Transceiver, Direct sampling from DC to 1.5GHz, from high performance EVM kit - Page 1
Forum section: electronics / rf-microwave

author: cnxunuo
author_rank: Regular Contributor
timestamp: February 16, 2019, 08:36:53 pm
content: later 2017, I am planning to build an direct sampling transceiver manpack for DC to at least up to 1.2GHz ham band.Before this decades, direct sampling only exist in the HAM world with shortwave,Now its time for VHF, UHF, 1.2GHz band to embrace direct sampling...........A good EVM board from TI suites this very well, the TSW40RF80EVM,14-Bit 3GSPS ADC,14-Bit 9GSPS DAC.These two RF ADC/DAC have built-in multi-channel DDC and DUC, which drastically reduces FPGA load.as i already have the FPGA EVM board when i bought my TSW54J60, a bit of fun in this thread:https://www.eevblog.com/forum/rf-microwave/play-with-1gsps-16bit-direct-sampling-receiver/msg2205279/#msg2205279I also uses this FPGA to USB3.0 board for ease of connection as TI has full software and register file for it.I found another EVM board which is extremely bargain (but still expensive if you only look at the price) considering it also contains 2 exotic chips with free supporting clock cleanup ICs and frontend ICs, and also came with a convenient FMC connector for use on FPGA development board.Get one and stick it in a box along with FPGA dev board such as VC707 or KC705, you get yourself an very nice direct sampling SDRThe heat sink on this EVM is strangely putted directly bellow thermal pads of the DAC, which still burns finger even with a fan on it.

author: cnxunuo
author_rank: Regular Contributor
timestamp: February 16, 2019, 08:40:57 pm
content: Did some very preliminary test last year, very satisfied for the performance,unfortunately I have little FPGA programming experience regarding jesd204b, TI's dll file is my only way of using it, what a waste.Wondering how long we have to wait untill an COTS HAM tranceiver utilizes direct sampling in UHF or even 1.2G band...

author: David Hess
author_rank: Super Contributor
timestamp: February 17, 2019, 11:26:59 pm
content: Quote from: cnxunuo on February 16, 2019, 08:40:57 pmExcept in special applications, I suspect never. Direct sampling without an IF based preselector severely compromises dynamic range compared to a superheterodyne design.

author: KE5FX
author_rank: Super Contributor
timestamp: February 17, 2019, 11:36:53 pm
content: Quote from: David Hess on February 17, 2019, 11:26:59 pmNot really, not if you're actually talking aboutdirectsampling in the first Nyquist zone with a suitable antialiasing filter. Random signals in other bands can actually serve as a useful source of dithering as long as the ADC's overall peak-to-peak input range is respected.In practice, there's little motivation to use these ultra-fast ADCs unless you actually need to capture hundreds of MHz of signal bandwidth. For a conventional transceiver application, it would be much more economical to obtain the data from cheaper ADCs in higher Nyquist zones with, as you suggest, a good preselector. Even a YIG filter is cheaper than some of these ADCs, especially when you consider the gate and power budget needed to support them.

author: cnxunuo
author_rank: Regular Contributor
timestamp: February 18, 2019, 06:04:39 am
content: I agree, randomely phased signal can hardly addup to overload ADC, and dynamic range even with minimum amount of preselection, is good enough, at least without co-site transmission.Folding 2nd or 3rd nyquist if done properly should also be fine, as many base station of LTE still do it...