Bolometer Power MeterThe standard p

Bolometer Power Meter
The standard power meter used in the Navy (Hewlett-Packard 431 C) is an automatic self-balancing instrument employing dual-bridge circuits. It is designed to operate with temperature-compensated thermistor mounts that enable you to measure power in a 50-ohm coaxial system from 10 MHz to 18 GHz and in a waveguide system from 2.6 GHz to 40 GHz. This power meter can be operated from either an ac or a dc primary power source. The ac source can be either 115 or 230 volts at 50 to 400 hertz. The dc source is a 24-volt rechargeable battery. A seven-position range switch allows full-scale
power measurements of 10 microwatts to 10 milliwatts or of -20 dBm to +10 dBm. These ranges can be further extended with the aid of attenuators. The thermistor mount (as shown in fig. 3-17) contains two thermistors: one in the detection bridge, which absorbs the microwave power to be measured, and the other in the compensation and metering bridge, which supplies temperature compensation and converts the measured rf power to a meter indication. Each bridge includes its respective thermistor element as a bridge arm.
Figure 3-17. - Power meter.

Basically, the power meter circuit consists of two bridges; each bridge includes one of the thermistor elements as a bridge arm. The bridges are made self-balancing through the use of feedback loops. Positive or regenerative feedback is used in feedback loop 1; degenerative (negative) feedback is used in feedback loop 2. Both bridges are excited by a common 10-kHz source. The 10-kHz amplifier-oscillator supplies 10-kHz power to bias the thermistor in feedback loop 1 to produce the resistance required to balance the rf bridge. An equal amount of 10-kHz power is supplied by the same oscillator to the second thermistor in feedback loop 2 through two series-connected transformers. Feedback loop 2 balances the meter bridge. When rf is applied to the thermistor in the detection bridge (but not to the compensation and metering bridge), an amount of 10-kHz power is present, equal to the rf power being removed from the detection bridge by the self-balancing action of the bridge. Since the rf power replaced the 10-kHz power, the detection bridge is in balance; however, the metering bridge must be balanced by its separate feedback loop. Sufficient dc power to equal the 10-kHz power lost by the metering bridge is automatically replaced, balancing this loop. Hence the dc power applied to the metering bridge thermistor is equal to the microwave power applied to the detection bridge. The meter circuit senses the magnitude of the feedback current. The resultant meter current passes through a differential amplifier to the indicating meter. The two thermistors are matched with respect to their temperature characteristics; therefore, there is only a very small amount of drift of the zero point with ambient temperature changes. When there is a change in temperature, there is a change in the electrical power needed by the thermistors to maintain constant operating resistances. This change is automatically performed by feedback loop 1, which changes the amount of 10-kHz power for both thermistors by the proper amount. The dc power in feedback loop 2 is not changed; and since it is this dc power that is metered, the temperature change has not affected the meter indication.

Results (English) 3:

测辐射热计功率计
用于海军标准功率表（休利特帕卡德431）是一个自动平衡电桥电路仪器采用双。它的目的是工作温度补偿的热敏电阻安装，使您能够测量功率在50欧姆的同轴系统从10兆赫到18兆赫和40兆赫至2.6兆赫的波导系统。该功率计可以由一个交流或直流电源供电。交流电源可以是115或230伏在50至400赫兹。直流电源是一个24伏的可充电电池。一七位置范围开关允许10毫瓦的功率测量，全面
10毫瓦或- 20 dBm的10 dBm。这些范围可以进一步扩展与衰减器的援助。安装的热敏电阻（如图3-17所示）包含两热敏电阻：在检测桥梁，吸收微波功率进行测量，并在补偿的其他计量的桥梁，提供温度补偿和转换测量RF功率表指示。每座桥包括各自的热敏电阻元件作为一个桥臂。
图3-17。基本上，功率表电路由两个桥组成，每座桥包括一个热敏电阻元件，作为桥臂。桥梁通过使用反馈回路进行自平衡。反馈回路中使用了正反馈或再生反馈1；在反馈回路中使用退化（负反馈）反馈2。这两座桥是由一个共同的10千赫的源激发。10千赫放大器振荡器提供10千赫功率偏置电阻在反馈回路1，产生电阻平衡的射频桥。在反馈回路中，通过2个串联连接的变压器，一个相等的10千赫的功率由相同的振荡器提供给第二热敏电阻。反馈回路2平衡表桥。当射频被施加到在检测桥（但不补偿和计量桥）的热敏电阻，一个量的10千赫功率是本，等于从检测桥的射频功率被拆除的自平衡作用的桥梁。由于射频功率取代了10千赫的功率，检测电桥是平衡的；然而，测量电桥必须平衡由它的单独的反馈回路。足够的直流功率相等的10千赫的功率损失由计量桥自动更换，平衡这一回路。因此，直流电源适用于测量电桥热敏电阻是等于微波功率应用到检测桥。仪表电路的感应电流的大小。所得到的表电流通过一个差分放大器指示仪表。两热敏电阻是相对于他们的温度特性相匹配；因此，只有一个非常小的量随环境温度变化的零点漂移。当温度变化时，这是由热敏电阻需要保持恒定工作阻力的电功率的变化。这种变化是由反馈回路1自动执行，从而改变为热敏电阻由适量的10 kHz的发电量。在反馈回路2直流电源是不可改变的；因为它是直流电源，计量，温度变化并没有影响仪表指示。