Tektronix 1705A instruction manual RF Input Diagram

Theory of Operation

regulated to become the + and -11.8 V supplies on the Main circuit board (regulator circuits are on Diagram 4).

The Power Supply schematic also contains the 11.8 V Post Regulators, whose outputs are used by most of the circuits on the Main circuit board. The Post Regulators do not appear on the block diagram.

High Voltage (Diagram 8) The unblanking signal from the Z-Axis Control drives the Z-Axis Amplifier. The Focus Amplifier, controlled by the front-panel control, provides a voltage to the crt focus ring. The crt is of the Post Acceleration type, which requires a relatively high potential difference between the cathode and post anode. The boost in 2nd anode voltage is provided by an encapsulated 4X Multiplier.

The High Voltage supply also provides the +100 V, required by the Horizontal

Deflection Amplifier (Diagram 4), to drive the crt horizontal deflection plates.

RF Input

Diagram 1

This diagram has both RF inputs for the Spectrum Monitor. The L-Band and

70 MHz inputs are both off-board subassemblies. The 70 MHz input consists of discrete components on a small circuit board that is totally shielded. The L-Band Tuner is also self contained, totally shielded, and contains no user-ser- viceable parts.

L- Band Input The L-Band signal (900 - 1750 MHz) is input directly to a tuner subassembly that contains the Tuned RF Amplifier, mixer, and Voltage Controlled Oscillator (VCO) required to produce the first Intermediate Frequency (IF) of 489.9 MHz. The 489.9 MHz 1st IF signal is cabled to the Main board.

The tuner subassembly is powered by the +11.8 volts, which is controlled by the Input Switching. (The +11.8 V to the L-Band Tuner is disabled when the

70 MHz input is selected.) In addition, the Block Down Converter +18 V supply (from Diagram 5) is fed to the tuner for output on its F-type connector. A recessed, rear-panel slide switch connects the +18 V supply to the L-Band Tuner. A pre-corrected ramp (from Diagram 3) to drive the VCO is also supplied to the tuner subassembly.

The /LBAND control line, from the microprocessor, is low when the L-Band is selected. When /LBAND goes high (indicating that the 70 MHz input has been selected) Q31 turns on and Q30 turns off, causing Q32 to turn off and disconnect the +11.8 V from the tuner.

70 MHz Input The input to the 70 MHz tuner has a 75Ω, 10 dB pad, which allows it to accept signals up to -20 dBm. The 70 MHz signal (45 - 100 MHz) is preconditioned

4- 4 1705A Spectrum Monitor