[ Most Recent Update: 24 Oct 2011 ( Minor changes to text and Revised WT40 Block Diagram. ) ]
WT40, Part 3: PACKAGING
By mechanical, I mean such things as:
[] front & rear panel layout
[] placement of parts on the circuit boards
[] arranging circuit boards on the chassis
[] a box to put it in
[] etc.
I have built some items that look pretty good, thank you, but most of my home brew projects are built by simply wiring the circuits without much thought or effort toward appearance. I even use ugly (point-to-point) wiring technique as opposed to printed circuit (PC) boards. If I were in the business of building stuff in great numbers to sell, I would, of course, use PC boards and assembly line techniques. Yes, I have made some PC boards, but for me they are more trouble than they are worth for one-of-a-kind projects.
Packaging is important, particularly if you are planning to use your home-brew equipment for many months or years, and it’s none too early for you to start thinking about what you want your 40 meter transceiver to look like when you have finished it. Whenever possible and/or practical, I will present alternative approaches for packaging.
Three packaging methods are briefly outlined, below.
[1] Build on a single, relatively large circuit board, then put it in a suitably sized box.
[2] Build the rig on two boards, one for the receiver section and one for the transmitter section, then put them into a suitably sized box or into separate boxes.
[3] Build and test each of the many module separately, then hook them together so they fit into a box or boxes, then connect the boxes together.
Building technique number [3] is the method that will be presented here, using three boxes:
[] Control Head, contains the Control Panel, plus VFO and VFO buffer.
[] Receiver, contains all receiver circuitry, except the VFO.
[] Transmitter, contains all transmitter circuitry, except the VFO. The transmitter box also houses the antenna switching circuitry.
The builder (YOU) are, of course, free to use any method you choose.
The Radio Shack 276-148 circuit board, or equivalent, is an excellent foundation for building a variety of electronic devices using dual inline pin (DIP) integrated circuits and discrete components, such as resistors, inductors, capacitors, etc.
At first glance, the 276-148 appears to be a ' Twin ' circuit board, with perforations across the middle so that the two halves can be easily separated. Close examination, however, reveals that the two halves are not identical. For most modules, this lack of symmetry is unimportant.
The left-hand side in the photo contains 213 perforations for mounting components and the right-hand side has only 228 perforations; for a total of 441 perforations for mounting components.
The ground buss, tie points, and mounting hardware, use up several perforations, so the number of perforations for mounting components is somewhat reduced, and this must be taken into consideration when arranging components on the boards.
I normally use the two halves as a single board. The dual boards measure 4.4 x 9.2 cm (about 1 3/4 x 3 1/8 inches).
A generic module will have Inputs for Power, Ground, Input, and Output.
Various modules will have different requirements for connections. For example:
[] An oscillator module would have connections for power, output, and ground, but no connection for signal input because the oscillator's function is to generate a signal that will be used as input to other modules.
[] Circuits, such as passive filters, would have input, output, and ground, but do not require a power input.
[] Some modules will have multiple inputs and/or outputs.
GROUND
The one connection that all modules must have is Ground. Ground is another way of saying a common connection.
Every module must be connected to every other module via a Ground connection.
Speaking of ground, the first step in preparing a 276-148 circuit board for all modules is to establish a ground buss.
The photo, above, shows the ground buss wire installed around the outer edge on the bottom (solder) side of the board, and standoffs have been placed at each corner. All modules will have a ground buss of #22 bare copper wire similar to that shown above.
The ground buss on each dual board requires about 11 1/2 inches of #22 bare solid copper wire. The ends are soldered together where they overlap to provide a continuous ground buss around the outer edge of the circuit board.
In addition to providing an anchor for the ground buss, the stand-offs also allow boards to be stacked, as shown below.
The modules we (YOU and I) will be building are similar to the ones shown, above.
Before we start building, perhaps it would be a good idea to take a quick look at what all this is leading to.
The simplified diagram below shows what we are aiming for - - -
Some things to notice in the diagram:
[] The title of the diagram, WT40 Block Diagram, means WannaTinker 40 Meter Transceiver (Now, you know the origin of the name, WT40.)
[] The VFO is used to determine the frequency of both the Receiver section and the Transmitter section of the Transceiver.
[] The transmit / receive (T /R) switching is done by relay. Yes, there are more elegant and clever ways to do T / R switching, but they are usually employed to implement break-in keying and/or to achieve minimum physical size for the unit. Neither minimum size nor break-in keying were high priority for this transceiver.
[] Voltage regulation and distribution will be detailed for each module, as required.
THE PLAN
First, we will build the Receiver section, including the VFO because the receiver can not function without a VFO. When all the modules in the receiver have been built and tested individually, the receiver modules, along with the VFO and VFO buffer, will be assembled and tested.
Next, the Transmitter modules will be built, tested and assembled.
Finally, the whole transceiver will be assembled and tested to verify it is working properly.
There is much work to be done, but first . . .
A FEW WORDS REGARDING RADIO RECEIVERS
We are bathed in electromagnetic radiation (radio signals) 24 hours each day. There is no escape. Some of this radiation is useful, most is not. That which is not useful is, by definition, “noise”.
A radio receiver has two primary functions:
[1] The radio receiver must have enough Sensitivity to be able to detect the electromagnetic signal of interest to the user.
[2] The radio receiver must have enough Selectivity to be able to eliminate most, if not all noise, and present only the signal of interest to the user.
For the WT40, the signal of interest is the Ham radio 40 meter band.
There are many auxiliary functions performed inside a radio receiver, but unless the receiver has good sensitivity and selectivity, the other functions are of little or no use.
SENSITIVITY is accomplished with amplification circuits of various kinds.
In order for a receiver to be useful for Ham Radio communications, it should be sensitive enough to detect very weak signals. Most communications receivers, including receivers built for the Ham bands, are capable of detecting signals of less than one micro-volt (uV). That’s 0.000001 volt, and that’s a pretty small amount of voltage. It is so small that it can’t be detected with ordinary test equipment such as your DMM. Having said that, I should point out that there are Ham Radio signals “on the air” that will present 5 to 50 microvolts (uV), or more, at the antenna input on your receiver.
SELECTIVITY is accomplished with filtering circuits of various kinds. The receiver section of the WT40 has two types of filters: [1] A radio frequency filter that attenuates all signals except the 40 meter signals and [2] An (optional) audio frequency filter built specifically for receiving CW (Morse Code) signals.
In order for a receiver to be useful for Ham radio communications, it should be selective enough to eliminate most of the “noise” that comes into the receiver along with the signal of interest. This is particularly important in receivers used for communications because the signals are usually much weaker than those of commercial broadcasts, and are sometimes packed much closer together.
It is good to have as much filtering as possible at the “front end” of the receiver - the first circuits encountered by the signal of interest when it enters the receiver. This will eliminate much of the "noise" before it gets into the following circuits and causes trouble.
The signal-to-noise relationship is more complex than the simple explanation presented above would indicate. For example, if the signal of interest is 10 uV and the noise is 20 uV, the signal will be obliterated by the noise. Not only that, but there is a certain amount of noise generated within your radio by the components simply doing their job. All those electrons rushing hither and yon can create quite an uproar!
Be that as it may, sensitivity and selectivity work hand-in-hand to grab the signal of interest from “thin air” and process it in such a way as to make it clear enough and strong enough for you to hear.
Magic!
Speaking of hearing the radio . . . the first module we will build is the Audio Amplifier module.
Why the Audio Module? (you might want to know).
Because that is where "the rubber meets the road", so to speak, or more exactly, the Audio Amplifier module creates the electrical signal that drives the earphones or speaker.
It is time to start building modules, beginning with gathering all the parts required for the the Audio Amplifier module, and that’s the first thing we’ll do in Part 4: The Audio Module (Introduction).
- END OF PART 3: PACKAGING -
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