At 09:34 PM 10/21/2009, you wrote: > > >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.
The SoftRock uses the same technology as the Flex radios, hence you have the same basic issues so it would be recommended that you have an IF removed several KHz away from the LO >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 advantage of a high conversion rate would be when followed by properly designed decimation is that it will increase your dynamic range of your AD converter. >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! The SoftRock bypasses such issues by not having any AGC, the dynamic range is determined by the quality of your sound card and further improved by decimation of the input stream. I cringe when I see and hear of people using 16 bit sound cards the improvement when using a decent 24 bit card is amazing. >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. You can easily get over 70 dB of image rejection and indeed even more with the proper cancelling algorithms >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? I would tend to think that the bus switches will give you better performance because of several issues; faster on/off times lower on resistance more closely matched devices The SoftRocks if you look at their schematics is that they are very simple DC receivers without many of the complications of more complex radios giving cleaner signals when compared to the multiple mixer and other stages of a super-heterodyne radios, yet with good software the overcome their weakness which lack of image rejection. A radio very similar to what you are locking at is the radio that Dan Tayloe designed for the Normal Club; < http://www.norcalqrp.org/nc2030.htm > It converts directly to the audio band pass (0 IF) but makes use of extensive filtering to improve the performance of the radio but it does not use any DSP devices. Cecil k5nwa www.softrockradio.org www.qrpradio.com < http://parts.softrockradio.org/ > Never take life seriously. Nobody gets out alive anyway.
