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Difficulty driving an ADL5350 (LO) directly with a AD9851

Question asked by bmackenzie on Feb 18, 2011
Latest reply on Feb 25, 2011 by bsam

Hello,

 

I apologize for including a question about a DDS device in the RF section, but the crux of my problem revolves around the performance of the passive mixer in question.

 

I am attempting to drive the LO port of a ADL5350 directly from an AD9851. Due to the nature of the DAC in the DDS, I am running its output through a DAC reconstruction filter before connecting it to the LO port on the AD5350.

 

I believe my problem is with the amplitude of the signal the AD9851 is producing. Like I said, i'm filtering the output through a lowpass filter that's effectively removing all of the harmonics, and obviously introducing some attenuation to the signal path. Additionally, the output of my reconstruction filter must also be AC-Coupled to the LO port of the ADL5350 -- again, this causes some attenuation.

 

I can't seem to get more than 0.5Vpp out of the DAC of the AD9851 when terminated into a 50ohm resistive load. Is this to be expected? Is there any way to squeeze any extra output amplitude out of the device itself? I'm trying to preserve the low phase noise characteristics of the device. After being pushed through my reconstruction filter (and terminated to another 50ohm resistive load), I can't seem to achieve more than 0.1vpp at the LO port itself.

 

Also, the datasheet for the ADL5350 is fairly unclear about the input levels for the biased LO port. That is, I couldn't find anything about the acceptable high-low amplitudes, or even the performance of the input buffer. Is it being driven into saturation/cutoff?. The only detail I could find on the drive level of the LO port was from the characteristic curves which dictate an experimental setup of +4dBm LO signal. Is this a typical input amplitude for the LO port?

 

Finally, despite LO port of the ADL5350 being AC coupled, I'm still finding that my input LO frequency has a DC-Offset of approximately 0.28V when applied to the port of the device. When the ADL5350 is removed, no DC-Offset is seen on the LO drive signal. Is this simply the bias level of the port and things are operating as expected? The simplified functional diagram in the datasheet of teh ADL5350 doesn't offer any details as to whether a DC offset should appear at this port.

 

Thoughts?

 

***EDIT #1****

 

After further experimentation, I've made some progress. I'm able to witness some functionality from the ADL5250. Without modifying any of the input LO drive levels listed above, I began experimenting with the value of the "LO Bias Inductor" (labeled L4 on page 17 of 24, Figure 58 of the ADL5350 datasheet).  Unfortunately, I had to swing the value of L4 way outside what is recommended in the data sheet. This is due to my LO drive frequency being significantly lower than what's illustrated. Here's some statistics of what I was able to some results with:

 

RF Input Frequency: 138.0MHz

LO Input Frequency: 28.35MHz

IF Frequency (Goal of downconversion): 109.65MHz.

L4 Inductor Value: 1.0uH (it's all I had on hand).

 

The datasheet doesn't explore the concept of either such a low LO frequency, or modifying the value of L4 so extremely. There's an "applications information" that depicts a suggested L4 value of 100nH for an LO frequency of 109MHz -- but that's as close as it gets.

 

I would assume that i'm making up for my low input amplitude by having such a large LO bias inductor, but, what is the impact of this? I can't imagine it's good for overall performance.

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