TestRF.com

Welcome to TestRF.com

Welcome to my T&M Blog – TestRF.com. Just a little somewhere to post and share information I find useful relating to my day to day activities.

My name is Darren Tipton and I’m one of several Rohde and Schwarz Product Engineers / Applications Engineers based in the UK having over 10 years experience in the T&M industry working in RF and Microwave along with cellular (GSM, WCDMA, LTE) and non-cellular communications technologies (Wireless LAN, WiMAX etc). I mainly work with Spectrum / Signal Analysis (swept and real time), Signal Generators and Power Meters but have been known to get involved with other interesting stuff.

If I post something here that you find helpful in your work with test and measurement equipment that’s great but you should be aware that this is my personal T&M blog. I hope to not post anything that’s too stupid – but it could happen! The views expressed on these pages are mine alone and not those of my employer.

Why we care about Residual EVM of a Signal Analyzer (802.11ac)

Error Vector Magnitude (EVM) is a measure used to quantify the quality or performance of a modulated signal from a transmitter or receiver. In simple terms, if we consider a constellation diagram the EVM is the magnitude of the difference between the measured vector and the ideal (reference) vector. This can be visualized as below.

It can be seen from this simple diagram that EVM is influenced by a number of parameters such as below:

• Phase Error
• Frequency Error
• Magnitude Error
• Noise that contributes to all of the above

Each of these areas are contributed to not only by the signal being measured, but also by the test instrument itself which has an effect on how well it can capture the signal, but also how it is able to generate an “ideal” reference signal to use for the calculation. If we take a look at this block diagram which shows a model of transmitter EVM contributions such errors also exist during the demodulation process. Therefor the limit of EVM demodulation performance can only be as good as the error contributions added during the demodulation process in the signal analyzer.

Many of the effects we are able to correct for in DSP as part of signal synchronization etc. Although phase noise is not so easy to correct for and has a direct impact on performance.

Below are some graphics showing a visual representation of the effects of the distortions described in the model on the constellation diagram.

EVM Performance when measuring the new 802.11ac 80MHz standard 256QAM

Below are two graphics showing screenshots taken from an R&S Vector Signal Analyzer on an 802.11ac signal with a specific EVM of the transmit signal. The good signal has 33dB of EVM and the bad signal only 30dB of EVM performance. It is clear to see on the bad signal that some symbols (constellation points) are much further away from the ideal point which would result in more error and a poorer EVM result.

Instruments with a worse EVM performance will contribute directly to this error and on signals over a wide bandwidth and such high order modulation schemes (256QAM / 1024QAM etc) EVM performance and phase noise performance of the test instrument becomes critical to measuring such parameters.

Graphics courtesy Rohde & Schwarz FSV-K70 user manual and some very helpful colleagues relating to the EVM plots above.

Test and Measurement Instrument control using PHP

There are various ways of remote controlling instruments such as remote desktop or VNC for getting access to the screen but often you want it to do something specific in an automated way. Many people use tools such as Labiew or even full on programming languages like Visual C, C#, VB etc with drivers (VXI or LXI) to achieve robust production solutions. Often when I am asked by customers to assist with some remote control, I turn to a web scripting language called PHP. Its not the normal scripting language people think of for instrument remote control but it has some benefits for me:

1. It has socket support allowing control to port 5025 by sending SCPI commands as ASCII to write commands and read results.
2. Its fast to develop, is very much like C but with simplicity.
3. Has built in support for Regular Expressions and Text Parsing meaning string manipulation is quick and easy.
4. As I’m familiar with the language I can knock up scripts quickly and send the SCPI command sequence to the customer.
5. It can be used with a web server. There are possibilities with PHP to build a scaleable instrument remote control solution that is becomes a web application, meaning easy control of instruments just by logging onto a website.
6. If required a complex system could be developed to easily store and recall results from a MySQL back end.

Of course there are other languages that could be used and are often used by customers such as Perl or Python to achieve the above tasks, I just happen to get on with PHP.

A simple example might be something like this:

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// Allow the script to hang around waiting for connections
set_time_limit (0);
 
$fsq_ip_address = '192.168.1.2';
$port = 5025; 
 
// open socket to instrument 
$fsq= fsockopen ($fsq_ip_address, $port, $errno, $errstr, 10); 
 
// open the output file for results 
$fp = fopen ("output.txt", "w+"); 
 
// query instrument for its details and output result 
writescpi($fsq, "*IDN?"); 
$result = readscpi($fsq); 
logger("Instrument: " . $result); 
 
// close sockets and file 
fclose ($fsq);
fclose ($fp); 
 
// Function Definitions 
// Write string to the socket (output errors if debug enabled) 
function writescpi($inst, $msg) 
{ 	
  global $debug; 	
  fputs ($inst, $msg . "\n");
 
  if ($debug == true) 	
   { 	        
    print ("\nCmd: " . $msg); 		
    fputs ($inst, "syst:err?\n"); 		
    $result = readscpi($inst); 		
    print ("Result: " . $result . "\n"); 	
  } 
} 
 
// Read string from the socket 
function readscpi($inst) 
{ 	
  $buf = fgets ($inst,100000); 	
  $buf = trim ($buf); 	
  return $buf; 
} 
 
// output to screen and to file at the same time 
function logger($result) 
{ 	
  global $fp; 	
  print ($result . "\n"); 	
  fwrite ($fp, $result); 
}

FSVR Transient Demo

This FSVR Transient demo really shows what realtime can do compared to a regular spectrum analyzer.

Multipath Diode Power Sensors vs Thermal Power Sensors for Modulated Signals

I am often asked about measurement of modulated signals using diode sensors / meters. There is a view that Thermal Power Sensors / Meters are more accurate than Diode Power Sensors / Meters when it comes to measuring modulated signals. There is a trust in the industry that drives this – of course, heat dissipated in a resistor is an absolute measurement of power. From an R&S perspective our thermal sensors (NRP-Z5x series) are able to get the match so good that our linearity is proven to the be just about the best there is with a thermal power sensor in the market today.

Within the square law region the detector provides a linear relationship between incident power and output signal. The detector works as an RMS (voltage) rectifier and delivers a signal that is proportional to the average RF power. Outside the square law region the relationship is non-linear and this causes the problem if you cannot compensate these non-linearities in real-time anymore.

Even if the sensor is a multipath device with switches – during the switch points, this can cause distortion and large in-accuracies in the switch points. A diode sensor should give a good average result, but like the Thermal sensor it will not track the envelope (rise / fall time etc). This is a topic for a different post! R&S multipath diode sensors have a patented design that does not include switches for each power range. All ranges are measured simultaneously and a soft weighting is applied to each path. Effectively the switch linearity can be corrected so the whole sensor is operating in a linear fashion.

Continue reading » » Multipath Diode Power Sensors vs Thermal Power Sensors for Modulated Signals

R&S UK and NPL to host Advanced Power Measurement Seminar

On 23 November 2011 R&S and NPL (National Physical Laboratories) will host a power measurement seminar at NPL’s high tech facility in Teddington, Middlesex. NPL are known as the UK authority in accurate power measurements.

Rohde and Schwarz were the first to introduce USB power sensors to the market and hold patents in the design of both thermal and diode power sensors making them the most accurate on the market.

The seminar will be headlined by Head of Power Sensor development at Rohde and Schwarz, Mr. Thomas Reichel who will discuss the technical challenges of designing sensors to make accurate power measurements at and above 50GHz.

A full agenda can be found here.

Register for the free seminar here.

Continue reading » » R&S UK and NPL to host Advanced Power Measurement Seminar