Programming FE-5680A Rubidium Frequency Sources (RFS)


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 ( 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.

Programming the FE-5680A

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.

Example Calculations

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.