My GPS disciplined 10 MHz square wave frequency reference did not work with
the Flex 1500 since the Flex 1500 reference input uses a bridge rectifier
as a frequency doubler and requires a sine wave source. Since a square
wave consists of a sine wave at the fundamental frequency combined with all
its odd harmonics, a low pass filter will convert a square wave to a sine
wave. I used RFSim99 to design a 3 pole Butterworth low pass filter. The
RFSim99 simulation model yielded a frequency response curve with 1.7 dB
loss at 10 MHz and 30 dB attenuation at the 30 MHz third harmonic. The
filter was built into Altoids box: two 270 pF capacitors and a 1.4 µH
inductor (17 turns of magnet wire on a T50-2 toroid core). The filter's
actual frequency response measured with a MiniVNA Pro corresponds with that
predicted by the RFSim99 model, and the Flex 1500 now works with this GPS
frequency reference.
Details at
https://plus.google.com/photos/114032640816757126398/albums/5902191645840048913
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