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 Post subject: RF amp circuit
PostPosted: Oct Fri 13, 2017 1:44 am 
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I'm looking for a small RF amp that is flat from 100KHz-140MHz with some sort of AGC if possible but if no AGC that may still work.

Input impedance can be whatever is best suited for the amp. Same with the output impedance and I will need an output for a frequency counter.

Will also need a separate output stage that can be modulated with the modulation input being high impedance and the output impedance should be 50 ohms.

The purpose is to develop an inexpensive drop in solution to make service grade RF generators be better than they were stock.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 2:18 am 
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Question: Why don't YOU come up with this cheap, fabulous RF amplifier, then you can sell it to us.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 3:07 am 
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You want an RF amp, what's the gain requirement? What's the desired power supply voltage and acceptable current drain?

"Better" than original? In what way? Noise figure? IM distortion? Third order intercept?

John

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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 4:57 am 
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I have a hunch that broadband RF amplifiers are available on a chip......have you done any searches?

Does a "service-grade RF generator" need more gain? In the case of my PACO E-200C, there's plentry of gain.....what is needed is a better attenuator. If I were to add this, I would first build a buffer amplifier with 50-ohm output, and then build ( or buy ) a proper attenuator. Along the way, I would add some shielding.

So, then what would I have? A relatively stable generator that produces a cesspool of harmonics, but with the level of all those harmonics precisely controlled. I might wind up wishing I had built a synthesizer.....or maybe just bought a modern generator......:)

Take a look at Lou Haskell's thread on the computer-controlled signal generator.

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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 12:41 pm 
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Johnnysan wrote:
Question: Why don't YOU come up with this cheap, fabulous RF amplifier, then you can sell it to us.


Because I haven't much experience designing with solid state and I have no experience designing anything tubed or solid state at RF frequencies.

OldWireBender wrote:
You want an RF amp, what's the gain requirement? What's the desired power supply voltage and acceptable current drain?

"Better" than original? In what way? Noise figure? IM distortion? Third order intercept?

John


Better as in providing a proper sinewave output. I know it's not really necessary for an RF generator to have a near perfect sinewave, but

1. The cathode follower of the service grade RF generators is usually the cause of a bad looking output.
2. I want a clean sinewave output.

The B+ requirement would depend on the desired output signal voltage.

If done right I can get + and - 12Vdc supplies from a single 6.3Vac winding by using two voltage doublers and reversing the diodes in one doubler. That would work for an OP-AMP.

Current draw would be whatever is required to put the desired output voltage into a 50 ohm load while keeping the sinewave clean.

pixellany wrote:
I have a hunch that broadband RF amplifiers are available on a chip......have you done any searches?


Not yet. Figured I'd ask here first.

pixellany wrote:
Does a "service-grade RF generator" need more gain?


No, but many need a better output stage between the oscillator and attenuator.

pixellany wrote:
I would first build a buffer amplifier with 50-ohm output, and then build ( or buy ) a proper attenuator.


That's the whole idea here.

pixellany wrote:
So, then what would I have? A relatively stable generator that produces a cesspool of harmonics, but with the level of all those harmonics precisely controlled. I might wind up wishing I had built a synthesizer.....or maybe just bought a modern generator......:)


If the oscillator in yours produces a clean or relatively clean sinewave over its whole frequency range you can add the RF buffer amp at that point and get a relatively clean output.

That's the whole purpose of this thread. An inexpensive way to make a service grade RF generator better without having to spend the big bucks for a proper RF generator.

That said one could easily buy a N3ZI DDS RF generator for just under $100 if one only needs 200mV output into a 1K load at up to 34MHz, but I think the RF buffer amp idea could be done for a lot less and be good to 140MHz so that one would then have a fairly decent RF generator for not a lot of money.

pixellany wrote:
Take a look at Lou Haskell's thread on the computer-controlled signal generator.


Will do.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 1:08 pm 
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Quote:
. Better as in providing a proper sinewave output. I know it's not really necessary for an RF generator to have a near perfect sinewave, but

1. The cathode follower of the service grade RF generators is usually the cause of a bad looking output.

.???
Why would that be true?

I think it's more likely that artifacts are caused by an unterminated coax connected to the output.

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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 1:48 pm 
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Tube Radio wrote:

"The purpose is to develop an inexpensive drop in solution to make service grade RF generators be better than they were stock."


When we are done with this, next project:

I want a simple drop in solution to my 24 year old Volkswagen Van. I has to cruise at 140mph, get 49 miles per gallon, and have wifi. Can't cost more than $20. Email when you have it.

Don't you think that if these "problems" were cheap and simple to eliminate, it would have done in the first place?


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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 5:36 pm 
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pixellany wrote:
Quote:
. Better as in providing a proper sinewave output. I know it's not really necessary for an RF generator to have a near perfect sinewave, but

1. The cathode follower of the service grade RF generators is usually the cause of a bad looking output.

.???
Why would that be true?

I think it's more likely that artifacts are caused by an unterminated coax connected to the output.


Not at all. I've done measurements and the signal is a fairly clean sinewave before the cathode follower and severely clipped on one peak of the waveform after the cathode follower. At least that's what I've noticed with both a Stark LSG-10A and EICO 315 RF generator.

Re-biasing the cathode follower which involves increasing the cathode resistor value and applying some DC voltage to the grid through a high value resistor does help, but it isn't enough.

The idea here is to find a point where there is a relatively clean sinewave and feed it to an amp of some sort that will provide a 50 ohm output and do it without pulling the oscillator any if possible.





Johnnysan wrote:
Tube Radio wrote:

"The purpose is to develop an inexpensive drop in solution to make service grade RF generators be better than they were stock."


When we are done with this, next project:

I want a simple drop in solution to my 24 year old Volkswagen Van. I has to cruise at 140mph, get 49 miles per gallon, and have wifi. Can't cost more than $20. Email when you have it.

Don't you think that if these "problems" were cheap and simple to eliminate, it would have done in the first place?


Yes they would have, but then again maybe not if it required extra components given these are service grade generators.

That said we have solid state stuff nowdays that would enable this to be done relatively inexpensively.

It's as simple as finding an amp design that can go to 140MHz, building it then building a power supply that can run on the heater winding and connecting it to the RF generator.

What is the typical maximum output of a good quality RF generator?


EDIT:

Basically what I need is a high impedance buffer.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 7:37 pm 
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High impedance? For RF....No

Cathode follower clipping? Bad design.

Here is a thought: it is often very difficult to improve systems by adding parts. Replace an entire subsystem with a modern replacement? Yes. Example: An E200C with a better attenuator.

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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 8:05 pm 
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Agreed.

The high impedance bit may be overzealous on my part.

Wanted something that wouldn't load the oscillator any and my first thought was high impedance.

I blame that on never having any RF design experience.

Then again I don't think there's a simple solution that will work without extensive redesign of the particular RF generator.

Cathode follower clipping? In a proper RF generator I would agree with you, but In a service grade RF generator it is more along the lines of a cheap overall design.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 8:26 pm 
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Hi TR. I know you have been working with signal generators and wanting a near sine wave output. The one I am linking to does not go to 140 but will go to 110MHZ. It will give up to 220MHZ on second harmonic. It is a service shop type. As you know these are not precision instruments and have no output filter. This does allow the modulation signal as well to apear on the output. I have one of these and they do work well. This is not my video but you can see what it will do.

https://www.youtube.com/watch?v=VjrsMhT222w

You could add a zener regulator to the B+ if you wanted to. If you made C20 the capacitor from the output tube plate to the attenuator pot smaller it would block more of the audio but might lower output a little depending on the value you use. It has a .01UF now. These generators are available for reasonable prices online etc. They are the heathkit IG102 series.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 8:57 pm 
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Tube Radio wrote:
Wanted something that wouldn't load the oscillator any and my first thought was high impedance.
.

The oscillator itself may be a fairly low impedance---depending on where the signal is picked off.

Probably be a good idea to peruse the schematics of the high-end generators to see how they do things

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 Post subject: Re: RF amp circuit
PostPosted: Oct Fri 13, 2017 9:07 pm 
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Could get one of these and play with it. That video shows that it is ahead of the service level pack. I do not have a camera so making that video is not possible.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Sat 14, 2017 1:04 am 
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The modulation issue is that they inject it at the cathode follower grid where the RF signal is injected so it doesn't provide proper modulation but it does work.

I recently used my unmodded Stark LS-10A with a digital frequency display to align my R-390 and it did ok for that.

Pixellany at the oscillator grid there is a 15K resistor to ground so it is low impedance.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Sat 14, 2017 1:27 am 
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Archive.org has John Markus' Modern Electronics Circuit Reference Manual, with 16 pagees of RF Signal Generator circuits, about 40-50 in all. All solid state:

https://archive.org/stream/ModernElectr ... 3/mode/2up

Markus has older reference manuals on the Archive.org site, some that have tube circuits.

https://archive.org/details/SourcebookO ... icCircuits

https://archive.org/details/ElectronicCircuitsManual

Do you need 2 types of modulation? With that kind of frequency spread, it appears you want to cover BC and FM bands and will need two solutions.

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 Post subject: Re: RF amp circuit
PostPosted: Oct Sat 14, 2017 1:53 am 
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I'd only need AM as I have an FM RF generator.


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 Post subject: Re: RF amp circuit
PostPosted: Oct Sat 14, 2017 4:41 am 
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Tube Radio wrote:
Pixellany at the oscillator grid there is a 15K resistor to ground so it is low impedance.

What counts in this context is the output impedance of the circuit---which in turn determines the ability to drive the next stage. Depending on the oscillator design, the signal might be taken from the cathode or the plate--or sometimes from a coupling coil.
Need to look at a typical circuit to get into this deeper.

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 Post subject: Re: RF amp circuit
PostPosted: Oct Sat 14, 2017 4:56 am 
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Howdy Tube,
Tube Radio wrote:
I'd only need AM as I have an FM RF generator.


Man, I'm really glad to see you working on this, dude. Even though I'm going to order up that HP signal generator for xmas, I'm still going to hack around on my Heathkit SG-7 and see what I can get her to do :D. I'm wondering if I can use one of those old synthesizer chips, flip a switch, and turn the thing into a function generator as well? I don't see why that wouldn't work. (Shrug.)

73,

Randy AB5NI

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 Post subject: Re: RF amp circuit
PostPosted: Oct Sat 14, 2017 4:59 am 
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ab5ni wrote:
......... turn the thing into a function generator as well? I don't see why that wouldn't work. (Shrug.)
Randy AB5NI

I do.......

{Post the schematic of an actual proposed creation, and we can talk....:)

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 Post subject: Re: RF amp circuit
PostPosted: Oct Sat 14, 2017 5:25 am 
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pixellany wrote:
Tube Radio wrote:
Pixellany at the oscillator grid there is a 15K resistor to ground so it is low impedance.

What counts in this context is the output impedance of the circuit---which in turn determines the ability to drive the next stage. Depending on the oscillator design, the signal might be taken from the cathode or the plate--or sometimes from a coupling coil.
Need to look at a typical circuit to get into this deeper.


In my stark it is taken from the plate through a 3pF cap to the grid.

Can't use that as bands A and B have a distorted signal at the plate.


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