Building a noise meter from the RAT noise tester. This design note describes how to build up "part of" the RAT noise tester, to achieve a log AC voltmeter that can be used to test your system for noise level. Specifications: Frequency Response: 20-20kHz Range: 1uV to 300uV (50 dB range), an input attenuator is described to provide 40 dB and 80 dB attenuation, extending the range to 30 mV and 3V. This may be further extended if desired. Input Impedance: 1 Meg. Modifications: Power Supply: The high tension voltage is not used at all, so the power transformer can be replaced with a 6.3V 1.2A transformer. Build the "filament" DC regulator section only. The supply will have only one output: regulated 6.2V. The LM309 does not need heat sunk. Noise Tester: The tester does not need the sockets, nor switches 3-20. It does not use HT voltage, so L1, C20, R41 and related circuitry is not used. Only the 4 IC's and the one transistor shown are retained. You may keep S21 (the function switch) as a convenient calibration tool. As R45 selects the heater voltage, it may be removed, but remember to wire the 6.2V contact on S21 if you want to use the switch as a calibration switch. Other circuit modifications: S12 is not used so connect the bottom of R10 to ground. C15 is not used. R9 and C13 are not used. The 60 Hz notch filter is not used so eliminate R20, 21, 22 and C14, 16 and 17. Connect the top of R32 to pins 1 and 2 of U2. Other circuit modifications: Change C2 from 5pF to 3.9 pF. Input attenuator: The noise meter input is the junction of D1, D2, and Q1 base. A suitable input attenuator is: input-----|(-------+------o<--o------to junction D1, D2, Q1 base 0.1u | 909k | | +------o | 10k | | +------o | 100 | attenuator: 0, 40, 80 dB | ground Calibration of the unit is accomplished as described in the schematic.