1/6 Langenbrettach, February 18th 2008 Matthias Bopp A precise reference frequency not only for your ham radio station Rev 0.4 A ham radio station needs a stable reference frequency if you want to move to very high frequencies or operating modes with very narrow bandwidths. Probably the worst case is reached if both aspects combine like it is the case in deep space satellite communication: 1.) Due to the very high distances between the satellite and the ground station the operating frequency is moved up to make use of high gain antennas with a reasonable size. 2.) At the same time the bandwidth of the signal is reduced to achieve higher S/N ratios. I intend to prepare myself for the reception of deep space satellite signals as the next generation of ham radio satellites is in design and will hopefully be launched in the next 2 years. Therefore recently I started to work on a suitable concept for the local oscillator generation of my system. I decided to buy a surplus Rubidium Frequency Standard and selected the model FE-5680A from Frequency Electronics Inc. After some search I bought mine from Guy Ovadia (guyorcon-) who provided a very professional and kind service. I have chosen a version of the FE-5680A module, which includes a DDS (direct digital synthesis) IC AD9830A from Analog Devices and thus provides a sinusoidal output signal of approx. 6 dBm with a user specified output frequency from 1 Hz to 20 MHz. This “Option 2” allows the remote adjustment of the output frequency by a computer through an RS-232 interface. Please note that there are multiple different versions of the FE- 5680A available. This module has the size .98 x 3.47 x 4.92 inches respectively 25 x 88 x 125 mm. The input power of 15 V DC is supplied through a 9-pin Sub-D connector. The current consumption is approx. 2 A during the start-up time and once the operating temperature of the module is reached it is reduced to 0.75A. My module did not supply the RF signal outside the module nor was the RS232 connection externally available. You will find instructions on the necessary modifications later in this article. Rubidium Frequency Standard FE-5680A Here is the pin-out of the 9 pin SUB-D connector: PIN FUNCTION NOTES J1-1 +15V DC power input J1-2 GROUND Provides DC return, RS-232 return J1-3 Loop Lock Indicator Indicates whether or not the output frequency is stabilized to the Rb atomic reference. Low output means it is locked. It should go low a few minutes after power up (it takes longer when the unit is cold). If you connect a 5-10 kOhm resistor from J1-1 to the anode of a LED and connect the cathode of the LED to J1-3, the LED will go on when the unit is locked. J1-4 NOT USED J1-5 GROUND Provides DC return, RS-232 return J1-6 1 PPS output 1 pulse per second output signal J1-7 NOT USED J1-8 RS-232 Rx True RS-232 signal with 12V J1-9 RS-232 Tx True RS-232 signal with 12V The parameters of the serial connection are: 9600 baud, 8 bits, one stop bit, no parity, no flow control. 2/6 Here are the technical data of my FE-5680A module: PARAMETER SPECIFICATION Output Frequency Programmable from 1 Hz to 20 MHz with a resolution of 1x10-12 Hz Type Sinusoidal Amplitude (minimum) 0.5 Vrms into 50 Ohm (+7dBm) Adjustment Resolution 3Vdc = unlocked, 1Vdc = locked Input Power ( 25C) 11 watts steady state, 27 watts peak DC Input Voltage/Current 15V to 18V 1.8A peak and 0.7A steady-state Ripple +15V: 0.1 Vrms Warm-up Time 5 minutes to lock 25C Size 25 x 88 x 125 mm / .98 x 3.47 x 4.92 inches Weight 434 grams respectively 15.3 oz. As my module did not supply the RF signal outside the module nor was the RS232 connection externally available I modified the module and routed these signals out. You can find pictures of the modification on the next pages. To route the signals outside the modules I had to make some modifications. I started with the lid of the cabinet where I added 2 openings around the Sub-D connector to route 2 additional cables to the outside. Modified lid to route the RF and RS232 signals out Left RF semi-rigid cable, right RS232 cable 3/6 This is the top view of the open FE-5680A module with the additional RF semi-rigid and RS232 cables. On top you see the physics package covered by foam for thermal insulation. On the bottom right you see the DDS (Direct Digital Synthesis) board where you find the RF- and the RS232 signals which have to be routed outside. Detailed view of the DDS board where the 2 cables are connected to the PCB Front view with closed lid and lock LED Front view with closed lid 4/6 Detailed view, where to tap the RF signal Detailed view of the RS-232 connection The centre pin of the semi-rigid cable is soldered to the centre pin of the Hirose RF jack. The shield of the semi- rigid cable is soldered to multiple ground connections of the PCB. The RS-232 signals are available at a 5 pin connector on the lower right of the DDS-PCB. Only the 3 leftmost pins ar