Over the course of people building autorangers, parts values can be refined, as follows:
1. The attenuator board layout shows an SMD 'C13' at the middle front. This capacitor is not fitted at the board and does not need to be fitted.
2. In some cases, the calibration range of the capacitor trimmers is not large enough. For the -48dB calibration, it is recommended to replace capacitors C5 and C17 (22pF) with a 47pF value if available, or just short them. With the -16dB calibration, C7 and C11 (22pF) can simply be left out.
All specifications are available in the Spec Sheet which is part of the Data Package which can be downloaded from the Autoranger page.
There are 3 calibrations that should be performed. The correct order is first to do the level meter accuracy and common mode adjustments, and then the frequency response adjustments for single ended and balanced inputs. To perform the calibration procedure, the cover of the unit has to be removed and test signals inserted and/or measured at internal test points as indicated in the calibration procedure.
In the calibration procedure the signal generator is called upon to supply output levels up to 10V or more, at frequencies up to 100kHz. An AC RMS voltmeter is needed for various tasks: For the display accuracy calibration, the AC voltmeter should be able to measure a 1V signal at 1kHz at preferably better than 1%. For the CMRR ration a null adjustments is done, and the AC voltmeter should be able to measure down to the mV level at 1kHz. For the frequency response flatness adjustment, the AC voltmeter should have a bandwidth extending to a high frequency, preferably 100kHz. Absolute accuracy is less critical here as the measurements are relative to levels.
For a complete description and possible workarounds see the Calibration Procedure in the Data package.