What generates less EMI: 74ACT series or 74LS

[Leo Bodnar]

I have been considering replacing a few 74LS series logic ICs with 74ACT in an old instrument from 80s (https://www.eevblog.com/forum/index.php?topic=3502.0) that has sensitive analogue section.
Mostly to reduce consumed power.

This got me thinking whether this would reduce generated digital noise or increase it?

74LS series has slower rise/fall times (~10ns) and higher consumed current (~50mA)
74ACT have faster edges (~5ns or so) but lower idle power (~50uA.)

What is the practical implication of switching from TTL logic to compatible CMOS technology?

[alm]

I don't think idle power has much to do with noise, it's just DC. Dynamic power drawn during switching is much more significant, since this is drawn in short bursts. My money would be on the 74AC(T) generating more EMI, but this is just a guess.

Any particular reason for choosing ACT over HCT? Faster edges are usually bad for EMI, everything else being equal.

[Leo Bodnar]

I don't think idle power has much to do with noise, it's just DC. Dynamic power drawn during switching is much more significant, since this is drawn in short bursts. My money would be on the 74AC(T) generating more EMI, but this is just a guess.

ACT has much lower input currents so even though edges are sharper the switching currents energy density might be lower...  This is still not very clear cut situation, isn't it?

Any particular reason for choosing ACT over HCT? Faster edges are usually bad for EMI, everything else being equal.

HCT has 4 times lower drive capacity and fan out ratio.  I am replacing ICs one at a time in a complicated multi-bus system so I don't want to risk.

[alm]

ACT has much lower input currents so even though edges are sharper the switching currents energy density might be lower...  This is still not very clear cut situation, isn't it?

Agreed. It may also depend on what part of the spectrum you're interested in, although fast edges tend to have a fairly broad spectrum. Noise from the LS parts is likely to be at lower frequencies, since they don't produce the fast transitions the AC(T) does.