10MHz distribution amp ideas
A while back, the UK Microwave Group magazine Scatterpoint published an article by Andy G4JNT on making a simple GPS disciplined oscillator based on the cheap Jupiter GPS boards. A unit was made as per the article, with a good quality 10MHz oscillator from John G8ACE as the ovened source then the simple circuit from Andy to lock the oscillator to GPS. The output was locked on 10MHz, and the results were very good. The 10MHz output was then checked on the spectrum analyser; At this point, it was noticed that the 10MHz output and its harmonics extended up over 1GHz it does make a nice marker generator as is, but since the unit was to be used to lock a receiver, it needed to be a little cleaner.
After some hours wasted trying to get good results with multi pole filters, it became clear that a proper 10MHz filter was needed. After some looking around for inspiration, a box of old of 10Mbps Ethernet LAN cards were located. Whilst looking at the isolated BNC sockets on these cards, the transceiver modules were examined and researched on Google, and sure enough one module was a multi-pole filter for the RX and TX paths. After removing one from the board, it was checked it on the spectrum analyser, and gave the following results;
(below) 10MHz output before filtering.
(below) 10MHz output after filtering.
The output from the G4JNT GPSDO is first buffered by a 74HC04 Hex Inverter, 2 gates in series, then the second gate driving the remaining 4 gates. 74HC04 4-output buffer. The 10MHz output from the 74HC04 of course contains harmonics. Each output is fed via a 27 ohm resistor, then into the filter network.
The filters on the Ethernet boards are marked YCL 20F001N. Each YCL 20F001N package contains one 7 pole TX filter and one 5 RX filter you can either use the two filters to drive 2 separate outputs,or wire both filters in series to get further rejection of unwanted harmonics. The pin outs for the filter block are on the schematic diagram below:
The other great thing about these filter blocks are the isolated outputs if you use the isolated BNC socket commonly found on the old LAN cards, you can get rid of problems caused by earth loops. After having built the 10MHz buffer, the outputs resembled a clean sine-wave on the oscilloscope, meaning a good quality 10MHz signal was now available to feed items of test equipment.