K2WS OCXO modification to the SDR-1000

click here for new Instruction guidelines document 11/06

Note: This modification is recommended only for experienced builders. Installation requires careful disassembly of the 4 board stack to have access to the DDS in order to swap jumpers and connect the new oscillator. A connection to the 13.8VDC+ supply is necessary to power the new OCXO. Care must be taken so that the transceiver is reassembled correctly. This is not a plug-and-play modification.


pdf version click here

for prices, contact Alan at <k2ws@hotmail.com> An OCXO for the SDR-1000© Alan Davis   K2WS February 3, 2006     My SDR’s  frequency stability needed some improvement.  These days has been  ESSB or High quality SSB. Using these modes, it is important that the rig be absolutely stable because a change of 10 to 30Hz will spoil an otherwise excellent quality signal.     I did a study on the effect of changes in temperature in the SDR cabinet with changes in frequency.  I found on my rig that the oscillator drifted (@ 15mHz) about 5Hz/ F° and that SSB transmissions would increase the internal cabinet temperature as much as  5° to 10° F.  This resulted in a see-saw, up-down frequency drift of  10 to 25Hz during a QSO!  The only time the rig was stable was during receive only mode and only when the room temperature  didn't change more then 1° F.       While 25Hz ‘drift’  might not even be noticeable for everyday SSB/CW operation,  I required more stability for my ESSB operation.   I suspect there are many other SDR users who also require higher stability such as VHF,UHF, microwave users and data mode users.     After looking at the Flex Radio reflector, it seemed that others were interested in solving this problem as well.  Although the solutions were elegant and  provide extraordinary performance, they seemed a bit extravagant  to me.  For example. a GPS-disciplined oscillator requiring a 2nd receiver and antenna,  or a device that phase locks the current 200mHz Valpey Fisher oscillator to a highly accurate standard using an external box. I didn't want the hassle of an  external box and antennas to produce  10 –12th  accuracy.     Looking at the high-end rigs on the ham market, we see mostly TCXO’s and 1  OCXO. The TCXO’s provide about 0.5ppm over 0° to 50° C and the OCXO in the Icom 7800 provides 0.05ppm over 0° to 50°  C.  Either method  should work  on the SDR-1000 and the resulting frequency stability should be  more in line with  other high-end rigs.      I searched for an OCXO that provided very low phase noise, availability within 2-4 weeks, a cost under $200 and the option to buy in small quantities.  To make a long story short, I found a 40mHz OCXO with frequency stability of  ± 0.03ppm  over –20° to +70° C with  low phase noise e.g., -100dBc/Hz @ 10Hz.  This unit could be acquired in 2-3 weeks and I bought  the  minimum  order of  3 pieces for $150 / OCXO.     In 2 days of spare time I produced a prototype 40mHz OCXO unit built on a small perfboard  that is mounted on standoffs behind the terminal block of the SDR’s chassis.  All that is necessary to use the new OCXO is to go to the Setup Menu and change the Clock Offset to 0 and the PLL Multiplier to 5.  After a 5+ minute warmup, adjust the Clock offset so that the rotating vector in the Phase Display Window slows down to a minimum –while tuned WWV.               My OCXO equipped   SDR-1000 is now within  0.05Hz of 15mHz WWV and it doesn't deviate more than  0.5Hz under varying temperature due to long transmissions!  I no longer need to ‘tweek’ the VFO to compensate for my rig drifting, what a relief!  For the 1st time I can rely on the frequency readout of my SDR-1000 – down to ONE HERTZ.   That $150 OCXO has made the SDR as stable as  the  $10000  Icom IC-7800. For complete info on my SDR-1000 OCXO project, including the stability study and photos of the OCXO board installed, please see: Improving the Frequency Stability of the SDR-1000.pdf 73, Alan Davis   K2WS for prices, contact Alan at <k2ws@hotmail.com> Note: This modification is recommended only for experienced builders. Installation requires careful disassembly of the 4 board stack to have access to the DDS in order to swap jumpers and connect the new oscillator. A connection to the 13.8VDC+ supply is necessary to power the new OCXO. Care must be taken so that the transceiver is reassembled correctly. This is not a plug-and-play modification. ©all information Copyright 2006 Alan M. Davis K2WS