Here’s where to get quality attenuators and connectors
Testing any high gain low noise amplifier requires a nice clean attenuator.
You need to drop the level of your function generator,
or that x1000 gain amp would need to supply 100 V output.
Pasternack Enterprisese sells a nice 30dB atten for about $42.
Their part number is PE7000-30.
If you put 2 of these in series, you have about x1000 attenuation.
(These assume a 50 Ohm load, so buy one of those too).
Here’s a link to their website: