All projects should begin with a set of requirements. The requirements for this project were:
THE BLOCK DIAGRAM
A functional block diagram of the approach I took is shown below. The shaded areas are the BAQSK. These are: the Power Supply, the Keyer, the T/R Switch, and the Control Circuit.
It's always tempting, when starting a new project, to begin with a clean sheet of paper and design everything from scratch. I've learned however, that this is not the most effective use of my time. I don't know how many projects have died on my bench because I ran out of time, or something with higher priority came along. So I've accepted the trade-off between having limitless fun and limited time. With this constraint in mind, I searched the Internet, and back issues of the various ham magazines for reusable technology, and I found a very nice T/R Circuit in Ham Radio (May 1976, p10). The article, "PIN Transmit/Receive Switch for 80 - 10 Meters" by James K. Boomer, W9KHC, was just what I needed. I also found a Keyer/Controller chip designed by Steve Weber, KD1JV. It's called the SKC (Simple Keyer Chip), and it's available for $2 each from Steve's website http://kd1jv.qrpradio.com/. This allowed me to combine the keyer and the contoller into a single functional block. Finally, Chuck Olsen (WB9KZY) of Jackson Harbor Press has a "Key-All" circuit that will key both cathode keyed and grid-block keyed rigs. It's available for just $12 from his website at http://home.att.net/~jacksonharbor/keyall.htm. Now that's what I call Reusable Technology! Isn't the Internet great?
The T/R Circuit uses PIN diode switching to switch the Antenna back and forth between the Transmitter and the Reciever. This provides fast, silent switching (as in no relay clicks). I used the exact circuit with a few parts substitutions to match what I had on hand (see "Circuit Modifications" later in this paper). The circuit works really well, and it's broadband enough to cover 80 - 10m.
The Power Supply comes from the same article as the PIN Diode T/R switch. It's a very straightforward design. The transformer with 125vac and 6.3vac outputs is probably going to be the hardest part to find. Transformers aren't as cheap as they used to be, so your best bet may be a hamfest, or a friend. I didn't have one of these in stock, but I did have a friend (thanks, Bob). Later, I found a great source for these transformers: the old Heathkit VTVM's.
This circuit consists of a Keyer/Controller chip called the SKC (Simple Keyer Chip), three FET output drivers for the SKC, and the Key-All transmitter keying circuit. The SKC is an 8-pin DIP IC which can be spotted at the center-left end of the prototyping board. It takes its input from the external paddle or key and creates two outputs: the Receiver Mute signal (which I also use to control the T/R Switch), and a delayed keying signal which keys the transmitter via the Key-All circuit. If a two-conductor plug is used for the external key, the SKC will detect this on power-up and will automatically go into the External Keyer Mode. This conveniently satisfies my requirement for being able to use an external keyer, or a straight key. My own preference is to use an external keyer/paddle combination, and the two-conductor plug. I find it easier to change the keying speed with a knob on the external keyer as opposed to using the menu in the internal keyer. But this adds another external "box" to the system, and your preference may be different. With this design, you can choose to use either one. The SKC chip also has a Sidetone output on pin 7. There is enough signal to drive a small (2") speaker directly through a 100 ohm resistor and a 0.1mfd blocking cap. A pot could be added to provide a volume control for the sidetone, but I didn't do this. The pitch of the sidetone is set by software, and is not adjustable. The Key-All circuit is built on its own little pc board, and is mounted on the right end of the prototype board. The SKC, driver FETs, and the opto-isolator on the Key-All board are powered by an LM105 5v, 100mA regulator. One of the output FETs on the Key-All board is connected to the center pin of the RCA phono jack labeled "TX KEY" on the back of the box, and the other FET is connected to the ground lug mounted under this RCA phono jack. It's possible to isolate both leads from ground if desired, but I do not expect to need this in my project so I just grounded one of the FET outputs. The only power needed for the KeyAll circuit is 5v for the opto-isolator.
In the T/R Switch, I substituted available devices for those called out in the original schematic. I used 2N2222's in place of Q2 and Q3, and a 2N3019 for Q1. Other substitutions are possible, but pay attention to the voltage rating on Q1. I also substituted 1N4007 rectifier diodes for the PIN diodes. Measurements showed they were an effective replacement, but performance did suffer a bit as a result of this substitution. The SKC Keyer Chip requires a 5 volt supply. I put a 5 volt regulator on the low voltage from the Power Supply, but the voltage occasionally dipped below what the regulator needed and the Sidetone sounded raspy. To remedy this, I changed the half-wave rectifier circuit to a full-wave voltage doubler, and the raspy tone was fixed.
Take a look at the schematic. The Power Supply provides about 12-14 volts to the T/R Switch for the switching transistors, and about 135 volts key-up (and 111 volts key-down) to the T/R switch to keep the switching diodes reverse-biased. The 12-14 volts is also used by the Power On LED, and is regulated down to 5 volts for the Keyer circuit.
The Simple Keyer Chip provides many of the keying functions needed, plus the sidetone, and T/R switching line. There's an FET buffer stage on the output of the SKC, which drives the KeyAll circuit. The SKC also contains an internal keyer which can be controlled by the MENU pushbutton and some taps on the key. See the detailed description of the SKC at http://kd1jv.qrpradio.com
The T/R Antenna Switch is an excellent design, and it performed better than I was anticipating. With 10 watts to a dummy load on 20m, the signal level at the receiver was only 18 millivolts rms (6.5 microwatts, or -22dBm). This is a very safe signal level to have at the receiver input. The insertion loss from the antenna connector to the receiver connector is 0.2dB, also a very acceptable specification. See the Insertion Loss Table for more details.
I housed the BAQSK in an old 8"x6"x3-1/4" metal enclosure that I got from Radio Shack. In fact, the enclosure was left over from a previous project - complete with extra holes. There is nothing critical about the layout of the Power Supply and the Cathode Keyer boards, but I would recommend following the layout for the T/R Switch that appears in the original article. As always with RF, leads should be kept reasonably short. The T/R Switch was built on a piece of copperclad board using the layout from the article, and the unwanted copper was removed with a Dremel tool. The Power Supply was also built on a piece of copperclad using "Manhattan Style" assembly. It could have just as easily been built on a prototyping board, or even perfboard. The Cathode Keyer was built on a Radio Shack prototyping board. This may not be available at your local store anymore, but it can be ordered directly from Radio Shack online (P/N 276-170). I just ordered some prototyping boards from Futurlec.com, and they look like they'll be a good substitute for the Radio Shack board.
I connected the BAQSK to my old Viking Adventurer and Drake 2-B. The antenna was a 20m dipole. I made a few contacts with good signal reports and no operational problems. Up to this point, I was operating crystal controlled on 14.061 MHz. I wanted to be sure the circuit worked as well on the other bands, so I dragged out an old Viking Model 122 VFO. Of course, this becamse a project-within-a-project, but with the wise suggestions from Glenn Zook (K9STH), the VFO was put into good operating order. Getting back to the BAQSK project... I checked operation on 80, 40, and 20m (the VFO didn't provide enough drive on 15m and 10m, and by this time, I didn't care). I made contacts on all three bands and everything was working great.
Now for the measurements. I wanted to see what the actual Transmit Insertion Loss was. A few dB here could be a problem. The measured loss was very nearly zero on all three bands. Next, I wanted to know how much of my transmit signal was getting to the receiver. I didn't want to blow out the front end of my precious 2-B. Happily, I was measuring 60 to 70 dB of isolation between the TX and RX ports with a key-down output power of 10w from the Adventurer. This translates to roughly 10 - 100 microwatts at the receiver, which the 2-B can handle easily without damage. Finally, I wanted to know how much of my received signal was being lost between the Antenna and Receiver ports of the BAQSK. This turned out to be about 1 dB, which sounds high but it's really quite acceptable. I think this could be improved by using the PIN diodes specified in the original article. The high junction capacitance of the 1N4007's is probably to blame for the 1 dB. A table of the Insertion Loss on all three bands is shown below.
|80m||0 dB||1.0 dB||76 dB|
|40m||0 dB||1.2 dB||70 dB|
|20m||0 dB||1.5 dB||64 dB|
A WORK IN PROGRESS
All of the original requirements were met. True QSK operation (being able to hear the receiver between the dots and dashes) depends a lot on the receiver recovery time. My little 2-B did just fine for my code speed which maxes out at 25wpm..
The Antenna-to-Receiver insertion loss can probably be improved by using real PIN diodes instead of my substitute rectifier diodes. I just found some MPN3700 PIN diodes for a very reasonable price of six diodes for $5 at Kits and Parts, but I haven't tried them yet. This is on my list for version 3 of this project
All the original requirements of this project were met using what I call "Reusable Technology". The Internet was a powerful tool for making this happen. This is truly a mering of the "Old" (1960's Boatanchors, and the 1976 article by W9KHC) and the "New" (the SKC chip, the Key-All circuit, and the Internet) to achieve a very smooth and versatile Boat Anchor QSK operation.
Power handling capability of the BAQSK can be as high as 122 watts in a 50 ohm system (see notes 1 & 2). But for safe operation with a 2:1 SWR, you should limit your power to 50w maximun. I operate QRP, so I'm nowhere near the point of forward biasing the 1N4007's. If you have any questions or comments on my BAQSK project, please send me an email at the address on my home page.
ed - k9ew
Note 1: I never ran more than 10 watts into the BA QSK circuit, and I am not liable for any damage if someone else uses this circuit.
Note 2: The reverse-bias on the Receive 1N4007 was measured at 110 Vdc during the key-down condition. With 110 Vdc of reverse bias on the 1N4007's, you would need 111 V peak RF voltage to forward bias the diodes. 111v of peak RF corresponds to 78 Vrms (111*sqrt(2)/2). This represents a power level of 122 watts (P = Vrms^2/R = (78)^2/50 = 122) in a 50 ohm system. A 2:1 SWR could result in an RF voltage of Vpkswr = sqrt(2) x Vpk = Sqrt(2)x [Vrms x sqrt(2)] = 2 x Vrms. To avoid having Vpkswr exceed 111 V, Vrms should be 55 Vrms (Vrms = Vpkswr/2 = 111/2 = 55). 55 Vrms corresponds to a power level of 60 watts (P = Vrms^2/R = (55)^2/50 = 60). So for safety, don't exceed 50 watts, and don't exceed a 2:1 SWR. If you can raise the level of the high voltage supply under key-down conditions, the allowable power level can be increased.