Adam finds the optimum 10 MHz reference level for his MiniKits GPS-9700 lock board.
Minimising the 10 MHz input level whilst maintaining reliable lock will help improve phase noise on the 49.152Mz output.
Shack 10 MHz reference is a Trimble Thunderbolt and feeds a Down East Microwave 10 MHz 4-way splitter, which delivers +12.5dBm on each output.
The GPS-9700 specifies an input level of -20dBm – 0dBm.
The board has 29dB of attenuation with a Pi attenuator consisting of R1, R2 and R3.
Using a stepped attenuator, it was found that lock was lost with 26dB of external attenuation.
Removing 5dB from that and we end up with 21dB of attenuation, and this provided reliable lock.
Total attenuation required is: 29 + 21 = 50dB
The value of R1, R2, and R3 will be changed to aim for 50dB of on-board attenuation.
This was easily done by piggybacking some 470 ohm resistors across the existing 56 ohm resistors R1 and R3
R2 (560 ohm) is replaced with 10k and 22k resistors in a parallel piggyback to arrive at 6875 ohms.
R3 is on the “bottom” side of the PCB.
This combination ultimately provided an attenuation of 49dB – close enough.
I have 6dB of headroom before the GPS-9700 loses phase lock, and an external attenuator is no longer required.
The amount of attenuation required to suit your own GPSDO 10 MHz signal level will most likely be different, and the resistors values will need calculating.
Links
MiniKits GPS-9700 https://www.minikits.com.au/gps9700
Le Leivre PI Attenuator Calculator https://leleivre.com/rf_pipad.html
All About Circuits Parallel Resistor Calculator https://www.allaboutcircuits.com/tool…
IC-9700 Discussion Forum on Groups.io https://groups.io/g/ic-9700/topics
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