GX2C digital low-power meter

1、 Overview

GX2C digital low-power meter indicatorEquipped with GX2B-T1GX2B-T2、GX2B-T3 probeLow power probes can be used to directly measure the average RF power of continuous or pulse modulation.

GX2C numberwordThe low-power meter is a modified product of the GX2 series low-power meters. It adopts digit display and header simulation reference indication.

GX2C digital low-power meterMaintained the originalThe GX2B indicator not only has the characteristics of wide measurement range, small temperature drift, and easy use, but also adds digital display to improve reading accuracy. At the same time, it also provides BCD code output, which can output BCD code parallel signals indicating the reading of the indicator according to the data transmission command of the external system. This can form a simple automatic testing system or microcomputer control system with external devices.

2、 Technical parameters

1. Power measurement range: 0.01 μ W～300mW (equipped with GX2B-T)₁、T₂、T₃probe）.

2. Frequency range: 0.05-12.4GHz(Equipped with)GX2B-T₁、T₂、T₃Probe),

When equipped with various waveguide probes, the upper frequency limit can reach2mm band.

3. Inherent error: ± 4% (full scale).

4. Working error: ± 5% (full range).

5. Proofreading factor shift error: ± 1% (85%)～100%, graded in 1% steps.

6. Zero stability: ± 0.5mW/1min.

7. Analog voltage output: Full scale 0.999V output impedance 1K Ω (Q9 socket).

8 BCD code output: parallel (2CA-24 socket).

9. Equipped with GX2B-T₁、T₂、T₃Probe technical parameters:

① Power range: GX2B-T₁：1μW～10mW；

GX2B-T₂：10μW～100mW；

GX2B-T₃：1mW～300mW.

② Calibration factor error: ± 5%.

stayCalibrate at 50MHz and provide calibration factor values for each probe at six frequency points: 1, 3, 6, 8.2, 10, and 12.4GHz.

③ Voltage standing wave ratio: ≤ 1.5.

④ Acceptable average power:

T₁Acceptable1-minute 25mW excess power test;

T₂Acceptable1-minute 200mW excess power test;

T₃AcceptableA 1-minute 600MW excess power test.

10. Various GX2 probes can be directly paired with TTA-1 connectors.

11. Preheating time and continuous working time:

① Preheating time: 30 minutes.

② Allow continuous work.

12. Power supply: AC 220V_～Allowable deviation± 10%, 50Hz tolerance ± 5%, power consumption not exceeding 5VA.

13. Dimensions: 160 × 320 × 250 (mm).

14. Weight:Not greater than6kg.

4、 Working principle

The GX2C digital low-power meter consists of two main components: a power probe and an indicator.

1. Power probe

Power probes are RF power sensitive devices. It linearly converts the input RF power into DC voltage and delivers it to the indicator for amplification and indication. The power probe can be divided into two categories: coaxial and waveguide. Its electrical schematic is roughly as follows:

GT₁byL16 coaxial connector or various types of waveguide flange connectors, which are connected to C₁、C₂、C₃The input transmission system is mainly composed of a DC path that connects the measured RF and isolates the thermoelectric element towards the signal source direction.

R₁、R₂The resistance value of the thermocouple. They are both terminal load resistors of transmission lines and electromagnetic fields→ Thermal energy → DC electromotive force conversion device. The device adopts a vacuum coating process, in which electrode leads and bismuth antimony thermocouples are sequentially coated on a dielectric film substrate (polyimide or mica). The selection of thermocouple patterns and substrates must take into account both the power capacity and conversion efficiency. R₁、R₂In coaxial systems, they are connected in parallel to meet the requirementsA transmission impedance of 50 Ω, R₁＝R₂＝100 Ω (practical value is 85-105 Ω). In waveguide systems, R₁、R₂It is connected in series, with a series resistance ofAbout 300 Ω.

The direct current electromotive force generated by the thermocouple is proportional to the measured radio frequency energy applied, and this potential isC₄、C₅andL₁、L₂After forming the filter, it is directly sent to the indicator for amplification. Calibration potentiometerWt adjusts the gain of the amplifier for calibration, and since the potentiometer is installed on the probe, there is interchangeability between the probes.

2. Indicator

The indicator is essentially a high-sensitivity device with a resolution ofA digital DC voltmeter with a voltage of 0.01 μ V. The whole machine is divided into three parts: amplification unit, A/D conversion, display unit, and BCD code output unit. The description is as follows:

① The principle of each part of the amplification unit is as follows:

Modulation and demodulation circuits. The entire amplifier is a modulated DC amplifier.3BG_1~4Four field-effect transistors form a series parallel chopper, which converts the input DC signal into the corresponding AC signal and sends it to the preamplifier.

from4BG_{1. 2}Composed of two effect transistors. The switching of the modulator and demodulator field-effect transistors is controlled by the same square wave generator. The two are synchronized, which can effectively avoid some interference noise. The square wave frequency is about800Hz.

Pre amplifier: consisting of three stages of amplification and an upper emitter follower, with a closed-loop gain of approximately10³The amplifier is composed of low-noise transistors and operates in a low current state to ensure that the overall noise meets technical requirements.

Active filter: using operational amplifiersF007（4JC₁）For active devices, to4R₁、4R₂、4R₄and4C₂、4C₃Forming a frequency selective component, its bandpass center frequency is consistent with the modulation square wave frequency, so that after pre amplificationSignals around 800Hz are amplified through frequency selection, while other interferences are suppressed.

AC amplifier: composed of4JC₂Construct a proportional amplifier with a closed-loop gain of μ WFile andDifferent mW levels are used to maintain appropriate negative feedback for the entire machine.

Integral amplifier and DC amplifier: The demodulated DC signal is sent to the integral amplifier, and the signal after integral amplification is sent to both the feedback network and the DC amplifier. The gain of a DC amplifier is determined by a correction factor networkPress 1% for 15th gearThe step level changes, with an amplification of approximately5.3 changes to 6.2. The offset voltage of the operational amplifier is adjusted by potentiometer 4W to zero when there is no DC input. The signal amplified by DC is sent to the analog output for measurement by external devices such as recorders, and to the analog-to-digital conversion circuit.

Range selection and zeroing. The range control of the whole machine is determined by the negative feedback of the whole machine, and the feedback amount is determined by the precision resistor. Therefore, changing the resistance value of the feedback resistor can control the range conversion. Zeroing circuit utilizes a local voltage regulator power supplyAfter ± 12V voltage division, it is adjusted by two potentiometers, coarse and fine, and a deviation voltage is added to the modulation stage to achieve it.

② The principle of A/D conversion and display unit is as follows:

This circuit uses two single-chip integrated circuits,XFC-3161 and XFC3162, three digit readout system. Among them, XFC3162 is for analog-to-digital conversion. The characteristic of this circuit is the use of dual integration A/D conversion, and the external integration capacitor adopts polyester capacitor, which has stable capacity and large leakage resistance. The reference voltage reference of the integrator is composed of the internal bandgap type ultra stable voltage of the integrated circuit. This ensures the accuracy and reliability of the integrator's operation. This circuit has internal timing and does not require an external clock signal, simplifying peripheral circuits.

XFC3161 is a BCD-seven segment decoding and constant current driving circuit, which can directly light up three LED type digital tubes after decoding and driving.

The above two integrated circuits adopt a scanning reading type.The BCD code output by XFC3162 is directly input to the BCD code input terminal of XFC3161, and the decoded seven segment drive output signal is sent in parallel to three LED digital tube displays. Simultaneously, XFC2162 outputs three selection pulses to drive three transistor 7BG transistors_{4. 5, 6}，4. Separate the original BCD codes into multiple channels to determine the actual display numbers of the three LED digital tubes. Integrated four-way switch C544B (7JC)₁）Control, the integration time constant for the high sensitivity level is approximatelyAbout 10 seconds. The following are 1 second and 0.1 second.

The simulated indication section is provided by the header7CB₁、resistor7R₂And potentiometer7W₁Waiting for composition,7W₁Used for adjusting the full scale of the header.

③ The principle of BCD code output unit is as follows:

becauseThe BCD code of the XFC3162 A/D converter is in scanning form, so this circuit adopts a two-level latch method. The latch control signal of one latch comes from the bit selection pulse of XFC3162, and the latch control of the two-level latch can be latched according to external transfer instructions. This does not require strict synchronization between the external system and the local BCD data bus, and allows the data content transferred from the first level latch to be read from the second level latch at any time.

The BCD code outputs a "1" level of+5V and a "0" level of ground.

The units of the A/D conversion unit and BCD code output unit are both+5V, supplied by an integrated regulated power supply. This power supply is on the same printed circuit board as the BCD code output unit.

5、 Structural features

The indicator adopts front and rear cast aluminum frames, and the front frame is equipped withA/D conversion and display unit header, range switch, calibration factor switch, coarse (fine)Zero potentiometer, power switch, and grounding pole. The rear frame is equipped with analog voltage output sockets（Q9) and power sockets, fuses. The BCD code output socket (2CA-24) is fixed on the BCD code printed board and leads out to the rear panel. The upper beam is equipped with a handle and a fixed amplification unit printed board. This board can be flipped and connected to various parts through various plug sockets. The insulation box of the transformer preamplifier is installed on the bottom plate. The insulation box is composed of two layers of cylindrical aluminum cover sandwiched with foam plastic and absorption cloth. There is also a reinforced aluminum plate installed between the front and rear frames and the upper beam and lower bottom plate, on which a BCD code output board is mounted. The left and right cover plates, as well as the bottom cover plate of the chassis, are fixed to the rack with screws, making installation and removal easy and the structure reliable. The bottom of the chassis is equipped with an upward movable bracket for easy observation.

The indicator is equipped withGX2B-T₁、T₂、T₃The probe is made of an extended aluminum thin shell, and some insulation measures have been taken to avoid the influence of external temperature on the thermoelectric element. The front end of the coaxial transmission line is made of low thermal conductivity stainless steel thin-walled material as the inner and outer conductors to reduce heat conduction. An insulated plastic disc is used to connect the outer shell and the outer conductor to avoid the influence of external temperature and human hand heat. The insulator near the thermoelectric element is made of boron nitride material with good thermal conductivity to make the inner and outer conductors as isothermal as possible. There is a small hole in the back of the outer cover where the calibration potentiometer is aligned to facilitate calibration, and it is usually sealed with a plug cover.