HB9UF: Repeater Calibration

In this video, I show the different building blocks of a 70cm FM ham radio repeater. I then use the test setup outlined here
• HB9UF: FM deviation, m...
to calibrate the repeater's frequency deviation. Lastly, I check the frequency response of the system with the same setup.

[@bgdwiepp]

Just a quick question, why do you need two filters, not just one? The TX doesn't care if a tiny RX signal goes into its front end.Or is the output from the TX not clean enough that some component will be on the RX frequency and pass through the filter and swamp out the RX signal?

[@MrCircuitMatt]

bgdwiepp Excellent question, thank you very much! I will cover this in a future video where I will run a de-sense test. I probably didn't spend enough time discussing those filters and I'll re-iterate over this.
In a nutshell, you are right in that while the filter in the receive path is critical to protect the receiver from the transmitter's power, the filter in the transmit-path appears to be useless at first glance. Some people say that the filter in the transmit-path is there to clean up 'broadband transmitter noise', but I sort of don't buy into this. I do however not have sufficient expertise to feel confident to claim that this does not happen at all. The transmit filter serves another purpose for sure, though: While RF is meant to go from the transmitter to the antenna, the reverse might happen just as well. This is particularly critical as you might find lots of RF at 'good'/popular repeater sites. This 'foreign' RF might get into your transmitter and cause all sorts of unwanted mixing products. Some people even put an isolator in the transmit path (a device that lets RF through from the transmitter to the antenna, but blocks the reverse path, similar to a diode) to mitigate this even further.
There is so much to say about these things and so many things I did not address in the video and in this reply. I'll keep this reply short-ish and say a bit more in future videos. Thanks again for asking the question though. I hope my reply was satisfactory.
Best regards, Matt

[@bgdwiepp]

MrCircuitMatt Great reply, It totally slipped my mind that at a TX/RX site there would be other high powered TX in very close proximity to the antenna used for the repeater, and the possibility that more than these two radios will be using that same antenna as well. Also good for cleaning up harmonics etc.

[@SkylerF]

@@bgdwiepp Indeed it IS broadband transmitter noise! It's very hard to detect this noise with normal test equipment, but even -115dBm of broadband noise is going to muck up your receive signal.