A7A2
A7A2 100 MHz Voltage-Controlled Crystal Oscillator (VCXO), Circuit DescriptionA7A2 consists of a 100 MHz voltage-controlled crystal oscillator, buffer amplifiers, and a frequency quadrupler. In conjunction with A7Al Reference Phase Detector, it forms the reference phase-lock loop. Outputs of 100 MHz and 400 MHz are used as frequency references by other assemblies in the instrument.
100 MHz Oscillator @
Q5 functions as a common-base amplifier with feedback to form an oscillator. The feedback includes the 100 MHz crystal Yl and varactor CR1 which are the principal frequency-determining components. CR1 is tuned by the output of A7Al and can vary the frequency by approximately fl kHz. The amplitude of the oscillations is limited by CR3 and CR4. TPl is accessible from the top cover; the voltage should be near -8 V dc for proper operation. The voltage is adjusted by tuning the oscillator with C4.
100 MHz Buffer Amplifier @
QS, Q7, QS, Q9, and Qll form a series of 100 MHz amplifiers. They buffer the 100 MHz outputs from each other as well as from the oscillator.
Quadrupler @
100 MHz from Q7 in @J is applied to T3. The secondary of T3 is connected to Q3 and Q4 to form a full-wave rectifier. Positive half cycles of 100 MHz to T3 turn on Q3 while negative half cycles turn on Q4. The collectors are connected together to sum the currents and produce even harmonics of 100 MHz. The output is tuned to 400 MHz by C3 and LlO to select
400MHz.
400 MHz Amplifier @
Ql and Q2 are two common-emitter tuned amplifier stages which amplify 400 MHz and filter undesired harmonics of 100 MHz. R67, R68, and R69 form a pad which is selected to give -10 dBm f2 dB output at 400 MHz.