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 75Ω loads.

As Figure 1 shows, the outputs use 75Ω 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 (www.hp.com) HP10811A component oscillator for the Colpitts oscillator. Connect the HP10811A's output through a 510Ω 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.

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