What is PSK31
Contents
1. A bright yellow bar with the red diamond shaped cursor in it is a PSK31 station To the left and right of the station select cursor are other PSK31 stations Some appear weaker and others will display brighter which indicates stronger signals Although some signals in the display appear weak they will still print perfect copy The most recent receive signals are displayed at the top of the PSK software waterfall display To the left of the cursor is another PSK31 station that is almost too weak to copy 1000 2000 1 i i Figure 3 Frequency is plotted horizontally and time is plotted vertically with the earliest at the bottom Strength is indicated by color increasing from blue to yellow To tune and receive any of these stations simply point to the signal with the mouse and click The diamond shaped cursor will jump immediately to the center of the station signal and the text from the station will begin to appear on screen in the receive window You can read the IMD of the selected station on the lower feature bar in most PSK software programs A good signal and ideal IMD is between 22 and 32 See Figure 1 In order for the program to provide a panoramic display with point and click tuning of stations it is necessary for your transceiver to provide both panoramic reception and transmission The receive band pass should be capable of receiving a large number of PSK31 stations at one time Hence the transceiver2. to receiver speaker j 3 3 uF 35V tantalum cap a i g A Eg 48 oam 1k ohm transformer J ga Pts 4N25 or Wgg0eeg00 ohm equivalent aa rmer 1N4148 J S j 47k diode R mS gt 1k pot lt v v y d COM port TX EED mien METTE m o E to soung Gara gt N INA Figure 10 Inside view of components My design is one cobbled together from a couple of circuits found on line some years ago It has an added LED to indicate transmitter activity The technology may be termed epoxy style The transformers are inverted and glued with standard two part epoxy the 5 minute stuff wouldn t hold and the opto socket glued to the side of one of them The plastic case prevents common grounds on the input and output jacks negating the transformer isolation 12 From The N3Uj COM Document Library Problem with Sounds Shut off those nice Windows sounds by going to Control Panel Sounds and Audio Devices Sounds and selecting No Sounds under Sound Schemes You really don t want to have them going out over the air every time you hit a wrong key or an e mail comes in Alternative Rigs If you don t own an HF rig then a low cost alternative is one of the single band rigs from Small Wonder Labs http www smallwonderlabs com in New Hampshire costing 100 each and 30 extra for the case If you just want something to experiment with they have a very inexpensive 80M kit called the Warb
3. 8125 From The N3Uj COM Document Library Software All of the brains for PSK31 are in your computer The transceiver provides a way to receive and transmit the signal but the filtering decoding and encoding uses the power of your sound card and the magic of software The following lists various software programs available for PSK31 Some also include other PC radio communication modes Some are free others are commercial and cost money Probably the most popular program is DIGIPAN DIGIPAN htto www digipan net Manual for DIGIPAN http kk7ug com html manual htm HAMSCOPE http www qsl net hamscope HamScope html MIXW http www mixw co uk product productmixw htm WINPSK htto www qsl net ae4jy winpsk htm WINWARBLER http www dxlabsuite com winwarbler W1SQLPSK http www faria net w1sql download htm Before you go on the air be sure you read the user s manual or the help files relating to the software you plan to use Hardware Several things are necessary for you to be able to operate on PSK31 The first you already have if you re active on HF your transceiver If you don t have an HF rig then see the section at the end Alternative Rigs Next you ll need an interface something you can build yourself or buy see Where to Get an Interface at the end of this paper Third is a computer theoretically anything running Windows 95 will do but realistically a Pentium 266 or faster with Wi
4. EZ PSK http www saratogaham com ezpsk Additional Software sites http www packetradio com freeware html http www qsl net wm2u psk31 htm Other information Article by Peter Martinez G3PLX _http det bi ehu es jtpjatae pdf p3 1 g3plx pdf Radio Amateurs of Canada links to digital modes of all kinds http Awww rac ca opsinto infodig htm From The N3UJ CQM Document Library Homebrew your own Part values are for the most part non critical and may be varied to fit whatever you have on hand as you will see from the examples that follow 1 1 600 ohm Audio Transformer 1 1 600 chm 1K Audio Transformer Rig Mike In Rig PTT In Figure 5 Typical interface From http www waypoint com users discobay Sound_Card_Presentation htm This one uses a fixed attenuator depending on the software to adjust To sound card Line In to computer serial port P2 To sound card Line Out To computer speakers sleeve tip O P4 Radio Ground Radio PTT Audio out Audio ground To Radio speaker jack Figure 6 Typical interface This one from http www jbgizmo com page28 htm Has pcb layouts and parts list This design has adjustments in both receive and transmit audio legs From The N3U COM Document Library Line In Tip of plug Audio Out Ti Ground input Ground Level Isolation M Transformer Line Out IK2 2 2uF 50v Tip of plug z l Mic Ground Ground
5. Led 1K Isolation 0 01uF Transformer Pin7 RTS 9 Pin We Com or 2 0 01uF IN4148 i Pin 5 a PTT NC J Ground 0 01uF Optocoupler Computer Radio Figure 7 Typical interface Schematic from http www hamtechnet com techtips This example has the adjustment in the transmitted audio path along with some bypass capacitors for RF on the COM port on the microphone side and the push to talk PTT circuit It also has a LED light emitting diode to indicate when the transmitter is keyed Still another site with a schematic and an article describing the hardware in pdf format is http www sparetimegizmos com Downloads Sound 20Card 20Buddy pdf As you can see the circuits designs are fairly standard with two transformers some sound level adjustment attenuator either on both sound from and to rig or just on the sound to the rig s mic circuits and an optocoupler of the 4N25 4N33 or equivalent series You could go cheaper by not using transformers and substituting a transistor for the optocoupler However you risk possible damage to your rig and a strong possibility of hum and noise which equates to a crummy signal 11 From The N3UJJ COM Document Library Mine Mine Line In Tip of plug Audio Out Ti Ground Taput Ground Isolation Transformer Line Out 3 3uF 35v Tip of plug Mic Ground Ground Led 1 2K Isolation Transformer Figure 8 Cover view to transmitter 4 nt ie microphone inp
6. What is PSK31 As the name implies PSK phase shift keying modulates the phase of a carrier and the number 31 references the actual bandwidth 31 Hz occupied by the PSK31 signal The software that first implemented PSK31 with a Windows PC and soundcard was written and developed by Peter Martinez G3PLX Two of the most significant features that make this the ideal mode for digital communications are first the extremely narrow band width and second the high immunity to noise and QRM PSK31 Window FB s d were on idle was 25db very good Yes your beam certainly make evid ence My I 1 3 6 6 7 8 _Tx _Spectrum Play oq fase Replca TnxRST senke ma Rx 1695 Rx2 1894 Tx 1695 IMD 140B Figure 1 PSK31 computer display showing frequency spectrum on the left and waterfall display on the right Phase modulation has more advantages than CW morse code CW uses amplitude On Off keying In a noisy or distorted propagation environment the amplitude of a signal will shift and vary much more than the phase of a signal When compared to CW PSK31 is a much more resourceful and robust operating mode The baud rate used by PSK31 is 31 25 baud This is fast enough to handle most operators manual typing capabilities a speed of about 50 words per minute It is intended as a means of keyboard to keyboard communication between two or more operators using a very small amount of frequency spectrum By comp
7. aring the small bandwidth of PSK31 and measuring its gain against a CW filter of 500 Hz 10 log 500 31 dB 12 dB quickly reveals that a CW transmitter must put out 15 to 18 times more power than a PSK31 transmitter just to achieve the same signal to noise ratio at the receiving station This means lower power transmitters and smaller antennas may be used with good results Fifteen to eighteen times relative to 100 watts CW translates into 5 7 watts PSK From The N3UJJ COM Document Library The Background and Philosophy of PSK31 The following is by Peter Martinez G3PLX developer of PSK31 PSK31 is the result of my belief that the present batch of data modes have left a gap in amateur radio operating the gap that was previously filled by AMTOR or even traditional RTTY in which two or more operators chat to each other on an open channel Modes such as packet radio Pactor and others are highly complex are unsuited to multi way conversations and in particular the long block lengths introduce an unacceptable delay in the processing of text such that even normal conversation is unpleasant and quick break question answer sessions are impossible The move to automated unattended message forwarding has left a gap in the person to person communication field and PSK31 is an attempt to remedy this situation with a simple but efficient code structure coupled with the narrowest possible bandwidth and with only enough error correction to m
8. atch typical typing error rates and with no time consuming synchronization changeover and ARQ processes The 31 baud BPSK modulation system used in PSK31 was introduced by SP9VRC in his SLOWBPSK program Instead of the traditional frequency shift keying the information is transmitted by patterns of polarity reversals sometimes called 180 degree phase shifts This process can be thought of as equivalent to sending information by swapping over the two wires to the antenna although of course the keying is more usually done back in the audio input into the transceiver A well designed PSK system will give better results than the conventional FSK systems that amateurs have been using for years and is potentially capable of operation in much narrower bandwidths than FSK The 31 baud data rate was chosen so that the system will handle hand sent typed text easily There is a problem with PSK keying which doesn t show up with FSK and that is the effect of key clicks We can get away with hard FSK keying at moderate baud rates without generating too much splatter but polarity reversals are equivalent to simultaneous switching off of one transmitter and switching on of another one in anti phase the result being key clicks that are TWICE AS BAD as on off keying all other things being equal So if we use computer logic to key a BPSK modulator such as an exclusive or gate at 31 baud the emission would be extremely broad In fact it would be ab
9. er by using the COM port on your computer or by sound operated switching built into the interface to switch the rig as the VOX circuits are too slow digitally speaking Less Is Better Than More Many of us will be using transceivers that require using the microphone connector for PSK31 input If this is the case and you plan to drive the microphone input with your sound card LINE OUT then the level from the sound card should be comparable to the output level of your microphone While many Sound Blaster compatible sound cards have the capability to be set for high or low level outputs these outputs are almost always much too high for the input level to the microphone port of your transceiver From The N3Uj COM Document Library If you cannot use the sound card Control Panel settings in Windows or your PSK31 software to decrease the output level of your sound card to meet the level requirements of your microphone input then you should use a resistive attenuator in your hardware interface to reduce the LINE OUT level to your transceiver MIC input The idea is to keep your audio signal at a low enough level to have a clean PSK31 signal on the air PSK31 Display PSK31 programs use digital signal processing DSP to perform very narrow filtering and then decode the resulting sounds into characters on the receive screen The display shown at figure 3 is how a typical PSK software spectrum panoramic waterfall window appears
10. filter setting of 4 KHz or greater for receive The effects of the transmitter filter will be seen in falling power output levels as you approach the filter edges From The N3UJJ COM Document Library 15m TARA Rumble Cortest DOL Hello R 599 599 HR Carlos Carlos CT1 CT1 BTU de CTIBQH KN STIBOH DE WOOL TU Carlos Name is Mark Mark OTH is CO CO GL in test 73 TIBQH DE WIOL SK Figure 4 A screen shot showing various PSK31 signals with copy on the trace with the diamond at the top The numbers at the top of the signal traces 1000 2000 amp 3000 indicate Hertz Hz relative to the receiver frequency From The N3UJ COM Document Library Where to Get an Interface The following are not endorsements for any particular brand or product merely a resource for your own use when searching for the interface that suits you best West Mountain Radio RlGblaster http Awww westmountainradio com Tigertronics SignaLink http www tigertronics com MFJ 127x series of Soundcard to Rig Interfaces http Awww mfjenterprises com Rascal Soundcard interface kits and more http www packetradio com Be aware that there have been complaints about parts quality and support from this company WAS8LMF Interface http members aol com wa8lmt ham tonekeyer htm ProData Interface http www ham kits com ProData1 htm MicroKEY USB Interface uses USB instead of serial port http www microham com mk html Saratoga Ham
11. ler for 49 I ve bought several qrp rigs from them over the years and find their service fantastic Look for good deals on used HF rigs Even slight vfo drift problems are compensated for by the automatic frequency control AFC feature in most PSK software Contents of CD This article What Is PSK31 both in Word format and Adobe pdf format Article by Peter Martinez G3PLX digipanzo 0 Docum i DigiPan Setup i ii DigiPan Tean ss hamscopev 154 Getting Started with Digipan Obe Acrobat 7 0 Docunient HamScopev 154 if a TPES 645 KB A EME UK Limited wisqlpsk5 winpsk213 3 774 KB 642 KB SE winpskuser211 t F Adobe Acrobat 7 iobe Ac i Pom 80 A WinWarbler Adho 0 Document a oS Ss 10 KE w WinWarbler Download and Installation Microsoft Word Document 13 From The N3UJJ COM Document Library
12. nd watch the power meter As you type the text you are sending should cause the meter pointer to have a slight jitter If you have the LINE OUT MIC IN level set properly you should see the power move slightly upwards At no time should you see the power level go above 50 of your transceiver s rated power output If it does go higher simply turn the Transmit Drive level down using the transmit level command in your software At no point should you see any ALC activity on your transceiver If you do be aware that you are creating unnecessary distortion Once the levels are set on your transceiver you should not have to touch the rig s controls again All future levels are set into and out of the sound card using the volume and wav controls built into the sound card control panel software In DigiPan these controls are easily accessed from the task bar at the top of the screen Simply click on configure and use the Transmit Drive see fig 2 command to properly drive the transceiver so that no ALC action is observed This avoids causing phase distortion or overdriving the inputs of your transceiver From The N3UJ CQM Document Library Select Transmit Drive k 4 3 U y Figure 2 Setting transmit drive Vox In PSK31 and other digital modes we do not use the VOX voice operated relay circuits in our transceiver to control changeover from transmit to receive and back Switching is done eith
13. ndows 98 or higher is better A fourth would be an antenna of some type This will be determined by your location but as always should be as high and in the clear as much as possible Having said that even attic antennas will work with PSK From The N3UJ CQM Document Library Transceiver Settings Be sure all speech compression is OFF Set the mode to single sideband USB Transceiver receive filter set to 4 KHz or higher if your transceiver will allow this independent of the mode setting Remember that the serious filtering is done by the sound card and software However there may still be problems caused by a strong signal within the pass band of the rig s filter affecting the AGC Turn the receiver AGC off if possible Open a window on your computer with the PSK software of your choice Connect a watt meter and dummy load to the output of your transceiver click on the PSK software TX menu button and adjust the speaker volume slider on the Windows taskbar tray for an output power reading of about one third to one half the power you would normally see when operating CW Transmitters these days for the most part are not designed for 100 duty cycle and digital modes like PSK31 and are on all the time you are transmitting like holding your key down continuously on CW DO NOT operate your transceiver at more than half its maximum power level rating Once you have the power level set type a few characters a
14. out 3 times the baud rate wide at 10dB down 5 times at 14dB down 7 times at 17dB down and so on the square wave Fourier series in fact The solution is to filter the output or to shape the envelope amplitude of each bit which amounts to the same thing In PSK31 a cosine shape is used To see what this does to the waveform and the spectrum consider transmitting a sequence of continuous polarity reversals at 31 baud With cosine shaping the envelope ends up looking like full wave rectified 31Hz AC This not only looks like a two tone test signal it IS a two tone test signal and the spectrum consists of two pure tones at 15Hz from the center and no splatter Like the two tone and unlike FSK however if we pass this through a From The N3UJ CQM Document Library transmitter we get inter modulation products if it is not linear so we DO need to be careful not to overdrive the audio However even the worst linear will give third order products of 25dB at 47Hz 3 times the baud rate wide and fifth order products of 35dB at 78Hz 5 times the baud rate wide a considerable improvement over the hard keying case If we infinitely overdrive the linear we are back to the same levels as the hard keyed system There is a similar line of reasoning on the receive side The equivalent to hard keying on the receive side is a BPSK receiver which opens a gate at the start of a bit collects and stores all the received signal and noise du
15. ring the bit and then snaps the gate shut at the end This process gives rise to the receive side equivalent of key clicks namely side lobes on the receiver pass band So although this integrate and dump method is 100 efficient in the task of sorting out signal from noise it will only reject signals by 10dB at 3 times the baud rate wide and so on the same spurious rejection figures that we got as spurious emission figures for the transmit side The PSK31 receiver overcomes this by filtering the receive signal or by what amounts to the same thing shaping the envelope of the received bit The shape is more complex than the cosine shape used in the transmitter if we used a cosine in the receiver we end up with some signal from one received bit spreading into the next bit an inevitable result of cascading two filters which are each already spread by one bit The more complex shape in the receiver overcomes this by shaping 4 bits at a time and compensating for this inter symbol interference but the end result is a pass band that is at least 64dB down at 31 Hz and beyond and doesn t introduce any inter symbol interference when receiving a cosine shaped transmission PSK Frequencies Band Digital Frequencies kHz 160 meter 183815 80 meter 3575 3585 3620 3640 40 meter 7060 7080 30 meter 10130 10145 20 meter 14065 14090 17 meter 18100 18110 15 meter 21060 21090 12 meter 24920 24930 10 meter 28110 2
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