Voltage-Controlled Oscillator 6~

The M/N voltage-controlled oscillator (VCO) consists of a resonator A7A4AlAl and Q2 with its associated circuitry. The oscillation frequency is determined by the bias on the varactors CR1 and CR2 which tunes the cavity resonator. Coarse tuning is provided by Cl, while

C5 is used to vary the output coupling to the cavity thereby varying the output power. The impedance looking into the emitter of Q2 has a negative real part which provides the conditions necessary for oscillation to occur.

Buffer Amplifier @

Ql is a common-emitter buffer amplifier which provides at least 0 dBm output over the 355 to 395 MHz range of the VCO.

LO Amplifier @

QS and Q7 form a buffer amplifier to assure that there is approximately 0 dBm to drive the ECL divide-by-2 IC (U2 in block 0). R24 and R25 set the proper dc level to drive U2.

M/N Output Amplifier @

Q3 and Q4 provide buffering between the ECL divider U2 and the output.

A7A4 M/N Output, Troubleshooting

The VCO tune voltage from the A7A3 M/N Phase Detector is amplified and applied to the Voltage-Controlled Oscillator (VCO) by the loop amplifier. The VCO output is amplified, divided by two, and applied to the AllA Sampler for phase locking the AllA YIG-Tuned Oscillator (YTO) in 10 MHz increments. A portion of the VCO output provides feedback to the A7A3 Phase Detector for phase locking of the M/N Loop.

Spectrum analyzer control settings for A7A4 troubleshooting:

( 2 - 2 2 )

(SHIFT) (MKRJREF LVL) (KSR)

(SHIFT) (BW) (KSF)

(CENTERFREQUENCY) . . . . . . . .

[FREQUENCY SPAN) . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.77 GHz

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 Hz

Loop Amplifier @

The tune voltage from the Loop Amplifier can be measured at TPl, located on the cover of the A7A4. The dc voltage at TPl, with the front panel settings indicated above, shoud be -14.2 V. If an unlocked condition exists, the voltage is approximately -0.5 V or -37.5 V. If the level is -0.5 V, the probable cause is no VCO output to the A7A3 Phase Detector.

A level of -37.5 V indicates that the 20 MHz reference to the A7A3 Phase Detector is not present.

2 A7A4