The low pass filter at the top
of the picture has four coils each having 14 turns wound on a
3.8mm former to produce a diameter of 5mm and a length of 10mm.
The input is coupled to the BNC connector via a 100nF capacitor.
The relatively high value is to reduce the input resistance to
long wave signals. The input of the first coil is decoupled to
chassis via a 200 pF capacitor and then the output of the first
and 2nd coils by 300pF capacitors. The output of the fourth coil
has a 200pF capacitor to chassis. The filter output connects
via a 100nF capacitor to the base of a transistor. At this point
in development, the base was self-biased by a 22kohm resistor.
The bias voltage results in current being drawn through the 91ohm
emitter resistor. The collector is tied to the transistor supply
voltage which is limited to around 12 volts by the temporary
collection of resistors you can see top right. RF output which
is substantially at the same voltage as the incoming signal is
coupled via a 100nF capacitor to the input toroid of the double
The 50MHz crystal oscillator
circuit is very simple and uses a small coil tuned by a small
trimmer capacitor to run the crystal on its 3rd overtone. This
is temporarily connected via a 100nF and 100 ohm resistor in
series with the output toroid.
You can see the diode ring chip
between the two toroids. The output toroid connects to a simple
bandpass filter covering about 50 to 80MHz. There are three coils.
The input coil has 12 turns and connects via a 22pF capacitor
to the shunt coil to chassis of 4 turns in parallel with about
180pF of capacitance. The output coil and capacitor are the same
as the input coil. Again, everthing needs tidying up and the
larger coils should really be at right angles.
When I'm happy with the
final result I intend to solder more tin over the low pass filter
etc to reduce pickup from local interference sources.
Below is a picture of
the converter output which will need a little explanation. I'm
using a tracking generator to produce the scan which goes from
100KHz to 100MHz. The left end shows the effect of the 30MHz
low pass filter superimposed on which is a 25MHz -40dBm output
from the crystal oscillator (I'm not sure that this should be
present if the oscillator is working correctly). Next is an area
of attenuation which extends to 50MHz where you can see the 50MHz
crystal oscillator signal sitting at about -32dBm. The "marker"
is resting on the baseline of the 50 to 80MHz band pass filter.This
filter is relatively simple and has a sag of 5dB. Then there's
an area of attenuation (at -80dBm) from about 82MHz designed
to reduce the effects of local FM broadcasts.
Below is a narrower scan
from 50 to 80MHz (=0 to 30MHz) where you can see the variation
in the bandpass filter characteristics. This shows a 2MHz signal
from my signal generator which is Tee'd into the tracking generator
output. The horizontal lines are 7dB apart, so the ripple is
7dB. As the signal generator is tuned upwards the spike moves
to the right where 30MHz is upconverted to 80MHz.
Here's a further scan
showing a signal of about 120KHz sitting on a scan from 50 to
51MHz (=0 to 1MHz)
Below is a scan of the
low pass filter taken from the emitter of the transistor buffer
after adding a 330nF decoupling capacitor at the collector followed
by an increased scan where you can see the small reverse leakage
from the 50MHz crystal oscillator. In these scans I'm using a
signal probe using a 1Mohm series resistor connected to a 10Mohm
input circuit. This basically adds insignificant loading to the
This next scan is made at the 50
ohm output connector. This shows 50MHz breakthrough with a small
amount of its second harmonic at 100MHz. You can see the shape
of the bandpass filter which rolls off at Marker 1 at 80MHz corresponding
to an upconverted frequency of 30MHz. By adding more filter sections
the attenuation could be increased to around 30dB.
Here's a screenshot of
the Lime SDR with the upconverter and tuned to about 57.160MHz
corresponding to the 40 meter band. The receiver is tuned to
a lower sideband signal registering -81dBm. I'm using an 80m
inverted V so signals are not particularly good but the level
of local interference is much lower than from a long wire.
Below is the latest circuit
for the G3PIY upconverter
Note that the length of coils
L1 to L5 is 15mm, L5 & L6 are 12mm and L7 is 6mm. L8 is 10
turns on a 3mm former.
The oscillator bias resistor
is now 47kohm not 120kohm. L8 trimmer is now a 22pF fixed capacitor.
The buffer emitter resistor is now 43ohms. Most changes were
done to reduce the supply voltage requirement to 5 volts. The
toroids are wound to suit their material. Mine are coloured yellow
and are a bit less than 5mm diameter and 2.5mm thick. See later
for the reason for the extra 50pF.
I decided to tidy up the
upconverter and see how it performs. Across the whole range to
30MHz I can easily resolve 5 microvolts from my signal generator,
in fact you can see a blip down to less than 1 microvolt. The
blip vanishes, as it should at 31MHz and above. Response is much
the same down through MF and LF . Plugging in a convenient dipole,
actually cut for 80m, results in strong reception down to ELF.
I moved the receiver to my main computer and tried a frame aerial
which I use for my SDR Play. This aerial gave me reception of
our local long wave station with a maximum indicated signal level
of -3dBm. This was achieved by fiddling with gain settings and
of course these are certainly not optimum. The very best I can
see with an SDR Play is around -22dBm.
Above, slightly tidier.
I modified it to work on 5 volts by substituting an RF choke
for the crystal oscillator collector resistor but the output
from the oscillator dropped and reduced the mixer performance
so I run the converter on 12 volts instead. I added a couple
of ceramic standoffs to make the construction more rigid and
increased the oscillator coupling capacitor to 330nF from 100nF
and removed the 100ohm series resistor. The oscillator bias resistor
is now 47kohm and I soldered the crystal to the tinplate. I also
reduced the buffer amplifier emitter resistor to 43 ohms from
1kohm. These mods were made so I could reduce the operating voltage
to 5 volts. It actually works on 4 volts but I found that pushing
this up to 12 volts increased overall gain.
Above is the upconverter rebuilt
into a small diecast box. I've added a screen along the centre
line and screens between the coils in the low pass filter and
placed a metal screen between the crystal oscillator and the
input toroid to the double balanced mixer. Also, I've put a screen
between the coils in the band-pass filter. I used veroboard to
minimise chassis currents between the input and output circuits.
The veroboard was cut to fit the box, then assembled and fitted
in place. Once this was done I soldered the BNC connectors to
the filters. I haven't checked yet to see if any improvements
are noticeable however, below are two screenshots using the Lime
SDR, one of the 30 metre band using a poor antenna. The receiver
is tuned to 9.570MHz. See further on to explain the reason
for the arrowed note.
The second is the band from zero
frequency to a little above BBC Radio 4 on 198KHz. A couple
of time signals can be seen in the waterfall. The aerial was
a short length of wire local to the computer with associated
chopper power supplies whose rough signals can be seen. Oscillator
breakthrough is at 50MHz.
Below is the same spectrum
for the Lime SDR, but without the upconverter and using the same
wire aerial as the picture above. Radio 4 is unreadable and the
only signals visible appear to be interference from chopper power
supplies. One noise source is producing spikes at 80KHz, 160KHz
and 240KHz with the second producing humps of noise at roughly
Once I'd familiarised myself
with the operation of the upconverter I decided to check it on
my spectrum analyser. By feeding the input with the tracking
generator and monitoring the output I could see the overall response
of the converter (picture will follow soon). The results are
a little puzzling because several things are going on simultaneously.
The input filter reduces the overall response above about
35MHz and the output filter reduces the response above 80MHz.
The level of the crystal oscillator is quite significant but
should only interfere with very low frequency signals. Having
checked the response I injected an RF test signal of 10MHz into
the input and this duly appeared as a 60MHz output. Initially
I was puzzled because the size of the upconverted signal was
switching between two values but this was corrected by turning
off the tracking generator. As with most home-brew projects you're
left with the doubt that it's working as best as it can. I'm
reminded of the radio designer a Mr Scott-Taggart who
produced receivers in the 1930s where almost every passive
component was variable resulting in a multitude of front panel
controls. I can also point you you to the
DST100 receiver designed for absolutely the maximum performance
in reading "almost" unreadably weak transmissions from
To this end I then checked the
sensitivity of the converter to the effect of adding 50pF extra
capacitance at various points in the circuit and found little
of significance except around the mixer coils. I discovered that
by touching 50pF across the primary winding of T2 pushed the
converter output up by over 10dB. Not only did the ouput rise
in amplitude but the "width" of the signal reduced,
looking much cleaner so I soldered the 50pF capacitor in place
(see circuit diagram above for its location). Unfortunately,
the extra output completely swamped both the SDR Play and the
Lime. The effect was to produce a comb of spikes spaced by about
17KHz around the 50MHz breakthrough signal and any tuned stations
so I snipped off the capacitor and all was well again.
By moving the RF test signal
up and down from 1MHz to 30MHz I found the corresponding output
51MHz to 80MHz followed as expected. The output remained pretty
well constant from 30MHz down to less than 1MHz when the output
increased (which is no bad thing as it will improve MF and LF
I then carried out a test on
the effect of the power supply. As I'd planned, the crystal oscillator
worked down to 4 volts before cutting out. Overall gain did improve
though as I increased the voltage (mainly due to the increasing
level of the 50MHz mixer local oscillator). To check nothing
untoward was happening as the supply voltage increased I checked
the crystal oscillator harmonics to check for unwanted spurii.
See the table below which shows the harmonics not worsening and
even reducing slightly as the supply voltage is increased, so
operating from 12 volts is fine.
50MHz level = f0
100MHz level = f2
I carried out some tests using
the Lime and found the following looking at the 50MHz breakthrough
signal from the upconverter...
I then tuned the Lime to the 40m
amateur band, tuning to 57.000MHz to 57.200MHz and compared the
results across the different inputs. All the inputs performed
much the same but I needed to individually adjust the overall
gain setting on each input to around -62dB give or take to prevent
overloading. Removing the upconverter and repeating the test
showed that only RX1 L was usable but it was considerably worse
than the upconverter results. Tuning down to Long Waves using
the upconverter (50.000MHz to 50.500MHz) I found results were
pretty good but as gain was increased and a tuned station became
stronger the breakthrough 50MHz signal became modulated from
the received station.
I discovered the reason for the 50MHz
signal being modulated by a tuned signal audio. I'd fitted the final assembly into a diecast
box with a pair of BNC sockets side by side at one end but I'd
forgotten to connect any of the ground connections from the PCB
to the outer case. The result was earth currents were getting
into the output cable and modulating any signals feeding the
output cable. I experimented by shorting various points on the
metal shielding to the case and found a solid connection at the
screen dividing the two BNC connectors almost completely removed
modulation. I'm sure I could improve things further by grounding
other sections of screening to the case also (See picture above).
Later I found that there was still some
residual amplitude modulation of the 50MHz crystal by incoming
strong signals. I also found some hum. By tuning the Lime SDR
to exactly 50MHz, listening to the receiver and prodding around,
it seemed to me that earth currents in the converter were responsible.
I tried shorting various earth points to the diecast box and
found varying results. The best result was found by connecting
the base of the RF transistor to chassis via a 3.3pF capacitor.
This eliminated the annoying hum on the 50MHz signal and which
was being transferred to one or two broadcast signals. This capacitor
did also dramatically reduce the modulation from the strongest
signal entering via the aerial (this was primarily Radio 4 on
198KHz) and was visible in the SDR waterfall. The remaining modulation
is now reduced to a very low level. A secondary improvement is
that the receiver noise baseline is less and a lot flatter. Typically
50MHz leakage is -44dBm whilst Radio 4 is -61dBm with the Lime
wideband input and gain set at 50dB.
Then, I decided to compare the results
using the SDR Play receiver. I found the 50MHz signal stood at
-41dBm with Radio 4 at -56dBm which are pretty close to the Lime
results. The SDR Play RF gain was set to Max and IF gain at -45dB
I then checked reception of Radio 4
on my Andrus. This registered -43dBm with RF gain set to 0dB,
-42dBm at -12dB and -24dB and -49dBm at -30dB.
an FM Rejection Filter to reduce breakthrough
Below is a first attempt to
build a filter designed to block FM broadcast signals breaking
through in shortwave bands.
The filter circuitry is fairly obvious
from the picture above, three bare coils between input and output
connectors using tiny coils with 100pF shunt capacitors and four
enamelled coils with series trimmers as reject circuits. The
bare coils are around 25nH tuned by 100pF capacitors and determine
the cut-off of the filter. These coils were initially slightly
larger resulting in a sharp cut-off at 58MHz but now they're
smaller the cut-off is around 80MHz. The reject coils are stagger
tuned to provide a wide reject band up to 108MHz. Tuning it proved
very difficult because I plugged in a coax lead that wasn't connected
to the spectrum analyser because of extreme untidiness on my
work bench. The correct lead feeding the analyser was picking
up an extremely weak signal displayed by using its "auto"
button. The pickup resulted in a picture which changed as the
trimmers were adjusted but then I noticed the vertical scale
was in small fractions of a dB. The penny dropped and a rummage
around the bench revealed the wrong lead was being used. After
discovering my mistake I switched the leads over and found the
filter worked perfectly (see the waveform below). The plan is
to rebuild the filter in a diecast box now that I understand
the interaction of coil sizes and capacitances. All the coils
are wound on a 3mm drill. Insertion loss is about 3dB.
Having tested the first attempt
I wasn't very happy with its performance so I made another but
using heavier wire to improve the Q of the coils. This is shown
below. Its made on a small piece of tin cut from the lid of a
box of Qualty Street chocolates. I used 50pF capacitors because
I have a large quantity of these, and 18SWG tinned copper wire.
The trimmers are something like 3-30pF. There are 5 coils in
series (4.5 turns on 5mm) with 3 shunt coils (12 turns on 5mm).
The performance is shown below the picture. The design probably
has a fancy name?
Marker 1 is set at 80MHz
and Marker 2 at 109MHz. Marker 3 is at 118MHz. The attenuation
over the FM broadcast band is about 50dB. The tracking generator
is set at -20dBm and some losses will be due to cables and connectors.
How did it work in practice but with the filter as shown and
unscreened? Classic FM dropped from -77dBm to -107dBm so in practice
I got an attenuation of 30dB. That station went from perfectly
clear and very strong to scratchy and very weak. Unfortunately
there were consequences. Testing the filter directly into the
Lime resulted in masses of extraneous noise spikes and lots of
out of band signals. I moved the filter outside into the antenna
feed line and this removed most of the local electrical noise
and revealed the out of band signals. Essentially one needs not
a reject filter but a band pass filter centred on your band of
interest. That might be my next step... a low pass filter for
say the AM broadcast band and a bandpass filter for each specific
band in which I'm interested.
When the filter is properly
enclosed the curve should be much the same shape but stray pickup
of RF will be reduced. Below is a test carried out with the screened
filter and the SDR Play tuned to 88.5MHz. First with the filter
Now, the filter removed
but with the SDR settings untouched.
Now with the settings
adjusted to position the noise floor.
What do we see? With the
filter installed the signal has much the same amplitude as peaks
of interference with the noise floor at higher frequencies raised
slightly, perhaps from a strong signal. Without the filter..
The noise floor is raised significantly from other and in-band
signals and finally the last picture shows the noise floor at
the same level but pushed down the display to give a better picture.
The received signal is -101dBm with the filter and -65dBm without..
indicating a rejection at 88.5MHz of 36dB. At this point I haven't
tested the filter on the spectrum analyser after fitting it into
its screened case so this figure might be improved a little.
The experimental filter in its case.
For convenience I'm using F connectors on flying leads rather
than connectors fitted to the case. The holes are for trimmer
adjustment.. two in the top and a third at the side.
During tests using my SDR Play
I saw virtually the whole spectrum full of intermittent spikes.
These were grouped in sets of half a dozen. I recalled I'd seen
this problem a few days ago and, guessing the cause, tuned to
the Citizens Band, which in the UK occupies 40 channels from
27.60125/27.99125 MHz (Channel 1 to 40 ). The CB craze has progressively
faded but there are still operators around, some of whom use
large aerials and very high power (both illegal). Unfortunately
local CB and an SDR are not entirely compatible due to overloading
of the SDR untuned input.
I inserted a variable attenuator in
the aerial feed and found that by adding 20dB of attenuation
the mass of 27MHz spikes disappeared and I could see the true
frequency of the CB operator. The breakthrough on other bands
was now much reduced, however all the other signals at the aerial
are also degraded by 20dB. The answer has to be a reject filter
for the band centred on 27.7MHz. Not too difficult to make because
its bandwidth needs to be only about 400KHz.
Below a plot of the cased Broadcast
FM reject filter, Markers..1=66MHz, 2=85MHz, 3=108MHz, 4=120MHz.
Ideally, one of the shunt coils needs compressing slightly to
reduce the air-band attenuation. The tracking generator has an
output of -20dBm so attenuation below 40MHz is virtually zero
so the filter is ideal for HF reception.
Now to transmit...
The BIG question...How does the Lime SDR work when used as a
driver for a transmitter?