dongle_comp.html

Comparison of
LP-PAN and SDR dongles

There has been a lot of talk lately about USB dongles for SDR. The most popular ones seem to be the Funcube Dongle Pro + at 165 Euros plus shipping from UK (~$220), and various inexpensive devices based on the Realtek RTL2832U COFDM demod chip with integrated USB. I have also included the SDRPlay RSP1 device, which uses different chips but a similar architecture. These can be very effective as wideband general coverage receivers, but have dynamic range typical of ham receivers of the 1970s. Similar results for RSP1 have been reported in ARRL lab testing.

The biggest shortcoming of all these devices is the blocking dynamic range, which causes desensitization of all signals when a strong signal is anywhere in the passband. To simplify, when the device is set to display weak signals (ones that you can hear on the radio), a single signal greater than about S9+20dB anywhere in the display will cause the noise floor to rise by the amount that the signal is greater than +20. This may or may not be an issue depending on your location and quality of your antenna.

Another issue is filtering. The cards have either no filtering, or very wide low pass filters that cover tens of MHz of spectrum, as opposed to sharp bandpass filtering that covers a small passband around the IF frequency, like LP-PAN or LP-PAN 2.

Below are some measurement numbers which quantify the dynamic range limitations.

73, Larry N8LP

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Comparison Table

Tested items...
Funcube Dongle Pro +
NooElec TV28T USB stick & "Ham It Up v1.2" upconverter
SDRPlay RSP1

Prices include shipping to US. They do not include cables/adapters. Software used was HDSDR and NaP3.

Here are the results of a couple USB "dongles". Note: FCD Pro + can be set for more gain to produce a lower noise floor than the settings I chose, but every dB of higher gain reduces the blocking point by a dB. For a typical noise floor setting of -130dBm, blocking starts at -45 dBm (S9 + 28dB), causing strong signals anywhere in the display to "modulate" the noise floor.

Device	Noise Floor Center / Edges	Blocking Dynamic Range	Cost New	Production Status	Notes
FunCube Dongle Pro +	-120 / -105 dBm	85 dB	~$225 in US	Produced in batches	OK with Win7 32b, spotty with Win7 64b Large DC spike. LNA ON, Mixer gain OFF. IF gain = +3dB.
NooElec TV28T & "Ham It Up" Upconverter	-104 / -104 dBm	80 dB	$70	In production	(Does not include cost of preamp, preamp power supply, filters, shielded case, etc.) Large DC spike.
SDRPlay RSP1	-130 / -130 dBm	80 dB	$150	In production	Tested with LNA ON and gain adjusted for maximum dynamic range. ZeroIF mode with DC/IQ on. ~10dB DC spike after nulling.
LP-PAN 2 w/Realtek ALC-892 built in PC sound card	-128 / -118 dBm 192 kHz	116dB	$250	In production	Full compatibility through Windows Mixer when using MME driver. ~20dB DC spike
LP-PAN 2 w/Asus Xonar U5 USB sound card	-130 / -126 dBm 192 kHz	119 dB	$315	In production	Driver support for Windows 7 thru 10. No DC spike.

Bottom line...

There is no comparison in dynamic range between LP-PAN 2 and the dongles. Blocking in the dongles causes the noise floor and all the signals to jump up and down in step with the loudest signals on the band. If the dongle is set to hear / see weak signals that the receiver can hear, then blocking would commence at about S9 + 20dB on quiet bands, like 20 - 10 meters.

Larry N8LP