[time-nuts] DDS on AD9852

[Bob Camp]

Hi

The amp you are using has about 18.5 db of gain. It’s a typical self biased transistor style amp. From the part number, it’s got a ~ 1.5 dbm output limit at 1 db compression. That gives you a max input level of  -17 dbm when the amp is in compression. If you want reasonable noise performance, you would stay 10 db below those numbers. That would put you in the -29 dbm range. -20 dbm is 31 mv peak to peak. That’s way below the levels you have running around on your scope displays. 

Because these amps are fairly simple in construction, when you drive them with large signals, you upset the bias. That changes the operation point. It can easily lead to all sorts of odd distortion. Before it does that it often leads to increased noise. Typically when you are after a quiet 7  to 10 dbm output (as in low phase noise and low harmonics) you go for at least a 16 dbm (1 db compression) amp and possibly one at 20 dbm. There are exceptions to this, but most of the simple little amps work this way. 

Bob




> On Jan 17, 2015, at 12:02 AM, d0ct0r <time at patoka.org> wrote:
> 
> 
> 
> 
> Hello,
> 
> I would like to ask some advise about the signal distortion I get from my DDS project.
> The T-Bolt GPSDO is in use as a REF signal source (10Mhz) for AD9858 DDS. This chip has two COS analog outputs which opposite to each other. I am using 20x reference multiplier in my project. And I am using Low Pass filter and RF amplifier connected to ONE of the COS output. The other output has no filter or op-amp on it. Attached is four files with OSC screen shots, where the signal which goes through the filter and amplifier somehow distorted. In contrary, the signal from another output (without filter and op-amp) has near perfect curve. I am using the same cable for connection to Oscilloscope.. And I set 5 Mhz output on DDS as an example. In this my project I am using Ten-Tec broadband pre-amp. The filter is standard Chebyshev Low Pass filter made by MINI-CIRCUITS. Model PLP-90 (81 Mhz).
> 
> Here is the link to Ten-Tec
> http://www.tentec.com/products/Universal-Low%252dNoise-Broadband-DC-to-1-GHz-RF-Preamp-%252d-Model-1001.html
> 
> 
> Is it normal to have that kind of distortions for the signal, or something wrong with design ?
> 
> -- 
> WBW,
> 
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