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 Post subject: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 2:50 pm 
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Hi I found my self trying to repair an old EICO because of its nasty looking sine wave then it occurred to me its the nature of the beast.
does anyone have a cure for this aside from replacement.
Rich


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 3:00 pm 
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That's normal....many generators in the "popular price range" use the distortion to generate harmonics for the higher bands.

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 3:10 pm 
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If you want RF waveforms that are pure sine waves with minimal distortion you probably need to invest in a laboratory grade generator such as a General Radio 1001-A. Service/hobbyist generators (which are fine for radio restoration use) depend upon a high harmonic content as noted by Pix.

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 3:45 pm 
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HI thanks, for the conformation.
:)
Rich


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 4:00 pm 
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If that is audio/modulation freq it should be cleaner. If it is RF then it is fine.... infact the distortion can be useful. Many service grade generators when that sloppy carrier gets AM modulated will actually generate a signal with both AM and FM modulation. If you ever troubleshoot an FM radio you may become grateful for the sloppy carrier and this effect.

BTW: I think we both have the same oscilloscope. :)


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 4:23 pm 
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You can clean it up a lot by passing the signal through a radio. The antenna coil and tuning capacitor, or the IF transformers in a radio will remove most of the harmonics, and that is how the signal generator was meant to be used anyway.

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 5:04 pm 
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Electronic Memory wrote:
Many service grade generators when that sloppy carrier gets AM modulated will actually generate a signal with both AM and FM modulation.

As a rule of thumb significant incidental FM occurs only when the oscillator is directly modulated and only when the AM modulation exceeds about 30%. That is why the most common service/hobby RF generators, such as the EICO 320, are limited to about 30% modulation. The more expensive service/hobby RF generators, such as the EICO 315, share a feature with laboratory grade RF generators - the oscillator is buffered, and the buffer is modulated to reduce incidental FM even at high levels of AM modulation. In my shop I use a General Radio 805-C Standard Signal Generator which has a specified worst case 0.05% incidental FM at 100% AM modulation, and it is significantly lower in most cases. I need that level of quality for work, but as I noted upstream even the basic service/hobby generators such as the EICO 320 are fine for radio service work. I use my 805-C for radio service, even though it is overkill for that purpose, because I already have it in the shop.

As a side note many inexpensive AM transmitters also use directly modulated oscillators and are also limited to about 30% amplitude modulation. Such is the case with most antique phono oscillators.

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 6:29 pm 
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Dale H. Cook wrote:
If you want RF waveforms that are pure sine waves with minimal distortion you probably need to invest in a laboratory grade generator such as a General Radio 1001-A. Service/hobbyist generators (which are fine for radio restoration use) depend upon a high harmonic content as noted by Pix.


My B&K E-200D makes pretty sine waves. They can be had for under $100.


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Wed 08, 2020 6:39 pm 
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Hi all, I started thinking about this because I was trying to save my If transformers from my Coronado 550 that were open that I rewound the primary, I was viewing the input and out of them.
thanks for all the great input!
Rich


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 12:18 am 
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These are from from my B&K 2000A, which are going for around $100 now. I use a Leader LDC-823S Frequency Counter with it connected to the frequency monitor output all standard equipment.
Pic 2812 1600 KHz Unmodulated
Pic 2811 1600 KHz Modulated
Pic 2814 10.7 MHz Unmodulated Top trace 10.7 Max sweep rate on the scope
Bottom trace Sweep rate magnified x5


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 1:37 am 
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Bill Eccher wrote:
These are from from my B&K 2000A, which are going for around $100 now.

You have some significant envelope distortion in the AM waveform. That B&K is significantly better than the basic directly modulated generators but far from the performance of the less expensive laboratory grade generators which can often be found used in the $100-$150 range.

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 1:51 am 
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True, but I never use modulation when I'm doing alignment on AM anyway, so who cares.


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 1:58 pm 
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thanks Bill, that was what I was thinking I should have what your B&K has but the EICO works for what it was intended..
I noticed while testing the IF transformer the input signal looked terrible and the output was pristine. I surely would rather have something better.
thanks all, I've learned something here
Rich


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 5:35 pm 
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That AM modulation envelop is actually pretty decent! The Heathkit SG-8 is absolutely horrid compared to yours.

30% modulation was pretty much standard at one time.

I spent some time trying to clean up a SG-8 when I was bored a few years back... improving the distortion involved
reducing the feedback on the lower band ranges... kinda like putting lipstick on a pig :)

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 6:19 pm 
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Quote:
I spent some time trying to clean up a SG-8 when I was bored a few years back... improving the distortion involved
reducing the feedback on the lower band ranges... kinda like putting lipstick on a pig


Same experience; same result! I even tried to put a short skirt on the pig (added a Zener to regulate the B+). Still a pig.

Rich


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 6:32 pm 
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Peter Bertini wrote:
30% modulation was pretty much standard at one time.

It was, and that was because directly modulated oscillators were pretty much standard at one time. The waveform displayed is pretty typical of the more sophisticated and higher priced service/hobby generators that were bought by the better service shops and hobbyists with deeper pockets.

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Thu 09, 2020 8:43 pm 
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Just a note from the days of (tube) TV service. A cheap signal generator was
capable of producing a usable tone on a TV set. I did that just as a quick test
many times. If the frequency was tweaked, a (sort of) bar raster was obtained.

These tests were made by connecting the generator to the antenna terminals of
the (NTSC) TV set.

Back to the present.

Out of curiosity, I just tried this with a cheap AM (and
short wave) , FM digital radio. I stuck the whip tip close to
the type N connector of my Measurements Corp model
80 . At 92.9 mHz the 1 kHz AM modulation tone was
heard clearly, and disappeared into the noise when I
cranked the attenuator back to about 8 micro
volts.

Chalk it up to crappy or non effective limiting.

I pulled the manual for a Conar (NRI) model 90
(from a kit) generator. These were part of correspondence
training courses for TV and radio repair.

The rather elaborate usage manual stated that an AM
signal generator will not be heard on a FM radio or a TV.

And surely the methods used by me, (and everyone else
in TV service back then) were not discussed.

Wobulators and TV sweep generators of course could do
proper FM work , especially S curve adjustments.

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Mon 13, 2020 3:36 pm 
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pixellany is correct. The distortion creates the harmonics that are used for the higher bands. If you clean it up, the higher bands won't work. If you need a clean sine wave, then this is not the tool for the task. It does what it was designed to do very well. That is a "normal" sine wave for the Eico's. Doc


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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Fri 17, 2020 6:22 pm 
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You will get a clean sine wave from one of these if you feed a resonant circuit, as you normally do in nearly any radio alignment. See the article here
https://people.ohio.edu/postr/bapix/SigGenFun.htm
Here's Figure 6 for the dual trace Eico view. Bottom trace is the Eico output. That's also how most Class C transmitters work, pulses into a resonant circuit. Also check the doubler action in the article (figure 7)

-- Rich

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 Post subject: Re: EICO signal generator output clean up
PostPosted: Jan Fri 17, 2020 7:06 pm 
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Cheap rf generators in AM mode modulate the oscillator. That results in 'incidental FM' which can be useful when working on FM receivers. The amount of FM is considerable and if you measure it with a deviation meter you can calibrate it.

Better generators have a buffer amplifier after the oscillator so that modulation is more pure. Better than that generators have separate AM and FM modulators with minimal interaction.

If you want accurate FM you need accurate gear. You can calibrate by means of Bessel nulls but that's getting very picky; it requires sophisticated gear, including a pure audio tone of known frequency.

I am less interested in FM servicing than I am in playing with equipment, so I enjoy calibrating generators for FM. Now that I have a calibated FM signal I have no use for it. Well, I do, in that I sometimes enjoy rechecking the calibration.

On a spectrum analyzer I see AM as three signals, the carrier and two side frequencies. But on FM even the smallest deviation results in many side frequencies. The energy for the side frequencies on AM comes from the modulator. In FM it's 'stolen' from the carrier, so at certain modulation levels the carrier actually disappears.


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