Those figures are interesting Dom ,thanks .
I think it shows why the output from the final stage is off by a margin , slightly low compared to where it should be .
To be honest Im not very wall practised in SMD rework , I look at the size of those resistors and the density of components I the area ,and get a bit scared by thinking about how easy it would be to mess up the board . Still I very much would like more accuracy from the amplitude display ,so Im strongly cosiddering upgrading all the resistors to 0.1% or better if I can through selection of components .
In the mean time ,I put together an enclosure for my post attenuator , it has a toggle switch with three positions , -10db, -20db, -30db , I also have a rotary two pole switch with 12 positions I'll set up as 10 single steps of 1db , a unity gain position ,and an off position that shorts the DUT input to ground through a 150 ohm resistor for noise testing . As far as precision is concerned ,if I can make the 10 db ranges accurate ,I dont really mind some 'slop in the gears' on the 1db range switch .
As you will see from the photos Im all done with the casework and constructional side ,she's looking good ,I have an idea the form the attenuators will take in my head but Im still not sure which is the most appropriate style of attenuation ,ladder or H network etc .
I also found a very neat stepped attenuator calculator ,you fill the blanks in terms of charachertistic impedences and attenuation you want and the software calculates the values , I think you can model several different kinds of attenuator topologies with it too . I'm unsure about the exact form both attenuation circuits need to be to function properly in series , and any advice people can offer would be welcome .
Its all a bit chicken and egg as far as choosing impedences , I need it for audio signals primarily ,but Id also like the attenuator to be usable at least up to a couple of hundred khz sine, I'll need to be able to feed single ended line level audio inputs of 10 or 20 kohms as well as high impedences directly from the output ,in the case of balanced connection being required a mumetal transformer can be used across the outputs ,I have a 1:1 10k-10k sowter transformer thats good up to about +10dbu and 50khz of bandwidth feeding 20 kohms input impedence. For mic level inputs which are around 3kohms normally, maybe a mic input transformer used in reverse(10k-600ohm) might work well.
I need more specific advice before I start building the attenuators really ,so I dont end up having to re-do it again ,you know the old saying 'measure twice cut once' , but my head is a little fuzzy on the maths
Pictures to follow soon , coffee time . I'l post too another forum about the specifics of the impedences and see if anyone likes the idea enough to fill in the blanks for me .