A7A4

A7A4 M/N Output, Circuit Description

A7A4 amplifies the output of the A7A4Al M/N VCO in two different paths. One path is amplification and buffering to drive the mixer in the M/N phase-lock loop. The other path drives a divide by two which, after further amplification, goes to the AllA Sampler.

The integrating loop amplifier which generates the tuning voltage for the M/N VCO is also included on A7A4.

LO Output Amplifier @J

Q5 is a common-emitter amplifier followed by a resistive pad to increase reverse isolation. Q2 is a common-base amplifier which is broadly tuned by L2, LlO, and associated capacitances. Ql is another common-emitter amplifier which is followed by a 400 MHz low-pass filter.

Divide By 2 @

U2 is an ECL divide-by-2 Integrated Circuit. U2 generates the M/N output frequency which is one-half that of the M/N VCO. For frequency spans of greater than 5 MHz, the M/N control line (LMNE) goes to a TTL high during the sweep. This disables U2, switching off the M/N output to the AllA Sampler. At the beginning of each sweep, U2 is turned back on and the M/N VCO is phase-locked to give the correct start frequency. During multi-band sweeps, U2 is turned on at the beginning of each band, but is turned off during the actual sweep.

Loop Amplifier @

Ul is connected as an integrating amplifier and generates -5 V to -35 V for tuning the M/N VCO. Cl3 is the integrating capacitor; Cl2 and Cl5 compensate Ul to guarantee stability. R33, R34, C14, and R15 compensate the entire phase-lock loop. The 200 kHz low pass filter rejects the sampling frequency (20 MHz divided by N) and its harmonics which are generated in the phase detector. CR6, R42, and R41 form an impedance breakpoint that increases the loop gain by approximately 5 dB when the tune voltage goes below approximately -7 V. This helps to make the varactor tuning curve linear.

For instruments with A7A4 M/N Output Assembly, part number 85660-60267, power for integrating amplifier Ul is provided by an additional +20 V connection (Pl-9, P24) and by QlO which generates -15 V from the -40 V supply. The output of Ul is buffered by common-base amplifier QS and by emitter-follower buffer Q9.

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