[NLRS] FunCube

[Doug Reed]

The Funcube was developed shortly before software developers found out
the DVB-T dongles could be used as SDR receivers. The Funcube uses the
E4000 tuner chip but instead of using the RTL2838 chip to generate IQ
data, it uses a stereo audio A-D chip to generate IQ data.  (That is
not the right terminology, but if you recognize how wrong the
description is, then
you know what I'm trying to say.)

As result of the E4000 tuner, the Funcube and DVB-T dongles cover the
same 64MHz-1700MHz range, with a hole around 1100MHz. As result of the
A-D IQ process, the Funcube provides 12-bit(?) data at a 96KHz data
rate versus the DVB-T stick providing 8-bit data at up to 3.2MHz(?)
data rate. The IQ data rate means the Funcube can show a 96KHz slice
of the band with about 90dB dynamic range, while the DVB-T dongle can
show up to a 3MHz slice of the band with 48dB dynamic range. (I didn't
verify these numbers but they are close.)

NOTE, the lower dynamic range does not directly translate as lower
receive sensitivity. It does determine if I can show the difference in
signal levels within the receiver's bandwidth as determined by the IQ
data rate. It may help you visualize the issue if you think of a
receiver S-meter that goes to S8 as the max rather than S9+30dB.
Within the limitations of the receiver (chip) you can hear the same
things but can't tell how much stronger one is over the other.

In practical terms, the higher dynamic range is a good thing, just
like it would be in any other high-performance receiver but the type
of applications you can run is limited by the bandwidth provided by
the IQ data rate. For any typical ham radio modulation, the 96KHz
bandwidth is entirely practical. Even for use as a 2M panadapter. But
you can't due a quick spectrum analyzer display of a transmitter
without using software to combine multiple sweeps. However the DVB-T
dongle can't display enough dynamic range to be useful for FCC
readings....

Bottom line, "you pays your money and you takes your pick."

If you are looking for your "first" SDR receiver, I still recommend
the DVB-T dongle using a R820T tuner chip. For under $20 you can get
your feet wet and learn what SDR can do by finding how the DVB-T
limitations effect what you want to do with it. In most cases, the same (or at
least similar) SDR software will work with either dongle and all other
SDR hardware. Once you have an idea what you want SDR to do, then you
can decide what SDR to buy for your next venture. You may decide the
$200 Funcube isn't what you want for a second step or you may be
entirely happy with it. .

It has been over a year since I did any research on what was new in
SDR hardware. The DVB-T and Funcube dongles are both "old news." The
last time I looked, some groups were trying to develop high-speed
high-dynamic range interface hardware to use for SDR. If there is a
new generation of USB3.0 interface in a 12-bit or 14-bit RTL2838-type
chip, we could see a really nice generation of pocket size SDR
hardware coming.....

The problem is that high-speed A-D chips are difficult to design and
more difficult to build in real hardware. That is one reason we tend
to have two "groups" of SDR hardware. I would call it the "audio"
group of SDR hardware that uses a stereo audio A-D to get 96KHz
bandwidth, or a "high-speed" group of SDR hardware that uses faster
A-D chips with less resolution.

Of course the real limitation of the DVB-T and Funcube is that they
are receive only. Wouldn't it be fun if they could transmit too????
But to be a practical transmitter I think you will need to use
something more than an 8-bit D-A since you need to control modulation
products more than 60dB down....

My suggestion would be to concentrate more on the applications you
wish to run and less on the hardware. What makes SDR fun is the
software can turn the hardware into almost anything. If any of your
applications require more than 96KHz bandwidth, you have eliminated
98% of the off-the-shelf SDR hardware. If you need to display more
than 50dB of dynamic range, you have eliminated the DVB-T and similar
hardware. If you want to display 90dB dynamic range and more than 1MHz
bandwidth, then you need deep pockets to buy a late model
Rhode-Schwartz or Agilent spectrum analyzer.  :-)

73, Doug Reed, N0NAS.