Tektronix 1705A instruction manual If Amplifier Diagram

Theory of Operation

by a 7-pole, 120 MHz, low-pass filter that rejects out-of-band frequencies. The Mixer, U190, with input frequencies of 45 - 100 MHz, and the Local Oscillator operating at [Fi + 130 MHz] provide a 130 MHz 1st IF. The signal output from the Mixer is terminated by a 75Ω, 3 dB pad made up of R291, R292, and R293. The IF signal is ac coupled into the amplifier, U296, with a gain of 20 dB.

R396, R391, and R392 form a 6 dB pad that outputs the 136 MHz 1st IF signal that approximates the L-Band output level for inputs 10 dB higher.

L393, L391, L389, C292, and C290 form a low-pass filter that is peaked at the 1st IF frequency (136 MHz) and approximately 20 dB down at the Local Oscillator frequency. Harmonics of the Local Oscillator frequency are at least 30 dB down.

70 MHz Local Oscillator The Local Oscillator (LO) is a Voltage Controlled Oscillator (VCO) whose output frequency range is 175 MHz to 230 MHz. The oscillator is a differential pair (pins 7-8 and 5-11) contained in U283. The oscillator employs positive feedback through L284 to the base tank circuit (L280 and C281). CR280 is a varactor whose capacitance is determined by the instantaneous level of the pre-corrected ramp from the Sweep Generator (Diagram 3). The center tap of T187 is the LO output providing an amplitude of approximately +7 dBm.

Q20 and Q21 form a switching circuit that turns off the VCO when the L-Band input is selected. When /LBAND goes low, Q21 is shut off, causing Q20 to unsaturate and disconnect the -11.8 V from the VCO.

The 70 MHz tuner assembly (A5) is contained in a rigidly mounted shield. Control signals and the -11.8 V supply are brought into this shielded tuner through feedthrough capacitors.

IF Amplifier

Diagram 2

This diagram shows the L-Band and 70 MHz inputs to the various conversion stages. It also shows the Local Oscillator/Tripler circuitry, Mixers, Amplifiers, IF filters, and Log Detector output.

2nd Local Oscillator Both Local Oscillator (LO) signals are derived from a single crystal, whose operating frequency is 119.8 MHz. The crystal is a 5th overtone type in a Butler oscillator configuration using Q13. Q11 buffers the oscillator and drives a 3 dB isolation pad on the LO input of Double Balanced Mixer (DBM) U25. Q12 is a cascode stage driving a tank circuit at the 3rd harmonic (359.4 MHz). The tank circuit is tapped down to drive a three-section helical resonator, FL1, tuned to 359.4 MHz. The output from FL1 is coupled into DBM U28.

1705A Spectrum Monitor 4- 5