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Old 20th Mar 2006, 2:42 pm   #61
YC-156
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Oskar and I are currently communicating through PM how to proceed from this point.
Quote:
Originally Posted by m1ecy
single earth points as close to the valve as possible are the norm, and short interconnects are a must!
A single ground point rarely works out well for HF transmitters, where the components are physically large. Short connections *straight* to the chassis plus care in layout, thus separating the ground currents, is usually all that is needed.

Aren't you having problems with the inductance of your ground returns in your VHF amps?

Best regards

Frank N.
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Old 20th Mar 2006, 2:47 pm   #62
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Default 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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Old 20th Mar 2006, 2:59 pm   #63
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by m1ecy
As for VHF amps, no, no problem with ground return there - all solidly bolted to deck using as much surface area as possible!
That is what I suspected.

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:
Originally Posted by m1ecy
Why discuss via PM? let us all know what you are doing!
We aren't really talking about technique at this point. I will let Oskar chime in here if he has something to add.

Quote:
Originally Posted by m1ecy
Im very concerned by lead length in Oskars design - lots of hassle there!
That must be the understatement of the week.

Best regards

Frank N.
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Old 26th Mar 2006, 8:13 pm   #64
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Default 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
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Old 26th Mar 2006, 9:55 pm   #65
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi again Oskar,

That looks very promising. I do have a 'few' suggestions, as you probably figured.
  • Loose the shield between the VFO and the 807 socket below chassis. The RF potential is the same on either side of it and it serves no purpose.
  • The second shield should have a few pieces of angle stock added, so that you can ground it to the base of the chassis at a few points between the front and the rear panel.
  • Turn 807 socket 90 degress counterclockwise as seen from below the chassis. This will make the G1 pin point toward 'From VFO'.
  • Move tie point for the grid blocking cap more in line between 'From VFO' and the 807 G1 pin. Aim to keep connections short.
  • Now you can connect Z1 straight from the G1 pin and to the grid block cap on the tie point. Other end of grid block cap still goes to 'From VFO'.
  • The 807 G2 decoupling cap must be located right at the valve socket, from the pin straight to ground. Just like the cathode decoupling cap.
  • Turn tie point, which carries G2 voltage dropper resistor, 90 degrees and move it closer to the point where the B+ connection passes through the remaining shield. This limits the opportunity for RF to be picked up or radiated from the 'cold' end of the resistor.
  • Move the 'cold' G2 decoupling cap up on the shield wall, decoupling right at the hole. Ground return must be boled to aluminium shield wall right at the hole. In other words forget putting any of the caps on the G2 resistor tie points.
  • Move R4 and its decoupling cap closer to Z1, which should now be pointing toward the right. The decoupling should return as closely to the hole for the valve socket as possible.
  • Make positively sure that you have an excellent connection between the top of the aluminium chassis and the copper foil on the PCB. Add more screws and locking washers around the valve socket if you feel like it.
  • There is no need to route the ground return of the PTT signal back to the PCB. Just bolt it to the chassis at the front panel.
  • The PTT connection from the 807 cathode pin (now pointing back toward the rear panel) should be routed up, right and down low along the shield toward the front panel mounted PTT switch. That way it will cross neither the 'hot' G1 nor G2 circuitry.
  • Add decoupling for PTT signal at the ext PTT socket right at the front panel. Bolt ground return straight to the chassis at that point using as short connections as possible.
  • Decide on one side of the 6,3V heater circuit, which should be routed through the chassis instead of through wires.
  • Connect one heater pin of the 807, the one closest to G2 would be my suggestion, straight to the PCB copper foil.
  • The other remaining heater pin should also be decoupled at the 807 socket. The wire will be routed back to the rear connection as it is now. Yes, that means two decoupling caps for that short piece of wire.
  • Resist the temptation to let the DC/heater power carrying wires hang up in the air. It is better to let them hug the chassis base or the shield. Lower RF field for them to pick up/radiate down low.
  • Wires intentionally carrying RF, like the G1 connection, should be spaced well away fromt he chassis, but this is less critical than the previous point.
  • Drop one of the 6,3V connections at the back, as you are now routing that part of the circuit through the chassis.
  • The wire to the grid current meter going topside (up into the nice and 'hot' RF environment up there ) must be decoupled right at the point where the wire passes through the chassis plate. Considering adding a high inductance/low current capacity moulded choke into the wire at that point as well, thus further enhancing isolation.
  • Depending on what the layout inside the VFO box looks like, then you probably need an internal shield inside that one as well. This should isolate the anode of the 6AG7 from the grid/cathode oscillator circuit and will hopefully prevent frequency deviations as you tune the 807 grid circuit. That means a shield from copper foil or a similar material across the valve socket.
  • Swap the location of the B+ connection and the RF output coax connector on the rear panel, possibly moving the B+ more to the right. It is OK to route the B+ wire a bit 'back' topside along the large RF choke for this to fit. Better than having the wires cross below deck.
  • Use a short piece of coax to connect the output loading variable cap to the output connector. Ground shield both at cap and output coax connector. This means less RF pickup by the B+ wires.
  • Decouple the B+ connection right where it passes through the chassis plate to the compartment below *in addition* to the existing decoupling cap at the connector on the rear panel. You know the drill by now.
  • Contemplate that in addition to providing electrical safety a wire mesh shield cage of the large 807 anode components topside would also largely prevent radiation of intense harmonic energy directly from the 807 anode structure. A class C stage generates intense amounts of hard harmonics, and those can severely disturb TV and radio reception nearby if allowed to spread.

    One reason why amateurs today have abandoned class C amps whenever possible are the difficulties in efficiently reducing the radiation of the strong harmonic energy RF class C amps generate at low VHF frequencies. An open frame transmitter like the one you are suggesting is downright illegal from an interference point of view, and the lack of proper RF shielding must be considered a fundamental design flaw. I would 'F'ail your design for that reason alone, since all electronic equipment designed today must by law ensure that they are 'nice neighbours' toward their surroundings.
Think about it and we can discuss all of this in the days to come.

...and keep up the good work.

Frank N.

Last edited by YC-156; 26th Mar 2006 at 10:02 pm.
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Old 27th Mar 2006, 6:30 am   #66
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

A few additional points, which I forgot the first time around:
  • All the coils should preferably be spaced at least their own radius from other components and any metal plate. That goes for the ends of them as well.
  • You should add an electrostatic shield to the 807 around its base, on the top of the chassis. It should be cylindrical and go from the chassis plate up to the point of the valve envelope, which is where the anode starts. Judging solely from a drawing of an 807 it looks like the dimensions should be something like 5cm in diameter and 5 cm in height. It shouldn't be a tight fit. Leave 5 mm of clearance at the widest point of the valve envelope.

    This shield should largely prevent pick-up of 'output' RF directly via the connection wires inside the vavle envelope, as they go from the valve base and up to the grid/cathode structure. It won't affect cooling of the valve either, as all the heat is generated above the point, where the shield stops.
  • You would be well adviced in moving the grid tuning cap back from the front panel a bit and isolating its frame from the ground. Use an isolated shaft extender. This will make adding neutralisation of the 807 stage at a later date much easier, if you ever need it.
  • I don't recall if we have discussed this, but since you have moved the TX to the 160m band, you *could* replace the VFO with a crystal oscillator. A 1.8432MHz crystal is easy to source at most electronic suppliers, and you wouldn't need the large shield box topside. Just a bit of local shielding around the 6AG7 socket, which could then be moved down onto the main chassis plate. Let me know if you want more details on this.
  • If you want to keep the VFO, then, if you want to be able to change and predictably set the frequency, you almost certainly need some form of mechanical reduction drive on the shaft for the VFO variable capacitor. Your layout doesn't seem to provide room for one, so maybe you have already mentioned this before.
Best regards

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.
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Old 28th Mar 2006, 11:27 pm   #67
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Default 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:
  • In this case the screen(!) grid, G2, is the dividing factor between the shield boxes. This is usually the case for tetrode or pentode gain stages. In theory this should ensure that the only coupling between the boxes is through the electron stream inside the valves. The physical structure of the valves used here doesn't actually allow us to realise this 100%, but we can try.
  • All signal wires are kept well clear of 'irrelevant' wiring for DC supplies, control circuits and heaters. We even saved a wire going into two of the shield boxes by grounding one side of the heater supply.
  • 'Irrelevant' components like meters, switches and power supplies are kept outside the main shield boxes. Presumably a larger exterior chassis provides structural strength and room for the rest of the components.
  • All ground returns are to the nearest chassis point. Star grounding be dd. This is not audio.
  • Signal ground returns near the valve sockets are returned as directly and as closely to the hole for the valve socket as possible.
  • All wires leaving the shield boxes, save for the antenna connector, are decoupled right at the holes in the shield boxes. The two sketches below the main drawing tries to demonstrate that is we let the decoupling happen a bit form the hole using a pair of caps, we might still generate a pickup loop on one side of a shield, and re-radiate the signal again on the other side. A few centimetres of wire in the wrong place can really ruin your day, if you are a bit unlucky.
  • Notice that some of the DC supply/control wires have two decoupling caps. The caps closest to the valves ensures a very short return path for the RF current, say for the 807 cathode. Short and low inductance RF ground returns are often critical to achieve stability and maximum gain from a stage.

    The cap at the hole ensures that no signal picked up by the wire on its way from the valve and up to the exit point can leave the shield box.
  • Control wires are kept close to the chassis plate at all times and well away from any signal carrying wires. Again this is an attempt at limiting stray RF pickup on the control wires. The gain from the 6AG7 signal grid and to the 807 anode is substantial. If we are unlucky, part of the 807 anode signal may leave via port (A) and travel back into the oscillator shield box via (B) or the heater supply. If the coupling between the 6AG7 screen supply wire and the 6AG7 G1 isn't negigible, we may have a problem on our hands. The total phase change as the signal passes through the two stages is 2x 180 degrees, 360, which is of course just perfect for sustaining an oscillation.
Hope this helps a bit.

Frank N.
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