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AT2380v1 //  2 Channels digitally controlled stepped attenuator
AT2380v1 //  2 Channels digitally controlled stepped attenuator
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Old 24th July 2017, 01:11 PM   #1
Frex is online now Frex  France
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Join Date: Nov 2009
AT2380v1 //  2 Channels digitally controlled stepped attenuator
Arrow AT2380v1 // 2 Channels digitally controlled stepped attenuator

Hello all,

Here is my new little DIYproject, the "AT2380".

As many DIYers here, i often needed to attenuate a signal to fit better with the input level of the target system.
This is a tool that i need to perform some tests on my other design, the AA2380 ADC.

So, i was thinking to building an attenuator for a long time.
There is of course some already built, but good attenuators are generally dedicated for RF application with 50/75 Ohms impedance.
This is a too low impedance for the main audio and instrumentation applications that i target.

I work on this project for some months now, in parallel with some others.
I designed it to meet my needs and to be a cost effective solution to share it with all interested DIYers.


The main specifications of the attenuator are :

  • full digital control (5M570 CPLD, VHDL coded).
  • 2 pseudo isolated attenuators Channels
  • Zin= 600 Ohms , Zout = ~ 0 to 300 Ohms (customizable)
  • Flat frequency response (± 0,05dB) from at least DC..1MHz (± 3dB to 10 MHz).
  • 0 to 127,5 dB attenuation in 0,5 dB step (8 bits / 256 steps)
  • Direct dB attenuation setting displayed on 4 digits seven segments LEDs.
  • Five selectable dB steps : 0.5 ,1 , 3, 6 and 10 dB by encoder increment.
  • Optional output level auto-tracking mode from +10 dBV to – 85 dBV (attenuation only) .
  • Full internal clock enable switch, for ultra quiet operation mode.
  • Calibration mode for dBV measurement
  • Relay fast refresh mode for audio volume control application
  • Bluetooth UART link to control attenuator and measure output dBV level
  • Shielded design to improve RF performance and noise immunity .
  • Low cost design and easy to find parts.
  • 100x160mm PCB size, fit in Hammond 1455 series extruded aluminum enclosure.
  • Front and rear panel made in PCB will all holes drilled and with gold silkscreen.
  • Through holes axial or SMD (1206) ladder attenuator resistors types.


These specs are preliminary as i don't have yet tested it, but it's the target.
Each attenuator channel use a ladder resistors design switched by 8 relays.
So, with 0.5dB resolution the attenuation can be controlled over 0 to -127.5 dB range.
(You can see how work a ladder log attenuator and calculate value here.)

The control of the attenuation is done by a rotary encoder with push button.
For easiest setting, the dB step by encoder increment can be choose between 0.5-1-3-6 or 10 dB.
The actual attenuation level is show on a 7 segments led display.

There is also the possibility to use it in tracking level mode.
When this mode is enabled, the output level of the attenuator track the desired level needed in the range of +10dBV to -90dBV (3Vrms to 30µVrms).

At this time, the prototype PCB has been send for manufacturing and i will have it in next week.
I will post a picture as soon it is in my hands.

You can show below the synoptic of the design :

Click the image to open in full size.

Others infos will follow soon, and i hope that the project will interest others DIYers.
Regards.


Update of the 20/11/2017
------------------------------------


FInal V2.0 released
The full schematic and the manual of the attenuator is available below :

Full schematics : AT2380v2_sch.pdf

Operating manual and specifications : AT2380V1_Manual_v0.pdf

Functional Synoptic :
AT2380v2 functional synoptic

Complete BOM file for both track and not rack, with Mouser price list :
AT2380v2 full bill of material with Mouser price list


I finalize the Gerbers files, and the group-buy for bare PCB will be open on next week-end.
More infos soon.




Update of the 26/11/2017 -- PCB GroupBuy started
---------------------------------------------------------------------------


The PCB group buy is started and can be found here :
AT2380v2 2Ch Digitally controlled Stepped Attenuator




Update of the 30/01/2018 -- Firmware Update. (v0.23)
---------------------------------------------------------------------------


I released today the new 0.23 CPLD firmware that solve some bugs and add new features :
- Version display at startup
- Calibration mode.
- Relays fast mode operation (for audio volume control applications)
- Bluetooth UART mode attenuator control

Because of these many modifications, i have made a new design folder (v2.1),
provided to all PCB owners.

Some files are available to download on the links below :

AT2380_SynopticV2.1
AT2380v2.1_schematics
AT2380_Manualv1.1
AT2380V2_Bill_of_material_Rev1



Note that bare PCB are available, if interested send me a PM.


Frex

Last edited by Frex; 30th January 2018 at 04:28 PM. Reason: update
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Old 25th July 2017, 03:44 PM   #2
jcga is offline jcga  France
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Hi Frex,

Subscribed, I'm definitively interested
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Old 31st July 2017, 04:07 PM   #3
Frex is online now Frex  France
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Join Date: Nov 2009
AT2380v1 //  2 Channels digitally controlled stepped attenuator
Hello,

Thank you Jcga, you are faithful to my projects

I received the prototype PCB's, and to show little more
what did it look like, some pictures of them below :


The complete PCB with main board and the attached front and rear panel :
Click the image to open in full size.

The PCB detached :
Click the image to open in full size.

A small mechanical verification with front panel :
Click the image to open in full size.

Same with rear panel :
Click the image to open in full size.



All seem to fit well for now, but it's just a fast test.
I must now order few missing parts, i will have all very soon.
So next step will be to solder all of that....

I will update soon.
Regards.

Frex
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Old 2nd August 2017, 05:07 AM   #4
1audio is offline 1audio  United States
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AT2380v1 //  2 Channels digitally controlled stepped attenuator
I'm interested too. Adding a USB interface could enhance its utility.
I would not expect the settings below -100 dB to be too accurate. At that level of attenuation stray coupling will dominate. The old GR's would you two sections for the higher attenuations and even then they have issues. At 50 Ohms you can get to higher frequencies but its still an issue. At RF its almost hopeless. On the HP Network analyzers you can put closed terminations on them and it still gets through all the stainless steel.

Here is how GR built their attenuator: http://i670.photobucket.com/albums/v...en/GR14504.jpg It shielded isolated elements with feedthroughs to the next stage. They get 200 KHz out of it. Those resistors are great but virtually unobtainable. The switch contacts are also really premium.
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Old 5th August 2017, 07:24 AM   #5
Frex is online now Frex  France
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AT2380v1 //  2 Channels digitally controlled stepped attenuator
Hello,

I worked a little this week on it and i made the 2 copper shield panels to separate
each attenuator channel. I also cut the red window for the Led display.
Missing parts are ordered, i will have all to be soldered next week.

Demian, I don't have planned to provide USB link, nevertheless it is doable.
There is some unused IO available on en header connector (test purpose)
that can be used with an ftdi chip for example to add usb control.
So, some code will have to be added of course and CPLD is not so big.

About the attenuation obtainable, as soon prototype build,
i would made extensive tests to verify attenuation performance versus frequency
at different attenuation levels. Defining full and clear specifications of
any electronics instrument is essential.

Frex
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Old 9th August 2017, 05:47 AM   #6
1audio is offline 1audio  United States
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AT2380v1 //  2 Channels digitally controlled stepped attenuator
You may need a shield can/copper around the -60 dB and possibly the -30 dB steps. Those are the ones where parasitics can dominate the attenuation.

Accurate measurement of attenuation at high frequencies is really difficult. There are many possible error mechanisms that make the process difficult. However once done it should be very stable. Also 10V -120 dB is 10 uV. Few instruments can measure that without noise dominating. Usually frequency selective meters are needed. You will need to plan for those challenges.

Lets see how it performs before getting into automation. However that plus a programmable oscillator would be ideal for checking an RIAA curve for example.
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Old 9th August 2017, 03:25 PM   #7
mediatechnology is offline mediatechnology  United States
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Location: Dallas, Texas
Great work Frex.
Subscribed to this thread.
Thanks also for the link to the stepped attenuator calculator.

Using the circuit board for a front and rear panel is also very clever.

If you have spare SPI port on the SM570 you might bring it out to a header if you haven't already done so.

Last edited by mediatechnology; 9th August 2017 at 03:27 PM.
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Old 11th August 2017, 05:41 AM   #8
Frex is online now Frex  France
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AT2380v1 //  2 Channels digitally controlled stepped attenuator
Hello,

Thank you for your comment Mediatechnology.
Any I/O of the CPLD can be used for a SPI link, and there is a 8 IO free connector
available on the PCB. I add it for test purpose.
So, adding SPI link to control the attenuation level could be done, just need to write
some code with taking in account available cells...

Demian,
As maybe that you can see on the PCB pictures, there is some (4) slot inside PCB.
These slot are intended to add 2 vertical copper shields made in PCB.
After insertion, they are soldered all along the PCB on the "golden line".
It's intended to reduce significantly HF impedance of ground planes ans also to shield channels.
I will post a picture with them inserted this week.

I don't have made much job on the design these day, DIY work in the house
is taking all my time for now...

Frex
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Old 16th August 2017, 06:52 PM   #9
Frex is online now Frex  France
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AT2380v1 //  2 Channels digitally controlled stepped attenuator
Post Parts soldered ! :o)

Hello,

I mounted all parts these days. All fit well.
Below, some pictures of the result.

Demian, you can see the two vertical copper shields between
each attenuator channel (not yet soldered).

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.



It has not been yet powered.Time to test now...


Frex
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Old 16th August 2017, 06:59 PM   #10
1audio is offline 1audio  United States
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AT2380v1 //  2 Channels digitally controlled stepped attenuator
That's really clean. You may need shields between some segments but that could be simple. Some thin sheet copper would work and it doesn't need to be an RF tight shield since this is audio.
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