ex SP2EBP, VK2EBP
Jan Jozef Oksiuta
|Australian Amateur Radio
DC to light, homebrewing,
minimalist antennas and projects, QRSS, QRPp and less
Noise Tracker, July 2014
Image on left - an SDR radio spectrogram of 630m band
showing persistent interference throughout most of the
band and extending well into the the NDB band below.
Similar pattern of interference lines repeats itself
throughout the spectrum from around 100kHz up to HF
bands. This screenshot was acquired with a sharply
tuned magnetic loop antenna, which accounts for the
marked increase in gain around the frequencies of
Recent introduction of the new MF amateur radio band of
630m (475 kHz) promised a lot new challenges and projects.
My excitement in monitoring the early activity on the band
was however somewhat marred by the usual scourge of any
suburban neighbourhood - a plethora of
unwanted man-made noise. My receiving setup consists of a
variety of active tuned loops and an SDR-IQ receiver. I
was able to get some improvement with a mast-head (or
"loop-head") preamp and balanced feed to the shack (Cat.6
cable). Some interference sources however still persisted,
with appearance on SDR spectrograms suggesting some
switch-mode power supplies being responsible.
Time has come to pin-point the sources, and to deal
with them. An idea was born for a weekend project to build
a simple "sniffing" receiver. Some of the design points
are listed below:
- a direct-conversion architecture,
allowing superior aural perception of broadband noise
when compared with simple AM detection, and easily
obtained selectivity. Single-sideband
selectivity is not required for this purpose thus
further simplifying the project.
- broadest tuning range possible, allowing
its use also in VLF range, and all the way to and
including AM broadcast band. For simplicity, the VFO
will have interchangeable plug-in coils to cover the
required bands, and tuning will be separate for the
front-end and the VFO.
- selective front-end, in order to reduce
intermodulation products, and a directional pickup
antenna, small in size but offering good sensitivity.
All these can be obtained by the use of a small FSL
(ferrite sleeve antenna). Tuning ranges will be
selectable by jumpers connecting sections of the FSL
coil in series or parallel, with additional fixed
capacitors as required.
- no automatic gain control - further simplifies
the project, and in any case the AGC would be
counterproductive in a device designed to detect
changes in signal intensity "by ear". A visual RSSI
would be nice but I considered it an overkill for what
was meant to be a cheap and quick project.
- last but not least - portable and
comfortable to move around, with battery power,
speaker and headphone output. This can double as a PC
sound card source for observation of spectrograms. I
ended up with a grossly oversized carry case from an
old disposed medical suction device, with lots of foam
filling up unused space and also cushioning up my
priceless ferrite rods.
schematic. VFO coil/capacitor plug-in shown (130uH/680pF) tunes the circuit between 450-510 kHz.
Most of my "quick" projects are built on
the fly - straight onto the circuit board. Proper
documentation with computer-generated drawings etc
would probably take me longer than the project itself -
hence the handwritten notes. Right-click on the image
to access it in full size.
The ferrite sleeve loop acts as a directional pickup and a
high-Q input tuning circuit. It consists of two
contra-wound 49 turns coils on a PVC pipe 111 mm O.D. and
using 0.43 mm diameter wire. Coils can be connected either
in series or parallel and are brought to resonance by one
two parallel sections of a common polyester 60 + 160
variable capacitor and an optional 220pF jumper-selectable
fixed capacitance. Inside the coil there is a sleeve made
of 29 ferrite rods 9 x 100mm in size, material S1.
Individual coils have 1.1 mH inductance each, with 865 uH
in parallel and 3.5 mH in series. With these values in
their various combinations the input circuit covers the
range of 130 to 960 kHz - from the 2200m, through the
630m, up to somewhere the middle of the broadcast band.
Two centre-tapped coupling turns around the main input
coil feed the signal in balanced mode to the NE602
Gilbert cell mixer. The mixer also accepts signal from
local oscillator. This contains two FETs, another tuning
cap, and a plug-in coil for the desired band.
Audio signal from the mixer is available on pins 4 and 5.
This is crudely low-pass filtered by the 10k/10nF RC
network before being passed to the SSM2019
audio preamplifier. The rest is straightforward - a
trimpot for initial volume adjustment, an LM380 audio
power amp, headphone socket, small speaker switchable with
a jumper. The lot is supplied from an SLA 12V battery
switchable in and out of the circuit with another jumper.
A three-pin regulator supplies 6V to the mixer chip and
also a bias supply to the audio preamp.
The sniffer was tuned to around 476 kHz - first the VFO,
then the input circuit. The buzzing noise was instantly
recognizable, sounding very much the same like the pesky
interference on 630m. A walk around the backyard quickly
revealed a quieter corner, previously missed in my
trial-and-error attempts with the main receiving loop.
(Doing portable tests with a laptop and SDR receiver were
fruitless, as the laptop itself generated sufficient noise
to mask all the others. One has to position the receiving
loop, move back inside to check results, then repeat again
and again.) The loop was duly repositioned and my
reception of WSPR stations from down South was instantly
More walking around the neighbourhood revealed that the
source of noise was most likely confined to my property.
It was narrowed down to the bunch of cables leaving the
shack and going to the earth stakes, antennas, ELF
equipment and whatever else I had connected. Separating
the cables and "sniffing" them one by one revealed that
the noise was carried by the earth cable. More walking was
done inside the house and eventually led me to the
entertainment unit in the bedroom, where the real foxhunt
begun, involving moving the furniture and then
individually unplugging all the appliances (and I barely
use any of them myself!). That included the TV, video
player, DVD player, TVSat box, cordless phone, video
sender, broadcast radio... All of them were buzzing very
happily at close distance. One of them however was not
content with just that - it also had to broadcast it
through the mains earth wiring! Finally, a trophy - a
respectable brand DVD player. Interestingly, it was rather
(electrically) quiet when turned on, but was coming back
to its secret life as soon as turned off into standby
mode. It is indeed very rarely used which would account
for my very rare noise-free periods of grace.
The offender was duly eliminated from service pending
further examinations, with a view to implement whatever
control measures might be recommended. I started my
630m reception with an SDR (now sooo beautifully clear),
plugged the subject in, ensured it went to its standby
mode, and..... nothing. Not a squeak. Then of course - the
player is Class II device, i.e. double-insulated, no
protective earth, and consequently the mains cable is
terminated with a two-pin plug. No earth connection. The
earth connection must be coming from the A/V cable that
connected it to the TV.
I then connected an earth wire to one of the output RCA
sockets and yes, the same wobbly bars started dancing
again on the receiver screen.
Quick negotiations with the spouse and the device will now
remain isolated until "absolutely" needed, in the meantime
I am going to probe it further. For now, I connected a 10
ohm resistor between the device's metal case and earth and
applied a portable oscilloscope across the resistor. I was
able to observe over 10 mV peak of broad spectrum noise -
a clear indication of 1 mA of AC RF currents. More than
enough to be seen by any sensitive loop antenna anywhere
in the backyard.
There is about 1
mA of RF currents flowing from the player's case to
earth with multiple frequency components
Interesting that this interference signal is ultimately
earthed yet still producing interference. Another
observation that puzzles me - the noise current appears
unchanged regardless of the mode the player is in - yet
the interference shows only when in standby mode. A lot of
things out there that I do not understand.
conquered at last
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and BlueGriffon. Last updated 2014-07-28