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Old 05-26-2017, 03:39 PM
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Checking Signal Generator Accuracy With A Frequency Counter and Scope

Hello all! It seems the more I learn about electronics, the more I need to know. Too bad none of it makes much money these days! Anyway, I do a lot of my radio and TV work with vintage test equipment. I do it because I like the stuff which I understand causes me to have to repair my repair equipment. It would be ludicrous to many, but I have a lot of fun with it.

Anyway, I have had an old Heathkit SG-8 Signal Generator for quite a while. It's great. It needed a .1mfd cap to get going real well and operates on it's 2 original Heathkit tubes. Not long ago I purchased a cheapo frequency counter to use mainly to see how accurate the generator is. My problem is that I don't get any kind of steady reading on the counter at all, or one that coordinates with what I should be seeing. SO, I hooked the output of my function generator to the frequency counter and I get normal readings. Then I tried connecting my Oscope to the Signal Generator and I get a sine wave. The trick for me is that moving the range knob on the generator only changes the amplitude of the sine wave.

Even that change isn't a lot and the bandwidth isn't changing much if at all. I'm willing to bet that this is caused some rudimentary of fundamental of electronics relating to RF that I haven't studied enough. It has to be as I know that the Heathkit works fine and the dial accuracy can't be off by more than about 5Khz. I've seen guys in YouTube vids do what I am trying to do, but I don't think they showed how to connect everything.. I know somebody here can set me straight as the always do. Thanks in advance!
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Old 05-26-2017, 06:01 PM
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"It needed a .1mfd cap to get going real well and operates on it's 2 original Heathkit tubes."
How do you know it's operating "really well"?

What is the frequency response of your oscilloscope? Are you running the SG-8 at a higher frequency than the o'scope can see?

Are you connected to the RF output (not the audio output)?

What do you mean by "bandwidth isn't changing?" Do you mean frequency isn't changing?
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Old 05-26-2017, 10:38 PM
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OK, I guess you are just asking the logical questions. I'm no electrical engineer from the good ol days, but I do OK.

Ok, my statement on it operating really well is only truly based on a couple of facts and/or observations. I can turn on an Amplitude Modulation receiver (which is all I'm really worried about at this stage) and make a loop of wire connected across the two conductors of a coaxial cable connected to the RF OUTPUT via an amphenol connector. The RF Output potentiomenter is quiet in action, and the RF STEPS wafer switch seems to do it's job in raising the RF signal in the steps to a satisfactory degree.

When I place the loop of wire around the antenna of the amplitude modulated receiver with it's tuning capacitor turned down low, I hear a distinct tone. This distinct tone is heard when the signal generators tuning capacitor position indicator is reading approximately 460 Kilo Cycles (Hertz). Most alignment instructions I have ever seen, if not all for A.M. radios recommend the use of a 455 Khz. signal.

My oscilloscope is a measly 20Mhz. Tektronix 2205 with an x1, x10 switchable probe. Perhaps I used the wrong term in "bandwidth" or application of the term. Although the Heathkit Generator seems to be doing exactly what it was built to do, and probably is; I would think that when I connect to a scope that when I change the frequency of the output of the generator, I should see a duplication of said change on the graticule of my scope. Not in amplitude though, but in the width of the waveform. That is what I see when I connect my function generator to the scope. Meaning that if I change frequency, I see the change of frequency on my scope. If I raise the voltage, I see a rise in amplitude of the waveform.

All of this is why I was honest enough to suspect my own ignorance. I suspect that there is something about an RF signal that I am not realizing that creates a need for it to be treated differently than that of a signal coming from a function generator. I mean I can produce a 455Khz signal from the function generator. In the end, all I really want to do is check the Heathkit's actual signal output compared to what the dial reads with a digital signal counter. Indeed, it's easy enough to center the "tone" and look at the dial and see 460Khz. But, messing with the good old analog stuff is really cool, but if I happen to have a boring digital counter, I guess I may as well see what the old stuff is doing.

Thanks for the questions and I hope we find some interesting answers.
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Old 05-26-2017, 11:04 PM
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OK - if you are set to a few hundred kHz frequency, your scope should see it. If you turn the frequency knob, you should see the width of the cycles change (this is called their period, not bandwidth.

If you are getting a tone in an AM radio, I presume you have the modulation set to INTernal, which is amplitude-modulating the 455 KHz. THIS may be the problem with reading the frequency on the counter. Try turning the modulation to EXternal with no audio connected to the input and see if your counter can read the RF output frequency. Also set the RF level to max and use a direct coax cable connection between the generator and the counter (I think you are doing this, the loop was only for the radio receiver (?), but it wasn't clear to me).
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Old 05-27-2017, 12:28 PM
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You definitely want to turn the modulation off while you check the RF frequency. Sometimes you will get an unstable or incorrect reading on a frequency counter if the signal level is too low and sometimes if the signal is so big it is causing the counter input circuits to distort. I have a SG-8 and like other generators of that vintage the amplitude varies a lot as you change the frequency. The signal also has lots of distortion on much of the frequency range, the harmonics can be used as a source of higher frequency. BTW, the modulation isn't very linear. I modified the circuit by adding another tube just for modulation. After working in a modern electronics lab I'm surprised that people were able to get radios adjusted right.
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Old 05-27-2017, 01:16 PM
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Quote:
Originally Posted by Notimetolooz View Post
The signal also has lots of distortion on much of the frequency range, the harmonics can be used as a source of higher frequency. BTW, the modulation isn't very linear.
Mine does not produce a good sine wave even at 455 kHz, and modulation is quite low, but it seems to be an adequate signal source for simple AM radio alignment. Is this typical, or does mine need some attention?
The frequency counter in my 'scope seems to measure it ok, but I suspect that some would have a problem with that waveform.

jr
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Old 05-27-2017, 07:57 PM
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Quote:
Originally Posted by jr_tech View Post
Mine does not produce a good sine wave even at 455 kHz, and modulation is quite low, but it seems to be an adequate signal source for simple AM radio alignment. Is this typical, or does mine need some attention?
The frequency counter in my 'scope seems to measure it ok, but I suspect that some would have a problem with that waveform.

jr
If it works OK for you then you could just leave it.
That IS a pretty bad waveform. It could be that the DC bias on a section of the 12AU7 is off somehow.
One thing I noticed from the schematic is that the output level control pot wiper doesn't have a DC blocking capacitor. That means that the changing that pot changes the DC load on the output section of the 12AU7. I added a 0.05 uF there. My SG-8 wasn't mint, the ON-Off switch was replaced with a toggle switch, so I wasn't concerned about keeping it stock. I did quite a bit of experimenting on it and modified the circuit. I got it to work better but it still is less than ideal. I just use it for AM BC adjustment, 540-1600KHz. I made a separate solid state 455KHz source with modulation.
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Old 05-27-2017, 09:41 PM
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OK, lots of good info here and I'm glad to see several people using this old a model. Actually, I'm just now getting where I mess with alignments as I've never had much that I believe was very far off as is. It's just cool (if you are a tech of any kind by trade) to be doing more than just shotgunning in caps and resistors.

Anyway, OldTVnut confirmed one thing I brought up misusing the term bandwidth. He says that I should see the TIME change on the graticule of my scope when I change the frequency selection of the SG-8. Well I don't for whatever reason, but I know it is changing. Maybe more to come on that topic. A good point he is inadvertently bringing up is that on a scope you are looking at time, not frequency. (Frequency = 1/T) My scope is old enough to not have freq. displayed on the screen, but it's new enough to do about any job I will do here. Not much going on in bench tech jobs around here that I know of. It's a throw away society!

Now I was in INT mode when making a loop and listening through a receiver. I don't know what it was on when I was connected to the counter. I will have to try that. Maybe that will cover both of these things. I am always blown away by guys like Notimetolooz who know this stuff so darn good that they can and do make their own improvements. I would guess that you really had to be lucky enough to work in the industry when the design and/or repair of this type stuff was a regular thing. When I was in college I would bring in a tube chassis and have everyone flabbergasted. A room full of ready to graduate techs that hadn't a clue even the concept of most of what goes on. This is a science that would be lost if it weren't for those of us still keeping it alive. Even though solid state and digital components are what is taught in secondary education, it's not as applied to radio and television. I guess post graduate engineers or whatnot are the guys that dream up what goes on inside say a flat panel TV set. I sure don't know. Never took one apart. But I know that somebody designed it. Heck, thermionic emission and vacuum tubes blow my mind! LOL! I mean it makes sense, but the fact that someone got paid to think up the different types to do specific things in a fairly precise manner is what makes me scratch my head. All the while, a lot of people thing I'm doing something really big, really skilled in my Frankenstein lab! LOL! There's a good side to everything! Thanks folks!
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Old 05-28-2017, 01:36 AM
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Let me get this clear... are you saying that when you change the frequency on the sg-8 you do not observe a change in the period of its output waveform didplayed on your 'scope?

What, then is the period of the waveform that you are seeing on the 'scope?

jr
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Old 05-28-2017, 11:25 AM
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The period (T) depends on the SEC/DIV setting on the scope. My thinking must be all screwed up. I am thinking that I should be able to observe the amplitude of a voltage, and as well if I change the frequency of an input voltage to the scope that it would be evident as well. Much the same as it is evident if I use my function generator as the output. If I change the range or frequency, I see it on the scope. How I see it depends on the SEC/DIV scope setting.
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Old 05-28-2017, 11:35 AM
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I may have stated the question poorly... I was not asking for a definition of period, but rather for the number that you measure. If the waveform on the 'scope does not change when you change the frequency of thr SG-8, what is the period of the unchanging waveform (in seconds or milliseconds or microseconds) of the waveform that you observe?

jr
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Old 05-28-2017, 01:25 PM
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Just to set the record straight, I wasn't taught tube circuits in school it was pretty much solid state stuff. Field Effect Transistor circuits are somewhat similar to tube circuits. I looked at information on several other RF generators like EICO 324 and Heathkit IG-42 and IG-102.
I took some pictures of the output of my modified generator.
The first shows the modulated RF output as well as the 400Hz signal.
The second shows output at about 500KHz on the lowest range, some distortion.
The maximum output amplitude varies from about 1Vpp at the top of the lowest range A, to about 40 mVpp at the top of range D (25 MHz). Range E never worked and couldn't find a fix.
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File Type: jpg SG8500KHzE01.jpg (87.0 KB, 7 views)
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Old 05-28-2017, 02:03 PM
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Wow! that looks pretty decent... might have to do some work on mine, although the output waveform gets better in the higher ranges.

thanks,
jr
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Old 05-28-2017, 08:00 PM
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The lowest range does have the worst wave shape. The oscillator section has almost 100V peak-to-peak on the plate at some settings. I think the distortion is mostly due to the large signal there hitting the limits of the tube operating range with the supply voltage. I am puzzled why they chose a audio tube (12AU7) rather than something that works better at high frequencies (12AT7,12AV7).
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Old 06-03-2017, 01:44 PM
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Quote:
Originally Posted by jr_tech View Post
If the waveform on the 'scope does not change when you change the frequency of thr SG-8, what is the period of the unchanging waveform (in seconds or milliseconds or microseconds) of the waveform that you observe?

jr
I did a lot of things based on all the great information given here. I am fairly satisfied with my results. The first good result was found when I isolated my frequency counter from line voltage with the modulation of the generator off. I was able to obtain a steady frequency. Oddly, (self hi five; lol) my UN-calculated guess of the generators output based on 455Khz ACTUALLY being 460 or so on the dial was spot on.

Also, I hadn't used my scope for obtaining specific information like frequency in so long I first had to realize the whole 1/time formula 10ms. For simplicity the UN-triggered waveform I obtain a sine wave in which a cycle takes 5 divisions of the graticule. So I can calculate for the reciprocal 1/.010 x 5 and I get 500 or 500Hz. Honestly, I was again making my own problems by forgetting the proper use of the reciprocal function of my TI36X calculator. I was multiplying .010 x 5 and then using 1/X. Duh! This has been a great thread for me and I hope helps someone in the future. Thanks to all!!!

These are things that I would only do at home working on communication devices. We never use bench equipment on the job keeping automation equipment going. When I quits, we try to locate it on the planet (Siemens) and then replace and test. It either works or it doesn't. Pretty basic. But they require some sort of undergraduate degree to do it.
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