Monday, 28 October 2013

Using the Raspberry Pi Wobbulator to test a RF Crystal and an IF Transformer

First of all I used the Raspberry Pi Wobbulator to examine the frequency response characteristics of an RF Crystal. One side of the crystal was connected to the output from the DDS module and the other side was connected to the input to the Buffer Amplifier / Detector. The outer metal casing of the crystal was grounded using a crocodile clip, and the crystal was tested with and without a small (10pF) capacitance in parallel. This capacitance was added simply by connecting the capacitor across the crystal using a pair of miniature crocodile clips.


 The nominal frequency of the crystal being tested was 14.070 MHz so the start frequency for the sweep was set to 14.060 MHz and the stop frequency for the sweep was set to 14.080 MHz and the increment was set to 100 Hz. Channel 2 was selected on the ADC Pi module and the PGA gain was set to 1x. The results are shown in the following screenshot.


The blue plot shows the response of the crystal on its own and shows the resonant frequency to be just above 14072000 Hz. The red plot is the response of the crystal with a 10pF capacitor connected in parallel across the crystal, and shows the resonant frequency to be a little over 14068000 Hz. This demonstrates nicely how the resonant frequency of the crystal can be “pulled” by adding a small capacitance in parallel.

Then I decided to use the Raspberry Pi Wobbulator to test the frequency response characteristics of a small IF transformer coil (TOKO Part RMC15002A - see photo below). This is a 455 kHz RF transformer and is typical of those used in the IF stages of many HF receivers. Proper alignment of the IF stages of a Superhet receiver is critical for good performance and it was for this task that a "conventional" Wobbulator was typically used, so I considered this to be an important test for the Raspberry Pi Wobbulator.



The frequency sweep was set to start at 200 KHz and finish at 700 KHz, with an increment of 1 KHz. Channel 2 was selected on the ADC Pi module and the PGA gain was set to 1x. The results are shown in the screenshot below.


The results show the response to peak at just over 450 kHz as would be expected. The results from this test and from testing the RF Crystal both demonstrate the effectiveness of the Raspberry Pi Wobbulator for measuring the frequency response characteristics of a component.

In my next post I'll discuss how I used the Raspberry Pi Wobbulator to test a multiband bandpass filter. 


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