Tektronix 1705A instruction manual Resolution Filter and Log Detector

Theory of Operation

Input Filter, Mixer, and IF Amplifier

Resolution Filter and Log Detector

DBM U28 is driven by the L-Band Tuner output at 489.9 MHz through a LPF/Notch filter consisting of L11, L12, and series resonant circuit C69 and W13. W13 is a shorted length of 50Ω coax forming a high-Q inductor. This provides a narrow notch at 590 MHz to eliminate a spurious output from the L-Band Tuner.

The output from DBM U28 drives U31 which provides about 8 dB gain to compensate for the mixer loss and optimize the system dynamic range. This signal is then combined with the 70 MHz tuner output with 3 dB combiner T1.

The band-pass filter, FL2, is a three-section helical resonator with about 8 MHz bandwidth.

U25 is a double balanced mixer that converts the 130.5 MHz 2nd IF frequency to the 10.7 MHz 3rd IF frequency. The LO frequency (119.8 MHz) is below the

2nd IF frequency by the 3rd IF frequency. J6 is a test jumper that can be lifted to evaluate prior circuitry or to insert a signal at the 3rd IF frequency to trouble- shoot the circuits that follow.

The 2nd IF Amplifier, U22, has a gain of 20 dB. U22 requires a 6 V Vcc; VR3 and R136 are used to derive the voltage from the +11.8 V supply. C31 and C32 are decoupling capacitors. The output signal level is -20 dBm maximum. J5 is another test jumper that can be used to access prior circuits or insert a signal at the 2nd IF frequency.

The 10.7 MHz 2nd IF signal passes through the 300 kHz Resolution filter or a combination of the 10 kHz and the 300 kHz Resolution filters. The switching of filters is accomplished by a combination of a control signal (/10KHZ) from the microprocessor (Diagram 5) and a switching network consisting of U23D, E, and F, Q9, and Q10, and switching diodes (CR22, 23, 25, 26, 27, 28, 29, and 30). When the control line is low, the signal path is through the 10 kHz filter. The 300 kHz filter is always in the circuit. Q10, CR22, and CR30 turn on to enable the 10 kHz filter; Q9, CR23, and CR28 turn off to switch out the bypass line. When the control line goes high, Q10 turns off to turn off CR22 and CR30; Q9, CR23, and CR28 turn on to shunt the 10 kHz filter and provide 300 kHz resolution. R175, the 300 kHz Gain adjustment, is set to match the loss of the 10 kHz filter. J10 is another test jumper provided to access prior circuits or insert a signal directly into the Log Detector.

The input to the Log Detector, U32, is amplified by U29 and pre-shaped by FL3 and FL4, a 300 kHz Resolution filter, and then ac coupled to U32. When the 10 kHz filter is selected, the signal is filtered by the 10 kHz filter and then applied to the Log Detector. The output current from the Log Detector is processed by Q24 and Q25 to the 15 kHz low-pass filter at U27 pin 3 (Dia- gram 4). The filter output then drives the Vertical Deflection Amplifier.

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