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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 RSP device, which uses different chips but a similar architecture. These can be very effective as wideband general coverage receivers, but have limited dynamic range.

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 RSP

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 RSP
-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.

For comparison, here are a couple entries from the LP-PAN sound card table using NaP3. The Realtek is representative of the sound cards that are built into the motherboards of recent PCs. The Asus is external USB.


Noise Floor:
Center / Edges
Dynamic
Range
Cost
New
Production
Status
Windows 7 64-bit Compatibility
Realtek ALC892 8ch HD Audio
motherboard card
-128 / -118 dBm
192 kHz
116dB $0 plus
LP-PAN 2
($250)
In production Full compatibility through  Windows Mixer when using MME driver. ~20dB DC spike
Asus Xonar U5 USB

-130 / -126 dBm
192 kHz
119 dB
$65 plus
LP-PAN 2
($250)
In production
Driver support for  Windows 7 thru 10. No DC spike.

Details...

LP-PAN 2. Of course, this is the one I have the most experience with. I use it with NaP3, which I believe is the best SDR app in terms of performance and user interface. The limiting factor on dynamic range with this setup is sound card clipping. If sound card gain is set properly, 115 - 125 dB dynamic range is easily achieved. This is 30-40 dB better than the dongles..

Funcube Dongle Pro +. This works well at lower signals levels, and the gain can be adjusted by changing the LNA and IF Gain settings. The limiting factor here is blocking. When signals get to be more than 85 dB above the noise floor, increasing the level increases the signal and noise floor by the same amount. 85 dB above the noise floor represents a signal level of approx. S9 + 28 dB when gain is set for a noise floor of -130 dBm to match a typical sound card. This is not a rare occurence. A strong signal anywhere on the display will cause "pumping" of the whole noise floor in step with the modulation on the signal.

It was generally easy to set up with HDSDR, SDR# or NaP3 on my laptop (win7 / 32b), but it was quite difficult getting it to work with my shack PC (win7 / 64b). The audio portion of the dongle interfaces through Windows Mixer, just like a sound card. It requires an SMA adapter for the input. There is a very large spike at 0 Hz, much higher than the spike that occurs with cheapie sound cards.

RTL2832U based dongle. This appears at first to be an inexpensive way to go, but has considerable limitations without adding a number of additional pieces of hardware. Because of the loss of the upconverter, the unit has marginal noise figure without an external preamp. The weakest signals that could be clearly displayed would be about S5. I found a Tuner Gain setting of +23dB to be about optimum. Raising the gain above this level just raised the noise floor. The noise floor is flat and has few spurs, but the ones I saw were very strong. Bandpass filters before and/or after the upconverter would probably help with this. That's a lot of pieces, and lots of adapters and cables.

I also tested the dongle without the converter and found the dynamic range to be a few dB better.

I had some difficulty getting usable RTL2832U drivers installed on my system, which should work with both SDR# and HDSDR. The default driver Windows installed automatically was not usable. The RTL2832U is not supported in NaP3.

SDRPlay RSP. This works well at lower signals levels, and the gain can be adjusted by changing the LNA and IF Gain settings. The limiting factor here is again blocking. When signals get to be more than 80 dB above the noise floor, increasing the level increases the signal and noise floor by the same amount. 80 dB above the noise floor represents a signal level of approx. S9 + 20 dB when gain is set for a noise floor of -130 dBm to match a typical sound card. This is not a rare occurence. A strong signal anywhere on the display will cause vertical "pumping" of the whole display in step with the modulation on the signal.

Setup of the SDRPlay RSP with HDSDR or SDR# is not trivial, and I did run into a couple problems.Overall setup complexity is on a par with installing a USB sound card and NaP3. I did not try it with SDR-Console. It does not work with NaP3.

Bottom line...

LP-PAN 2 provides more than 30dB better blocking dynamic range performance, even with the Realtek HD Audio built-in PC sound card. Cost is competitive, and there is no spike in the middle with the Asus external cards. LP-PAN 2 also benefits from the rig's bandpass filters, which are much steeper than the ones in these devices (the NooElec does not have have any filtering at all).

In my opinion, SDRPlay RSP is the best choice among the various "dongle" devices if you want a flexible SDR for general coverage reception on any frequency. But it is not well suited for panadapter use in a high performance station because of the dynamic range limitations.

In my opinion, RTL2832U based dongles, with the added complexity of needing the up-converter, good quailty preamp and lots of cables/adapters are not worth the effort. Throw in a shielded case and filters, and the apparent low cost can approach the SDRPlay RSP with a lot more work involved. And the driver installation/configuration was harder in my experience than installing a sound card (especially if it is already installed on your system).

Larry N8LP