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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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20th Mar 2006, 2:42 pm | #61 | |
Retired Dormant Member
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Oskar and I are currently communicating through PM how to proceed from this point.
Quote:
Aren't you having problems with the inductance of your ground returns in your VHF amps? Best regards Frank N. |
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20th Mar 2006, 2:47 pm | #62 |
Dekatron
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,518
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi Frank,
A point well made! As for VHF amps, no, no problem with ground return there - all solidly bolted to deck using as much surface area as possible! Why discuss via PM? let us all know what you are doing! Im very concerned by lead length in Oskars design - lots of hassle there! Cheers Sean
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Engineers make things work and have spare bits when finished |
20th Mar 2006, 2:59 pm | #63 | |||
Retired Dormant Member
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Quote:
I used to own a homemade 2x 813 amp, which I had bought from someone else. It looked like it dated back to the fifties, and it used a common ground point. Truth be told I never managed to get the efficiency up very high, not even at low frequencies. Since then I have always built my amps 'VHF style', no matter the frequency, and have been pleased with the results. Quote:
Quote:
Best regards Frank N. |
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26th Mar 2006, 8:13 pm | #64 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi all!
My problems with the test setup were likely (obviously? ) caused by bad layout / wiring. I have tried to put down a sketch of a better layout. Observe that the layout is for the chassis intended for the "real" transmitter, not for the test chassis. I am not sure I should waste more time messing with the test chassis. Maybe I have learned all of the most important lessons. Here is the chassis I will use: http://www.gargnas.net:3000/bilder/d...l/P1290003.JPG The placement of the components shown in the above picture is still valid. Given the size of certain things, this transmitter will not exactly use space in this chassis very efficiently. Here is my layout sketches: http://www.gargnas.net:3000/bilder/d...yout_above.pdf http://www.gargnas.net:3000/bilder/d...yout_below.pdf Ok, now it's submitted for "Frank-approval" Edit: The obvious "bad" about this layout is safety, unless the chassis topside is covered with some kind of hood, it is leathal! /Oskar |
26th Mar 2006, 9:55 pm | #65 |
Retired Dormant Member
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi again Oskar,
That looks very promising. I do have a 'few' suggestions, as you probably figured.
...and keep up the good work. Frank N. Last edited by YC-156; 26th Mar 2006 at 10:02 pm. |
27th Mar 2006, 6:30 am | #66 |
Retired Dormant Member
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
A few additional points, which I forgot the first time around:
Frank N. Edit: By convention the 'flow' of a transmitter circuit mostly go left-to-right, but it doesn't really matter, of course. Last edited by YC-156; 27th Mar 2006 at 6:36 am. |
28th Mar 2006, 11:27 pm | #67 |
Retired Dormant Member
Join Date: May 2005
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
As I promised Oskar B. last week here is finally my little introduction to RF circuit layout techniques. Please note that the rules below should be taken as very conservative starting points, which can often be relaxed somewhat in the real world. However I decided to go the full mile, since I feel that makes everything much clearer.
The need for RF screening. When designing and building RF equipment, valve or semiconductor, there are at least three main reasons why the builder should go to the effort of adding efficient RF screening to the layout. They are: 1 Avoid instability. If RF energy can somehow travel from the output of a gain stage and back to its input, or any of the preceeding stages, instability and oscillations can occur. These parasitic oscillations can be either on or off the working frequency, and oscillations in the lower VHF range are not uncommon. So it is usually not sufficient to provide shielding, which is only effective on the operating frequency. Aim for wideband shielding whenever possible. 2 Avoid interference. If RF gain or mixer stages are left unshielded from their surroundings, they may either radiate unwanted signals or pick up airborne RF energy, causing difficulties along the way. Class C valve stages are particularly troublesome, since the physical valve structure is often tall enough to allow it to function as an antenna in its own right. The output tuned circuits are of no help here in reducing this radiation, since it has already taken place before the harmonic RF energy has a chance to be attenuated. In receivers and other low level stages the lack of shielding can be really troublesome due to pickup of intereference from nearby computers and other electronic gizmos. Even simpler, then different parts of the circuit may interfere with each other as well. 3 Avoid burning yourself or your surroundings. RF energy, if improperly shielded and grounded, can cause severe burns and outright destruction to attached semiconductor equipment. Multilegged fuses are particularly sensitive to this. Anyone, who has used a microwave oven, will be aware that RF energy can cause heating in lossy insulators. Touching a transmitter chassis with poor shielding and insufficient RF grounding can be a very painful experience. Fortunately it is rarely fatal, but permanent scars are a definite possibility, since RF burns goes deep down into the tissue. As little as 1W can be enough to cause burns in exceptional circumstances. Been there, done that. Fortunately it almost takes less time to explain how to do it properly than to justify the need for RF shielding. The fundamental tool for RF shielding is the enclosed, conductive box, a Faraday cage. No electromagnetic field can escape from or enter inro a perfectly unbroken electrically conductive box. If we prevent unwanted signals from entering or leaving via wires breaching the walls of such a box, we can achieve very good isolation of the processes inside toward the immediate surroundings. A circuit with shielding applied consists of a number of such conductive enclosures, more often than not sharing one or more walls. The (strictest) rule for how many boxes we need is simply that no signal amplification may take place inside any box. Amplification as found inside valves and sand state devices may only occur at the boundaries between two adjacent boxes. Another way of thinking about this is to say that the signal level must be the same anywhere inside a given shield box. In reality the shield boxes doesn't have to be rectangular nor anywhere near airtight. The wavelength of most RF energy hobbyists are interested in ensures that boxes with even fairly large holes and areas covered in wire mesh will provide excellent isolation. It is probably obvious that if we have X active devices, we will need X+1 shield boxes. However sometimes less than this will do, and the art of RF design is to decide when you need to play by the rules and when you don't. Anyone, who has worked on, say, a vintage radio, will know that each valve, save perhaps for the VHF dual triode, isn't accompagnied by a pair of metal boxes. By distancing gain stages, careful placement of passive comonents and careful use of IF transformer casings, an adequate shielding effect can often be achieved. Unfortunately for the hobbyist experimenter they rarely have the resources of the large electronics manufacturers, and it may often be faster and easier to solve the shielding problem by brute force, than having to experiment with component layout and similar techniques. Below I have attached a fictitious physical layout for a 6AG7 + 807 transmitter, where all sections operate at the same frequency. Some components have been left out for clarity. Apart from all the rules already mentioned in this thread, I have added a few more to the mix:
Frank N. |