Detecting very low power RF pulses with AD8310

I'm working on a design using the AD8310, attempting to detect very low power RF pulses, on the order of -75dBm with pulse widths ranging from 400 to 500-nsec.  My log detector output is buffered with a differential op-amp (in a single-ended to differential configuration) and then digitized with a 12-bit ADC.

When I disconnect the log detector from the buffer, the ADC reads out +/-2 counts.  With the log detector connected to the buffer input (log detector input 50-ohm terminated) I am seeing a noisy waveform with peaks around +/-150 ADC counts.  I disconnected all other circuits in the signal chain up to the log detector from the power supplies, so I'm convinced that what what I am seeing is coming from interactions among the log detector, buffer, and ADC.

I probed the log detector input and output with a spectrum analyzer, and did not see any frequency content above the noise floor of the analyzer.  When I probe the buffer output, I see many harmonics of the 25-MSPS sampling clock.

When I probe the log detector output with a digital scope, I am seeing ripple with fast (~20nsec) rising edges and slower (100s of nsec) RC-looking falling edges and amplitudes on the order of 10's up to a couple hundred mV.  I acquired an AD8310 evaluation board, and see the same fast-rise/slower-fall waveform when probing the evaluation board output with the scope, leading me to think the choppy waveform may be caused by my power supply or the scope.

Has anyone else seen this behavior and found a good way to mitigate it?

Karl

  • 0
    •  Analog Employees 
    on Feb 22, 2013 11:57 PM over 8 years ago

    Do you have a signal applied at the input of the AD8310 while you are probing the output?

    The evaluation board has some supply decoupling, but not much.  Referring to Figure 40 in the datasheet, you could try populating C5 with a 0.1 uF cap or even a 1 uF cap and see if that helps reduce any signal that may be getting on the supply.  Adding a little bit of resistance for R5 might help as well; something on the order of 2 to 15 Ohms. 

    Another thing you can try is populating C3 to decrease the output video BW (refer to the Output Filtering section on page 18).  I would start by slowing the output way down by putting a 1uF cap on BFIN, and see what happens. 

    Let me know if this helps.

    Joel

  • I saw the oscillation behavior both with and without a signal applied.  I did notice at some level of input signal (around -75 dBm @ 70-MHz input drive) the oscillation seemed to drop in magnitude.

    I populated C5 with a 4.7uF cap, and tried both a 10-ohm resistor and a ferrite bead in the R5 position.  I can't imagine those hurt, but did not make a noticeable improvement to the output stability.

    Placing a large (1uF) capacitor on C3 cleans up the response, but slows the rise time to many milliseconds.  My application requires rise time in the 10's of nanoseconds for detecting pulses on the order of a few hundred nanoseconds.

    In addition to the fast oscillation, I found a slow oscillation by switching the scope over to line trigger.  This slow oscillation had a 60-Hz component as well as some higher harmonics, when the eval board was connected to the signal generator.  The scope has a battery powered mode of operation, so I disconnected the scope from the wall, and the 60-Hz component remained, leading me to think it's coming from the signal generator.  The product is a wireless receiver, so we should not be subject to 60-Hz coupling from signal generators in the field, but our sensitivity performance standard test is based on a measurement with a signal generator hooked up to an external antenna connector.

  • 0
    •  Analog Employees 
    on Feb 25, 2013 10:34 PM over 8 years ago

    Is there a way that you can power the AD8310 eval board with a battery?

    Also, can you send or post the schematic of your application?  Have you altered the eval board in any way (beside the supply decoupling and the output filtering you tried)?

    Thanks,

    Joel

  • I did power the eval board from a battery.  It didn't make things worse, but did not solve the problem either.  I thought my best bet was to run the eval board and scope of batteries, so the only ground connection would be the signal generator (Agilent MXG series).

    Another change I tried was loading a C-L-C narrow-band matching network as described on pages 16 and 17 of the datasheet.  It appeared to lower the floor, but not eliminate the oscillations.  The oscillations were present on the board as it was delivered here, with the broadband 50-ohm match and default component population.

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:45 PM over 2 years ago
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin