SDR Equipment
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I decided to pull together
here my experiences with this new branch of radio development
(currently dotted around my website) to help spread the word
about this completely different branch of radio and show you
various examples of equipment.
You get what you pay for in
this branch of radio and of course you'll need some sort of computer
and suitable software to make it work, although some relatively
expensive offerings include a built-in computer with firmware.
First the UK-developed SDR
Play
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Next the Lime SDR
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The Lime SDR is a bit
different to the SDR Play because not only is it a receiver but
it can also transmit. It can run from a standard PC using appropriate
software over a publicised frequency range of 100KHz to 3.8GHz.
You'll notice from the pictures below it has three Receive and
two Transmit input/output options as well as two separate receive/transmit
connectors. It also carries a special USB connector and a socket
for using an external power supply. The USB connector is the
type used on external USB 2.5 inch hard drives and DVDs which
carry dual USB plugs for power+data and power. Ideally external
power should be used in transmit. Click
either picture to read more. |
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Now, my favourite SDR the
Andrus Mk1.5
This, now very scarce SDR, is described below, complete
with my attempt at repairing it.
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This rather fine looking
"Software Defined Radio" was made in Estonia and was
designed for reception of radio frequencies from 5KHz to about
32MHz. It's on its way here but I bought it untested at about
20% of its usual price so whether it will work or not I don't
know... It was described as "house clearance" and "can't
test because I don't have a power supply". A rather risky
purchase. Sure enough.. I plugged it into my computer and there
wasn't a request for a driver. After a few minutes the case began
to get warm so I unplugged it. The next day I removed the circuit
board from its case and found that it has three LEDs inside.
One lit green and so did the second. Pressing the two buttons
"Reset" and "HWBE" caused the second LED
to flash three bursts of five which looks to me like an error
message? I measured the internal power supply (there are several)
One was about 3.3 volts and another 1.8 volts but the other was
trying very hard and failing to reach 3.3 volts. I checked and
found the circuit drawing too much current and causing the regulator
to run exceedingly hot was labelled VCC3V3ETH/AVCC3V3ETH and
fed almost exclusively the ethernet controller, a chip labelled
KSZ8851SNL. This fed an ethernet socket incorporating two LEDs.
I removed this and also a tiny choke feeding the two ETH supplies.
Connecting a power supply to the ETH circuit proved it to be
consuming 1.5amps at 1.5 volts with the chip running too hot
to touch. Maybe someone connected a cable carrying DC (power
over network cable) and blew up the chip? The chip is dreadfully
difficult to remove and just as tricky to fit a new one. See
the picture below..
Without the ETH supplies connected,
I plugged in the USB cable and after pressing Reset and HWBE
the third LED lit and the computer requested a driver...
click
the picture to see its schematic |
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Although I've swapped
lots of surface mounted chips this one will be the trickiest
so I've ordered a new rework station. Up to now I've struggled
with a normal soldering iron but the so-called QFN package with
32 pins is not going to be easy. It's size is 5mm square x 0.85mm
high and the 32 pins are on the underside of the chip. Some QFN
chips do not have side access to their pins but this one may
because it's defined as a Micrel MLF package. Above, you can
see the code number in full (often this isn't the case and you're
left puzzling over exactly what it is from the sparse information
printed on it). In fact the KSZ8851SNL is no longer made because
the KSZ8851SNLI (the version with a wider temperature spec of
-40/+85C) has displaced the 0/70C version.
Why did the original fail? As
it connects to an ethernet network it might have suffered from
a high voltage from lightning or even an experimenter playing
around with Power-over-Ethernet. I removed the ethernet socket
in case this was responsible for the problem. This is coded TE-6-6605851-1 (click to see the spec)
and, because it includes some parts I'll need to check it out
for damage before refitting it. |
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The rework station arrived and I
detached the faulty chip without any trouble. To see if the chip
had been preventing the board from booting up I plugged the board
into a USB port. Two green LEDs came on but it was only when
I pressed the Reset and HWBE that one of the green LEDs went
out and a red one come on that it was detected by the computer
(exactly as before). My guess is the ethernet chip is checked
by the main processor and failure to respond will prevent completion
of bootup. |
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Before fitting the new chip.
As there was a decent quantity of solder
present on the pads I didn't add any fresh solder but a good
smear of flux was added before applying hot air at around 380C
to the board to preheat it especially the solder on the centre
pad which I checked was fluid before accurately placing the new
chip above its pads. |
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Once the chip was placed in position
with its marker dot correctly orientated I continued to apply
hot air. At no stage can you see whether it has been soldered
correctly as the pads are mostly underneath the chip. After around
10-15 seconds I removed the hot air gun, waited for the board
to cool then refitted the ethernet socket which I'd removed for
access then refitted the inductor removed to break the short-circuit
on the power rail. Just a few spots of flux to remove and the
radio looks as good as new... |
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Now came the proof of
whether (a) the chip has been soldered properly and (b) whether
this was the only fault...
The board is powered via a mini-USB
lead fitted with two USB plugs for USB3 sockets. Because the
SDR program deals only with the Andrus via its ethernet adaptor
I also plugged in a patch lead. Plugging in the mini-USB lead
made the computer respond which it had failed to do previously.
With the ethernet chip removed I had to press the reset button
to enable the board from being detected and at this point it
couldn't find the right driver. The aerial plugs into the inner
BNC socket and then I opened SDR Console and searched for and
selected Andrus... whereupon it was detected correctly, and starting
the receiver proved all was fine. Radio 4 came in loud and clear
with 40m noisy but full of stations. |
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Here's a couple of screenshots,
the second an 80m QSO between G3OQD and G4HJW.. |
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The Adrus SDR is
best operated using its RJ45 interface because this enables it
to be remotely located and minimise local interference. There
are two means of supplying power.. first via its mini USB connector
and secondly via its circular coaxial power socket. Ideally the
former should use a USB lead having only power connections. Commonly
these cables are supplied with external DVD or hard drives where
a Y-cable splits their required power between two USB plugs,
of which only one carries signals. What's the power requirement
of the Andrus SDR? I measured this using the power plug and found
that about 350mA was drawn at 5 volts for a few seconds, then
once the RJ45 LEDs came on this increased to about 550mA at 5
volts. |
Add-on Hardware
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Now some ideas about practical
aspects. Anyone trying out an SDR will be rapidly become aware
of its drawbacks as well as its fascinating features. Most radios
are designed to be tuned precisely to a broadcast of interest
and simulateously "detuned" from broadcasts that are
of no interest. Most SDRs are wide open with no tuning other
than their local oscillator (except perhaps some sort of passband
filter). Ideally then, to make the most of an SDR you should
use an appropriate filter in the aerial lead. For example, if
you're listening to amateur radio broadcasts you do not want
interference or degraded reception due to strong local Band II
FM transmissions or a strong local medium-wave transmission.
Another add-on, which mainly
concerns the Lime SDR, is the use of a converter. The Lime SDR
is optimised at frequencies in the VHF and higher bands and is
pretty poor at MF and HF. To receive frequencies below say 30MHz
an up-converter is recommended and if you'd like to transmit
below 30MHz a downconverter is desirable. |
VLF Reception
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Frequencies below 100KHz
are quite interesting although nowadays these can be plagued
by interference. Many transmissions are associated with the Military
and are relatively narrow band. These can be identified by their
musicality, if that's a sensible term, rather than the random
hash normally associated with chopper power supplies.
click
the picture to read more
If you don't
have an SDR you can use your PC sound card instead! |
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To get the best reception
at VLF you need a decent aerial and the easiest to make will
be a long "random" wire. Unfortunately this will pick
up any strong local signals outside the band of interest so you
can make a low pass filter to improve matters. Without a filter
you may get unwanted AGC action which reduces signal strength
or cross modulation resulting in jittery reception or even speech
appearing on say a teletype broadcast.
click
the picture to read more |
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A simple Upconverter for 0 to 30MHz
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Here's a converter that
transposes zero to 30MHz to the range 50-80MHz. I chose that
VHF range because it's relatively free from strong broadcasts.
click
the picture to read about it |
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Transmit with a Lime SDR
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Reject strong FM broadcasts
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Build a Loop Aerial
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To minimise interference
and to get really excellent reception you can build a loop aerial.
This can be located at a point furthest away from local interference
and, because it's directional, it can be rotated to get the best
results. I carried out a few experiments and found medium-wave
varicap diodes are ideal for the job and still available. Essentially
one of these diodes can take the place of a 500pF tuning capacitor
and can be adjusted remotely allowing you to tune a station from
in front of your PC.
My example has three large coils
selectable via a couple of miniature relays so I can select and
tune Long Waves, Medium Waves and the 80m band. Connection between
the operating position and the aerial is via multi-way CAT5 cable
which gives me enough conductors for coil selection, varicap
tuning, power and RF output.
Another solution which I haven't
tried is to use a ferrite rod, again tuned by a varicap diode.
This would be a lot smaller than the loop and could even be mounted
high up on a cheap rotator.
click the
circuit to read more |
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Listening to medium or
long waves at night reveals many more broadcasts. I looked on
long waves last night then in the morning in daylight and you
can see the results below. Click
the picture to see a clearer view. The results are not ideal
because I used an 80m inverted V dipole rather than a very long
wire. The receiver used was my Andrus SDR. The times were midnight
(upper scan) and 10am (lower scan) on the 24th August 2021. |
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