Since I do not have an expensive high frequency A/D converter, I resorted
to the normal PC sound card, which usually can sample upto 48 KHz with
16 bit resolution. Some good sound cards are capable of sampling upto
96KHz with much better resolutions.
Use of sound card limits the bandwidth one can get. According to the famous
Nyquist theory, the maximum frequency I can sample using a sound card is 24KHz.
So the maximum bandwidth will be 24 KHz, from DC to 24KHz. But sound cards have input filters, so it will a little narrower. To fully utilize this sampling capability, we need to convert the center of an RF signal bandwidth to 12 KHz. In my case, from 455 KHz to 12 KHz. We need 467 or 443 KHz signal source and a mixer. If we use 443 KHz, the spectrum will be flipped.
I used the converter by I5XWW, which he sell for 15 Euro. It uses an NE612
balanced mixer IC. It is similar to
the standard DRM converter, less the
one TR input buffer amp. It is a nicely built unit with a voltage regulator
and a diode-based reverse polarity protection. It converts 455 KHz signal
to 12 KHz.
The input to this mixer is tapped from the base of Q33. This portion of
circuit is only active when the mode is set to FM. At this stage, the signal
has already filtered by the ceramic filter F5. It's bandwidth is 15 KHz (6dB).
Since the skirt is not very sharp like mechanical filters, the actual usable
bandwidth is about 20KHz. Since the center frequency is 12 KHz after
conversion, the signal spectrum will occupy from 2KHz to 22KHz. Aliasing
will be reduced because of the ceramic filter.
Many commercial HF radios with DSP feature also convert signal
down to about 10 KHz before any DSP processing. Note that R-2000's LO1 frequency shifts in USB, LSB, or CW mode.
For more flexible SDR, we must sample the signal before filtering. Since
the second IF filer, F1's bandwidth is about 150 KHz, sampling before 3rd
IF filters will give us this wide signal. But the A/D converter must be
able to sample at 1.2 MHz or higher. Also, there will be considerably more
information to process. However, we will be able to see 150 KHz of spectrum
at once and tune to whatever looks intersting. Or simultaneously process
multiple streams. For instance, we can cover the entire 40m band in Korea
(7.0-7.1MHz) without retuning the RX.
[The mixer]
[The RCA jack for 12 KHz output]
[The RX board]
As for software, I used open source Dream (http://drm.sourceforge.net) on
Linux. I do not use the DRM feature but AM/SSB/CW/FM mode. It is not
perfect but good enough to test the idea. More advance and flexible
design will be possible with GNU Radio project.
[Dream on Linux]
The following rpm packages were built on RedHat 9. As long as there is
no error while installation, they will also work on later Fedora versions.
Dream requires more libraries such as hamlib, faad2, and journaline, but I
disabled them in my version. I also changed some code to make it more
usable (for me, but might be the case for you too). It is also configured
to use ALSA. I had to configure it this way because
my sound card does not work in a full-duplex mode with the OSS driver.
Dream.ini (The config file. Copy it to your home directory)
Dream.ini
I tuned to 80m band and could see multiple SSB transmissions in the
20KHz-wide spectrum. I could also choose any one and listen to without
retuning the RX. When R-2000 PC control is complete, SDR portion
can be integrated with the control. It will also allow us to overcome
the limitation of 50Hz tuning step, since SDR can tune in 1 Hz step.
The accuracy will depend on the 10 MHz reference frequency of R-2000
and the reference clock on the sound card.
Vittorio De Tomasi, IK2CZL has also
developed a nice Windows software that does DSP and demodulation.
The first version works great, and he will soon release much more
improved new version.
Crispino Messina, I5XWW, the maker of my mixer board also makes
Windows version of Dream available here. You need to get the ZIP file and the QT DLL file.
For more serious experiments,
GNU Radio is the way to go. But the version 2 requires newer libraries
and tools. So plan on updating your linux box.
Copyright (C) 2005 Kihwal Lee
