schematic and board layout revised 2/10/2008
PSK transceiver prototype and Pocketdigi software running on PDA.
The usual method of Transmitting a PSK-31 signal is to use a SSB transmitter. This is fine for most situations, but SSB rigs tend to draw a lot of current and are fairly large. However, for field or emergency use, you might want a PSK transmitter which uses a lot less current, is smaller and lighter. The circuit presented here is a method of using the standard PSK audio modulation from a PC sound card to transmit a PSK signal without the need for a SSB transmitter. Since the RF Power Amp runs in Class C mode, it is much more efficient than a linear amplifier normally used with PSK transmission. A simple Direct Conversion receiver with a VXO makes this a complete PSK transceiver. The rig is built with common, easy to get and inexpensive parts. Receive current is a modest 30 ma and transmit current averages 450 ma, with a peak output power of about 3.5 watts.
There are of course, some limitations to this novel PSK transceiver design. The most significant being the transmit frequency can not be set by the usual method of clicking on a signal in the PSK waterfall display. A station you wish to communicate with must be manually tuned to the 1 kHz marker on the waterfall display. This is because the transmit frequency is fixed at 1 kHz above the receiver zero beat frequency. For example, the receiver would be tuned to 14.0700 MHz in order to receive a signal being transmitted at 14.07100 MHz. Since the receiver shares the same oscillator as the transmitter, the oscillator frequency must be shifted by 1000 Hz (1 kHz) in order to match the received stations transmit frequency. This T/R offset is preset and is chosen to be 1 kHz for reasons which will be explained later in the circuit theory section.
A PSK-31 signal is a serial data stream which uses a 31 baud rate and 180 degree phase shifts to distinguish between 1's and 0's. In addition, the signal is amplitude modulated between phase shifts to reduce bandwidth and help the decoder program recognize the transitions between the phase shifts. If you look at the PSK output of a sound card, you will see an audio tone with a low frequency AM modulation, similar to what is shown in Figure 1. When used with a SSB transmitter, the audio tone is mixed with the various oscillators in the rig to produce the output frequency and the superimposed AM modulation controls the output level.
We can transmit a PSK signal with a Class C amplifier provided we do a couple of things. These things would be modulate the PA with the low frequency modulation and change the phase of the signal driving the PA at the appropriate times. The most difficult part of this is extracting the modulation envelope from the audio carrier in order to determine when phase shifts occur and modulate the PA.
How the circuit works:
The diagram below shows the basic block diagram of the audio PSK demodulator, phase detector and shifter and amplitude modulator.
The first step is to remove the audio carrier from the PSK modulation signal generated by the PC sound card. This is done by using a sample and hold circuit which detects zero crossing of the audio signal (U2d) which triggers a flip-flop "one shot" and activates an analog switch (U3). The peak audio voltage at the time the analog switch turns on charges C6. Because of the fixed pulse width of the one shot flip-flop, this circuit is only effective around a fairly narrow range of audio frequencies. It has been optimized for a 1 kHz signal.
U2b buffers the recovered modulation signal and is then further amplified by U5b/Q1/Q2 to have sufficent current and amplitued to modulate the PA. Not shown is U5a, which is used to detect when the audio level produces 100 % modulation of the PA. 100 % modulation occurs when Q2 is driven into saturation by the op amp U5b.
Phase shifts of the transmitted signal occur each time the envelope goes to 0 volts. This condition is detected by U2a and is used to trigger a flip-flop in order to change the phase of the transmitted signal at each transition. An XOR gate is used to phase shift the transmitter carrier and drive the PA FETs, which are three BS170 MOSFETS in parallel.
The op amps require a negative supply. Note that the Vss of U3, the 4066 is also connected to the negative supply. This is to keep the output of U2c from being clamped to ground through the diode on the analog switch inputs. The negative supply is generated on board using a 4049 hex inverter, U4. In this way, an external negative supply is not required, which simplifies the external supply, especially if battery power is used, such as a gel-cell. Two of the inverters are used as a free running R/C oscillator which is then buffered by the remaining four inverters wired in parallel for increased current output. When the inverter outputs go high, C8 is charged to the supply voltage through D3. When the inverter outputs go low, the charge on C8 is dumped into C10 through D4, creating a negative voltage on C10. Because of the two diode drops, about a -3.5 voltage is obtained.
Receiver and VXO circuits:
The receiver is a very simple Direct Conversion type, comprised of a SA612A mixer and a LM386 audio amp. The LM386 gives a fixed amount of audio gain with very few extra components. There is sufficient gain for good sensitivity, enough that a little bit of signal attenuation is needed to keep from overdriving the PC's sound card microphone input. The exact sensitivity is a little hard to determine. Best I can figure, it is about 0.5 uV for minimum good copy. C47 across pins 1 and 8 of the LM386 improves sensitivity, but it also increases the amount of noise on the output, so its debatable if it realy should be used or not. Q10 is used to mute the audio output during transmit.
The required 14.070 MHz local oscillator signal is generated by mixing a 5.0688 MHz crystal with a 9.00 MHz crystal. The internal oscillator of a SA612A is used for the 5.0688 MHz crystal. C52 is used to set the T/R frequency shift to 1 kHz. C52 is switched in and out of the circuit by D13 and Q11. R33 provides forward bias current to turn D13 on when Q11 is on. C52 is active during receive and shifts the frequency lower.
The 9.00 MHz crystal uses a separate oscillator circuit. Two crystals are used to form a "super VXO" so sufficient frequency tuning range can be achieved. The frequency is tuned by C31. A poly-variable like those found in AM/FM radios can be used. If you have an air variable cap and vernier dial, that would be better than the poly-variable, as fine tuning would be easier. Having very fine control of the frequency is helpful if you want to respond to a station calling CQ, as you need to match thier frequency very closely.
Ideally, you will have an Oscilloscope to test and align the circuits. It can be done without a Scope, but is a lot quicker and easier with one, especially if any troubleshooting has to be done. Testing starts with the local oscillator.
Everything should now be working properly and you can connect up an antenna and try to talk to someone. Note: The BS170's can be damaged is the SWR is high. The BS170's are running near their maximum current and voltage rating when operated from a 13.8V supply. High SWR can cause the BS170s to draw excessive current. Make sure you have a low SWR (below 2:1) before transmitting! Use another transmitter is you need to adjust an antenna tuner for best match. Using Tune mode in the PSK program generally produces a lower amplitude tone than what is normally used and will reduce power output, so this might be safe to use as long as the SWR is not too greatly out of whack to start with.
If you have a second PC which you can set up as a second PSK station, it would be a good idea to verify proper transmission by talking to yourself. This is also the best way to verify and fine tune the T/R offset to ensure your receiving and transmitting on the same frequency.
Although you can see all the signals in the pass band of the receiver, remember you have to manually tune a station you want to communicate with to exactly 1 kHz on the waterfall. Only then will your transmit frequency match that station. Since tuning is a bit touchy, it is a lot easier to simply find a clear frequency and call CQ and let other stations come to you. If you did not get your offset set quite exactly to 1000 Hz, a responding station maybe slightly off your receive frequency. It is permissable to fine tune the station by clicking on it's waterfall, provided you don't have to deviate from 1 kHz by more than +/- 50 to 100 Hz. If your off by more than that, you should go back and set the offset frequency closer.
The best way to build this rig is on a printed circuit board using the above layout. This layout is sized to fit into a standard 5 x 5 x 1.5" plastic box and has board mounted input and output jacks. The layout was done using CirCad 98, of which a demo version can be down loaded from http://www.holophase.com The CirCad 98 file for this board can be down loaded here: [Board file] Once you instal CirCad 98 onto your PC (it will run fine on XP, but if you do a test print, you will likely have to close the program and re-open it to get a proper print a second time. At least, this is what I have to do using an old h-p Laserjet 5L printer) Be sure to set the proper layers for printing, you will need the pad master and bottom tracks selected. Also, be sure the print to scale box is checked. Because toner transfer film inherently inverts the image, do not mirror the image.
When building the board, note there are a number of point to point jumpers required, indicated by the black lines between pads. It is also very easy to make solder shorts from the pads to the ground plane.
It is also possible to build the rig dead bug, Manhatten or ugly style. I would suggest the RF section be built on a copper clad board for ground plane and the analog/digital sections on perferated board.
If you need to buy all new parts, the total cost less shipping about $53.00. This includes a plastic box, but not the tuning cap and circuit board. The board you will have to make yourself.
40 meter and 30 meter versions:
It is possible to make the rig work on 40 meters. This I have tried and works great. I have not yet tried the 30 meter version, but should work fine also. In addition to the part values changes listed below, it would be a good idea to add a second diode in series with the QSK diodes D11 and D12 to keep strong SWBC stations from causing them to conduct and cause intemod.
40 Meter values:
X1 = 4.00 MHz, X2 and X3 = 11.059 MHz
C29, C50 and C51 = 47 pfd and the internal caps in the IF cans are left intact.
R27 = 1 K ohms
C22 and C24 = 330 pfd, C23 = 680 pfd, C19 = 68 pfd
L2 has 16 turns on T37-2 core and L3 has 18 turns on T37-2 core
L5 is not used and is jumpered out
L4 becomes 12 uhy and C26 becomes 47 pfd
The T/R offset direction is now reversed due to the High - Low = operating frequency mixing schem. R36 is moved to the R37 location to reverse offset direction.
The rig will just bearly tune into the PSK sub-band of 40 meters. The Tuning cap C31 will need to be a much smaller value. Adding a series cap of say 33 pfd to a poly-variable will help reduce the tuning range and keep it more or less in the sub-band.
30 meter values:
It should also be possable to put the rig on 30 meters. The PSK frequency for 30 is 10.140 MHz. If X1 is made 4.00 MHz and X2, X3 = 6.144 MHz, that works out to be 10.144 MHz. Using lower sideband offset shift, this will put the operating frequecny in the right ball park.
The IF transformers are not modified and C29, C50 and C51 are no longer needed.
C22/C24 are now 220 pfd, C23 is 560 pfd and C19 is 47 pfd.
L2 has 13 turns on -2 core and L3 has 16 turns.
L4 = 10 uHy and C26 = 33 pfd
Some Mouser part numbers:
IF transformers (3) : 42-IF123-RC remove internal cap in bottom of can. ($0.94 each) (2.82)
BNC rt angle ant jack: 571-5227161-2 ($1.85)
2.5mm power jack : 163-5003-E ($0.64)
3.5mm stereo pc mount audio jack (2) : 161-3507-E($0.79 each)(1.58)
Plastic box board will fit into: 616-71886-510-000 (black) or 616-71887-510-039 (bone)($8.00)
SA612A and toriods are available from www.kitsandparts.com
Poly-variable for tuning available from www.qrpkits.com
If you have to order most or all of these parts in the small quanities required, it would be nice to do it on line instead of phoning it in. They aren't going to make much money on this order, as it will take a lot of time to fill the order!
Parts list: (quanity) (value) (Mouser part number) (price,each)
(1) LM324 quad op amp 512-LM324AN ($0.35)
(1) LM358 dual op amp 512-LM358AN ($0.28)
(1) LM386 audio amp 513-NJM#386BD ($0.42)
(1) 4013 dual flip-flop, CMOS 511-4013 ($0.29)
(1) 4066 quad analog switch, CMOS 511-4066 ($0.18)
(1) 4049 hex inverter, CMOS 511-4049U ($0.23)
(1) 74HC86 quad NOR gate 512-MM74HC86N ($0.40)
(2) SA612A mixer/oscillator ($2.00 each)
(1) 78L05 5V regulator, TO-92 512-LM78L05ACZX ($0.20)
(1) TIP-42G or MJE-2955E pnp power, TO-220 512-MJE2955TTU ($0.50)
(4) 2N3904 NPN small signal 512-2 n3904TA ($0.21 each)
(4) 2N7000 N-FET TO-92 512-2N7000 ($0.11 each) (0.77)
(3) BS170 N-FET TO-92 512-BS170
(1) 1N4756A 1W, 47V zener 512-1N4756A ($0.08)
(1) 1N5817 shottky rectifier 512-1N5817 ($0.19)
(10) 1N4148 silicon diode 512-1N4148 ($0.02 each) (0.20)
(1) red LED - I'm sure you can find one of these with out having to buy one!
Resistors Mouser 291-(value)-RC 10/$1.00 min order
(4) 100 ohm, 1/4W, (7) 1 K ohm 1/4W, (1) 1.5 K ohm 1/4W, (1) 2.2 K ohm 1/4W, (1) 4.7 K ohm 1/4W
(5) 10 K ohm 1/4W, (7) 22 K ohm 1/4W, (1) 47 K ohm 1/4W, (2) 100 K ohm 1/4W, (4) 150 K ohm 1/4W
(1) 470 K ohm 1/4W, (2) 1 MEG ohm 1/4W
(3) 10 pfd NPO 140-100N2-100J-RC ($0.09 each) (0.18)
(6) 22 pfd NPO 140-50N2-220J-RC ($0.06 each) (0.36)
(1) 33 pfd NPO 140-50N5-330J-RC ($0.06)
(4) 47 pfd NPO 140-50N5-470J-RC ($0.06 each) (0.24)
(2) 150 pfd C0G 80-C315C151J1G ($0.20) (0.40)
(1) 330 pfd C0G 80-C315C331J1G ($0.20)
(1) 70 pfd trimmer 659-GKG70015 ($0.20)
(5) .001 ufd 140-50Z-102M-RC ($0.07 each) (0.35)
(6) .01 ufd 80-C315C103M5U ($0.12 each) (0.72)
(13) 0.1 ufd 80-C315C104M5U ($0.09 each) (1.21)
(4) 1 ufd/25V 140-XRL25V1.0-RC ($0.06 each) (0.24)
(3) 10 ufd/16V 140-XRL16V10-RC ($0.06 each) (0.18)
(1) 330 ufd/16V 140-XRL16V330-RC ($0.11)
(1) 5.6 uhy RFC 43LS566 ($0.41)
(1) 18 uhy RFC 434-23-180J ($0.20)
(1) 5.0688 MHz crystal 520-HCA506-20X ($0.46)
(2) 9.000 MHz Crystal 520-HCU900-SX ($0.53 each) (1.06)
(1) FT42-37 $5.00/25 min order from kitsandparts.com
(2) T37-6 $5.00/25 min order
73, KD1JV Steven Weber 1/07/2008 revised 1/18/2008