Plumbing a laboratory with a standard frequency makes a lot of sense if the lab uses multiple frequency counters, spectrum analyzers, and other frequency-dependent test equipment. Rather than spending time keeping all of the instruments' oscillators in calibration or buying expensive, high-precision oscillators, you can use the circuit in Figure 1 to distribute a single calibrated frequency source to the external-reference input of each instrument. The circuit represents a simple, 10-MHz source and distribution amplifier. The output comes not from the emitter or collector of the Colpitts-oscillator transistor, Q1, but rather from the current flowing in the 10-MHz crystal. The common-base stage, Q2, converts this current into a voltage and establishes the correct dc level for the output amplifier, IC1. This IC contains four gain-of-two buffers with 110-MHz, 3-dB bandwidth and can drive double-terminated 50 or 75 Ω loads.
|Figure 1.||A laboratorywide distribution system is an alternative to multiple frequency sources.|
As Figure 1 shows, the outputs use 75 Ω impedance levels to take advantage of inexpensive F-type connector hardware and low-cost video coaxial cable. IC1 also provides good isolation between its outputs, so that changes in loading on one output do not affect the other outputs. The circuit delivers more than 6 dBm to each termination. If high accuracy and low drift are critical needs, you can substitute Hewlett-Packard's HP10811A component oscillator for the Colpitts oscillator. Connect the HP10811A's output through a 510 Ω resistor and a 10-nF coupling capacitor, directly to the emitter of Q2. If you need more than four outputs, you can duplicate the IC1 stage as many times as necessary.