1. Base Unit Requirements
The NJ QRP MicroBeacon is a low parts count, low cost keyer beacon with attenuator control of the transmitter's RF output levels. It can be used as a normal iambic keyer with memories during normal operation, or as a short-term beacon which can transmit a message over and over.
This specification is for review and comment; everything presented here can be changed. The requirements are:
1. Full iambic keyer support with self completing dits/dahs and both dit and dah memories. The speed range will be approximately 5 to 40 WPM.
2. Multiple user-programmable memories. Eight memories will be included.
3. An optional computer control port. The keyer can operate without this interface installed.
4. Memories can be programmed from the computer port or the paddle.
5. Three programmable output bits for control of the attenuator. These bits can be controlled from either the paddle, computer port, or the memories.
6. Simple user interface.
7. Ability to auto-replay any of the memories. This is how the beacon is implemented.
8. The ability to turn the output on or off directly from a memory. This is used to turn on a solid carrier when operating a beacon. This, combined with the variable attenuatoin, makes a very good propagation beacon.
9. Low parts count.
10. Low cost. We can probably produce a kit that can be assembled for less than $20 with all new parts, or less than $10 with a reasonably stocked junk box.
11. The keyer will be built on a PC no larger than 2 by 4 inches. The goal will be to reduce this size as much as possible. The PC board will include appropriate test points to aid in the construction and test.
12. The keyer should draw no more than 20ma of current at 12 VDC for normal operation. The attenutor may require some additional power. If possible, we will reduce the power requirements.
13. The keyer will drive the target rig via an open collector transistor. When "key down," the output will be grounded.
The project will be an open one. Anyone interested in the project may get involved in the design of it. We will almost certainly use a microcontroller for the "guts", and the design of the code will also be public, as will be the resulting code. Anyone with access to appropriate development tools should be able to take our software and improve it, or at least experiment with it.
2. User Interface Requirements
* 1 analog input for the speed control (ie, a knob connected to a pot).
* A pushbutton to toggle program/play mode. This will also have an LED next to/above it that is lit when in programming mode.
* 8 pushbuttons to select which of the 8 memories is being played or programmed. Each of these will also have an LED associated with them to indicate which is playing or being programmed.
* A toggle switch to select ONE-TIME or REPEAT mode.
Basic operation of the keyer would be:
* The paddle is normally active as a regular iambic keyer.
* To program a memory, the user presses the PROGRAM button. The PROGRAM LED will light. The user presses one of the 8 memory buttons to select which memory he is programming. The LED associated with that button lites. When the user is finished programming, they can either press the PROGRAM button again, in which case the LED goes off, or they can select another memory.
* To play a memory, just press the button. The LED lights up for as long as the memory is playing, then goes out. The user can cancel the play by pressing the same memory button again (ie, when you press the CQ button instead of the exchange button).
* To set up a beacon, flip the switch to REPEAT, and then play a memory. It will automagically repeat until the switch is flipped to ONE-TIME.
Unfortunately, in order to allow advanced options like controlling the attenuator, the keyer must also "decode" the characters so it can recognize commands. We can probably get away with a very limited set of "smart" logic and look for a single special character (4 dahs?), then a simple character after it to indicate the attenuator setting. Maybe something like this:
---- (four dahs starts program mode)
xxx three elements; dit turns off a bit, dah turns it on. So, for full attenuation, you'd send ---.
We also need several special characters to send long key-downs and long key-ups. By default, the memories should clip leading and trailing dead time.
One other thing to consider is to have a telephone style keypad instead of the pushbuttons. It would require fewer I/O bits (only 7 needed), but more overhead to do the scanning.
3. Attenuator Requirements
This is a list of requirements for a means of achieving variable RF output power adjustment for the NJQRP Microbeacon project. While Informally referred to as a programmable power attenuator, use of a physical attenuator is only one means of obtaining a settable output power. A short tradeoff section at the end of this document compares several methods of implementing the programmable function.
o Adjustable QRP HF transmitter power output via a beacon microcontroller.
o TBD power levels or attenuation levels. To minimize control leads required from the microcontroller, a maximum of 8 levels is recommended, which could be accomplished with three control leads. Alternatively, two leads could control a maximum of four output levels.
o Output levels should be stable and repeatable within approximately 1 dB.
o DC operating power should be a commonly available value. Since most QRP transmitters operate from 12 volts, dc, this is the recommended value. Since battery operation is likely, minimum power drain is desirable.
o The power control means should be capable of operating on any HF amateur radio frequency band.
o If a programmable attenuator method is used, SWR reflected back to the beacon transmitter should not exceed 1.5:1 for protection of the transmitter and to ensure predictable output RF power.
o Small size (no more than a few cubic inches) and light weight (under 1 pound) are desired to facilitate portable operation.
o A weather cover is desired to allow operation out-of-doors.
The current practice in beacon transmitters seems to be the use of no more than a watt as the maximum power level. This is in keeping with the spirit of QRP in using the minimum power necessary. In addition, since the beacons are often used on the more popular amateur bands, it is important that they not generate excessive QRM. With good antennas these 40 meter beacons have been received over distances of several hundred miles during daylight and several times that distance at night, down to low milliwatt outputs.
Beacons operated recently by AA4XX and WA3NNA have used power levels which decrease in steps of 10 dB. On HF, the minimum easilydiscernible difference in power levels is about 3 dB, and since one S-unit is 6 dB, it is recommended that the increments (decrements) be no smaller than 6 dB. However for beacons at VHF or higher, smaller steps may be desired.
Potential Implementation Methods
So far I've come up with two categories for power control using an existing beacon transmitter. There are other ways if a special-purpose transmitter is used. The latter will not be discussed at this time.
1. Reducing RF power by using a switched attenuator.
2. Reducing RF power by controlling the DC supply voltage to the transmitter's output amplifier.
Switched Attenuator Design thoughts
Benefits of using a switchable attenuator are that the transmitter need not be modified in any way and the conceptual simplicity of the method. Disadvantages are:
a. The need to dissipate nearly the full output power of the transmitter under some circumstances.
b. The possibility of a physically large device, depending on the power dissipation, the type of attenuators used and the means used for switching the attenuator sections.
c. A potentially high power dissipation for the attenuator switches. Probably the best type of attenuator to use is a "pi" type of attenuator composed of carbon composition or carbon film resistors. 50 ohm attenuator pads are very easy to design and will give reasonably accurate results using inexpensive 5% resistors.
The two easiest types of switches are PIN diodes and relays. PIN diodes seem attractive at first glance, but tend to be fairly complicated in a multi-attenuator configuration. Their microsecond switching speed is not needed in this application. Power dissipation can be on rather high, on the order of 100 mW or more switch. In addition, isolation may be only 30 to 40 dB, requiring special techniques if high-value attenuators are used.
More detailed practical considerations will be presented in a later discussion.
Relays may be the best choice. They are relatively small (a candidate is the Omron G6H-2-DC-12, available from Digikey for $4.22 each. It is a dpdt relay that is only 5x14x9mm and has a low operating power requirement of only about 12 ma (1028 ohm coil resistance at 12 volts).
The simplest way of arranging the attenuators to be switched is to put them in cascade, and arrange the dpdt relay to either bypass them individually when powered off, or put the selected one in line when its associated relay is energized. This is identical to the way that manually switched lab type attenuators are built.
A means of reducing operating current for the attenuator even further is to use a latching type relay. This type has two coils, one to toggle back and forth between the two relay states when pulsed briefly. The disadvantage is additional circuit complexity, but that is minimal. A candidate latching relay is the Omron G6HK-2-DC-12 which has the same dimensions as the non-latching type mentioned above and is also available from Digikey for $5.22.
Using Supply Voltage to Set Transmitter Output Level
The use of supply voltage adjustment for setting the beacon RF power output level was alluded to in an earlier discussion. Here are some thoughts on that technique.
RF output power from a class-C amplifier is approximately proportional to the square of the DC supply voltage. Thus an output stage that supplies 5 watts at 12 v DC will produce about 2 watts at 7.6 volts and 1 watt at 5.37 volts.
1. For a 5 watt transmitter, pretty good output control can be maintained down to about 100 mW.
2. DC control can be easily accomplished by means of an integrated circuit linear voltage regulator and switched resistors to set the voltage.
3. Power levels can be accurately calibrated for a given transmitter using switched potentiometers and fairly good stability and repeatability are expected.Disadvantages
1. With a linear voltage regulator, there will be wasted power of as much as several watts in the regulator. This must be traded off against the constant dissipation of a 5 watt transmitter with attenuators.
2. Below about 100 mW, accurate RF output control becomes difficult with a simple regulator and RF leakage through the output stage becomes appreciable.
For accurate control at lower levels an external attenuator is required in conjunction with DC supply control.
3. This technique is usable only with a beacon transmitter in which the supply voltage to the final amplifier can be split off from the rest of the transmitter stages.
Using a linear regulator is not the only means of producing a variable supply voltage. There are a number of pulse-width modulated switching regulators available that can do the same job with much higher efficiency.
1. Same as the linear regulator with wasted power in the milliwatt region.
1. Same as the linear regulator plus:
2. Added complexity. Parts count would probably double, although size may be the same or less since a large heat sink is not required.
3. Added filtering must be used to lessen switching transients on the supply line to acceptable levels. These ultrasonic signals could create unwanted spurious outputs from the beacon transmitter.
4. Supplementary attenuation will still be required for power levels below about 100 mW.
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