Automatic Level Control Techniques for DDS-based Designs - TODAY 8PM

[knology wb4jfi]

It’s good to see that the Chat With The Designers is back!

I’ve been adjusting the output level of DDS chips for a while now.  Typically, I use an I2C connected digital pot, such as the MCP4551.  You can adjust the DDS output level by varying the 3.9k resistor used to adjust the DDS output current level.  I’ve found that you can drop that 3.9k resistor to almost half that value without too much issues, and then use a digital pot to adjust the actual resistance value above and below 3.9k.

There is also the possibility to actually AM-modulate the 3.9k resistor pin with a voltage from a D/A converter.

I’m on vacation in Myrtle Beach right now, so I will probably miss tonight’s event.  Rats!!!
73, Terry, WB4JFI


From: Karl W4KRL 
Sent: Tuesday, May 06, 2014 3:31 PM
To: tacos AMRAD 
Subject: Automatic Level Control Techniques for DDS-based Designs - TODAY 8PM

http://www.cwtd.org/May_6-2014.html

 

"Automatic Level Control Techniques for DDS-based Designs" will be the topic on this Tuesday's episode #68 of "Chat With The Designers" at 8pm ET on Teamspeak.

 

Most homebrewers know that DDS boards suffer from output level roll-off due to the sampling theorem (sinc function), as well as due to gain-product bandwidth limitations of amplifiers used after the LPF on the DDS boards. And sometimes that roll-off can result in as much as five times lower voltage level at 60 MHz than at the lower frequencies. For applications using a DDS in accurate measurement applications, designers usually correct for this frequency-dependent level change using some sort of calibration technique ... which can sometimes be a challenge.

 

We have developed a relatively simple ALC circuit that appears to be working splendidly at keeping the output of the DDS-60 at a relatively constant output level from 1-60 MHz, and we'll be discussing the circuit and techniques in-depth, along with different approaches to calibration in measurement instruments benefiting from constant signal source levels.

 

The whiteboard used for each episode is at http://www.cwtd.org/May_6-2014.html ... it's somewhat under construction at the moment, but will be completed in time for the show on Tuesday.

 

Hope you can join us

 

!73, George N2APB & Joe N2CX www.cwtd.org

 

PS: It's easy to get on Teamspeak ... http://www.cwtd.org/TeamspeakInstructions.html 



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