The heart of the 436A Power Meter is a patented logarithmic analog-to-digital converter (Figure 3-10). This circuit, as distinguished from the usual dual slope technique popular in digital voltmeters, allows readings in watts or dBm with the same accuracy and with a minimum of extra circuitry. Unlike analog approaches to the problem of log conversion, no warm up time is required and there is very little temperature drift (<0.001 dB/°C). 

At the start of the measurement cycle switch S1 closes and connects the unknown to the integrator IC1. After a fixed period of time switch S1 opens leaving a voltage VI on the output of the integrator which is proportional to the unknown input voltage and hence the input power. 
In the watts mode an appropriate reference voltage is connected to the integrator through switch S2, to discharge the integrator at a constant rate. The discharge continues until comparator IC2 detects a zero crossing. The discharge time T is proportional to V 1 and hence to the unknown input voltage. The time is counted to get a digital result representing watts of input power. 
In the log conversion mode the first part of the conversion cycle is the same, so that voltage V 1 is on the integrator when switch S1 opens. Then switches S3 and S4 are closed so that the capacitor C1, initially charged to V1, discharges exponentially through Rl according to 

	vo(t) = V1 e -t/RIC1		 (3-1) 

where vo(t) is the discharging output voltage on CI. The discharge continues until Vo (t) reaches threshold VT, as sensed by the comparator. The time lapse T is given by 
	T/R1C1 = ln V1/Vt = 2.303 log V1/Vt		(3-2)

The discharge time. or the time registered on the counter, is seen to be proportional to the log of V1. RI and VT are adjusted at the factory to provide precise conversion to dBm. 
At the end of the measurement cycle, switches S5 and S6 are closed and a feedback loop is formed that places a charge on C2 to remove any remaining voltage from IC1. 
Digital counters normally have an output uncertainty of ±1 one count. This is due to the internal time-base oscillator being out of synchronism with the start of the measurement interval by, at most. one count and also being out of sync with the end of the measurement by at most, one count. Thus there is a maximum possible total uncertainty of two counts or ±1 one count. The analog-to-digital converter described here only has an ambiguity at the end of the measurement cycle. The beginning of the discharge cycle is triggered by the internal time base and therefore has no ambiguity. Thus the ambiguity in output indication is a total spread of one count or ±1 one-half count. 
The HP 436A automatic power meter is capable of working with several different thermocouple mounts covering different power ranges and also of working with a diode detector power sensor (discussed in the next chapter). Each power sensor contains a resistor that indicates to the power meter the sensitivity of the sensor. With this resistor the 436A Power Meter can determine the mount sensitivity and adjusts the output indication appropriately. 
In summary, the thin-film, semiconductor, thermocouple power sensor, because of its low reflection coefficient, can yield the most accurate, broad-range measurement system. When used with a modern digital power meter, fast and automated measurements are also possible.