I am designing a DSP based portable transceiver (no PC) and have a few questions concerning the quadrature sampling detectors.
Flex Radio uses an 11khz offset from the LO. This is apparently to avoid LO phase noise, 60hz pickup, and 1/f noise which occurs at near zero LO offset. Although I would not expect phase noise to be a factor since the bus switch does not turn on until the enable signal is above 1v. Has anyone measured the output of the QSD with a grounded input switched at 14Mhz? just what is there? I can offset from the LO up to 115 kHz using the selected CODEC but at the expense of an additional FIR filter to perform a frequency translation. The downside of the offset is the loss of selectivity. I am not planning on tuning with a spectrum display and hence there is no need to convert such a large bandwidth in the ADC. The ADC has approximately 100db dynamic range and I had planned on using an analog AGC (-20db to +10db) ahead of the ADC .. another reason to use a low or no offset from the LO is to prevent the AGC from being swamped by nearby strong signals outside the intended bandpass.. the classic receiver problem! What is the noise figure for the common bus switches? I didn't see anything quoted in the TI, Fairchild, or ONsemi data sheets I am designing to use the same switching devices in the transmit lane but note the crosstalk is about -50db for several bus switch devices and am also curious what contribution this adds to the opposite sideband suppression. If the switching crosstalk and noise figures are significant in the integrated bus switch devices has anyone considered using discrete FETs for the switching function?
