZ cancellation is not possible. In such cases, a slight improvement in the high counting rate performance can sometimes be achieved using the manual PZ adjustment mode to arrive at a compromise solution.

A front-panel switch on the Model 672 provides the choice of either a triangular or a Gaussian pulse shape on the UNIPOLAR output connector. The noise performance of the triangular pulse shape is equivalent to a Gaussian pulse shape having a 17% longer shaping time constant. In applications where the series noise component is dominant (short shaping time constants), and the pile-up rejector is utilized, the triangular shape will generally offer the same dead time and slightly lower noise than the Gaussian pulse shape. A front-panel switch permits selection of the optimum shaping time constant for each detector and application. Six time constants in the range from 0.5 to 10 µs, and the TRIANGULAR/GAUSSIAN switch combine to offer 12 different shaping times. A bipolar output is also provided for measurements requiring zero cross-over timing.

To minimize spectrum distortion at medium and high counting rates, the unipolar output incorporates a high-performance, gated, baseline restorer with several levels of automation. Automatic positive and negative noise discriminators ensure that the baseline restorer operates only on the true baseline between pulses in spite of changes in the noise level. No operator adjustment of the baseline restorer is needed when changes are made in the gain, the shaping time constant, or the detector characteristics. Negative overload recovery from the reset pulses generated by transistor-reset preamplifiers and pulsed optical feedback preamplifiers is also handled automatically. A monitor circuit gates off the baseline restorer and provides a reject signal for a multichannel analyzer until the baseline has safely recovered from the overload. 

Several operating modes are selectable for the baseline restorer. For making either a manual or automatic PZ adjustment, the PZ position is selected. This position can also be used where the slowest baseline restorer rate is desired. For situations where low frequency noise interference is a problem, the HIGH rate can be chosen. On detectors where perfect PZ cancellation is impossible, the AUTO baseline restorer rate provides the optimum performance at both low and high counting rates.

An efficient pile-up rejector is incorporated in the Model 672 Spectroscopy Amplifier. It provides an output logic pulse for the associated multichannel analyzer to suppress the spectral distortion caused by pulses piling up on each other at high counting rates. The fast amplifier in the pile-up rejector includes a gated baseline restorer with its own automatic noise discriminator. A multi-color pile-up rejector LED on the front panel indicates the throughput efficiency of the amplifier. At low counting rates the LED flashes green. The LED turns yellow at moderate counting rates and red when pulse pile-up losses are >70%. 

When long connecting cables are used between the detector preamplifier output and the amplifier input, noise induced in the cable by the environment can be a problem. The Model 672 provides two solutions. For low to moderate interference frequencies the differential input mode can be used with paired cables from the preamplifier to suppress the induced noise. At high frequencies a common mode rejection transformer built into the Model 672 input reduces noise pick-up. The transformer is particularly effective in eliminating interference from the display raster generators in personal computers.

All toggle switches on the front panel lock to prevent accidental changes in the desired settings.

Performance

Note: Unless otherwise stated, performance specifications are measured on the unipolar output with 2-µs Gaussian shaping, the manual PZ mode, and the AUTO BLR mode.

Gain Range	Continuously adjustable from 2.5 to 1500. Gain is the product of the COARSE and FINE GAIN controls.
Unipolar Pulse Shapes	Switch selection of a nearly triangular pulse shape or a nearly Gaussian pulse shape at the UNIPOLAR output.
Bipolar Output Pulse Shape	Rise of the BIPOLAR output pulse from 0.1% to maximum amplitude is 1.65 times selected SHAPING TIME. Zero cross-over of the bipolar output pulse is delayed from the maximum amplitude of the Gaussian UNIPOLAR output by 0.33 times the selected SHAPING TIME.
Integral Nonlinearity (Unipolar Output)	<±0.025% from 0 to +10 V.
Noise	Equivalent input noise <5.0 µV rms for gains >100, and <4.5 µV rms for gains >1000 in manual PZ mode, or <6.0 µV for gains >100 in AUTO PZ mode.
Temperature Coefficient (0 to 50°C)	• Unipolar Output  <±0.005%/°C for gain, and <±7.5 µV/°C for dc level. • Bipolar Output  <±0.007%/°C for gain, and <±30 µV/°C for dc level.
Walk	Bipolar zero crossover walk is <±3 ns over a 50:1 dynamic range.
Overload Recovery	Unipolar and bipolar outputs recover to within 2% of the rated output from a X1000 overload in 2.5 non-overloaded pulse widths using maximum gain.
Spectrum Broadening†	Typically <8% broadening of the FWHM for counting rates up to 100,000 counts/s, and <15% broadening for counting rates up to 200,000 counts/s. Measured on the 1.33-MeV gamma-ray line from a 60Co radioactive source under the following conditions: 10% efficiency ORTEC GAMMA-X PLUS detector, 8.5-V amplitude for the 1.33-MeV gamma-ray on the unipolar output.
Spectrum Shift†	Peak position typically shifts <±0.018% for counting rates up to 100,000 counts/s, and <±0.05% for counting rates up to 200,000 counts/s. Measured on the 1.33-MeV line under conditions specified for SPECTRUM BROADENING.
Differential Input	Differential nonlinearity <±0.012% from –9 V to +9 V. Maximum input ±10 V (dc plus signal). Common mode rejection ratio >1000.
Pulse Pile Up Rejector	Threshold  Automatically set just above noise level on fast amplifier signal. Independent of slow amplifier BLR threshold. • Minimum Detectable Signal  Limited by detector and preamplifier noise characteristics. • Pulse Pair Resolution  Typically 500 ns. Measured using the 60Co 1.33-MeV gamma ray under the following conditions: 10% efficiency germanium detector, 4-V amplitude for the 1.33-MeV gamma ray at the unipolar output, 50,000 counts/s.
	†Results may not be reproducible if measured with a detector producing a large number of slow rise-time pulses or having quality inferior to the specified detector.

Controls and Indicators

Fine Gain	Front-panel, 10-turn precision potentiometer with locking, graduated dial provides continuously variable, direct reading, gain factor from 0.5 to 1.5.
Coarse Gain	Front-panel, eight-position switch selects gain factors of 5, 10, 20, 100, 200, 500, and 1000.
Shaping Time	Six-position switch on the front panel selects shaping times of 0.5, 1, 2, 3, 6, and 10 µs for the pulse-shaping filter network.
UNI Shaping	Two-position locking toggle switch on the front panel selects either GAUSSIAN or TRIANGLE pulse shaping for the UNIPOLAR output.
Input	Front-panel, four-position switch accommodates either + or – input polarities, and selects the differential (DIFF) or normal (NORM) input modes. In the NORM mode only the NORM input connector is used. In the DIFF mode the preamplifier signal cable is connected to the NORM input, and a cable having its center conductor connected to the preamplifier ground through an impedance matching resistor is connected to the DIFF REF input. The impedance matching resistor must match the output impedance of the preamplifier.
BAL (Differential Input Gain Balance)	A 20-turn potentiometer mounted on the PC board inside the module allows the gains of NORM and DIFF REF inputs to be matched for maximum common mode noise rejection in DIFF mode.
PZ Auto/Man Switch	Locking toggle switch selects either the AUTO (automatic) or MAN (manual) pole-zero cancellation adjustment mode. Both modes permit PZ cancellation for preamplifier exponential decay time constants from 40 µs to ∞.
Auto PZ Button	With PZ switch in AUTO PZ position, momentarily pressing AUTO PZ button turns on the BUSY LED and initiates automatic adjustment of the PZ cancellation circuit. BUSY LED turns off when adjustment is complete. Once completed, the PZ adjustment is held until the button is pushed again. Memory of the last PZ adjustment is protected against unforeseen power outages.
Manual PZ Adjustment	20-turn potentiometer on the front panel permits screwdriver adjustment of the PZ cancellation. The screwdriver-adjusted value is effective whenever the PZ switch is in the MAN (manual) position. For transistor-reset preamplifiers or pulsed optical feedback preamplifiers, use manual PZ adjustment set fully counterclockwise.
Limit Pushbutton	Inserts a diode limiter in series with the front-panel UNIPOLAR output connector and test point. Prevents overload distortions in the oscilloscope when observing accuracy of the PZ adjustment on the more sensitive oscilloscope ranges.
BLR Rate	A front-panel, three-position, locking, toggle switch selects the baseline restorer rate. PZ position offers lowest fixed rate, for adjusting PZ cancellation. AUTO position matches the rate of the PZ position at low counting rates, but increases the restoration rate as the counting rate rises. HIGH rate position is provided for suppressing low frequency interference.
PUR Accept/Reject LED	Multicolor LED indicates percentage of pulses rejected because of pulse pile-up. LED appears green for 0–40%, yellow for 40–70%, and red for >70% rejection.

 Inputs

Norm	Front-panel, BNC connector accepts preamplifier signals of either polarity with rise times less than the selected SHAPING TIME and exponential decay time constants from 40 µs to ∞. For the – INPUT switch setting, the input impedance is 1000 Ω on a coarse gain of 5, and 465 Ω at coarse gain settings ≥10. For the + INPUT switch setting, the input impedance is 2000 Ω for a coarse gain of 5, and 1460 Ω for coarse gains ≥10. Input is dc-coupled, and protected to ±25 V.
Linear	Rear-panel connector. Identical to NORM input.
Diff Ref	Front-panel BNC connector is used for the preamplifier ground reference connection when operating in the differential input mode. Operative only with the INPUT switch in the DIFF mode. For the + DIFF INPUT switch setting, the input impedance is 1000 Ω on a coarse gain of 5, and 465 Ω at coarse gain settings ≥10. For the – DIFF INPUT switch setting, the input impedance is 2000 Ω for a coarse gain of 5, and 1460 Ω for coarse gains ≥10. Input dc-coupled; protected to ±25 V.
Inhibit	Rear-panel BNC input connector accepts reset signals from transistor-reset preamplifiers or pulsed optical feedback preamplifiers. Positive NIM standard logic pulses or TTL levels can be used. Logic is selectable as active high or active low via a printed circuit board jumper. INHIBIT input initiates the protection against distortions caused by the preamplifier reset. This includes turning off the baseline restorers, monitoring the negative overload recovery at the unipolar output, and generating PUR (reject) and BUSY signals for the duration of the overload. The PUR and BUSY logic pulses are used to prevent analysis and correct for the reset dead time in the associated ADC or multichannel analyzer.

Outputs

Unipolar, UNI	Front- and rear-panel BNC connectors provide positive, unipolar, shaped pulses with a linear output range of 0 to +10 V. Front-panel output impedance <1 Ω. Rear-panel output impedance selectable for either <1 Ω or 93 Ω using a printed circuit board jumper. Outputs are dc-restored to 0 ± 5 mV and short-circuit protected.
Bipolar, BI	Front- and rear-panel BNC connectors provide bipolar shaped pulses with the positive lobe leading. The linear output range is 0 to ±10 V. Front-panel output impedance <1 Ω. Rear-panel output impedance selectable for either <1 Ω or 93 Ω using a printed circuit board jumper. Baseline between pulses has a dc level of 0 ±10 mV. Short-circuit protected.
CRM	The Count Rate Meter output has a rear-panel BNC connector and provides a 250-ns-wide, +5-V logic signal for every linear input pulse that exceeds the pile-up inspector threshold. Output impedance is 50 Ω.
Busy	Rear-panel BNC connector provides a +5-V logic pulse for the duration that the linear signals exceed the positive or negative baseline restorer thresholds, or the pile-up inspector threshold, or for the duration of the INHIBIT input signal. Useful for dead-time corrections with an associated ADC or multichannel analyzer. Positive NIM standard logic pulse is selectable as active high or active low via a printed circuit board jumper. Output impedance is 50 Ω.
PUR	Pile-Up Reject output is a rear-panel, BNC connector. Provides a +5-V NIM standard logic pulse when pulse pile-up is detected. Output also present for a pulsed reset preamplifier during reset, and reset overload recovery. Output pulse is selectable as active high or active low by means of a printed circuit board jumper. Output impedance is 50 Ω. Used with an associated ADC or multichannel analyzer to prevent analysis of distorted pulses.
Preamp	Rear-panel standard ORTEC connector (Amphenol 17-10090) provides power for the associated preamplifier. Mates with power cords on all standard ORTEC preamplifiers.

Electrical and Mechanical

Power Required	The Model 672 derives its power from a NIM bin supplying ±24 V and ±12 V, such as the ORTEC Model 4001A/4002A Bin/Power Supply. The power required is +24 V at 90 mA, –24 V at 170 mA, +12 V at 330 mA, and –12 V at 190 mA.
Weight	Net  2.3 kg (5.0 lb).   Shipping  3.6 kg (8.0 lb).
Dimensions	NIM-standard double-width module, 6.90 X 22.13 cm (2.70 X 8.714 in.) front panel per DOE/ER-0457T.