Friday 22 November 2013

Finalizing the design of the Raspberry Pi Wobbulator PCB

I've been experimenting with the AD8307 logarithmic amplifier from Analog Devices as a potential replacement for the discrete buffer amplifier / detector stage I used in the Raspberry Pi Wobbulator prototype.

The following screenshot shows the frequency response of an RF crystal as measured by the AD8307 logarithmic amplifier. The horizontal line near the top of the screenshot is the plot obtained when the output from the DDS module was connected directly to the input of the AD8307 log amp.


Compare the above plot to the plot shown below which was obtained when the same frequency sweep was performed but the response was measured by the discrete buffer amplifier /detector stage.


The plot obtained when using the discrete buffer amplifier / detector stage has much more pronounced peaks, but it should also be noted that the frequency of the primary response peak is noticeably higher than when using the AD8307 logarithmic amplifier. I can only assume that the coupling capacitors used on the input to the AD8307 effectively "pulled" the resonant frequency of the RF crystal lower. The horizontal line in the above plot was obtained when the output from the DSS module was connected directly to the input to the discrete buffer amplifier / detector stage.

From the results above It is clear that the AD8307 and the discrete buffer amplifier / detector stage both have their own strengths and weaknesses, so I've decided to include both on the Raspberry Pi Wobbulator PCB! The proposed schematic for the Raspberry Pi Wobbulator PCB is shown below.


The proposed design has an output from the DDS module ("RF OUT"), and two inputs, one of which is connected to a discrete buffer amplifier / detector stage ("RF IN LIN") and the other of which is connected to a AD8307 logarithmic amplifier ("RF IN LOG"). All connections are in the form of PCB mounted SMA sockets, as well as PCB pins.

The outputs from the discrete buffer amplifier / detector stage and AD8307 log amp are connected to channels 1 and 2 on the MCP3424 ADC chip. The other two channels on the ADC chip are terminated in PCB pins so that these channels can be used with an external detector, or just used as general purpose analogue inputs.

I'd welcome any comments on (or corrections to) the above schematic because I'm currently putting the finishing touches to the PCB layout, and once this is completed I will be placing an order for a batch of PCB's to be manufactured.

6 comments:

  1. I would prefer that the Raspberry Pi GPIO PIns used were link selectable.

    ReplyDelete
    Replies
    1. I haven't made the GPIO pins user selectable because the software needs to "know" which pins to use. If the GPIO pins used were selectable, the user would have to set the pins on the board and in the software. This would of course be possible to implement, but I've tried to make it as easy to use as possible.

      Why not join the "rpiwobbulator" Yahoo group for the latest information and to contribute to the project.

      Kind Regards,

      Tom

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  2. Any chance of making this stackable between the Pi and a PIFACE CONTROL & DISPLAY unit ?
    That way data can be collected and recorded as a 'portable' unit for later analysis.

    ReplyDelete
    Replies
    1. This should be possible, and I can certainly see the attraction of going "portable". There may be some GPIO pin reassignments necessary, but that shouldn't be a problem - perhaps a good reason to make the GPIO pins selectable (see previous comment).

      Why not join the "rpiwobbulator" Yahoo group for the latest information and to contribute to the project.

      Kind Regards,

      Tom

      Delete
  3. Hello Tom, the difference in the "response peak" can be explained by the nearly linear response of the the diode rectifier on one hand, and the logarithmic response of the AD8308 on the other hand.
    While the diode circuit has a dynamic range of about 30 to 40 db, the AD8307 will have about 90 db.
    The DC bias of the diode has the benefit of increasing the sensitivity, but with the drawback of increased temperature sensitivity. For me, the best would be the AD8307 plus a FET probe with a low input capacitance.
    Best regards, Rudolf DL5FA

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    Replies
    1. Hello Rudolf,

      Thank you for your very helpful comment. I realize that this explains the difference in the heights of the peaks, but what I don't understand is why the peak when using the AD8307 is at a slightly lower frequency. As I mentioned above, I con only assume that this is due to the capacitance on the input to the AD8307 chip.

      Why not join the "rpiwobbulator" Yahoo group for the latest information and to contribute to the project.

      Kind Regards,

      Tom

      Delete