Jim:
In a direct conversion system, the only intermod products are between the modulating audio and the carrier. Interstage transformers cannot filter that out. For a push pull output stage, the strongest harmonic is the 3rd harmonic, which as a rule of thumb is -10dB or more from the carrier frequency. As long as the LPF is designed to suppress the 3rd harmonic by at least 33 dB the system is usually withing spec. A 5 pole Chebychev 1% ripple is more then adequate. Of course you need to validate the performance with a spectrum analyzer.
The ASUS is certainly a better USB adapter fir the application then the one I mentioned, but it costs 3x as much, more then the RXTX is you buy it new.
Howard
