Command block with /testfor doesn't send output through comparator

Command block with /testfor doesn't send output through comparator

Command block doesn't recognize comparator as output, in "Previous output" box it show: "[03:19:23] /testfor is only usable by commandblocks with analog output" message.
When placing redstone dust next to command block it sends out signal only if command block is powered, so with use of clock will output be also clock.