SU1599672A1 - Method of measuring signal-to-noise ratio of valve having hollow cathode - Google Patents

Abstract

The invention relates to technical physics and can be applied in spectral measurements, for example, in assessing the accuracy characteristics of atomic absorption spectrometers or certification of spectral lamps. The purpose of the invention is to improve the accuracy of measuring the noise of a hollow cathode lamp by taking into account the noise of a photodetector. The method consists in measuring the luminous flux of a hollow-cathode lamp, measuring the luminous flux of an additional light source, such as an incandescent lamp, an LED, or a deuterium corrector. Before this, the luminous flux of the additional source is aligned with the luminous flux of the lamp being measured. Then the noise of the hollow cathode lamp is calculated, and at the same time the noise of the photodetector is compensated. The invention of the luminous flux of a hollow cathode lamp and an additional source may occur alternately, for which they must be interrupted by a modulator. 1 il.

Description

The invention relates to technical physics and can be applied in spectral measurements in evaluating the accuracy characteristics of atomic absorption spectrometers.

The purpose of the invention is to improve the accuracy of measuring the noise of a hollow-cathode lamp (signal-to-noise ratio, then s / w) by taking into account the noise of the photodetector.

The drawing shows a block diagram of a device that implements the method.

The device consists of a hollow cathode lamp 1, a monochromator 2, a photodetector 3, a recording system 4. In addition, the device includes an additional source 5 optically connected to the input window of the photoreceiver 3 (the power supply sources and the photodetector are omitted for simplicity; in addition , not shown modulator of light fluxes of radiation sources.

it can be installed in a known manner).

The method is dried as follows.

A lamp with a hollow cathode serves a working current. The luminous flux of the lamp through the monochromator (or other selection device) is fed to the photodetector. A specified number of times (usually at least 30) measure the signal from the photo-receiver. Then, the signal / noise ratio p1 is calculated in a known manner. This relationship can be written as

L

where I is the total noise of the photodetector and

hollow cathode lamps; (/) - the average value of the lamp signal with

hollow cathode.

Then the lamp with a hollow cathode is turned off, a light is sent to the photodetector after

cn to

the current of an additional light source, the noise of which is substantially less than the noise of the photodetector. It is convenient to use an incandescent lamp, a LED or a deuterium corrector as such kind of sources. It was established experimentally that the listed sources have a sufficiently low level of intrinsic noise and are suitable for the considered measurements.

The photodetector signal is measured and the c / w ratio is calculated in the second case. Obviously, it can be written as

 / 2 - the average value of the luminous flux of an additional light source;

|, p - photodetector noise. From the ratio of () 2 and pi

follows

 .X. 7s

).

where lpk is the noise of a hollow cathode lamp. We put now. Then the rms noise of the hollow cathode lamp is

I

Forestry complex

E, V / 1,

where the refined signal-to-noise ratio XR is

, /.

Thus, after measuring the luminous flux of the additional light source, the noise of the photodetector is determined, which is then taken into account in the further calculation. In order to obtain extremely small errors, the luminous flux of an additional light source before measurements should be equalized with the luminous flux of a hollow cathode lamp. However, this equalization, or rather the error of alignment of the luminous flux of the additional light source and the flux of the hollow cathode lamp, depends on the required measurement error. Since the photodetector noise depends on the luminous flux (i.e., current) according to the law of degree 1/2, the imbalance of the flux by 2 times gives an error (all other conditions being the same measurement) by only 40. If the luminous fluxes differ by 10-20 %, then this error is reduced to a few percent and it can be neglected. Consequently, a distinctive feature of this method is the weakening of the requirements for precision alignment of the luminous fluxes of a hollow cathode lamp and an additional light source.

0

These measurements can be carried out alternately, for example, by interrupting the luminous flux from radiation sources by means of a mechanical modulator, as is widely used in atomic absorption spectrometers. Electronic modulation of the light flux is also possible. This approach helps to reduce measurement errors associated with random interference, for example, from the side of the photodetector's power source.

When using a deuterium corrector, it becomes possible to measure the noise of a hollow cathode lamp in the ultraviolet region of the spectrum. In this case

The 5 deuterium corrector should be installed in front of the monochromator, and the light fluxes of both sources should be reduced to a monochromator in a known manner, for example, by means of a translucent mirror.

Example. Noise study conducted

0 lamps with a hollow cathode type LT-2. The current pulse duration was 200 µs with a duty cycle of 10. With a delay of 50 µs relative to the rising edge of the light pulse and a current amplitude of 218 mA,

5 values of U, 1483 units,, 91 units, 0, 51.49, / 2 1493 units,, 57 units, р 154.96,, 95.

It can be seen that the actual signal / noise ratio of a hollow-cathode lamp was much higher than when directly determined together with the photodetector noise. As a photodetector, a PMT-130 was used with a supply voltage of 1300 V. With a delay of 100 µs relative to the front of the light pulse, the signal-to-noise ratio for a hollow lamp

5 cathode increased to 237.59, while the c / w ratio in total with the photodetector increased only to 54.91.

This method was considered at a zero approximation of the dark noise of the photoreceiver. In this case, the dark current of the photodetector is approximately A, while the operating signal is usually up to 10 A or more, i.e., it exceeds the dark current by several orders of magnitude. By virtue of this rms value

5 photodetector noise in the dark mode (i.e., without illumination) is very small and can be neglected. If it is not possible to neglect this noise, then the calculated formula will take the form

v -V where 1 is the dark noise of the photodetector.

To take it into account, the photodetector should be shielded from the radiation, measure the dark noise and then subtract from the photodetector noise when measuring an additional light signal from an additional light source. These operations are known.

Thus, the advantage of the proposed method is the high accuracy of measurements due to the suppression of photodetector noise.

Claims (1)

Invention Formula The method of measuring the signal-to-noise ratio of a hollow-cathode lamp, which consists in measuring the light flux of the lamp under investigation by a photo-receiver multiple times, determining the average value of said light flux, determining the total noise g of the photo-receiver and the hollow cathode lamp, and calculating the value of the pi signal / noise from pi (/,) |, characterized in that, in order to increase the carry the measurements, additionally direct the light beam of the auxiliary radiation source to the photodetector, the noise of which is much less than the photodetector noise, repeatedly measure the light flow of the auxiliary source, determine the average value of the specified light beam / 2, determine the noise of the photodetector, and calculate the updated value of the p signal l / noise of a hollow cathode lamp from the expression where p, j,) / §. V (pVr) -l, Pl / P2