A classic combination (for the practical array sizes of an SPAA) is S band for search (less attenuated by weather) and something like Ku band for engagement (higher resolution to track target and own projectiles) 

X band is in between.  If we want our SPAA to do C-DRAM and CBR roles, the X band's higher resolution over S band would be advantageous in the search role to track the smaller targets like incoming projectiles.  It would also be able to provide mid course guidance for the missiles, so your engagement radar really only needs to work the guns. 

I would choose a two face rotating 3D X-Band GaN AESA array for the search radar, and Ku Band for the engagement radar, plus an EO suite.  Also an APS is still desirable for ATGM and drone protection, and that'll have MMW and a staring EO suite as part of the MAWS

Refleks said:

I would choose a two face rotating 3D X-Band GaN AESA array for the search radar, and Ku Band for the engagement radar, plus an EO suite.  Also an APS is still desirable for ATGM and drone protection, and that'll have MMW and a staring EO suite as part of the MAWS

Are there any modern Ku band radars? And sorry - Abbreviation explanations? MMW and MAWS?

Also wondering: My support platoon would have 1-2 SPAAG and 1-3 AMOS. would 1-2 detached search radars that both provide air search and fire finding not be more useful?

MMW is millimeter wave radar, higher resolution than Ku and used in active protection systems for ATGMs (would also work well against really small drones) but also more weather related attenuation 

MAWS is missile approach warning system, which might use a variety of sensors from MMW radar, and multi spectral EO, typically several around the vehicle in fixed mounts for continuous  360 coverage

I was talking more of a Gepard / Tunguska setup where dual radars are common on one vehicle, but you could of course offload the search radar to another platform to cue the SPAA and feed CBR data to the AMOS

Refleks said:

MMW is millimeter wave radar, higher resolution than Ku and used in active protection systems for ATGMs (would also work well against really small drones) but also more weather related attenuation 

Do we then still need the Ku band radar if we have EO and MMW? SkyRanger doesn't seem to think so

It depends.

Even if MMW radar attenuates faster than Ku and X band, it still has advantages over EO when there is poor optical visibility (ie, fog, dust, smoke). Some sources say it's useful out to about 5km which sounds reasonable and this exceeds the effective range of the SPAA cannons so it should theoretically be fine as an engagement radar as an alternative to Ku (around 8 km) as long as its cued by an onboard or offboard search radar so it knows where to look if the inbound is fast, like a HARM. 

That being said, the radars used as part of an active protection systems may not be enough to reach that 5km; they may be operating at lower power for instance, for LPI (low probability of intercept) reasons, so it's harder to detect the emissions.  In that case, it might warrant a separate engagement radar.

I don't know of any SPAA system that uses MMW as an engagement radar currently other than in the APS or weapon role (brimstone). 

well, one thing one could do is to simply make the front sensor of the (presumably flat panel) MMW more powerful to provide the necessary target designation?

Yep that should work

The other thing I am wondering about is sticking the search radar on the SPAAG and feeding CBR data to the AMOSs.

Korean SPAAG added a anti drone secondary radar and optional laser , its likely the only way to go , SPAAG itself with gun only is kinda hard to keep as an antidrone defense as modern optics enable drones to stay out of spaag range fairy easy . 

But i also imagine with all the gizmos this will be a 15+mio$ SPAAG

Primary Armament: Anti-aircraft gun
Secondary Armament: Surface-to-air missile (short / medium range)
Tertiary Armament: Anti-tank guided missile or Laser (optional)
Primary Sensor: 3D air search radar
Secondary Sensor: Counter-drone radar
Tertiary Sensor: Electro-optical sensor (option)

The concept that looks like its a german turret , the mockup is quite different.