Author Topic: OSH Audio THD instrument and live BOM  (Read 4304 times)

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Offline rhb

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Re: OSH Audio THD instrument and live BOM
« Reply #25 on: February 12, 2019, 12:10:18 am »
What I really wanted to call attention to was his methodology. not copying the circuit.
 

Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #26 on: February 12, 2019, 01:40:48 pm »
I will have to do a little thinking about two of the things in that article, first the use of a CdS cell/LED pair to substitute for the JFET, and the other is the use of two adjacent 1N4148 diodes to somehow thermally stabilize the circuit.  I suppose that involves why a series reference diode also needed to be there.
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Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #27 on: February 12, 2019, 05:42:41 pm »
for what it is worth this is the oscillator module prototype.  The fact is that for whatever design changes I want to make in the future, the cost is mitigated by the modular design method employed here.  That lowers the bar to adoption for this instrument to what I hope is a manageable level.

The main point of this thread was not any specific circuit design decision, but an overall management strategy using what I hope is a useful spreadsheet tool that allows for project visibility from parts purchases to fabrication and assembly.  This instrument is intended as an example of a multi-assembly project employing such a tool.
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Offline rhb

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Re: OSH Audio THD instrument and live BOM
« Reply #28 on: February 12, 2019, 06:22:16 pm »
Very nice. My interests lean a lot in the RF direction. I eventually want to assemble a collection of modules that can be used for prototyping a design.  For that, having everything in  metal boxes with standard connectors, dimensions, etc is desirable.  So very much in the same vein as your efforts.

But I'm still buying test gear so I haven't got to the PCB and BOM yet.

I suspect that to really understand Jim's paper you should review the detailed physics of the devices.  Williams, Pease and Widlar were renowned for their understanding  of subtle details that less thoughtful people just blundered by and which were the difference between a circuit working and not working.  I also think the inclusion of the A2 output trace in the figures is a strong hint.
 

Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #29 on: February 14, 2019, 01:23:40 am »
These two pictures are the notch filter module and a view of the EAGLE image for the main board (the rev A board is coming back from fab sometime later this month, so the pretty parts picture is not available yet).  This picture is after I decided to allow for a seventh module to contain the track-and-hold circuit and another bypass toggle associated with it.  I am breadboarding the track-hold now and will test it externally for now, but allowance is made for the next (rev B) run to modularize it.  It might have to be done with SMD parts to fit the form factor, but specifying the parts for the ultra-stable construction I want is way more problematic in the 0805 or 0603 sizes I typically use.
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Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #30 on: February 14, 2019, 01:45:34 am »
I suppose, rhb, that for your purpose then something like my modular system could be used, except the board interconnect would incorporate some sort of SMA and rigid coax arrangement instead of the Molex KK type?  What about UFL?  I can see modules (37x50mm) each containing a mixer, attenuator, pre-amp, and the like.  The power rails are +/- 12V which is fine for the JFET op-amps I use, but would that be usable with your kind of RF circuitry?
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Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #31 on: February 14, 2019, 01:48:41 am »
Actually that broadens my questions a bit further.  I don't have the instrumentation to measure it, but does anybody know what are typical insertion and return losses from a right-angle THD Molex KK type connector with 2.54mm geometry?
« Last Edit: February 14, 2019, 02:02:38 am by asgard »
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Offline rhb

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Re: OSH Audio THD instrument and live BOM
« Reply #32 on: February 14, 2019, 03:39:02 pm »
I am planning on using SMA connectors for RF with 90 degree cables for most connections because I was able to get lots of them cheap.  I'll make up other cables as needed.

I plan to use boxes made of galvanized steel.  It's easy to solder, has fairly good H and E field shielding properties and i can get all the scraps I want from an HVAC shop.  I'll probably build some small bending brakes of a piece of steel channel and angle for a few standard sizes so that all I need to do to make a box is cut the sheet, clamp it in place,  fold it and solder the corners  The board will mount to the lid using standoffs with connectors, controls , etc on the underside of the the circuit board.  Each size will require two brakes, one slightly larger  than the other so the two nest properly

Once a module is finished it will be soldered shut with connector labels on the top and a schematic on the bottom.

Power and control is still  an open question.  I want feed thru capacitors on power and signal lines but have not identified a suitable item.  They are difficult to find at affordable prices.  Generally power would be 12 V  but other levels as needed.  I'm only interested in very low power (QRPp) operation.  In some cases e.g. an MCU control block, there will be an internal 3.3 V regulator in a subcompartment.

MiniCircuits sells connectorized version of their parts which package the device in a metal enclosure with the connector of your choice.  But at eye watering prices.

U.FL is only good for RF test points as they are only last for a very small number of connections.  I plan to place them in strategic locations that can be jumpered in w/ zero ohm resistors.

So that's the *theory*.  It will be interesting to see what reality is like.  I'm sure it will evolve over time.  I got the general idea when I opened up my HP 8601A sweeper.  It's a standard set of die cast boxes which are sized to fit a standard chassis.  The connectors are on the lids and the boards are edge mounted to the lids.

With regard to measuring insertion and return loss for the connectors you're using I *think* that you could do that  with an  impedance bridge and an appropriate test jig.  However, I really don't think there is much reason to even consider connector  insertion and return losses at these frequencies.  Surface oxidation resistive effects probably dominate. In any case, to be useful you'd have to be maintaining a constant  impedance level everywhere.  It looks a lot like a rabbit hole that would consume a lot of time.  Before you go too far in that direction, set up the algebra and look at the magnitude of the effects at audio frequencies.  If the connector losses matter you'll have to calculate the impedances all along the signal path.

I just bought an HP 8753B VNA which covers 300 KHz to 3 GHz so I shall be going to VNA school for a while.  I've also developed a bit of an OCD issue with EMI, so it is likely to get augmented with another instrument that covers lower frequencies so I can properly characterize EMI filters. That gets *very* messy because power system impedances are not well defined and definitely not 50 ohms.
 

Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #33 on: February 14, 2019, 09:20:52 pm »
This is the layout intended for the track-hold module.  It also sits in 37x50mm 4-layer construction, albeit with offset board interconnect and not centered.
« Last Edit: February 14, 2019, 09:44:57 pm by asgard »
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Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #34 on: February 20, 2019, 05:09:42 pm »
Here are links to the PCB's at OSH Park:

https://oshpark.com/shared_projects/BGlH40tE
https://oshpark.com/shared_projects/OEj5Q82Z
https://oshpark.com/shared_projects/SFhUT8oh

The Track/Hold module will be last but that will require more bench testing before I can release that.
Klaatu Barada Nikto!

J.R. Stoner Bifrost Development Group asgard@jeffnet.org
 

Offline asgardTopic starter

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Re: OSH Audio THD instrument and live BOM
« Reply #35 on: March 14, 2019, 01:32:30 pm »
Here is a potential trap for yourng players.  Electromechanical datasheets are the worst.  Every vendor seems to have their own method of doing mechanical drawings that are, unfortunately, unique.  Also, never attempt to interpret them at 2:00 in the morning.  Main board needs a respin because the rotary switch gazoutas won't go into the gazintas.
Klaatu Barada Nikto!

J.R. Stoner Bifrost Development Group asgard@jeffnet.org
 


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