Hi chuckey,
Thanks for your reply. Actually, my upmixer output connects the driving amplifier input. So, I could not put 50 ohms as my upmixer output. I have tried to simulated the DA input and find the equivalent input circuit. It is 4pF // 60-ohm. I have tried to connect this equivalent circuit one by one and see the effect. As I put 4pF, the gain will shift down. After I added 60-ohm in parallel with 4pF, the gain drops 8dB. I find that if the resistor value is increasing, the gain is increasing too. However, the DA is designed by my colleague. I could not find any method to solve it. Do you have any suggestion?
Actually, the transformer is quite hard to use it. As I tune the 2nd coil, it will affect the 1st coil side. Why do I say that? As I make the lost of the transformer is ~ -5dB, the buffer gain will drop extremely (just could get 3dB only). However, as the buffer gain could get 17dB, the transformer will lost -17dB. It is hard to tune it. Does it any technique to tune it? The circuit is same as found in ieee paper. The passive mixer connects with a DA. But for my case, I make DA as buffer and the transformer output connect to my colleague DA input. Why do I need to do that way? It is because my mixer requires to give out different gain stage.
Thanks,
wccheng