Quadrature generation with counter/timer card

[Henrik_Volkers]

Here is a hardware solution:

http://www.wb5rvz.com/sdr/ensemble_rx_ii_vhf/04_div.htm

just a 7474 logic ic

Spoiler

I know that a software solution seems more elegant 😉
but in remembering Bob Pease:

Spoiler

[Kevin_Price]

I would very slightly tweak the ideas that Travis referenced and linked to.  The problem I see with making separate software calls to change the freq on your 2 counters is that the unknown & unpredictable software latency between those calls is likely to get your simulated quadrature out of phase, definitely getting away from 90 degree phasing and possibly looking like a direction reversal.

With the hw you have, I'd opt to use a 3rd counter pulsetrain as the timing source rather than a dummy AI task's sample clock.  Then you can change the freq of the 3rd counter on-the-fly, and the simuluated quadrature will maintain proper phase relationship.   (Note: you can also change the sample rate of an AO task on-the-fly, and could use a dummy AO task.  An AI task can't have its sample rate changed on the fly if you use the default internal clock for sampling.)

-Kevin P