THE FOLLOWING COMES WITH NO WARRANTY OF ANY KIND. TRY THIS AT YOUR OWN RISK – THERE IS A RISK OF DAMAGE TO YOUR EQUIPMENT BY COPYING THE EXPERIENCES DISCUSSED BELOW.
One of the members of SARA has managed to get the external clock working on his HackRF One using a cheap 12 USD OCXO from Banggood. The issue was these cheap OCXOs output 5V TTL, while the HackRF and other SDRs like the Airspy R2 expect a 3.3V clock input, so connecting it directly will not work and can damage the SDR/CLK chip. The absolute max on the HackRF CLKIN is 3.8V for the Si5351 clock chip inside.
His solution was a 10 dB SMA attenuator in between. Since CLKIN is high impedance, the pad only attenuates around 5 dB instead of 10, which lands the 5V TTL right in the valid window.
HackRF successfully detected it as:
/mnt/extra$ hackrf_clock -i
CLKIN status: clock signal detected
He jerry-rigged an oscilloscope connection with 2 bare wires poked into the BNC, sonce he did not have an SMA to BNC adapter. The oscilloscope showed a 10 Mhz square wave.
He gave this important advice: If you try this: do not use the sine output of the OCXO, it swings negative, and the Si5351 does not tolerate below -0.5V.