Re: Very basic tests of step attenuator
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Alan
Fantastic!? That information is what I was looking for when I posted the original question. Now to work up some ADC code for an Arduino so the testing process can be automated.?
This is all related to making a few step attenuators using SMT parts.? Tolerances (and
probability of errors) seems to warrant a testing procedure to verify each attenuation step. Higher attenuation steps may be beyond the Arduino ADC resolution range but I can
work around that by adding DC amplification.
Thanks,
Arv? K7HKL _._
On Fri, Mar 29, 2019 at 6:08 AM Alan < g8lco1@...> wrote: The attenuation at DC is exactly the same as at LF as the stray capacitance? and inductance will? be insignificant up to around 10MHz .? I would suggest that a 50 ohm resistor is inserted in series with the power supply to the? input?? and the output has a 50 ohm resistor to ground.? I would use two 1% 100 ohm? ? W ??wire ended metal film resistors in series for each termination. ? If the direct voltage is 2V to the? input side termination and you use a DMM , set the voltage across the 50 ohm series R to 1.000V then the voltage across the input to the atten gives? the ratio of the input resistance to the 50 ohm resistor ?to around 1% accuracy. The calibration of the DMM is unimportant, the linearity of the DMM should be very good, much better than it¡¯s accuracy. ? Then you can check the voltage ratio between the input and output to get the attenuation.? Turn the attenuator around for? checking the output side input resistance. ? The limitation of this method is thermal voltages adding or subtracting from voltages. As these are around a few micro Volts and you are using around a Volt of drive? the errors are very small for home use. Thermal emf¡¯s ?can be cancelled ?by reversing the supply Heating in the resistors is easy enough, with 1V across 50 ohms it¡¯s 1V * 20mA=20mW. At ?2.236V you reach 100mW, 3.535V is 0.25W,? 5.0V is 1/2W ( 5*0.1 W)? 7.707V( root 50)= 1W? so 2V should be safe without too much thinking. ? It¡¯s the linearity of the DMM that matters not the attenuator ( if you don¡¯t change ranges ). That should be better than 1 in a thousand? reading around half scale on a 1999 count meter. ( cheap meters are dual ?slope) so reading a 20dB pad to about? 1%.of ratio. A 19999 count would read the attenuation to 0.1% of ratio ?. I would be inexpensive to invest in some 0.1% 100 ohm resistors for many reasons! ? Alan G8LCO ? ? ? ? ? Sent from for Windows 10 ? ? Can I test the DC attenuation of a step attenuator by using just a DC source voltage and DC voltmeter across the 50 ohm termination? This would not validate LF to HF performance but just the DC characteristics. The math seems to indicate that this is possible and that it is maybe a rather simple
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