What does SDR TX look like in the time domain? - Page 1
Forum section: electronics / rf-microwave

author: AlexI
author_rank: Contributor
timestamp: March 03, 2018, 06:28:44 am
content: I have a test and measurement application where I need to produce certain kinds of signals. I have no experience with SDR but I'm wondering if SDR or DDS chips could produce what I need. For example, I could use something like LimeSDR, or a DDS chip like AD9910.As I understand it the SDR input is a series of I/Q values that could change at up to the symbol rate. For now, ignore Q, say it's always zero. When the I input is 0, 0, 0, max value, 0, 0... - what does the radio output in the time domain?I assume the signal doesn't go from zero to max amplitude instantly; the amplitude ramps up and down at a certain rate. Is there a spec for this? Could someone who has a SDR radio record this on a fast scope and post here?

author: awallin
author_rank: Frequent Contributor
timestamp: March 03, 2018, 09:05:48 am
content: LimeSDR spec says "Bandwidth: 61.44 MHz" - if we assume that goes around both sides of a carrier then the changes in amplitude will happen on roughly a time-scale of 1/30 MHz = 30 ns - give or take a factor of pi... (as long as the analog part of the RF chain has enough bandwidth).sorry don't have TX hardware to test atm...

author: Kalvin
author_rank: Super Contributor
timestamp: March 03, 2018, 09:11:21 am
content: To make a long story short: First, the PC will set the carrier frequency in the SDR which will be used by the mixer, then the PC will compute and feed the time-varying I & Q signals into the SDR which will be fed further into the mixer, the mixer will then multiply and sum the I&Q signals with the carrier signal, and finally the SDR will output the modulated carrier to the TX output.w2aew has nice videos on modulation. Here is a link to the first in the series to get you started.#170: Basics of IQ Signals and IQ modulation & demodulation - A tutorial

author: Kalvin
author_rank: Super Contributor
timestamp: March 03, 2018, 09:41:39 am
content: Quote from: AlexI on March 03, 2018, 06:28:44 amIn your example, you are creating AM-modulation, bursts of carrier wave in which the carrier is turned off and on by the I signal. For example, if your carrier is set to 100 MHz and the I-signal is 0,0,0,MAX,0,0,0,MAX,0,0... and the symbol rate of the I signal is 1 kHz (ie. one symbol lasts for 1 millisecond), the TX output will contain bursts of 100 MHz carrier in which each burst lasts for 1 millisecond, and the pause between the bursts is 3 milliseconds. In practice you may not want to produce this kind of CW-modulation as it will create a lot of wide bandwidth interference as the carrier is turned on and off abruptly. Thus, the SDR radios has filtering for the baseband I & Q signals which will limit the rise and fall times (ie. limit the baseband bandwidth), which will thus limit the output spectrum. However, the SDR radios may have baseband bandwidth of 10 MHz or more, which means that your TX signal may be also be > 10 MHz wide around the 100 MHz carrier unless you specifically limit the baseband bandwidth.

author: AlexI
author_rank: Contributor
timestamp: March 03, 2018, 11:12:15 am
content: Thanks Kalvin. I guess I just want a time-domain translation of "TX signal may be also be > 10 MHz wide around the carrier". I guess that means the ramp up and down time is on the order of half of 1/10MHz or 50ns.But what exactly does the amplitude envelope look like? It's probably not a linear ramp up/down. Gaussian? Flat-top Gaussian? If so, how wide is the flat part? Something else like a sinc function? etc.

author: Kalvin
author_rank: Super Contributor
timestamp: March 03, 2018, 01:02:00 pm
content: Here is a simple quick & dirty simulation for the LTSpice. The carrier is set to 100 kHz so that you can see the carrier cycles. The I-signal is modulated by a pulse with amplitude 1 Volt and duty cycle 1 ms on and 3 ms off. The I & Q signals are fed into a mixer through a simple RC low-pass filter. The mixer is created by a mathematical formula for simplicity.

author: ogden
author_rank: Super Contributor
timestamp: March 03, 2018, 01:51:08 pm
content: Quote from: AlexI on March 03, 2018, 06:28:44 amCould you possibly tell us - what exactly you want to produce? Could be so that there are many ways to achieve what you want and SDR is not the best. Who knows..

author: dkozel
author_rank: Regular Contributor
timestamp: March 13, 2018, 01:26:22 am
content: Hi Alexi,I have several SDRs which support baseband operation, essentially high speed DACs/ADCs with either buffers or baluns depending on requirements. I'd be happy to attach one to a scope if you wanted a real-world picture. The ones I have interpolate the time domain signal up to 800 MS/s with multiple half-band filters along the way to remove high frequency artifacts. That guarantees no sharp transitions between samples, but I'd have to model it to get more precise a picture.Most SDRs you're probably thinking of will be mixing the baseband samples (your time domain samples) with an oscillator to produce the RF output as have been described by ogden and Kalvin. The architecture of the SDR will have an impact on exactly what the waveform looks like at the output.