(Figure 3-9) uses FET switches that are in intimate thermal contact. This is essential to keep the two FET's at exactly the same temperature to minimize drift. To eliminate undesired thermocouples only one metal, gold, is used throughout the entire dc path. All these contributions were necessary to help achieve the low drift already shown in Figure 3-6. 

The chopping frequency is chosen at 220 Hz as a result of several factors. Factors that tend to djctate a high chopping frequency include lower l/f noise and a larger possible bandwidth. Limiting the chopping to a low frequency is the fact that small spikes from chopping inevitably get included with the main signal. These spikes are at just the proper rate to be integrated by the synchronous detector and masquerade as valid signal. The fewer spikes per second, the smaller this masquerading signal. Since the spikes are also present during the zero-setting operation, and remain the same value during the measurement of a signal, the spikes are essentially removed from the meter indication by zero-setting and cause no error. The spikes do, however, use up dynamic range of the amplifiers. 
One way to minimize noise while amplifying small signals is to limit the bandwidth. Since the noise generating mechanisms are broadband, limiting the amplifier bandwidth reduces the total noise power. The narrowest bandwidth is used for the weakest signals. As the power meter is switched to higher ranges the bandwidth increases so that measurements can be made more rapidly. On the most sensitive range the time constant is roughly 2 seconds while on the higher ranges the time constant is 0.1 seconds. A 2-second time constant corresponds to a 0 to 99 percent rise time of about 10 seconds. 
Reference Oscillator 
A frequent, sometimes well-directed, criticism of thermocouple power measurements is that such measuremen.ts are open-loop, and that thermistor power measurements are inherently more accurate. The increased confidence in the thermistor measurements results from the direct substitution of dc power for rf power in a closed-loop system. The bridge feedback of substituted dc power compensates for differences between thermistor mounts and for drift in the thermistor resistance-power characteristic without re calibration. With thermocouples, where there is no direct substitution, sensitivity differences between sensors or drift in the sensitivity of a sensor means a different dc output voltage for the same rf power. Because there is no feedback to correct for different sensitivities, measurements cwith thermocouple sensors are said to be open-loop. 
The 435A solves this problem by including a 50 MHz power-reference oscillator whose output power is controlled very accurately. To verify the accuracy of the system, or adjust for a sensor of different sensitivity, the user connects his thermocouple sensor to the powerreference output and, using a front-panel screwdriver adjustment, sets his 435A to read 1.00 mW. The reference oscillator is applied to the input port of the power sensor just like the rf to be measured; the same capacitors and conductors are used in the same way for measurement and for calibration adjustment. This feature effectively transforms the system to a closed-loop, substitution-type system. 
Automatic Measurements 
The introduction of the modern thermocouple sensor corresponded in time with a demand for automatic measurements. This demand not only required a digital output of the power level being sensed, but complete digital control of the power meter as well as regular front panel control. This control had to include the ability to zero-set, to change ranges, and to indicate the units of power (e.g., mW, dBm, etc.). The HP 436A Power Meter was designed especially for this purpose. 
The analog portion of the 436A is very similar to the 435A discussed in the previous section. The two instruments use the same power sensors, amplify the small dc voltages from the sensor by first chopping at a 220 Hz rate, then amplifying that ac signal, and synchronously detecting the ac signal just before metering. The synchronous detector is different to enable automatic measurements at the highest speed practical for rapid data gathering.