Transmitter upconversion or not?

Semiconductor manufacturers sometimes suggests circuit arhcitecture for their IC's. In the case of I/Q modulators one can see two approaches:

1. Direct conversion where the quadrature IC modulates a VCO on the output frequency.
2. An intermidiate frequency VCO in the range 50-250MHz is modulated and then upconverted to output frequency.

Number 1 is obviously much simpler (less filtering) - that is why I suspect it to have some serious drawbacks!

Is there a short answer to why these two approaches both are used?

Jan

The drawback with at direct launch IQ modulator is the out of band noise floors. For GSM, you need to hit ~ -160 dBc/Hz of phase noise at the input to the PA. If the IQ modulator noise floor is -150 dBm/Hz, then you are going to have trouble meeting that spec.

Dave
www.keystoneradio.com

Dave, thanks.
In your reply you mention both dBm/Hz and dBc/Hz

Was that an error or not?

Jan

Not really. I made an assumption that the input power to the GSM pa is ~ 0 dBm, so the dBm/Hz and dBc/Hz is about the same. That's what I get for posting before my first couple of cups of coffee...

The specification you get from the GSM spec is at the PA output, and ends up being about -130 dBm/Hz. To determine the Phase noise at this input, you need to make some assumptions about the PA's gain and noise power, that's how you get back to a dBc/Hz spec, which is what the VCO guy's like.


Dave