RX Performance

We tested the RS-HFIQ's receive performance with four different USB sound cards:

Creative X-FI SB1090

ASUS Xonar 5.1

Startech ICUSBAUDIO2D

SIIG CE-SA0011-S1

All of the devices were capable of capturing audio at 96 Ksps with a resolution of 24 bits so that's where we tested. The Creative and the ASUS are capable of sampling at 192 Ksps. The signal generator is and Agilent E8257C which is very quiet so we can really see the performance of the cards.

The tests were conducted on the 10, 20 and 80M ham bands at frequencies of 28.3, 14.2 and 3.8 MHz respectively. A screen shot of the HDSDR RF spectrum was captured with input signal approximately 10 dB above the noise, 50 dB above that and again 40 dB higher for a range of 100 dB. The specific values are different for each band because the RS-HFIQ has more front-end gain at higher frequencies. The resolution bandwidth for the spectral display is 5.9 Hz and HDSDR was running on my old Dell Core I5 laptop running Windows 7 Pro.

Here is a representative plot showing the Creative XFI digitizing signals from the RS-HFIQ on 80M with an input signal at 3.799 MHz, -110 dBm:

The input signal is in the light blue band and we can see that it's almost 20 dB above the noise and the rest of the spectrum is free from spurious signals with the exception of a small artifact about 1 kHz below the desired signal.

In the next plot we have increased the input signal 50 dB to -60 dBm.

This time we see a good solid signal, approximately 70 dB above the noise floor with good clean spectrum.

In the final plot for this sound device on this frequency we've increased the input another 40 dB to -20 dBm and we are starting to see some distortion products appearing close to the desired signal.

What this means is that if you are trying to work a station that's coming in at -110dBm you could have some trouble if a station 90 dB stronger is within 5 or 10 kHz. It also shows that the LO in the RS-HFIQ, the SI5351, is generating a a signal with relatively low phase noise and the Agilent signal generator is making a very clean signal.

You can see the data for all of the sound devices and all bands in the files below. It's interesting the there isn't all that much difference between the performance of the various sound devices. The Creative XFI is arguable the best of the bunch with low noise and spurs and noticeably lower distortion products. The SIIG seems to be the worst with higher spurs and distortion. The difference is relatively small and it would be difficult to imagine an on-the-air situation where the variation in performance would be detectable. The ASUS's noise flor bends downwards at the right and left edges of the plot, indicating roll-off in the audio spectrum but when the sampling is changed to 192 Ksps no roll-off was observed.

In all cases, increasing the input by another 10dB caused clipping the the RF front end creating distortion severe enough to prevent normal operation.