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Author Topic: Cheap noise source with speaker output for testing FM deviation ?  (Read 2123 times)
G7IVJ
Member

Posts: 82




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« on: November 13, 2012, 02:46:43 PM »

Gents,

I have a Motorola test set with deviation meter, and need to find a cheap device to play audio from its speaker to input into a transceiver mic (via coupling not direct injection) whilst I check the FM deviation on the Motorola and make changes to the radio

Upto now, I have been speaking into the rigs mic, but find it impossible to get a constant leel of voice out for long enough whilst I make the changes.

Any ideas where I can obtain a battery powered noise generator with speaker, so I can hold the mic to the speaker whilst I am making the changes ?


Thanks for your help in advance
73

G7IVJ
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N4CR
Member

Posts: 1701




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« Reply #1 on: November 13, 2012, 02:58:14 PM »

Tone generator in your computer?
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73 de N4CR, Phil

Never believe an atom. They make up everything.
WB6BYU
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Posts: 13573




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« Reply #2 on: November 13, 2012, 03:25:28 PM »

HT with the squelch open?

I've found a number of toys that have sound generators, but you'd have to see what
is available locally (perhaps in Disposals shops).

A DTMF ("TouchTone") generator with one key pushed would give you two audio tones
that might work better than a single one.

Otherwise a NE555 timer used as an oscillator will easily drive a speaker to sufficient
volume.  Using two of them you can sweep the frequency or toggle between two
different tones if that helps.
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W6EM
Member

Posts: 900




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« Reply #3 on: November 13, 2012, 05:38:01 PM »

Years ago, when I worked in a comm shop, the techs were all issued "bobby" police whistles.  They put out a two-tone sound circa 400-600 Hz and were used to set deviation with.  Simply blow into the whistle in close proximity to the mike and it worked like a champ.

Being as how you are a lot closer to the source than we were in California, perhaps the London police surplus outlets might have one or two for sale at a reaonable price.....  :-)

73,

Lee
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KA4POL
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Posts: 2125




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« Reply #4 on: November 13, 2012, 10:18:07 PM »

Many years ago I built a 1750 Hz generator using a XR2209 chip. If you want to be really accurate you would have to inject the correct level.
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G4AON
Member

Posts: 545




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« Reply #5 on: November 13, 2012, 11:55:47 PM »

I repaired 2 way radios for about 20 years and we never used anything but voice, a loud "four" into the mic is all that is needed.

If you really need a noise source, and I doubt it is necessary, any FM radio with the squelch open will provide plenty of noise.

73 Dave
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G7MRV
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Posts: 481


WWW

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« Reply #6 on: November 15, 2012, 08:45:15 AM »

Most of the UKs emergency service radios were serviced by the company i work for, and that was my job for many years. Our test sets (Marconi 2955's) had an audio generator which we would set to 1kHz  at 100mV to drive the mic input directly. For testing that the radio deviation was correct via the internal mic, we simply whistled into it!

You know how to whistle, don't you?  Wink
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G3RZP
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Posts: 4957




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« Reply #7 on: November 15, 2012, 01:30:16 PM »

I do wonder if that would meet the EU enforcement requirements if tested in a different Test House..........

Of course, since EU enforcement is practically non- existent, it wouldn't really matter...
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WB6DGN
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Posts: 619




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« Reply #8 on: November 16, 2012, 12:04:46 AM »

Quote
Our test sets (Marconi 2955's) had an audio generator which we would set to 1kHz  at 100mV to drive the mic input directly.

That's the way its done.  Most manufacturers specify modulation to be set with a 1kc tone but the input level may vary with the type of mic. supplied with the radio.  My procedure was to set the IDC or peak limiter with the 1kc tone at the specified level, then set the tone generator to 400 cycles to check the limiter action.  Then reconnect the mic. to verify that peaks don't exceed the specified limit and that average audio is maintained at around 60% of peak.  Only once, in a nearly 60 year career did I get a "pink slip" from the FCC for modulation and that was found to be a component failure (a 6AL5 shunt limiter had failed).
Tom
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KE3WD
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Posts: 5689




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« Reply #9 on: November 16, 2012, 06:43:56 AM »


There is an old "quick and dirty" trick which involved holding the Mic against the receiver of the common telephone and using the Dial Tone to modulate the mic. 

Not calibrated, of course, but does have the advantage of precision, being quite close the same amplitude from day to day and unit to unit. 

With a bit of practice, one can get this to work. 

But I'm talking the good old fashioned telephone models, like the famous old Western Electric Model 500 here, with the electromagnetic receiver and carbon mic.  I doubt seriously if one of the dinky modern solid state telephone sets would be of much use here. 


73
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WB6DGN
Member

Posts: 619




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« Reply #10 on: November 17, 2012, 12:24:06 AM »

Quote
There is an old "quick and dirty" trick which involved holding the Mic against the receiver of the common telephone and using the Dial Tone to modulate the mic.

Problem with that is that you have a frequency selective amplifier in the audio chain that shapes the frequency response at 6db/octave preemphasis from 300 to 3000 cycles.  That's why modulation must be set with a specific reference frequency as the center point and a sine wave signal.  Setting it with a complex waveform such as voice or a dial tone signal will always produce uncertain results.  All manufacturers that I'm familiar with specify 1000 cycle sine wave as the reference signal.  The dial tone reference will produce inconsistent results depending on the characteristics of the audio shaping network used in a particular radio.  Same with voice.
That said, you DO check with a 400 cycle tone and also with voice after setting the modulation to CONFIRM that the audio chain is working properly (confirm that peak modulation STAYS withing specified limits and average modulation is proper) BUT this only applies AFTER it has been properly set using the specified correct reference.
Like anything else, there are NUMEROUS ways of doing something but, usually only ONE RIGHT way.  Just listen on most ham repeaters; you'll HEAR what I mean.
Tom
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KE3WD
Member

Posts: 5689




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« Reply #11 on: November 17, 2012, 05:45:42 AM »

All Tom proves is a lack of knowledge concerning the makeup of the dialtone frequencies. 

The standard dial tone in the USA is compposed of two frequencies that are the A above Middle C and the F above Middle C from the standardized tempered scale referenced to A = 440.  (I have also used the dialtone reference tune musical instruments from pianos and organs through guitars.  It is always locked on frequency and as near as your closest twisted pair landline phoneset.)

So you have a dual tone that composes the dial tone,

The higher of which is 440Hz

And the lower tone is 349.23 Hz

This is indeed within your passband.  

And one must understand that the term used, "quick and dirty" -- is just that.  

But it can really be used in a pinch, I once worked at a communications repair facility where the three of us, all licensed as far as the FCC would go with all endorsements, actually used this method when nothing else was available.  

This suggestion was not meant to replace a properly calibrated mic/modulation test set, but that should be obvious.  One would think...

73
« Last Edit: November 17, 2012, 05:48:06 AM by KE3WD » Logged
WB6DGN
Member

Posts: 619




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« Reply #12 on: November 17, 2012, 05:34:00 PM »

Quote
All Tom proves is a lack of knowledge concerning the makeup of the dialtone frequencies.

Geeesshh!  OK!  Have it your way, however, from your angry reply, I see a few things that you don't understand as well.  I will "watch out" for your posts in the future, GUARANTEED!  Have a nice day.
Tom
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