need to make a switchbox for testing audio gear

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..and need more eyeballs to go over it.

I'm about to embark on a PCB layout, but would like a second opinion on
the circuit to see if I need more 'feature creep'.

I have all this Tektronix 5000 series test gear and they all support
GPIB control to automate their behavior. The missing link is a
switchbox to change routing programmatic-ally to allow tests that need
external routing changes.
Some of those tests would be:
  • input CMRR (both 50 ohm balanced and 600 ohm unbalanced)
  • input impedance
  • input shorting for testing noise floor
  • line output impedance
  • line output CMRR
  • line output balancing tests to see that it unbalances properly
  • microphone phantom power range
  • amp output routing for different loads (8,4,2) including open-circuit
    and an alternate speaker.

My Tek MI5010 is loaded with three 50M30 DIO cards and can drive up to
48 relays directly, so I don't have an issue if I need to add more
switching. Looks like I'm up to *just* 27 now ;) A routing matrix for
doing an entire console at once is something I can do later/separate.
I'm just concerned about the single ins/outs right now.

So the question is... What did I forget?

Also, I tried looking at using a CPLD to data compress the states down
to 16-bit (thus just one 50M30), and even wrote the VHDL source for it.
The problem, or I guess my fear, is that I had to go with a Xilinx
XC9572-7-PC84. That's 84 pins!! Though I need 40 I/O lines, I barely touch
its capabilities... That, and a $230 programmer, too, but it is cheap @
$7.35 in singles
Digi-Key - 122-1444-ND (Manufacturer - XC9572-15PCG84C)


  • swtch_jig.pdf
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  • input_logic.vhd.txt
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yup. The Tektronix MI5010 with three 50M30 cards can directly power 48 relays and is controllable over GPIB. So a test suite summary example for measuring input CMRR would be something like this:

> switch bypass
< switch OK
> osc do sine;1000Hz;rms(1.228V)
< osc OK
> dist do measure db
< dist measure ready
> getdata dist -> $a
> switch input bal-CMRR
> dist do measure db
< dist measure ready
> getdata dist -> $b

Input CMRR equals $a-$b all done by the computer and it should take 0.75 seconds or less.

Input impedance would be this off hand:

> switch bypass;inputZ-V2
[INDENT]^- relay0 enabled on the input brd[/INDENT]
< switch OK
> osc do sine;1000Hz;rms(100mV)
< osc OK
> dist do measure volt
< dist measure ready
> getdata dist -> $v2
> switch inputZ-V1
[INDENT]^- relay0 and relay1 enabled on the input brd and relay5 enabled on the output brd monitoring across R1.[/INDENT]
> dist do measure volt
< dist measure ready
> getdata dist -> $v1

input impedance would then be $v2/(($v2-$v1)/1000ohms) and it should take 0.75 seconds or less to complete.

The question I have is... Have I missed any common tests in the switching unit that require routing changes? I think I have everything, but knowing myself.. as soon as I build it I'll realize something I forgot. I'd go over this with the other folks at the job, but none are qualified to help. That's why I'm here.

My test gear isn't that helpful to me if I don't ever follow through and automate it the way it was meant to be.. and frustration is the true mother of all invention because all the qualifying tests I have to do take forever and this isn't how things are supposed to be.


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more complete schems

more complete schematics of my switcher project idea. Not sure yet of the proper grounding scheme, but will follow AES54-2-2008 as best I can.. one input test does need a terminated pin1 at the DUT.

Usually my indicator of when a design is done, is when there is nothing more to take away. In this case, I guess I do need to churn out a unit and stop over-thinking it to discover missing features by using it <sigh>

I'll keep it modular (each schem will be PCB) so it'll be easy to mod.

The whole logic board will be the part that'll take the CPLD if I ever go that way. I'm up to 37 relays now. 4 more and I'll have to get a larger CPLD than that 84 pin one. bwaa haa haa.


  • patch_AES54-2-2008.pdf
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  • swtch_jig1.pdf
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  • swtch_jig2.pdf
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