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Winter 2005 Issue |
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MICROWAVE The Qualcomm “Omnitrack” DRO Synthesizer By Chuck Houghton, WB6IGP |
Photo A. Bottom of the synthesizer PC board with pins of the synthesizer chip isolated by dremel-cutting around each pin to isolate them for experiments on other frequencies. |
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The Qualcomm DRO (dielectric resonant oscillator) synthesizer normally operates on a frequency of 2620 MHz and has an output power of +10 dBm. Frequency stability is under control of a dielectric resonant controlled oscillator (DRO). This ceramic puck stabilizes the oscillator, and synthesizer phase-locked-loop (PLL) circuitry must be modified to change the output to another frequency more desirable for amateur microwave use. Unfortunately, the DRO synthesizer is controlled by its divide-by reference of 1.25 MHz (random-access [RA] counter set to divide reference 10 MHz by 8). A solution was to change the RA counter to divide the stock frequency of 10 MHz by 64 for a new reference frequency of 156.25 kHz. There are many possibilities, but for this application the irregular frequency steps limited those frequencies that could be reached by the PLL chip. Modification of the DRO took the form of adding solder—bits of copper or short lengths of copper wire soldered to the top of the DRO, stretching the DRO resonance. This could be a small coupled pF capacitor, or an inductance used to tweak the DRO to a new frequency (free-running frequency near desired operation point). The above is where this project lay for some years. Then while cleaning out the shed and trying to reorganize material stored there, I came upon a large box of DRO synthesizers. At the same time, I revisited the file of material and modification notes that John Stevens, WB2BYP, and I had worked on previously. I had done the original modification work using the stock reference frequencies of 1.25 MHz and 156.25 kHz, but John saw a much better use for these DRO synthesizers, working his plan around a new reference frequency of 1 MHz. This allowed many other possible frequency combinations that were capable of being reached by reprogramming the PLL dividers and constructing a new divider (divide by 10) to produce a 1-MHz reference from the 10-MHz TCXO (temperature-compensated crystal oscillator).
John put together a great modification
procedure and gave me his permission to cover it here. I am sure it will
be met with great interest, especially because the synthesizer PC boards
are still available from the author (WB6IGP) for modification to other
useful microwave frequencies.
The sequence of images and comments builds
upon my article entitled “Above and Beyond,” which appeared in the June
1994 issue of 73 Magazine. I had known about these synthesizers for a few
years, and only recently started thinking about them as an LO (local
oscillator) or marker generator for microwave experimentation.
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