Category: performance testing

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Performance testing of Software Defined Radios

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Performance testing of Software Defined Radios

Do SDRs perform better than conventional Superhet architecture radios? A big problem is that many of the traditionally used tests, which compare radios on league tables and in reviews, are not very relevant to SDRs due to fundamental differences in technology.
A very good article on this topic by Andrew Barron ZL3DW
Performance testing of Software Defined Radios

We already published a post on the sensitivity of the Quadrus SDR here:
http://spectrafold.com/quadrus/digitizer-hardware/sdr-receiver-sensitivity-test/

The published results are:
SSB -111 dBm at 10 dB S+N/N with 2.1 kHz bandwidth
CW  -119 dBm at 10 dB S+N/N with 400 Hz bandwidth

According to Andrew:

MDS (Minimum Discernible Signal)

MDS is a measurement of how sensitive the receiver is. It represents the weakest signal you can hear. You need it to be good if you want to hear very weak signals on a quiet band, for example when sunspots are poor or the band is closing. If the band is noisy the noise level coming in the antenna port will often be higher than the MDS so sensitivity is not as relevant.

In the test a signal is input to the receiver and the MDS is the input signal level when it shows as 3dB above the receiver noise floor. The MDS is better if the bandwidth of the receiver is reduced because a narrow bandwidth allows less noise in. So it is usually measured using a typical CW bandwidth of 500Hz and using a typical SSB bandwidth of 2.4kHz. It is normally checked on several bands as well. In most SDR receivers especially direct sampling (digital down conversion DDC) receivers you would expect the same performance on all bands. When you compare results relating to different radios, check that no attenuators or preamplifiers are in use. Most SDR receivers have an MDS better than -125dBm in 500Hz bandwidth and better than -115dBm in 2.4kHz bandwidth. Excellent receivers can achieve an MDS better than -130dBm in 500Hz bandwidth and better than    -120dBm in 2.4kHz bandwidth. SDRs with 8bit analog to digital conversion (ADC) will probably not be able to achieve that level of performance because of limited dynamic range.

It is not easy to compare the results, because we’ve measured in 400 Hz / 2.1 kHz bandwidth instead of 500 Hz / 2.4 kHz, and we’ve measured a 10 dB signal above the noise floor instead of 3 dB. However, we can do a very simple calculation of adding 7 dB to the 3/10 dB measurement level difference.

So our performance testing comparable to this reference article is:
SSB -118 dBm at 3 dB S+N/N with 2.1 kHz bandwidth
CW  -126 dBm at 3 dB S+N/N with 400 Hz bandwidth

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Sensitive SDR receiver

Sensitive SDR receiver for sensitive information

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[Sensitive SDR receiver for sensitive information]

Sensitive SDR receiver based on the DRU-244A digitizer SDR hardware platform

The DRU-244A digitizer board was designed and implemented for building radio receivers. It contains a 16 bit resolution low-noise Analog-to-Digital Converter (ADC). It has a low-noise input preamplifier and an input attenuator in front of the ADC. The low-noise preamp provides input sensitivity as low as -111 dBm in SSB reception mode with 2.1 kHz bandwidth and 10 dB SNR. With external preamplifier it could be as low as -122 dB. If you would like to see more detailed test results on SDR receiver sensitivity click here:
http://spectrafold.com/quadrus/digitizer-hardware/sdr-receiver-sensitivity-test

Practical results for receiving weak signals

I’ve already made some practical tests with my very simple inverted-V shaped wire dipole antenna. It was used in the country side, so there was less human made noise than in the crowded city or urban area.

Sensitive SDR receiver as long range DRM SDR

If you are not a professional radio enthusiast with a vast knowledge of exact broadcasting carrier frequencies, one of the easiest ways to find some DRM radio stations is to just look for its unique spectrum in the HF band. The DRM spectrum is a very typical, noise like, wide band signal. This is exactly what I did; I simply connected the antenna to my receiver through a 30 MHz low-pass input pre-selection filter, and started visually looking for DRM radio signals in the spectrum. I was really surprised when one of the first signal received turned out to be be coming from India and the next one was from South Africa. Not bad. The SDR receiver seems to be real sensitive if I am able to decode these signals in the middle of Europe.

drm01 drm02

If you are interested in how the SDR receiver was connected to the Dream DRM decoder software, please read these posts:

http://spectrafold.com/quadrus/radio-software/virtual-audio-connection/

http://spectrafold.com/quadrus/radio-software/sdr-drm-receiver/

Sensitive SDR receiver for secret transmissions

One of my other favorite signals in the HF spectrum is the UVB-76 Buzzer on 4555KHz.

https://en.wikipedia.org/wiki/UVB-76

http://priyom.org/military-stations/russia/the-buzzer.aspx

http://www.wired.co.uk/magazine/archive/2011/11/features/enigma?page=all

As the frequency is well known, I could simply dial it in to the SDR receiver software, and immediately see the transmission on my screen and hear the famous “Buzzer” tone on the speaker with very high Signal-to-Noise Ratio (SNR).

Receiving the UVB-76 Buzzer on 4625KHz with sensitive SDR receiver

There are a lot of mysterious signals in the HF spectrum. The sensitivity of our receiver – especially with an external, low-noise per-amplifier and a good antenna – makes it possible to listen to even the most remote signals.

listener Contact014

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SDR receiver sensitivity test

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Receiver sensitivity specifications

One of the most important features of a radio is its ability to receive low level signals, in other words, its sensitivity. We have lot of different definitions for receiver sensitivity. Some excellent descriptions can be found on radio-electronics.com. For linear modulation formats, like AM, SSB, and CW, the Signal-to-Noise Ratio (SNR) or one of its variants, e.g., the signal plus noise to noise ratio ( =(S+N)/N ), are most commonly used.

In the first case, we can measure the signal level and the noise level separately. This may be done with a spectrum analyzer employing a simple sine wave test signal. In the second case, we resort to measuring the signal and the noise together, because can’t separate the noise from the signal. For this we utilize a wide band power meter or a Root Mean Square (RMS) voltmeter.

If the difference between the signal and the noise level is greater than 10 dB, the above defined two ratios are practically equal. When we look at the specs of different receivers, sometimes it is hard to immediately compare the performance of different models, because they are specified differently. For example, the SNR may be defined with different bandwidths in mind. More specifically, 10 dB or 12 dB SNR values represent vastly different receiver sensitivity based on whether it is defined for 500 Hz, 2.1 kHz, or 2.4 kHz bandwidths.

Practical receiver sensitivity test of the DRU-244A-based SDR

Receiver sensitivity test setup

The test setup is very simple. We need to use a calibrated test generator to feed -80 dBm and lower signal levels into the input of the receiver, while we measure the audio output level with an RMS voltmeter.

Receiver sensitivity measurement procedure

Switch on and tune the receiver to the test frequency (F) with a given bandwidth (BW). First, we disconnect the signal source and measure the output noise level. Secondly, we connect the RF signal source, and increase the signal starting from a very low level, until we have an audio output voltage with a given level. The signal level on the generator (P) shows the receiver sensitivity for a given bandwidth and the SNR level. Instead of traditional voltage meter, like the venerable HP-400, you can use a sound card-based scope and audio analyzer. Usually, it has built in SNR measurement capability. For my last measurement, I used the Multi Instrument software by Virtual Instrument Technology.
You can download the 21 day free trail from this page:
http://www.virtins.com/downloads.shtml
Or you can use other similar audio analyzer program from Daqarta where you can download a 30 days trial of the latest version:
http://daqarta.com/dqdown.htm

We already have digitized samples in the SDR radio, so, it is possible to skip the DAC/ADC sound card conversion, and with the Virtual Audio driver we can send the samples directly from the SDR radio software to the measurement software. I’ve used this audio driver to connect the SDR receiver to the DRM decoder in one of my last post.
http://spectrafold.com/quadrus/radio_software/receving-drm-broadcast-sdr-radio-receiver/

SDR receiver sensitivity test results

I’ve tested the DRU-244A at F = 10.1 MHz, BW = 2.1 kHz, and S+N/N = 10 dB with and without a pre-amplifier. During my tests, I’ve used a ZX60-P103 amplifier from MiniCircuits with fixed 23 dB gain and less than 3 dB noise figure. It is specified from 50 MHz, however, it can be used down to 2 MHz.

The following pictures show the different steps of the SDR receiver sensitivity measurement for SSB, CW, AM, and FM signals.

SSB (2.1 kHz) and CW (400 Hz)
sdr receiver sensitivity noise sdr receiver sensitivity noise 2
sdr receiver sensitivity noise o sdr receiver sensitivity noise 2 o
sdr receiver sensitivity signal sdr receiver sensitivity signal 2
sdr receiver sensitivity signal o sdr receiver sensitivity signal 2 o
sdr receiver sensitivity ssb sdr receiver sensitivity cw

AM and FM with signal display on the SDR receiver, noise and signal out, and the generator.
sdr receiver sensitivity signal 3 sdr receiver sensitivity signal 4
sdr receiver sensitivity signal 3a sdr receiver sensitivity signal 4a
sdr receiver sensitivity signal 3c

sdr receiver sensitivity noise 3 sdr receiver sensitivity noise 4
sdr receiver sensitivity signal 3o sdr receiver sensitivity signal 3o
sdr receiver sensitivity AM sdr receiver sensitivity FM

Receiver sensitivity results and conclusion

As you can see from the receiver sensitivity measurement results, the sensitivity is
SSB -111 dBm at 10 dB S+N/N with 2.1 kHz bandwidth
CW  -119 dBm at 10 dB S+N/N with 400 Hz bandwidth
AM -105 dBm at 10 dB S+N/N with 30% modulation
FM -108 dBm at 10 dB S+N/N with 3 kHz deviation

The sensitivity can be improved with some external low noise preamplification and additional external gain to reach -122 dBm sensitivity in SSB operation mode.

sdr receiver sensitivity noise sdr receiver sensitivity signal and noise
sdr receiver sensitivity noise sdr receiver sensitivity signal and noise
sdr receiver sensitivity 2

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