Rapco 1804P9
GPS Disciplined Precision Frequency Source

Last month I was exceptionally fortunate to be given a professional GPSDO … A Rapco 1804P9. Up until now I have been slaving my 9cm and 13cm transverters to my G3RUH-Clone GPSDO, which I have to admit is a very competent little box. However, over the years it has suffered a few failures. The ex-junk-box mains power module failed and no less than two Isotemp 134-10 OCXOs failed. So, latterly it has been running from one of the shack DC PSUs whilst the OCXO is now a Isotemp 143-141.


I will be honest; up until last week I had not heard of Rapco (now part of Spectracom) or their GPSDOs. A search on the Internet will turn up various Ebay auctions and several on-line discussions … all relating to the 1804M variant. However, detailed information is scant to say the least and the ONLY links purporting to ‘point’ to downloadable manuals turned out to be booby-trapped with malware … most curious! However I was able to identify a potential source of information and within a few hours I had a clutch of pdf manuals for various flavours of 1804 but not the P9 variant.

August 2016:

The Rapco 1804M provides a single 5MHz 50-ohm compatible sine-wave output which is GPS-Locked. The Rapco 1804P9 is a different beast altogether in that it provides no less than 5 distributed 10MHz 50-ohm compatible sine-wave outputs which are GPS-Locked. Apparently the 1804L17 is also a 10MHz source, but like the 1804M, it is single-output and the Trimble GPS RX is piggy-backed onto the Main PCB.

The photograph on the left shows the inside of the Rapco 1804P9. The large PCB on the left embodies the PSU (bottom), the RS-232 interface (4800 Baud 8N1), Hitachi CPU, 10MHz OCXO, 5 x 10MHz outputs,1pps Timing o/p and Alarm o/p.


The smaller multi-output board provides 5 x 1Hz TTL outputs (Square-Wave) slaved to the 1pps signal on the Main Board.


Finally the Trimble GPS RX is piggy-backed onto the square PCB bottom right. This appears to be a Lassen SKII.


Connection to the active antenna is via an N-Type socket on the rear panel.