I purchased a number (4) of these atomic frequency standards on eBay. The specific versions of the FE-5680A I purchased come equipped with a DDS (direct digital synthesis) IC AD9830A from Analog Devices, which produces a sinusoidal output signal at approximately 6 dBm. Users can remotely adjust the frequency - within limits - through an RS-232 interface using a computer. There are various versions of the FE-5680A available on the market and so information given may not be applicable to all units. The particular models bought require just a 15 V DC input power supplied through a 9-pin Sub-D connector - except for one unit which requires an additional +5 V input to enable a RF signal out. During start-up, the current consumption is around 2 A, but it drops to 0.75A once the module reaches its operating temperature.
All the information for the usage of these units at HawkRAO came from Matthias Bopp (DD1US) in a PDF document which can be accessed from his website (https://www.dd1us.de). Due to the variations between units no specific connection information is provided on this webpage - the PDF just mentioned should be consulted for that information.
After disconnecting the power supply, connect the RS232 cable to both the computer and the FE5680A module. Then, reconnect the power supply to the device.
Open a terminal program, such as Window's HyperTerminal, and configure it to open the COM port to which the cable is connected. Set the terminal to 9600 baud, no parity, 8 data bits, 1 stop bit, and no flow control. To test the connection, type a capital "S" into the terminal and press Enter. If the connection is successful, you should receive a response that looks like this:
R=50255055.299544Hz F=32F0AD99E3D4E4400
If you do not receive a response, ensure that your terminal program is set to output CR/LF characters when you press Enter. NOTE: the last 4-byte hex words (the greyed out E3D4E4400 in the example given above) can be ignored - both when writing down the 'F' value, and when programming using the 'F' command.
To program the output frequency, use the command structure "F=abcdefgh" followed by the Enter key. The variable "abcdefgh" represents a 4-byte hex word, such as "32AB56DF". The output frequency is calculated by the formula:
N / (2^32) * R
- where "N" is the 4-byte hex word that you entered (ranging from 0 to (2^32)-1), and "R" is a reference frequency of approximately 50.255 MHz obtained above in the "S" command response.
Credit: Don Latham for discovering the commands and Guy Ovadia for providing them.
These examples are for two RFS unit on hand (#41757 and #52970) and show programmed values for 10 MHz.
Note that the output frequency setting resolution is about 50.255 MHz / (2^32) - about 0.0117 Hz. Consequently the closest frequency setting to 10 MHz is 9.999999998 MHz - which is about 0.002 Hz low. The next step up is 10.000000009 MHz - which is further away at about 0.009 Hz high. These offsets from 10 MHz are -0.0002 ppm and +0.0009 ppm respectively. So - although 10.000000009 MHz looks 'nicer', 9.999999998 MHz is actually closer.
NOTE: although the short-term stability of the FE-5680A source is of the order of 0.0001 ppm, there is a drift specification of 0.002 ppm/year. Assuming an age of 20 years, this amounts to about 0.04 ppm. The actual setting resolution is 0.0117 Hz or 0.0012 ppm @ 10 MHz. It seems reasonable to assume that the accuracy of the 10 MHz output frequency is within +/-0.05 ppm.