Let me know if you plan to visit here ... a worthy stop ...
Enjoy your plane, I have not flown since high school ... actually got my student license and soloed prior to getting my auto license at 16.
Flying was big fun ... what was it ... I think a Piper Cherokee?
Landing in a cross wind for first solo flight ... sliding the plane in side ways ... what were they thinking of letting a kid fly their planes???
To check the input impedance with the instrument powered up, drive the input with a signal generator that has a known 50Ω output impedance. While watching the output voltage level from the generator with a Hi-z scope using a BNC T, connect the coax to the unknown input. If before connecting, the voltage was say 1Vpp, and as you connect it, the voltage drops to ~1/2V, then the input is truly 50Ω. If after connecting it, the voltage stays near 1Vpp, then the input impedance is much, much higher...
Mike ML, thank you ... hooked it up ... and did your test above ... the voltage level only dropped 10mV ... from 5V to 4.9V.
So it would appear the this gate input is much higher than the 50 Ohm noted in the operator's manual.
Based on this, do you expect that the circuit (of yours) would drive this gate input OK?
The final two output IC's are the 74HC&08 & 74HCT11.
They publish and output drive current of 25mA max.
5V/25mA = 200 Ohms.
I measured an gate input impedance of 1k Ohms when the unit was powered off.