Why is PTT ground separate from Mic ground? Don't they both go to the same bus on the radio?
Thanks
They do. From the mic, analog audio (shield lead) and the PTT return lead should connect at one point inside the transceiver, typically very close to the mic jack. In addition to what others have said, the point is to control current back to a common ground point. To help ensure low audio noise, current between functions is split into two pathways back to the transceiver. Otherwise, joining the two at the mic end can result in high noise current on the audio shield when the PTT button is activated, especially with older transceivers that use high-current PTT circuits. Keep in mind that the two returns are at equal ground potential, but currents are appropriately diverted.
The designers of older gear like Heathkit, Drake, Collins, etc. used one common mic ground for audio and PTT return current on the mic cable. Back in the day, it was very common to hear an annoying click in the mic audio as the op activated the mic.
Have a look at some of the best test equipment made and you'll see perhaps a half-dozen grounds: audio ground, multiple digital grounds, chassis ground...all of which are on isolated paths to a common ground point.
Sometimes, it's necessary to bifurcate two or more audio ground paths. For example, on my QRZ page is a low-noise audio amp that has two separate ground planes, both for audio. The idea is to ensure that the high-current return path feeding a speaker doesn't modulate the high-gain, low-noise stage's ground plane. The planes are separate but come together at exactly one point: the voltage regulator ground pin.
So, in a nutshell, it's all about low-noise return path current management.
Paul, W9AC