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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
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
73, Larry N8LP
Funcube Dongle Pro +
NooElec TV28T USB stick & "Ham It Up v1.2" upconverter
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
Note: FCD Pro
can be set for more gain to produce a lower noise floor than the
settings I chose, but every dB
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.
Center / Edges
Dongle Pro +
/ -105 dBm
with Win7 32b, spotty with Win7 64b
Large DC spike. LNA ON, Mixer gain OFF. IF gain = +3dB.
TV28T & "Ham It Up" Upconverter
of preamp, preamp power supply, filters, shielded case, etc.) Large
/ -130 dBm
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
/ -118 dBm
compatibility through Windows Mixer when using MME
driver. ~20dB DC spike
|LP-PAN 2 w/Asus Xonar U5 USB sound card
/ -126 dBm
support for Windows 7 thru 10. No DC spike.
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.