Resistive noise generator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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This generator (see figure) allows you to determine the noise figure of VHF receivers operating in the frequency range 140 ... 1300 MHz. Noise figure measurement limits 1,05...3. The SWR of the device does not exceed 1,4. Dimensions - 106x55x35 mm.
Noise EMF occurs in this generator due to the increase in the temperature of the conductive layer of the resistor, the resistor is heated with direct current. Resistor R1 (with a resistance of 75 or 50 ohms, depending on the required output resistance of the generator) receives a constant voltage through a filter consisting of a choke L1 and capacitor C2. The current flowing through the resistor R1 (and hence its temperature) is controlled by the variable resistor R2. The SA1 switch connects the generator to the built-in (10 Kvant elements) or external (12 ... 14 V battery) power source. Structurally, the generator is made in a rectangular case made of foil fiberglass 1,5 mm thick with a removable bottom cover. The handle of the resistor R2 with a scale is displayed on the front panel of the case. Capacitor C1 - unpackaged, K10-9. It is soldered directly to the central pin of the XW1 connector, which was previously shortened flush with the body. Resistor R1 - MLT-0,125. Choke L1 contains 15 turns of enameled wire with a diameter of 0,6 mm, wound on a mandrel with a diameter of 4 mm. The winding pitch is variable, from "turn to turn" at the beginning to 3 mm at the end connected to the resistor R1.
The receiver noise figure is measured as follows. The receiver is set to the telegraph mode of operation, the maximum bandwidth, and the AGC is turned off. The noise generator is connected to the input of the receiver. The slider of the resistor R2 is set to the position of maximum resistance. A voltmeter is connected to the output of the receiver and the voltage value (Uout1) is determined. Then turn on the power of the noise generator and smoothly reduce the resistance of the resistor R2 to a minimum and fix the voltage Uout2 at the output of the receiver. When measuring, it should be taken into account that the heating time of the resistive layer of the resistor R1 is usually 5...8 s. The received data is substituted into the expression
where Tx is the effective noise temperature of the receiver, K; T - ambient temperature, K.
The noise figure n of the receiver is determined by the formula
n=Tx/T+1.
To reduce the measurement error of the noise characteristics of the receiver. due to the instability of the power supply, a fixed current must flow through the resistor R1 for all measurements. It is set by resistor R2. To control the current in series with the resistor R2, it is advisable to include a milliammeter. In addition, it should be taken into account that the measurement error is also affected by the design of the device (how the temperature regime inside the case is set).
Authors: A. Bondarenko, A. Baryshev; Publication: N. Bolshakov, rf.atnn.ru
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