[time-nuts] FE-5680A FAQ update: question about frequency synthesizer architecture

beale beale at bealecorner.com
Fri Jan 27 13:27:32 EST 2012

I added a bit to the "electronics" section of the FE-5680A FAQ as below.

(Note- until today, I had the 8 and 6 digits transposed, calling it the fe5860a. But no one noticed :-)

The updated section is below. I measured the 20 MHz input and 5.3 MHz output of the DDS, but I'm puzzled by how the tuning resolution (4.6 mHz) of the DDS output is divided by such a large factor to achieve 0.18 uHz resolution at the final 10 MHz output. Can any frequency synthesizer gurus explain how this is done?

The main digital electronic parts are:

    Maxim DS80C323END (8051, 44TQFP, -40/+85C, 18 MHz, 4 8-bit ports, 64K/64K ROM/RAM )
    STMicro PSD813F1V-20UI (1Mbit flash, 256Kbit EEPROM, 16Kbit SRAM, 3k PLD gates, ISP)
    Xilinx XC9572XL VQ64BN (64-pin CPLD, 178 MHz, 72 macrocells)
    Analog Devices AD9832BRU (25 MHz Direct Digital Synthesizer, on-chip 10-bit DAC)

Other ICs on digital side of PCB:

    Maxim MAX708 CPU supervisor
    Maxim DS1832 CPU watchdog, brownout detect
    Maxim MAX882 3.3V LDO (5V input)
    Maxim MAX1246 4 ch. 12-bit ADC
    Maxim MAX3232 RS-232 transciever


A 60 MHz sine from the VCXO enters CPLD pin 64 and it generates 3.3V square wave outputs at 30 MHz (pin 22), 20 MHz (pin 1), and 10 MHz (pin 49). The 20 MHz output goes to the clock input of the AD9832 DDS chip, which generates a 5.3 MHz sinewave output (nominal, when the RS-232 offset is set to 0). The frequency resolution of the DDS itself is (Fclock)/2^32 and since Fclock=20 MHz, the 5.3 MHz output is tuned in steps of 4.657E-3 Hz. However, the FE-5860A 10 MHz output step size is 1.7854E-7 Hz, so the DDS output frequency must be effectively divided in the overall system by a factor of about 26000 at the 10 MHz output (or a factor of 4333 at the 60 MHz VCXO frequency). The Rb hyperfine transition is at 6.835 GHz which is about 683x larger than 10 MHz.

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