Tektronix 1705A instruction manual Theory of Operation

Theory of Operation

IF Amplifier Circuits (Diagram 2)

Sweep Generator Circuits (Diagram 3)

driven with a pre-corrected sweep ramp. Pre-correction is required to make up for the inherent VCO nonlinearity.

A +18 V supply is connected to the L-BAND input connector so that it can be used to power a Block Down Converter (usually at the antenna). The supply can be switched on and off by a recessed slide switch on the rear panel. The voltage is generated on a separate circuit board that is mounted inside the 1705A rear panel.

The L-Band Tuner output passes through a 1st IF filter which contains a notch at 590 MHz to eliminate a spurious mixing product. It is then mixed with an L.O. of 359.4 MHz to produce a 2nd IF output at 130.5 MHz. This output is amplified by an 8 dB gain MMIC and is combined with the 70 MHz tuner output.

The 70 MHz input consists of a 7-pole, 120 MHz, bandwidth low-pass filter; a VCO (which, like the L-Band VCO, is driven by a pre-corrected sweep ramp); a mixer; and a 20 dB amplifier. The 70 MHz input circuits also output a 136 MHz 1st IF with a gain of 0 dB ±3 dB.

The 136 MHz 2nd IF is converted a third time to produce a 3rd IF frequency of

10.7MHz. The crystal-controlled Local Oscillator operates at 119.8 MHz to provide the 10.7 conversion. The oscillator’s output is tripled to 359.4 MHz to provide the 130.5 MHz conversion for the L-Band Tuner output. A three-sec- tion helical resonator is used for the 130.5 MHz IF filter.

An additional band-pass crystal filter, centered at 10.7 kHz, with a 10 kHz bandwidth, can be added by front-panel selection, to provide narrow resolution. The 300 kHz bandwidth filter is always in the circuit regardless of the front-pa- nel RESOLUTION selection. Maximum bandwidth of the 1705A is 300 kHz.

The resolution filters drive a FSK receiver IC. Only the meter output of this IC is used to provide a voltage proportional to the log of the input power. This drives a selectable video filter and the Vertical Deflection Amplifier.

The output of the Ramp Generator drives the Horizontal Deflection Amplifier (Diagram 4), Gain Control (SPAN/DIV), and the Marker Generator. The Ramp Generator free runs with its repetition rate controlled by the front-panel SWEEP SPEED control. The amplitude of the ramp remains constant.

The Gain Control, which provides the ramp that is eventually used to drive the VCOs, consists of an operational amplifier with selectable input resistances. The resistance selected is dependent upon the SPAN/DIV setting selected from the front panel. The output ramp from the Gain Control circuit drives the Sweep Shapers.

The CENTER FREQUENCY control provides an offset to the sweep ramps in all SPAN/DIV settings except FULL. In the FULL SPAN/DIV setting, a

4- 2 1705A Spectrum Monitor

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