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.