PLL Oscillators for Microwave Transverters

PLL 1136 Two synthesizers were developed for 10 GHz fig. 1 and DB6NT fig. 2, fig. 5, fig. 6 transverters to replace the existing crystal multiplier chains fig. 3. This is an inexpensive project, with easily available parts. The original transverter oscillators are quartz controlled, with simple temperature stabilization. This type of stability is not good enough over temperature extremes that are often encountered in the field. This is very frustrating especially during weak signal QSOs and contests. The new DB6NT transverters, do not even contain crystal heaters for stabilization.

oscillator demi Many of the operators of these transverters never know exact frequency, and are unable to successfully make QSOs. The lack of accurate frequency adds another unknown to the process of making contact. Many rovers are working in extreme conditions, under very hot, cold, dry or wet conditions, truck vibrations, and they have a limited time for operations. They need to have accurate frequency readout, and they don't want to wait oscillator to stabilize. During the 2002 microwave contest, we observed more than 30 KHz drift over temperature while roving in the Central Valley of California. Temperatures ranged from 65F in the AM to 109F in the PM, and back to 70F in the late evening. While we were quickly able to calculate the KHz/F (because of known mountaintop stations) there was always significant doubt at each setup.

One good solution is to lock the crystal, but this is not the easiest way. Another solution is to inject a locked high frequency signal, bypassing a few of the multiplication chains. The phase noise of new VCO's for 1200/2400 MHz is very good.

oscillator demi

The performance of synthesizers, phase noise and stability, highly depends on the quality of the reference oscillators. Both synthesizers are designed for 5 MHz or 10 MHz reference oscillators fig. 7. There is a jumper on the board to make this selection possible. There are many relatively inexpensive high quality reference oscillators available on the surplus market. There are typically two versions, with electrical or mechanical tuning.

Electrical tuning needs an external potentiometer. Such pots must be of good quality, with low temperature drift. However in many oscillators the tune voltage will be very close to the reference voltage provided. This means temperature drift of the pot will be approximately equal in both legs of the resistor. Ten turn 10K pots work very well. Mechanical tuning is with small screw, actually trimmer capacitor for fine frequency tuning (see picture with reference oscillators). The output level provided by the reference oscillator should be higher when the reference is below 10MHz (sensitivity of the PLLs to noise on slow signals is greater)

PLL 1278

Naturally, the synthesizers' phase noise is worse than with a quartz crystal oscillator and multiplier chain. On prototypes built for 1136 MHz and 1278 MHz, phase noise at 1KHz offset is -90 dBc and -85 dBc respectively. On original DEMI crystal oscillator board we measured is -95 dBc at 1kHz. It should be mentioned that every time the frequency is doubled, phase noise increases 3 dB. Definitely, this design is a trade off between phase noise and stability. These synthesizers are designed for mid-level radios and rovers, for a few watts and small dishes, up to 1 meter, but not for EME station level.

PLL 1278 bottom

To install the PLL 04 -1136 board in DEMI transverter is easy. It is necessary to remove the original crystal oscillator board fig.3, and replace it with the PLL 04 and reference oscillator fig. 4. No additional tuning is necessary, except to calibrate the reference frequency with pot or trimmer cap. During the power on state, the LED blinks once. This indicates that the program is loaded in the PLL.

reference tcxo

On 1278 MHz, the PLL LED will blink two times. The first blink indicates that the program is loaded in the PLL, and second blink indicates that the PLL is locked. If the PLL is unlocked, the LED will blink until the PLL locks. After the PLL is locked, the microcontroller enters the sleep mode. There is also a manual reset switch on the board. For the DB6NT transverter, it is necessary to remove three resistors and two capacitors, and install an SMA connector on the transverter wall.

phase noise test

Two of the PLL 04-1136 boards for the DEMI transverter were used during the 2003 microwave contest by roving stations in the Central Valley again. Temperatures were much more mild, only reaching 104F! During the entire contest the frequency stability was better than 100Hz at 10GHz. No more worrying about frequency stability!


demi modified

                          Shematics:

                          1136 PLL diagram
                          1278 cnt controller



     Modified Demi Transverter


Special thanks to Jim N9JIM for his contribution on this project and Dave AD6A for the pictures.
September 2003
 
 
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