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 Post subject: Question about frequency response of homebrewed projects
PostPosted: Feb Thu 09, 2017 5:57 pm 
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Location: Memphis, TN
I think this might be my first post in the homebrewed section. Hopefully not the last! I just got an oscilloscope and some other test equipment, and along with that came a TON of various tubes. Many are old TV tubes, and from spot checking many of them they sell for $1-$2 each. They seem to be prime candidates for experimentation and learning. So, I figure I can use that oscilloscope, frequency counter, and frequency generator to just try things. For example, I can't think of any better way to learn more about this than to build some amplifiers and maybe a transmitter and receiver or two from 'junk' tubes, and see what works and what doesn't. I've already gotten some good advice on amps to build from people on here. Now, to my question:

For an amp, or maybe a transmitter, I figure one good test of how well it works is to see how well it translates the input signal to the output. So, is it really as simple as feeding various frequencies into the input and seeing how they come out the other end?


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 Post subject: Re: Question about frequency response of homebrewed projects
PostPosted: Feb Thu 09, 2017 6:12 pm 
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If you believe all the theory, then electronics is easy----eg just put in a signal an measure the output. The devil is in the details.

For example, your DUT (device under test) will put a load on the signal generator, and your scope---or other measurement tools---will put a load on the output. But--don't panic!! Part of the learning and experimentation process is to discover the nuances of making measurements.

the single most important general concept is being aware of the impedance** in a circuit. If you are checking a battery, the impedance is pretty close to 0, with no reactive components. It's pretty hard to get an error due to loading by the measurement equipment. At the other extreme is something like the grid of a tube. It might have an impedance on the order of 2 megohms DC, plus a reactive component. Even using a low-capacitance 10X scope probe will alter the operation of the circuit.

Impedance is also important when using a signal generator. At RF frequencies, the signal output at the end of the cable can be strongly dependent on the load---which includes the termination of the test cable. The best practice is usually to terminate the cable with its "characteristic impedance"--typically either 50 or 75 ohms.

Let me know if I've lost you somewhere---post more specific questions, etc.


**"impedance" is the general term for the ratio of voltage to current--- AC or DC. DC resistance is a special case of impedance.

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 Post subject: Re: Question about frequency response of homebrewed projects
PostPosted: Feb Thu 09, 2017 7:55 pm 
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Joined: Oct Thu 02, 2014 5:57 am
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Location: Memphis, TN
That makes perfect sense! After reading on here, I realized the circuit doesn't care/know if its a probe tip or what that's been added. All it 'sees' is a modification, and that's one of the areas I want to start experimenting with. I want to make sure my measurements are really measurements, and not numbers incorrectly assumed.

Out of curiosity, last night I hooked my sstrans up to the scope, just to see what the waveform looked like. I hooked the scope up to ground on the sstrans, but on the antenna I just hooked the scope probe around the insulated wire. It still read just fine, and I figure that way the sstrans isn't trying to transmit into whatever circuitry is in the scope. Then, I hooked up my signal gen to the input on the sstrans, and it was really neat to see the waveform as I varied the frequency, gain, and modulation. It was probably pretty elementary for many on here, but I've never used a scope before.

How would I go about terminating a cable with 50 or 75 ohms? I'm guessing it's not as simple as using a resistor... :wink:

I sincerely appreciate the advice! It's been a long time since I was a new guy to a hobby, but I want to learn literally anything and everything I can. I once read that the only difference between science and screwing around is whether or not you take notes. I'm taking notes. 8)


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 Post subject: Re: Question about frequency response of homebrewed projects
PostPosted: Feb Fri 10, 2017 12:01 am 
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Quote:
How would I go about terminating a cable with 50 or 75 ohms? I'm guessing it's not as simple as using a resistor... :wink:

At low frequencies (up to the low MHz range), a resistor works fine if the leads are short. At higher frequencies (10's of MHz or more), "terminators" are available with standard connectors that attach to the end of the cable and pass the signal through (but watch out, many don't pass it through). Be aware, though, that 50 or 75 ohm outputs are generally found on lab grade equipment. And even then, it isn't universal. Some lab grade audio generators have 600 ohm outputs. Occasionally, especially with antique test equipment, other values show up.

Some hobbyist and service grade equipment does have a standard output impedance, but a lot of it doesn't, it isn't specified, or it isn't even constant. RF signal generators in particular fall into this category. When this happens and you want to measure the gain of your circuit, you have to actually measure the input voltage instead of relying on the generator dial.

Note that small receiving tubes will operate into the MHz region, many into the hundreds of MHz. They don't cause frequency response issues. Frequency response is dependent on the circuit they are used in. On the other hand, tubes, along with their circuits, do cause distortion and noise. These are harder to measure than frequency response.

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 Post subject: Re: Question about frequency response of homebrewed projects
PostPosted: Feb Fri 10, 2017 1:28 am 
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On my signal generator, the big issue is harmonics and how they vary with the attenuator setting. I simply guessed at a terminating resistor, and it made a significant improvement. I did not feel any urge to optimize.

What you will see in "lab-grade" equipment is a low-impedance output stage with a matching network that presents the cable with the right impedance. How this is done---and how well it works---depends on the frequency range being covered, and how much you are paying for the instrument.

The advantage of this careful matching is that the generator can be calibrated in terms of the signal level it delivers to a matched load ( eg the termination at the other end of the cable)

Don't go too deep too fast....the first step is simply to be aware of how source and load impedance matters in various situations.

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-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Question about frequency response of homebrewed projects
PostPosted: Feb Fri 10, 2017 1:30 am 
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Location: Annapolis, MD
Side note:
Maximum power transfer occurs when source and load impedances are the same. Sometimes you care about this, and sometimes you don't...:)

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-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Question about frequency response of homebrewed projects
PostPosted: Feb Mon 13, 2017 1:39 am 
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Joined: Oct Thu 02, 2014 5:57 am
Posts: 416
Location: Memphis, TN
Do you have a picture of how your probes are setup? I've got oscilloscope probes on my frequency counter and generator, but those are 1/10 meg ohm I believe. I'm wondering if I need something different,

I did have some luck playing around with the scope and my sstrans though. I moved the frequency a little higher up the dial, and found out exactly where it would start over modulating. I had a radio playing on the other side of the room to hear what I was doing.


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