Theory of Operation
Publication 21555, Rev. E, September 2002
3-5
The operation of the various functions of the Calibrator Module can be understood more easily if the
circuits are discussed individually. The functional sections of this module include the following:
1.
The 50 MHz oscillator (Q4) and its current control circuit consisting of U4D, Q5 and U2C.
2.
The RF output circuit consisting of the low pass filter, the stepped attenuator, and the connector
and cable to the front panel of the power meter.
3.
The oven to maintain the control thermistor at a constant 60 °C. It is located on the small board
attached to the bottom of the Q1 heater transistor. The board contains the RT1 and RT2
thermistors and the Q7 control transistor.
4.
The thermistor bridge to measure the RF power by DC substitution. It consists of RT1, U1 and Q6.
5.
The track and hold circuit that remembers the ambient bridge voltage, using U2B, U8D and U3A.
6.
The 14-bit DAC and reference supply to measure the ambient bridge voltage and control the RF
output level, made up of U11, U7, U8C&D, U4, U14A, U2A&B, U3A, U5, U10, U16B&C,
U13B&C and U9B.
7.
The correction circuit to measure the temperature of the PIN diode attenuator so that a correction
for the temperature dependent loss of the diodes can be corrected, consisting of RT2 and U14C.
8.
Calibrator NVRAM control circuit, U26 and U18.
3.4.1
50 MHz Oscillator
The first section of the Calibrator Module consists of a colpits oscillator circuit with a controllable
power output. The output power is measured by the thermistor bridge and set by varying the DC current
through Q4. This current is supplied by a voltage to current converter consisting of U14D, Q5 and U4.
The power generated by Q4 is linearly related to the current through it. Thus, the voltage from U4 that
is converted to current by U14D and Q5 is linearly related to the RF power generated. When the
calibrator is set for 0 dBm, the voltage at U4-6 is near 0 volts.
3.4.2
RF Output
The 50 MHz oscillator output is capacity coupled to the low pass filter, L13, L14, L15 and associated
capacitors. The resultant harmonic-free RF is applied to the switched PIN attenuator, CR8 - 14 and
associated resistors and control amplifiers U19 and U14B. The first section is 10 dB, the output section
is 20 dB, and a resistor between sections adds another 10 dB. Thus, the output power can be
programmed from +20 to -30 dBm.
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