So what makes this buzzing-radio sound anyway?

[Tim Williams]

www.seventransistorlabs.com/QRZ.mp3
(...QRN? I don't remember the Q-codes.)

Recorded from a Theremin-in-progress, but the same sound shows up all over
the airwaves, LW, SW, professional receivers or homemade. In this case,
it shows up when connecting only a foot of wire to a 5MHz oscillator (yes,
rather high for a Theremin, this is for S&G's at the moment). Said
oscillator seems to be acting in an autodyne capacity.

The waveform seems to be a frequency modulation (advancing at least one
cycle) every 8.3ms. Hence the buzzing sound, and hence the FM synth
character at different frequencies.

It must be line driven, somewhere, but it's not my power supply, and
anyway, that wouldn't explain it being everywhere on the dial. It also
seems rather dubious (or impressive?) that it's having such a strong
effect on my oscillator, despite using only a one foot antenna.

Tim

 

[Tauno Voipio]

www.seventransistorlabs.com/QRZ.mp3
(...QRN? I don't remember the Q-codes.)

QRZ? = who is calling?

QRN = man-made noise.

Recorded from a Theremin-in-progress, but the same sound shows up all over
the airwaves, LW, SW, professional receivers or homemade. In this case,
it shows up when connecting only a foot of wire to a 5MHz oscillator (yes,
rather high for a Theremin, this is for S&G's at the moment). Said
oscillator seems to be acting in an autodyne capacity.

The waveform seems to be a frequency modulation (advancing at least one
cycle) every 8.3ms. Hence the buzzing sound, and hence the FM synth
character at different frequencies.

It must be line driven, somewhere, but it's not my power supply, and
anyway, that wouldn't explain it being everywhere on the dial. It also
seems rather dubious (or impressive?) that it's having such a strong
effect on my oscillator, despite using only a one foot antenna.

Tim

It could be e.g. a Chinese energy-saver lamp switcher fed with raw
full-wave rectified supply, with the power line filter components
saved from the BOM.

 

[[email protected]]

www.seventransistorlabs.com/QRZ.mp3
(...QRN? I don't remember the Q-codes.)

QRM, I think.

Recorded from a Theremin-in-progress, but the same sound shows up all
over the airwaves, LW, SW, professional receivers or homemade.

From 0 to about 0.7 sec, it sounds like a "carrier", so to speak. From
about 0.7 sec to about 1.5 sec, it sounds like there is "data" modulated
on it somehow. Then, the theremin starts up.

A quick analysis in Audacity shows some peaks at roughly 17.5 kHz, 14
kHz, 10 kHz, and maybe 6.5 kHz.
http://birdbird.org/tmp/sed/williams-spectrum-begin.png

Of course, doing an FFT on an mp3 is somewhat dubious. Record to .wav
(or other lossless format), grab a copy of Audacity, and see for
yourself locally. Not real-time but the price is right.

From about 12.0 sec to the end of recording also seems to be free of
theremin sounds. I still think it sounds like "data", but the frequency
is different. The spectrum doesn't have peaks, either.
http://birdbird.org/tmp/sed/williams-spectrum-end.png

It must be line driven, somewhere, but it's not my power supply, and
anyway, that wouldn't explain it being everywhere on the dial.

Hit breakers until it stops. If it never stops, try it in the daytime
(when streetlights and other lights are off) and and at night. If you
have any overhead power lines in your area, wander the immediate
neighborhood and look and listen for small arcs at the poles. Is there
any (possibly unannounced) broadband over powerline in your area?

Matt Roberds

 

[Martin Brown]

QRM, I think.


From 0 to about 0.7 sec, it sounds like a "carrier", so to speak. From
about 0.7 sec to about 1.5 sec, it sounds like there is "data" modulated
on it somehow. Then, the theremin starts up.

I'd hazard a guess that it is a very badly behaved switched mode PSU,
speed controller, CFL light or dimmer somewhere. To make such a mess it
seems likely that the outward and return paths are not close together so
that you have a room bathed in magnetic loop coupled RF noise.

A bit like the method that the T setting on hearing aids uses - might be
worth seeing if you can pick it up with a pancake coil and a crystal
earpiece then you might be able to metal detect to the source.

A quick analysis in Audacity shows some peaks at roughly 17.5 kHz, 14
kHz, 10 kHz, and maybe 6.5 kHz.
http://birdbird.org/tmp/sed/williams-spectrum-begin.png

Of course, doing an FFT on an mp3 is somewhat dubious. Record to .wav
(or other lossless format), grab a copy of Audacity, and see for
yourself locally. Not real-time but the price is right.

Daqarta (sp?) will do it in realtime if you can feed the signal into a
soundcard. I have used it for public demos of Sound and Music. ISTR You
have 30 days free use before you have to register and it still works as
a basic signal source after your evaluation period expires. It isn't
outrageously priced so if it does what you want worth buying.

 

[Tim Williams]

From 0 to about 0.7 sec, it sounds like a "carrier", so to speak. From
about 0.7 sec to about 1.5 sec, it sounds like there is "data" modulated
on it somehow. Then, the theremin starts up.

The theremin action is continuous of course, the different periods are
just where I put my hand to give you an idea what it sounds like at
various places.

Like I said, the waveform looks like frequency modulation -- no matter
what the carrier frequency; it seems like constant displacement FSK. I
don't get what kind of signal is pulling the oscillator like that, nor how
it can do it so strongly from so little antenna. (Is it really that
sensitive? Why don't theremins in the MF range do it?)

Hit breakers until it stops. If it never stops, try it in the daytime
(when streetlights and other lights are off) and and at night. If you
have any overhead power lines in your area, wander the immediate
neighborhood and look and listen for small arcs at the poles. Is there
any (possibly unannounced) broadband over powerline in your area?

Beats me. Unfortunately, all noise abatement advice is N/A -- I'm in an
apartment, suburban area. The elephant in the room is obviously the
computer (for which without, how do I record..), but based on experience,
I don't think that's the source anyway.

Tim

 

[Jasen Betts]

www.seventransistorlabs.com/QRZ.mp3
(...QRN? I don't remember the Q-codes.)

Recorded from a Theremin-in-progress, but the same sound shows up all over
the airwaves, LW, SW, professional receivers or homemade. In this case,
it shows up when connecting only a foot of wire to a 5MHz oscillator (yes,
rather high for a Theremin, this is for S&G's at the moment). Said
oscillator seems to be acting in an autodyne capacity.

The waveform seems to be a frequency modulation (advancing at least one
cycle) every 8.3ms. Hence the buzzing sound, and hence the FM synth
character at different frequencies.

It must be line driven, somewhere, but it's not my power supply, and
anyway, that wouldn't explain it being everywhere on the dial. It also
seems rather dubious (or impressive?) that it's having such a strong
effect on my oscillator, despite using only a one foot antenna.

Could your oscilator be squegging ?
Vackar's topology is particularly prone to this.

 

[Tim Williams]

Could your oscilator be squegging ?
Vackar's topology is particularly prone to this.

Waveform at the osc (before the mixer) is nice and clean. Of course, I
can't see the FM/noise at frequency.

Tim

 

[[email protected]]

<[email protected]> wrote in message news:[email protected]...

The theremin action is continuous of course, the different periods are
just where I put my hand to give you an idea what it sounds like at
various places.

Since there aren't any liner notes, I can only go by what I hear.

I don't get what kind of signal is pulling the oscillator like that,
nor how it can do it so strongly from so little antenna.

Like Martin Brown said, maybe whatever it is has a huge loop area for
some reason.

Beats me.

Jeff Liebermann gave the link to look up BPL deployments.

Unfortunately, all noise abatement advice is N/A -- I'm in an
apartment, suburban area.

You can, almost for sure, find out if it's coming from something inside
your apartment. (You might not be able to turn off things like a fire
alarm system.)

Apartment buildings tend to have main circuit breakers for each
apartment at the meter. This may be an unpopular approach, though.

Since you said it shows up in commercial receivers too, maybe grab a
radio and take a stroll. Or, if your theremin can be made portable,
take it along with you.

The elephant in the room is obviously the computer (for which without,
how do I record..),

You could get a piece of plastic 5/32" wide and 5,000 feet long, put
some iron oxide on it, and drag it at 1.9 inches per second past a coil
with the signal you want to record on it. Naah, it'd never work.

but based on experience, I don't think that's the source anyway.

The ARRL page that Jeff mentioned is http://www.arrl.org/sounds-of-rfi ,
but nothing there really sounds like yours. On the other hand, all of
those recordings were made with things that were designed to be radio
receivers, so they may sound different than your theremin.

The thing is, *lots* of equipment can generate radio frequencies. At
least a handful of people have now listened to your sound sample and
nobody has yet said "I know exactly what that is", so it seems likely
that you'll have to do more snooping around to find out.

Matt Roberds

 

[Spehro Pefhany]

www.seventransistorlabs.com/QRZ.mp3
(...QRN? I don't remember the Q-codes.)

Recorded from a Theremin-in-progress, but the same sound shows up all over
the airwaves, LW, SW, professional receivers or homemade. In this case,
it shows up when connecting only a foot of wire to a 5MHz oscillator (yes,
rather high for a Theremin, this is for S&G's at the moment). Said
oscillator seems to be acting in an autodyne capacity.

The waveform seems to be a frequency modulation (advancing at least one
cycle) every 8.3ms. Hence the buzzing sound, and hence the FM synth
character at different frequencies.

It must be line driven, somewhere, but it's not my power supply, and
anyway, that wouldn't explain it being everywhere on the dial. It also
seems rather dubious (or impressive?) that it's having such a strong
effect on my oscillator, despite using only a one foot antenna.

Tim

Have yout tried turning the lights off, especially overhead
fluorescent lights? Even some non-electronic ones can make a buzz on
the radio (I think the old 8' ones were particularly bad).

 

[Tim Williams]

Have yout tried turning the lights off, especially overhead
fluorescent lights? Even some non-electronic ones can make a buzz on
the radio (I think the old 8' ones were particularly bad).

Yes -- all I have nearby is a 20W CFL; no effect.

Update: I've put in proper coils (now operating ca. 2.23MHz) and changed
around a few things (mixer ports, gain, filtering); I think filtering much
over ~5kHz between AF stages has helped (including getting rid of the hiss
and whine from the volume section, which uses a slope detector).

It still does it, but only when... get this... it's completely
*un*grounded. I can put ferrite beads on all cables (i.e., power and
signal out) and it buzzes; ground it with only one (e.g., clip scope probe
to the chassis) and it goes away. This sounds like a grounding problem,
but it's build on copper clad, ground plane everywhere, oscillators
partitioned. Also, ceramic caps on all outside connections. No
opportunity for RF ground loop.

Visual aid: the design is very similar to this one I built a while ago.
http://www.seventransistorlabs.com/Theremin/
Main difference, I split the pitch mixer so there's a separate VGA so I
can add effects to the CW pitch signal. (Yes, as a "7 Transistor"
project, it's all discrete; this one uses 24 transistors in total, only a
little over.)

Tim

 

[Don Lancaster]

Could your oscilator be squegging ?
Vackar's topology is particularly prone to this.

A dimmer would be highly suspect with an 8.3333 millisecond time period.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Martin Brown]

Yes -- all I have nearby is a 20W CFL; no effect.

Update: I've put in proper coils (now operating ca. 2.23MHz) and changed
around a few things (mixer ports, gain, filtering); I think filtering much
over ~5kHz between AF stages has helped (including getting rid of the hiss
and whine from the volume section, which uses a slope detector).

How are you doing the amplitude modulation? Mine was via a slowish LDR
optoisolator to avoid the two RF signals fighting each other.

I also had a bit of fun and games tuning it so that it was linear in
distance for logarithmic changes in pitch - the hardest part of all. It
drifted like hell depending on room temperature but you could play by
ear well enough that people could sort of recognise the tune.

It still does it, but only when... get this... it's completely
*un*grounded. I can put ferrite beads on all cables (i.e., power and
signal out) and it buzzes; ground it with only one (e.g., clip scope probe
to the chassis) and it goes away. This sounds like a grounding problem,
but it's build on copper clad, ground plane everywhere, oscillators
partitioned. Also, ceramic caps on all outside connections. No
opportunity for RF ground loop.

In case it is squegging you might want to try decoupling capacitors at
strategic points. Mine proved very problematic when I came to putting it
in a nice screened box which was tedious in the extreme until I managed
to make it stable again. Something as wild and woolly as a theramin is
sat on the edge of wild instability so you have to fight pretty hard to
get both the pitch control and the amplitude modulator to behave
themselves at the same time.

It could easily be that it needs to be properly earthed to work and
protect it form local interference. Mine went through a period where it
would only work when earthed too. Eventually it did accept living in a
metal box, but it came very close to being put in a shoe box!

 

[Tim Williams]

How are you doing the amplitude modulation? Mine was via a slowish LDR
optoisolator to avoid the two RF signals fighting each other.

Single balanced mixer. Whereas the schematic at the end of my link shows
volume (as a control voltage from the slope detector) being fed directly
into the pitch mixer (and hence performing gain control), this has
separate mixers for each (pitch alone and volume control alone). Pretty
basic circuits, seems to work well, and everyone else seems to be doing it
(a little searching on VCAs turned up a couple standard circuits following
the Moog synthesizer, which is part of the envelope generator).

I also had a bit of fun and games tuning it so that it was linear in
distance for logarithmic changes in pitch - the hardest part of all. It
drifted like hell depending on room temperature but you could play by
ear well enough that people could sort of recognise the tune.

How was that, pitch into CV into nonlinear function into VCO?

V-to-F's being another synth topic that's notoriously drifty, no surprises
there. One thing where monolithic is pretty much required.

Log pitch would be wonderful. These things are so tricky to play because
the 'width' of a note varies depending on your distance to the antenna,
plus geometry. It's hyperbolic (roughly; there'll be some (x + d)^-3/2
sort of action going on with near fields too), then polynomial or power
series (freq goes as sqrt(LC)), then finally log (or exp) for pitch. It's
still a one-to-one function, but absorbing all of it analytically (even
given a suitable approximation for the antenna field and geometry) into
one invertible function is unlikely. An approximation (PWL or whatever)
would be possible, but might still require considerable skill to operate
because even a 1% accurate correction is off by... about a major sixth?
Getting that as clean as a few cents would be miraculous by any means!

In case it is squegging you might want to try decoupling capacitors at
strategic points. Mine proved very problematic when I came to putting it
in a nice screened box which was tedious in the extreme until I managed
to make it stable again. Something as wild and woolly as a theramin is
sat on the edge of wild instability so you have to fight pretty hard to
get both the pitch control and the amplitude modulator to behave
themselves at the same time.

Strange, I haven't had much trouble with them (or radio stuff in general,
well, a little, but nothing I couldn't iron out).

The present model is happy right now -- good clean tone (while grounded,
at least), and playable, though the volume is a bit too sensitive (i.e.,
you have to move your hand rather far to go from 'on' to 'off'!), and I
need to install fine tuning adjustments (the trimmer caps are tunable, but
not very user-friendly).

It could easily be that it needs to be properly earthed to work and
protect it form local interference. Mine went through a period where it
would only work when earthed too. Eventually it did accept living in a
metal box, but it came very close to being put in a shoe box!

Obviously it's being more sensitive to noise somehow -- but it doesn't
make sense that it's more sensitive when the chassis is less grounded.
The only obvious 'mode' would be dipole action across the width, but at
2ish MHz and a couple feet across, it's hardly a resonant length. And
being a balanced mode, that wouldn't change with grounding (much). So,
clearly it's an unbalanced mode. Dunno.

Tim

 

[josephkk]

QRZ? = who is calling?

QRN = man-made noise.

I am afraid i am forced to split this hair:

QRN: Noise created by inexperienced (novice) operators
QRM: Other man made noise.

Upon checking is got this wildly wrong. See:

http://en.wikipedia.org/wiki/Q_code

Oh well.

?-)

 

[josephkk]

Waveform at the osc (before the mixer) is nice and clean. Of course, I
can't see the FM/noise at frequency.

Tim

All said and done it still sounds like a hacked arc welder to me.

?-/

 

[John S]

I am afraid i am forced to split this hair:

QRN: Noise created by inexperienced (novice) operators
QRM: Other man made noise.

Upon checking is got this wildly wrong. See:

http://en.wikipedia.org/wiki/Q_code

Oh well.

?-)

According to:
http://www.arrl.org/files/file/Ham Radio License Manual/Comm w Other Hams-Tech Band chart back.pdf
(watch out for the wrap)...

QRM Your transmission is being interfered with _________
(1. Nil; 2. Slightly; 3. Moderately; 4. Severely; 5. Extremely.)
Is my transmission being interfered with?

QRN I am troubled by static _________. (1 to 5 as under QRM.)
Are you troubled by static?