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+--- Thread: ATTEN F2700-C FREQ COUNTER OCXO UPGRADE (/thread-1100.html)
ATTEN F2700-C FREQ COUNTER OCXO UPGRADE - VK6ZFG - 13-11-2021
ATTEN F2700-C OCXO UPGRADE
The Atten F2700-C frequency counter uses a 13 MHz TXCO reference as this provides a greater counting range than 10 Mhz. A 13 Mhz output is available at the rear of the counter but it has no provisions for the use of an external reference source.
The TCXO reference is quite stable but I have been able to get it close but not exactly on the frequency to my satisfaction. I decided that an OCXO may be a better way to go as this option would provide for a smoother precise frequency adjustment along with much improved frequency stability accuracy.
The existing TXCO was removed from the counter and a machined 14 pin socket installed with the unused pin cut short and spaced so as to clear the PCB. This is to avoid short circuits on any of the circuits using pins the plug in PCB.
A premade prototype PCB was used to mount a 16 pin IC socket with extended leads (closest size available) so that it would plug into the IC socket on the counter PCB along with the OXCO and a 5K multiturn pot for fine frequency adjustment. An additional 14 pin IC socket was also mounted on this PVB as a storage location for the TXCO for when the OXCO was in use. (A switch or jumper could be added to select between the two if desired). All components were sourced from Altronics.
The plug in PCB has been wired and OCXO ordered by slow boat. In the interim the TXCO is plugged into the socket on the plug in PCB so as to make the counter operational. Note: Photos were taken prior to delivery of the 13MHz OXCO.
The original standard frequency counter current varies from 0.2A with four digits to 0.3A with all the digits on.
As the OXCO requires some additional power (250mA stable state, 600mA startup) the power supply was upgraded as a precaution by replacing the 1A diodes with 3A ones and the 1A voltage regulator changed from a LM7805 to a 5A LM184IT. The voltage regulator heatsinking was improved by the addition of a length on angle aluminium.
The new regulator has a different pinout than the original one. This was catered for by drilling a new mounting hole and mounting the regulator on an insulator to one side so that two of the existing PCB holes could be reused and only one jumper lead needed.
The transformer is probably adequate for the task as is would appear to similar to a 2152 transformer (15V CT, 1A, 15VA) based on the iron lamination cross section area used. To be on the safe side a 75mm x 50mm x 6mm block of aluminium block with a suitable shim and heat sink compound was was inserted between the transformer and the case to improve the heat transfer away form the transformer. On the other side of the case a thick aluminium channel with heat sink compound was mounted. While far from an ideal heat sink arrangement it does nevertheless result in increased heat transfer away from the transformer.
Alternatively a small 240v ac to 5V dc SMPS could be installed instead of the existing power supply system. Cheaper option probably, but the noise and isolation properties of this would need to be taken into consideration.
One option considered but not taken up at this stage was to add a switch to select either internal OXCO operation or external 13MHz reference.
Igor
VK6ZFG