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Very low sensitive of AD9874

Question asked by gurovegor on Dec 16, 2014
Latest reply on Dec 22, 2014 by PMH

We are using your product AD9874 for wideband high quality TV tuner and narrow band high sensitive radio and have some questions about it.

Firstly we are interested at narrow band system. Frequency is about 145 MHz.

Electric circuit of connection is figured on the picture 1. Program settings locate in file “Registers.txt” and table 1. Here used an external clock signal (20 MHz, -5 dBm by Agilent generator) and external LO signal (142.5 MHz, 0 dBm by Rohde&Schwarz generator). The external RF (radio input) signal is unmodulated sine (140.0002 MHz, -30 dBm by Rohde&Schwarz generator). Using the output data we plot graph and see sine wave 200 Hz as expected, figured on the picture 2. Reducing the level of input RF signal to -60 dBm we can see distortion, figured on the picture 3. As a result we have very low sensitive about -60 dBm when expected is about -90 dBm with 7 kHz band on 140…145 MHz.

To clarify this low sensitive we have measured IF signal spectrum in order to see two frequencies 2.5 MHz and 282.5 MHz and there amplitude. On the picture 5 is figured the spectrum of IF signal when the CLK signal is off and on picture 6 is figured figured the spectrum of IF signal when the CLK signal is on. We see a lot of frequencies unknown origin. But the most important 2.5 MHz signal have a very low level -92 dBm (and -70 dBm when CLK off).

We can’t explain these results and have no idea how to repair it. Why we see LO signal in the IF signal with a so high level? What is the 129.16 MHz frequency? Moreover we are don’t understand where is the filter between mixer and ADC. Probably needs an external filter after (or before) 100 pF capacitors?

 

Picture 1

Picture_1 - AD9874_mak_SCH.png

Picture 2

Picture_2 - RF signal -30dBm.png

Picture 3

Picture_3 - RF signal -60dBm.png

Picture 4

Picture_4 - IF spectr (clk off).PNG

Picture 5

Picture_5 - IF spectr (RF -60dBm).PNG

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