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| raypsi... |
 Posted: Sat Oct 31, 2009 10:53 am |
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Guest
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Hey OM:
ASK is CW pure and simple. What you have is infinitely variable speed
CW. And some times the speed varies so much you can't tell if it's a
dit or a dah.
You can't beat CW, no way, no how, for the best source, of signal, in
an EMI environment.
Towit you can't beat ASK either.
Just keep on doing what you are doing.
As for square wave for audio, we are looking at an infinite number of
harmonics in a square wave, and that may be a good thing, but is it
what you want?
73 OM
de n8zu
On Oct 30, 12:40 pm, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]I'm using the wein bridge to generate audio tones that are fed to the
transmitter for digital radio communication (basically a homebrew ASK
radio modem). I'd be open to using that, but I'm not aware of any
radio modems that output square waves.
This oscillator also has 2 digital pots attached: 1 to trim the space
tone, and 1 to trim the mark tone. There's a high speed switch
between the two pots to rapidly switch between mark and space tones.
I could use this same setup with a 555 and trim the resistor to vary
the pulse duration, but I'm not sure what impact a square wave would
have on transmission, reception, decoding, etc...?
Has anyone heard of a square wave being used as an audio tone for
digital radio communication?
Thanks in advance,
Dave
On Oct 30, 12:06 pm, George Herold <ggher... at (no spam) gmail.com> wrote:
Hmm I have no idea if that will make any difference. What are you
using to control the feedback? Can you use a more robust oscillator
(as John suggested) Some type of 'bang-bang' rather than sitting on
the 'knife-edge' of oscillation with the Wein bridge. I needed the
low harmonic distortion of the Wein bridge... Is that what you need?
George H.[/quote] |
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| George Herold... |
| Posted: Sat Oct 31, 2009 4:51 pm |
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On Oct 31, 3:17 pm, whit3rd <whit... at (no spam) gmail.com> wrote:
[quote]On Oct 30, 11:49 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:> On Oct 30, 1:26 pm, whit3rd <whit... at (no spam) gmail.com> wrote:
The amplitude-stabilizing part of a Wein bridge oscillator has a diode
and capacitor.
(I was thinking of a FET/voltage-controlled-resistor stabilizer)
The amplitude-stabilizing part of the wein bridge I constructed uses 2
diodes (opposite directions) to limit amplitude
http://radiocom.kunsan.ac.kr/lecture/ewb/analyses/wien_bridge.JPG
Would a bypass cap across or between the diodes help?
Yes, the addition of 200 pF across the diodes might do some good;
RF reaching those diodes will modulate their ON threshold, which
will lower their impedance, and that's what turns the oscillator
amplitude
down. The sensitivity of this kind of network is lower than the one
I was thinking of, though.
Tin-plated soft steel (like tin cans) is an excellent shielding
material; I
think it's time to snip open some of the recycle-bin chili containers
and apply shielding.
[/quote]
If you don't mind the size, I've seen circuits built on the top lids
of metal one gallon paint cans.
George H. |
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| George Herold... |
| Posted: Sat Oct 31, 2009 4:58 pm |
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On Oct 30, 2:49 pm, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]On Oct 30, 1:26 pm, whit3rd <whit... at (no spam) gmail.com> wrote:
On Oct 30, 7:43 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second).
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or
The amplitude-stabilizing part of a Wein bridge oscillator has a diode
and capacitor. You've got a little crystal radio there, and it
detects
the transmitter and turns the oscillator down (WAY down) thinking
it has detected the oscillator amplitude. To check this, put a
voltmeter on that capacitor while you key the transmitter.
Put a bypass capacitor across the diode (to conduct RF) and
shield that sensitive part of the circuit. A ferrite bead in series
with
the diode won't do any harm, either.
The amplitude-stabilizing part of the wein bridge I constructed uses 2
diodes (opposite directions) to limit amplitude rather than a diode
and a cap. The amplitude limiting circuit I'm using is almost
identical to the one shown here:
http://radiocom.kunsan.ac.kr/lecture/ewb/analyses/wien_bridge.JPG
Would a bypass cap across or between the diodes help?
Thanks!
Dave- Hide quoted text -
- Show quoted text -
[/quote]
I used a light bulb. Lots of circuits on the web, I started with one
by Jim Williams.
George H. |
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| miso at (no spam) sushi.com... |
| Posted: Sat Oct 31, 2009 9:37 pm |
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Guest
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On Oct 30, 7:43 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second). If I hold the transmitter
further away, it continues oscillating, but I'd prefer to reduce the
EMI to have a more robust design.
I have a few questions I was hoping some people with EMI experience
might be able to shed light on:
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
2. Would a low-pass filter on any of the op amp inputs be a good
option, or would shielding be a better option?
3. If I used shielding, would standard aluminum foil be adequate?
What general guidelines are there regarding form and allowable holes
in shielded enclosures?
Thanks in advance!
Dave
[/quote]
How about a cap between the + and - inputs of the op amp.? A small one
to short it at RF, but not effect the circuit where it operated. |
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| Tim Shoppa... |
| Posted: Sun Nov 01, 2009 6:42 am |
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Guest
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On Oct 30, 9:40 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]I'm using the wein bridge to generate audio tones that are fed to the
transmitter for digital radio communication (basically a homebrew ASK
radio modem). I'd be open to using that, but I'm not aware of any
radio modems that output square waves.
This oscillator also has 2 digital pots attached: 1 to trim the space
tone, and 1 to trim the mark tone. There's a high speed switch
between the two pots to rapidly switch between mark and space tones.
I could use this same setup with a 555 and trim the resistor to vary
the pulse duration, but I'm not sure what impact a square wave would
have on transmission, reception, decoding, etc...?
Has anyone heard of a square wave being used as an audio tone for
digital radio communication?
[/quote]
I think you mean "AFSK". At least that's what I think you mean. ASK Amplitude Shift Keying.
Something in the transmitter chain stops (or should stop!) the square
wave from being square with all the odd harmonics going out to
infinity.
This something may be in the audio stage (example: low pass audio
filter), or the IF stage (example: bandpass filter), or in the RF
stage (example: tuned circuits).
It is very doubtful that after going through the transmitter and
receiver that it'll come out as a square wave on the other end.
Choosing to transmit a square wave audio waveform is usually a poor
choice because you know that it can't come out looking that pretty on
the other end. This sort of design decision might be made for a very
low-end radio control transmitter of the 60's or 70's out of
ignorance, but today we know how to do far better with little extra
effort.
Most designs make a conscious choice to be a friendly transmitter, and
limit splatter and unnecessary bandwidth that would be in violation of
the FCC rules, by running any square wave through a low pass audio
filter AND additionally using a rational choice for the IF filtering
too. Way better than nothing, is a simple RC low pass in the audio
stage. Still to be nice the resulting audio level has to be carefully
set to not cause splatter in subsequent stages.
Tim N3QE |
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| ChrisQ... |
| Posted: Sun Nov 01, 2009 5:57 pm |
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Guest
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dave.harper wrote:
[quote]On Oct 30, 1:26 pm, whit3rd <whit... at (no spam) gmail.com> wrote:
On Oct 30, 7:43 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second).
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or
The amplitude-stabilizing part of a Wein bridge oscillator has a diode
and capacitor. You've got a little crystal radio there, and it
detects
the transmitter and turns the oscillator down (WAY down) thinking
it has detected the oscillator amplitude. To check this, put a
voltmeter on that capacitor while you key the transmitter.
Put a bypass capacitor across the diode (to conduct RF) and
shield that sensitive part of the circuit. A ferrite bead in series
with
the diode won't do any harm, either.
The amplitude-stabilizing part of the wein bridge I constructed uses 2
diodes (opposite directions) to limit amplitude rather than a diode
and a cap. The amplitude limiting circuit I'm using is almost
identical to the one shown here:
http://radiocom.kunsan.ac.kr/lecture/ewb/analyses/wien_bridge.JPG
Would a bypass cap across or between the diodes help?
Thanks!
Dave
[/quote]
Have you decoupled the psu rails close to the device with caps and rf
chokes ?. The rf must be getting into the op amp somewhere, so keeping
the layout as compact as possible, with short decoupling paths from
device to ground plane may help. It doesn't take much lead length to
resonate at 144Mhz. The other question is what power level is the tx
running at and how close is the op amp to the op stage ?. If you are
running at anything more than a few 10's of milliwatts, you will need
screening around any adjacent low level stuff, especially at vhf.
I built some hf linear amps in the 100 watt region some years ago and
the rf just gets everywhere if you don't pay attention to decoupling and
screening...
Regards,
Chris |
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| raypsi... |
| Posted: Mon Nov 02, 2009 5:58 pm |
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Guest
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On Nov 1, 11:42 am, Tim Shoppa <sho... at (no spam) trailing-edge.com> wrote:
[quote]On Oct 30, 9:40 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
I'm using the wein bridge to generate audio tones that are fed to the
transmitter for digital radio communication (basically a homebrew ASK
radio modem). I'd be open to using that, but I'm not aware of any
radio modems that output square waves.
This oscillator also has 2 digital pots attached: 1 to trim the space
tone, and 1 to trim the mark tone. There's a high speed switch
between the two pots to rapidly switch between mark and space tones.
I could use this same setup with a 555 and trim the resistor to vary
the pulse duration, but I'm not sure what impact a square wave would
have on transmission, reception, decoding, etc...?
Has anyone heard of a square wave being used as an audio tone for
digital radio communication?
I think you mean "AFSK". At least that's what I think you mean. ASK =
Amplitude Shift Keying.
Something in the transmitter chain stops (or should stop!) the square
wave from being square with all the odd harmonics going out to
infinity.
This something may be in the audio stage (example: low pass audio
filter), or the IF stage (example: bandpass filter), or in the RF
stage (example: tuned circuits).
It is very doubtful that after going through the transmitter and
receiver that it'll come out as a square wave on the other end.
Choosing to transmit a square wave audio waveform is usually a poor
choice because you know that it can't come out looking that pretty on
the other end. This sort of design decision might be made for a very
low-end radio control transmitter of the 60's or 70's out of
ignorance, but today we know how to do far better with little extra
effort.
Most designs make a conscious choice to be a friendly transmitter, and
limit splatter and unnecessary bandwidth that would be in violation of
the FCC rules, by running any square wave through a low pass audio
filter AND additionally using a rational choice for the IF filtering
too. Way better than nothing, is a simple RC low pass in the audio
stage. Still to be nice the resulting audio level has to be carefully
set to not cause splatter in subsequent stages.
[/quote]
Hey OM
I looked at this guys profile.
I would say he's into telemetry.
If you want to stay away from EMI take it to the 2.4Ghz band.
I don't think he's a ham..
I think he cross forumed this post.
My best guess is he is running into trouble on the VHF/UHF spectrum
where there is tonnes o EMI. QRN and QRM.
73 OM
de n8zu |
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