One of them uses the PICAXE AXE401 shield with a socket for the DDS oscillator to be inserted. The shield was placed inside an aluminum chassis and is held up by insulating standoffs. Though there are no ground planes in the AXE401 shield, I was able to create a controlled impedance channel to surround the RF path with ground up to the transition into the coax which is pigtailed onto a SMA(F) cable mounted to the chassis.
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| Figure 2-1: Chassis assembly with display, tuning knob, and buttons. |
The LCD is a White Letter with black background and is backlit for ease of viewing. The LCD is hardwired to the AXE401 because I ran out of jumper length. I got it from MicroCenter in Fairfax, but you can get it directly from www.sparkfun.com. The two switches are normally-open momentary SPST switches which are tied high with 10k resistors to 5V.
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| Figure 2-2: Front View of Oscillator with Freq Counter next to it. |
The dial controls the frequency while the two buttons move the cursor back and forth. If the cursor is moved onto the MHz Scale, the band will switch 80m>40m>30m>20m>17m>15m>12m>10m>80m, or in reverse depending on which way the dial is rotated. As figure 2-2 shows, the display and oscillator are in fairly good agreement. I later recalibrated the MFJ to match the spectrum display as well.
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| Figure 2-3: Front View of Oscillator/Freq Counter - tuned to 10.118MHz |
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| Figure 2-4: Spectrum of Oscillator Tuned to 30m Fc = 10.118MHz Span = 100kHz Resolution BW = 100Hz |
My next Blog post will show the other fixture I designed with my own PCB layout.
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