SynthDescription

Both synthesizers require a phase reference. These signals originate from the hydrogen maser in the control building and are transmitted on fiber optic cable to each antenna. Both synthesizers require 128 MHz. The L302 requires it for clocking the DDS which creates the references used internally for phase locking and the L301 requires it for it's phase locking reference. Microwave harmonics are provided by the L300.

L300 Synthesizer Reference Generator

The L300 receives 128 MHz and 512 MHz and creates harmonics of those frequencies for use as references in the L301 and L302. The 512 MHz harmonics are also utilized to produce 1024, 2048 and 4096 MHz for use in down conversion and as clocks.

The YIG oscillator output is electrically tuned to approximately 128 MHz from the desired harmonic. The 512 MHz harmonics output from the L300 are mixed with the YIG oscillator output to produce a nominal 128 MHz "beat note", or more proper, IF or intermediate frequency. The phase locked loop circuitry then takes over and forces the YIG oscillator to adjust it's phase and frequency to exactly (N*512) +/- 128 MHz. The synthesizer can operate, therefore, in 256 MHz steps. It's has been designed to tune to 11.904 to 20.096 GHz. In phase lock, the synthesizer takes on all the attributes of the reference, such as phase noise and frequency stability for all offsets from the carrier up to the bandwidth of the phase locked loop. For offsets outside the PLL bandwidth, the YIG oscillator exhibits it's characteristic low noise. Wide tuning ranges and low noise make the YIG oscillator the only practical device for wide band radio astronomy and interferometry.

The L302 is a dual loop design that enables the synthesizer to continuously tune to all frequencies between 10.8 and 14.8 GHz. This is accomplished, in part, by the dual loop design and the sub hertz tuning capability of the DDS references. The control loop is similar to the L301 synthesizer, except that the harmonics are of 128 MHz. The control loop can tune to any frequency +/- 22 to 42 MHz from a harmonic. This generates a 22 to 42 MHz IF where phase locking occurs. The Phase reference is a direct digital synthesizer. The main loop, or output loop is essentially another synthesizer like the control loop, except that it's reference is now the control loop output. The main loop YIG output mixes with the control loop output to produce a 1 to 22 MHz IF. This IF is compared to another DDS signal for phase locking the main loop. Interferometry requires that each antenna be at a slightly different phase to compensate for the different arrival times of the incoming electromagnetic waves. This is accomplished through precision timing and control of the main loop's DDS reference. We desire phase coherence and this is not natural for a digital synthesizer, so pains have been taken using timing signals and careful programming to ensure this.