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The FLuke PM6685 is a benchtop frequency counter, part of the PM668X family of counters. It features a resolution of 10 digits per seconds on all frequencies with an additional 2 digits with the overflow function enabled made possible by its high-accuracy rubidium counter. Included also is auto-triggering with autosensitivity and autowaveform compensation. The display also features a bar-graph level monitor showing signal level on a dB scale. The PM6685 features two signal inputs (A and E, both with +/- trigger slope), an external reference input D, and reference output G. The primary input A (AC coupled) is a BNC connector with a selectable impedance of 1 Mohm or 50 ohm. Input frequency ranges from 10 Hz to 300 MHz and a level sensitivity of 10 mVrms at 10 Hz to 50 MHZ, 15 mVrms at 50 to 100 MHz, 20 Vrms at 100 to 150 MHz, 30 Vrms at 150 to 200 MHZ, and 50 mVrms at 200 to 300 MHZ;pulse sensitivity is 50 mVpp with a 3 ns minimum pulse width. Input A dynamic Range: 30 mVpp to 70Vpp. For manual triggering, the sensitivity ranges from 10 mVrms to 10 Vrms, variable in 3 dB steps, and trigger level is selectable on waveforms with duty factors of <0.25, 0.25 to 0.75, and >0.75. In automatic trigger mode, the minimum frequency is 50 Hz with a sensitivity range of 10 mVrms to 25 Vrms. Low-pass filter: 100 kHz nominal 3 dB point, and a minimum of 40 dB of attenuation at 1 MHz. Secondary input E is DC coupled and uses a BNC connector, used in ratio with input A and external arming/gating with a frequency range of DC to 80 MHZ. Input E allows for a minimum pulse width of 6 ns and a minimum slew rate of 2V/micro-second. The external reference input D is AC coupled with a BNC connector and an input frequency of 10 MHz and a voltage range of 200 mVrnms to 10 Vrms. The impedance is approximately 1 kohm. The reference output G is also AC coupled with a BNC connector, outputing a 10 MHz sine wave at a level of >1 Vrms into 50 ohm. Auxiliary functions include external signal on input E to inhibit start and/or stop triggering; nulling frequency offset, measurement time selection, local/preset modes, restart (starts new measurement), display hold, analog output (select digits and scaling factor), and display backlighty toggling
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