Negative voltage generator. I originally designed this circuit in order to repair an Icom 701. It is a fairly standard application of the ubiquitous 555 chip. The output is dependant on the supply voltage and the load, you will not get -9v out with 10v in for example. There is no such thing as a free lunch, and the diodes drop some volts. Replacing silicon diodes with Schottky types will improve things a little. With a bit of imagination it can be constructed in a very small volume.
The available current is quite small, about 20mA maximum into a 500R load. Efficiency around 30%. If I recall correctly, the 701 needed just a few uA, so well within the capability of this circuit.
Having recently purchased an SSB i/f PCB that needs -ve volts on the mixers and op-amps, I searched my notebook for
the schematic and made an electronic copy on my Samsung tablet, using the amazing app, SCHEMATIC.
This is a work in progress., and some progress has been made
Due to information from the Yahoo group and others (see below) I have updated some capacitor values, and added a capacitor that I omitted. There was a resistor missing from the original drawing that I worked from, this has been corrected. I have added the test-point numbers, and a couple of notes about the audio filter.
I purchased this board from eBay. The vendor was based in Israel. At $37 NZ I considered it to be good value, as the circuit included 3 Crystal filters and 3 mixers. Searching Google produced so little information I was concerned that I would have to reverse engineer the board in order to produce a schematic. I eventually found a German language site ( forgotten the address ) with a link to a drawing. It appears to have been done in Eagle Lite. There were no component numbers or values. Some reverse engineering was going to be needed after all.
I spent some considerable time working my way through the circuit and identifying the component values. I sincerely hope that there are not too many mistakes. The drawing would be very cluttered if the part numbers were included, so I have omitted them, except for the values around the LPF. A couple of the transistors were oriented wrongly in the drawing, I have corrected them in my schematic. I have also separated the AGC and the AUDIO modules from the rest of the circuit, along with the voltage regulator. This makes it a little less confusing to my eye. I hope this works for you too.
The signal input is on pin 16, where it passes through a couple of resistors before being split to the FET and the Low Pass Filter. The FET circuit buffers the signal, rectifies it and routes the resulting DC to pin 13. Purpose ? Unknown.
Following the LPF, the signal is coupled into the mixer by a 22uF capacitor. Yes 22uF. This is a Tantalum part, as are most of the polarised capacitors on the board. Top left corner of the pic. Something tells me this is not an RF signal. The output of the mixer is undoubtedly DSB at 9MHz. The following ( roofing ? ) filter is AM bandwidth at 9MHz. So the local oscillator input at pin 10 could well be a carrier, and the signal could be audio. The DC at pin 13 then maybe acts like a VOX or squelch circuit.
Following the AM filter the 9MHz I/F is amplified in the MC1350. It then passes to both SSB filters, only one is selected, then to the second mixer.
The stages following the second mixer are at audio frequencies. The required sideband is selected by the voltage level on pin 7 which gates the 4016. So only one of the 2 sideband mixers or filters, is actually used at any one time. The CIO or BFO is applied via pin 9. This must surely be at 9.000MHz in order to demodulate the SSB signal from the filter.
The audio is passed to an Op Amp filter circuit, it looks like bandpass. Comprised of high pass / lowpass. Then the signal is routed to the 600 Ohm output transformer at pins 5 and 6. Pins 1,2,3 and 4 carry the same signal at different impedances along with a couple of grounds. A sample of the audio is also fed to the Plessey SL1621 AGC generator. The capacitor values around this device are approx 50% of those in the data sheet. This would make the AGC response quite fast I believe.
The overall circuit had me confused for a while. The Yahoo group has certainly helped by proving that the board receives SSB. Not so sure about AM. AGC is applied to the MC1350. So the signal level must be expected to vary, as we expect in a receiver. The switches in the AGC circuit appear to be able to select normal, delayed or off. I would have expected the AGC to also drive an S meter, but there is nowhere in the circuit to sample the AGC for indicating purposes, that has been brought out to a connection point. Test point three looks like a good candidate.
I have had the board powered up, but no signals yet. The negative voltage generator measured 9.5v off load and 6.5 when connected to the PCB. Not enough current available I assume. A better solution is needed.
Ultimately some modifications around signal pin 16, changing the 22uF capacitor and removing the LPF would enable the first mixer to accept RF. The Carrier input would then be the LO. It might make an interesting receiver.
If anyone has made any more progress than this I would be more than happy to hear from you. gw4sae at AOL dot com
Thanks to Chuck K8LBH for the link to his YAHOO group, they are actively building radio’s using the PCB.
Having recently acquired an FT-817 I was interested in trying it in WSPR mode. It didn’t take much research to discover that the DATA connector on the rear panel has bi-directional audio, labelled as data. PTT is also available on the same connector. Unfortunately there is no VOX capability when using this port.
I already had a couple of USB soundcards ( $5 on eBay ) so I decided to utilise one of them as an audio interface to the USB port on the laptop. I removed the USB plug and replaced it with a socket. A standard USB to USB connects the interface to my laptop. The audio jacks were also removed and the mini DIN was wired directly to the exposed pads.
Since the soundcard can produce a few volts of audio during transmit, I decided to use a simple voltage doubler to convert some of the audio signal to DC for the transistor switched PTT control. The audio is delivered directly to the voltage doubler, but is potted down for the transmit drive to the radio. A series resistor also helps protect the mic input of the soundcard. You should set the mode to DIG.
To my surprise, the whole thing worked first time and spots from Australia and USA were reported during the first 500mW transmit cycle. WSPR really is an amazing mode. The most difficult part was getting it in the box, neatly !
The box is the smallest from Jaycar. Shown without the lid. The braid simply bonds all the ground points with a low impedance path.
Took a little time to get the layout ‘safe’, so that none of the parts touched.
I am forever tinkering with circuits of one kind or another. Usually receivers. Etching a PCB for a one off project is too time consuming and hardly cost effective. On the other hand, strip-board is not the best medium for RF construction. I am also not a fan of “ugly construction”. So for many years I manually cut squares and oblongs onto my PCB’s using a craft knife and a steel straight edge. Then I discovered ‘Me Squares’ and ‘Me Pads’. My first few project using these pads took up more space than I had bargained for as the pads were spaced out to accommodate the components. Also it was a bit messy getting just that tiny amount of super glue on the pad. Then one day I accidentally knocked over a half full bottle of glue. It was difficult to put things down for a while !
Later, while building a classic post mixer amplifier, I drew the pad layout on squared paper. The individual pads were just a few mm apart. I quickly realized that if I stood some of the resistors upright I would not need to separate the pads at all. A little further planning and I had the whole circuit module on pads, in one piece. I recommend spending a little time on this, as the result is well worth the effort. Just follow the schematic layout from left to right. Try to get at least one grounded component on each side of your module.
This example uses the 8 pin audio amplifier the LM380-8. Here is the schematic.
Here is the module built on pads and squares.
Here is the same module mounted on the PCB. There are usually sufficient components going to ground that NO GLUE is needed to hold the module down to the copper substrate.
The grounded components are holding everything in place. I’m sure there will be the odd occasion when a pad will need to be glued down. For me this is a last resort, and I now use double sided adhesive tape if really needed.
Most of the projects posted here are constructed in this way. You should try it, it really works well, and it undeniably look good.
There was an error in the feedback resistor value on the schematic near the NE5534. It was shown as 1k5. This resistor sets the gain of the op-amp stage and should be 220k, giving a gain of 146, or 43db.
As shown in the heading pic, the transmitter will interface to the previously published receiver, via the changeover relay, which is VOX operated. The audio from the PC, or laptop, is buffered by the external USB sound-card. Since the computer does all the timing and data generation via the WSPR software, the only thing that needs to be taken care of is that the PC clock must be accurate. There are a number of applications available that will synchronise your PC clock to an atomic standard every time you start your system. Here is a good place to start http://www.worldtimeserver.com/atomic-clock/
This will ensure that your WSPR activities are within the standard 2 minute time slots. I recommend setting your transmission percentage to an odd value. This will tend to randomise your 2 minute listening and transmission slots over a period of time.
There doesn’t appear to be much merit in selecting a variable voltage regulator for this circuit. However, my initial design incorporated a FET PA which needed a bias voltage. Not having had much luck, or any experience, with the FET design, I resorted to a pretty standard bipolar circuit. Just set the output voltage to 8v before connecting the mixer supply Pin 8, to this supply node. The mixer device will surely destroy itself if you exceed 9v supply.
You can of course simply fit a fixed voltage regulator from the LM78xx 100mA range. Anything between and including 5v and 8v should work fine.
Audio Input and Mixer.
The WSPR audio input to the transmitter is controlled by the power slider control in the WSPR software. This control should be set to 75% to start with. I found the signal at the transmitter input, which is also connected to the VOX input, to be near 1V p-p. You may find a different level depending on your sound-card capability.
The preset pot at the mixer input should be set for 200-300mv on pin 1. If you can’t measure this voltage, set the pot to the ground end. Later, during the transmit cycle, adjust the pot until the signal can be seen in Spectran, or heard on your receiver. With no audio there should be no output as there will be no sidebands. Increasing the signal to more than about 300mv will not improve the output power and may well lead to distortion.
The 1nF capacitor is insurance against any RF getting into the audio port of the mixer, along with the audio, and generating unwanted products in the output.
The audio arriving at the transmit mixer, is ‘mixed’ with the oscillator frequency. The oscillator frequency is determined by the crystal, and the other components at Pins 6 and 7. In this case it is 10.140MHz. The NE602 ( SA612 etc. ) data sheet shows the formulae for calculating the values. They are not too critical so I used the nearest preferred values. The addition of TC1 and L1, enables the frequency to be ‘pulled’ slightly, in order to put the transmitter exactly on the 1500Hz marker in Spectran. See Alignment section.
You can of course fit almost any crystal here and by changing the LPF components you will move the transmitter to another band. Replacing the crystal with a VFO or synthesiser will also make it multi-band, again depending on the LPF components. This would be a great opportunity to construct a versatile CW transmitter.
The mixer output at Pin 5 produces the DSB signal that we will use. With 7.5v supplied to the mixer, the output pins carry in excess of 6v DC in my prototype, so a capacitor is used to couple the signal into the gate of the driver FET. There is no need to match the output of the mixer to the gate impedance of the FET, as we are trying to transfer maximum signal voltage not power. The source resistor provides some 2v of gate bias via its voltage drop, and is bypassed at RF in order to maximise the signal gain. You should find over 2v p-p at the drain.
The drain transformer is bifilar wound and is connected as a centre tapped transformer providing a 4:1 impedance transformation whilst feeding RF to the base of the output transistor. I experimented at length with the number of turns and various tapping points. There was little difference in the output, unless I chose ridiculous ratios. However, using a 22uH choke for the drain load and coupling with a capacitor reduced the output considerably. So I stayed with the bifilar solution.
The PA is broadband and runs in class AB, biased by 680R and the 1n4001 diode. Not the best bias configuration, but more than capable at this power level. A ferrite bead on the base leg guards against possible VHF parasitics which destroyed a couple of earlier devices.
The output devices may be any of a number of common NPN types. My choice of 2N3904 was determined by the fact that there were at least 100 of them in my spares box, and I got through quite a number of them. I experimented with a couple of other transistors before I realised there was a spurious VHF problem. As previously mentioned a ferrite bead cured that. I tried various resistor values in the emitter circuit, and reached the conclusion that, in this configuration at least, 4R7 is a good compromise between power output and thermal protection. At 2R2 the transistor gets hot. At 10R and higher, the output power naturally reduces.
A small heatsink is recommended. I used a 2 small piece of scrap tin plate formed around the transistor and soldered to the PCB. The whole heatsink assembly is about half the size of a postage stamp. It gets warm during a 2 minute transmit cycle. If your device runs too hot then increase the surface area of the heatsink. Or raise the value of the emitter resistor at the cost of a reduction in output power.
The PA collector load is a trifilar broadband transformer. The 9:1 ratio transforms the collector impedance hopefully a bit closer to 50R in order to match into the following Low Pass Filter. Again I tried a number of different ratios here, and also a bifilar transformer as in the driver stage. The trifilar configuration appears to offer the best performance.
The LPF at the output is essential for removing the unavoidable harmonics. This filter is of a standard design, I have selected the values in order to use off the shelf components. This means that the LPF impedance is not actually 50R, but it is fairly close. At 1uH the -3db corner frequency is 10.7 MHz, the response is 40db down at 21MHz.
If you roll your own, the toroids should be FT37-6 wound with 18 turns. The response now will be very slightly higher due to the fractionally lower inductance of ( theoretically ) 0. 97uH.
I believe there is a 5% manufacturing tolerance with these toroids so minor manipulation of the number of turns may be required. i.e. If the measured output is considerably lower than 12v p-p or 400mW, remove one turn and measure the output again. In the vast majority of cases there should be no problems here.
This is simply an audio amplifier with a gain of 10, followed by a rectifier / doubler circuit. The electrolytic capacitor determines the ‘hang’ time and the resistor fixes the maximum base current to just over 1mA. Almost any 8 pin DIL op-amp will work here. Also just about any small signal NPN transistor that can handle the relay operating current. A shocking 40mA in my case. Pun unintended.
Download and install the free software, Spectran. http://www.sdradio.eu/weaksignals/spectran.html
Or use your favourite Spectrum viewer.
Check that there are no low resistance paths from all supply pins to ground. Ensure the supply polarity is correct. A couple of 100R 1/2 Watt resistors, wired in parallel across the output of the LPF will be a suitable 50R dummy load.
The amplifier may not withstand an open or short circuited load for any length of time, so you should ensure that either a 50R dummy load or a suitable resonant antenna is attached at all times when in transmit mode.
Connect a multimeter in series with the supply, switched to a suitable current range. Expect to see between 40mA and 50mA consumption at power on. The bias voltage at the bases of the output devices should be very close to 0.7v.
The simplest way to align the transmitter is to connect the audio from the PC sound-card, to the PCB, and run the WSPR software. This should have previously been setup with your callsign and locator. Select 50% transmit. As soon as the transmit cycle begins, the VOX relay should switch and the Tx LED will illuminate. Then monitor the transmission in Spectran via a receiver connected to another PC. Alternatively, if you have a tablet, there are apps available for IOS and Android devices that will allow you to drive the transmitter with WSPR data. I borrowed my wife’s laptop for this purpose. ( Thank you Maria )
Adjust the crystal oscillator trimmer, CT1, until the received signal appears at the 1500Hz marker on the Spectran waterfall. Close Spectran and now launch the WSJTx software, on the second PC, and check that you are receiving your own signal.
If you don’t have access to a second PC then you can unbalance the mixer by keying Pin 1 to ground via a 10k resistor. This should be removed after setting up.
Keying the mixer input pin produces a carrier at the crystal frequency. A simple crystal controlled CW transmitter results, this can easily be monitored on an oscilloscope for clean waveform and amplitude. Or you can listen for it on an SSB receiver tuned to the carrier frequency, NOT the WSPR frequency, which is 1500Hz higher due to the sideband generation. A receiver and monitoring software such as Spectran, are used as above. You will need to tune 1500Hz lower on the receiver because there is no USB to demodulate, just the carrier. Switch in your attenuator, as the local signal is quite strong. Over 12v p-p should be observed on the scope, or more than 400mW into 50R at 110mA or so.
You will still need an audio source in order to test the action of the VOX circuit. With 1v of audio drive, the output of the op-amp is 8v p-p clipped sine wave, and there will be at least 7v DC after the voltage doubler. The 4k7 resistor in the VOX transistor base circuit allows about 1mA or so of base current. With a nominal gain of 100 there will be more than enough collector current to drive a relay.
Current consumption during the transmit cycle should be in excess of 100mA, or about another 70mA more than standby. Indicating about 850mW power input with a 12v supply.
You should find around 18 to 20v p-p on the ‘scope, at the collectors of the PA. This will include lots of harmonics. After the Low Pass Filter the output power is in excess of 450mW, or 13v p-p, indicating about 55% efficiency.
450mW doesn’t sound like much power but believe me worldwide coverage is certainly possible. CW enthusiast have known this for a long time. On the first evening during a short test period, a couple of USA stations and some Aussie neighbours spotted my little signal from ZL.
If you have previously built the receiver, you can at this point wire the Rx and Tx modules together via the relay, as shown in the VOX schematic. Note that the VOX circuit is always powered, but that the receiver and transmitter are powered up depending on the state of the relay, which itself depends on the WSPR software tx/rx cycle. The relay also transfers the antenna to the correct board.
Connect the USB sound-card to your PC and set up the WSPR timing to your preference.
A suitable antenna cut for 30m is recommended. Such as a half wave dipole or a quarter wave vertical, with radials or groundplane. You don’t want to waste any of that meagre power in antenna losses through a random length of wire, while transmitting, using this exciting and interesting weak signal mode.
Joe Taylor K1JT invented WSPR and other weak signal modes. My thanks go to him for my renewed interest in ham radio.
Ver 1.2 Corrected some spelling mistakes. Added better image of receiver. Updated schematic.
Having developed an interest in WSPR last year, I purchased a very neat transmitter kit from QRP-LABS. The Ultimate3S. Mentioned elsewhere on this site.
( WWW.qrp-labs.com )
The popular kit worked as soon as it was powered up. After a couple of days of successful ‘beaconing’ on 30m WSPR, and despite owning a couple Elecraft transceivers, the urge to construct a suitable WSPR receiver overcame me and I began researching the project in earnest. Most designs seem to be direct conversion architecture mainly based on the ubiquitous LM386 and NE602 et al. I built a couple of these designs and wasn’t happy with the performance. The 602 is not known for its signal handling, and the LM386 is another problem area as it is usually pushed to its limit in an effort to simplify the circuit. I would happily trade simplicity for proportionally better performance.
The 602 is an elegant part even with its limited performance. This got me thinking about a really narrow Band Pass Filter in the antenna circuit, thus limiting the energy reaching the mixer input. I decided that a narrow bandpass crystal filter would be ideal. A superhet design, although more complex, would give single signal reception, avoiding the usual image problems associated with direct conversion, apart from other known issues. I was surprised to find that some Elecraft QRP rigs use the NE602 as a receive mixer, and even the very well respected K2 uses it as a product detector. Suitably encouraged I pressed on.
I already had a number of 10.140MHz crystals and hundreds of others. While checking through my collection of crystals, in order to select a suitable I/F to work with, I found some 10.7 MHz parts. A quick calculation determined that the VFO or local oscillator would need to be 560 kHz. Oscillator stability shouldn’t be a problem at that low frequency. Something about the frequency jogged my memory about remote controls for TV, Hi-Fi and such devices. A quick search of the web revealed some 560kHz ceramic resonators for sale on Ebay. The seller was in Germany. A pack of 5 of the 560kHz parts were ordered that same day. Shipping cost was reasonable, and the parts arrived within a few weeks. In the meantime a number of circuit designs were sketched out.
I reworked the circuit topology on paper and decided to use the resonators in the I/F and also in the Carrier Insertion Oscillator driving the product detector. The 10.7 crystal would then be used as the local oscillator. This was beginning to look interesting.As soon as the parts arrived I quickly determined that the NE602 would oscillate well with the resonator,using a small inductor and a trimmer capacitor.
The first part of the circuit that was built is shown here. The product detector and Carrier Insertion Oscillator based on a SA612A. Applying a 560kHz signal from my home-brew synthesised signal generator, to the mixer, produced an audio tone in the high Z earpiece that I used for monitoring the very low level output from pins 4 and 5.
The inductor is 470uH and the capacitor is 15pF fixed, in parallel with a 60pF variable. These are parts that were available at the time and a better balance of values should be selected. The oscillator could easily be ‘pulled’ high or low as witnessed by the changes in audio tone.
I/F Phase splitter.
The novel part of this circuit is the phase splitter feeding the input pins of the product detector. I have not seen this done before and I’m not sure if it is more effective than a transformer. However it appears to work fine. This takes the single ended I/F signal and drives the mixer in push-pull. The source and drain resistors are 1k5 in an effort to match the input impedance of the mixer. I removed the internal capacitor from a 455 kHz I/F transformer ( yellow core ), then measured the inductance and from that calculated the required capacitor value. Despite my efforts I could not peak the signal. I then accidentally discovered that the transformer was self-resonant at 560kHz, so no capacitor was fitted. I then easily found a peak in the signal, which was audible even off tune. The FET gate resistor is 1M, making the input very high impedance and less likely to affect the transformer circuit preceding it. An inductor and capacitor were selected as a 850kHz LPF, and fitted between the transformer and the gate of the FET. This removed a 1MHz approx, spurious signal that I could see on the output pins when monitoring with an oscilloscope.
The first mixer.
This has the 10.7 local oscillator used to convert the antenna signal to the 560kHz I/F. The output of this stage is DC coupled into the base of the emitter follower I/F stage. The emitter resistor is 1k5, in an effort to match the impedance of the mixer. The 560kHz resonator is used as an I/F filter, driven from the emitter circuit, and feeds the low impedance winding of the I/F transformer. I later added another resonator. See the alignment notes. The front end
The antenna input circuit uses a ready wound transformer of 2.6uH, available from ( WWW.GQRP.COM ) as 2u6L. This needed 100pF to resonate at 10.140 MHz. It’s fairly broadband, but the tuned circuit’s main function is to match the low impedance of the antenna (hopefully 50R) into the high impedance of the crystal. It will of course provide some filtering thereby protecting the crystal from out of band high energy signals. The wanted signal filtering will be done by the crystal.
An alternative antenna circuit could be built around a T-37-2 toroid holding 26 turns, 50pF fixed capacitor and 60pF variable in parallel. The primary winding could be 4 turns. The toroid is best mounted clear of the PCB. I moved the voltage regulator to enable short routes to all parts of the circuit.
Finally the audio stage, which is also driven in push pull. The NE5534 is fairly low noise and at this low signal level is unlikely to overload, despite the high gain of the circuit. A preset resistor is fitted at the output in order to facilitate level setting of the audio into the PC or laptop. In many years of experimenting with computers and radio interfaces, I have never damaged a PC. However I suggest you fit an isolating transformer if you have any misgivings.
As an alternative to an audio isolating transformer I have used a USB sound-card with great success. These devices are very cost effective ($3NZD) considering their utility, and certainly cheaper than a transformer. Windows 10 recognised them immediately and automatically loaded the requisite driver. No further software installation is required. You will need to select ‘PNP USB Device’ or something similar in the WSPR-x audio setup. On the PC or laptop you will need to set the USB microphone gain to at least 50, maybe more.
The NE602 maximum rated working voltage is 8v. On no account exceed this. A low noise linear power supply, rather than switch mode, is highly recommended. I used a SLA 12v battery.
You will need an accurate signal source of 10.140MHz. I recommend using the filter crystal as a temporary test oscillator. Fit a 4p7 capacitor in its place in the receiver circuit. Replace it with the crystal once alignment is done. If you have a transmitter, such as the QRP Labs Ultimate3S already running WSPR things will be a lot easier
First, it always pays to check that there are no low resistance readings from supply pins to ground. Ensure your supply polarity is correct. ( ask me how I learned this ! ) Check that the CIO variable capacitor is set to half mesh. Connect a pair of headphones. Apply 12v DC. If you can power it up with a milliammeter in circuit, the reading should be less than 30mA. My measurement was 23mA.
I have used this oscillator circuit many times with no problems. Just place the oscillator close enough to the receiver so that it can be heard in your headphones and can therefore be detected by the monitoring software later. Set the audio output pot to about 3/4. Tweak the antenna coil for loudest signal. Use the correct tool or you WILL break the core. Similarly tweak the I/F transformer. Connect the audio output from the receiver to your sound-card. I monitored the audio in Spectran.
My transmitter was sending the WSPR messages every 2 minutes. By adjusting the variable capacitor in the product detector oscillator circuit, it was possible to line up the waterfall signal in Spectran exactly on the 1500Hz marker. The waterfall in WSPR-x is unsuitable for this. Switching from Spectran to WSPR-x and waiting a few minutes confirmed reception of my own signal. At this point I strongly suggest you put the receiver in a screened enclosure and surround it with some polystyrene foam in an effort to stabilise the temperature. The device that I found to be most temperature sensitive is the 470uH inductor in the CIO. I might replace this with a different type of coil. Maybe a toroid. It may be necessary to re-tune the CIO after boxing up and allowing the temperature to stabilise.
Later testing using a signal generator and headphones revealed that the opposite sideband was not very well suppressed. So I added a further resonator, in series with the first, and a 180pF capacitor to ground at their junction. This greatly improved things. Best dx heard on the first day using a 35ft vertical antenna, was EA, at over 19,000 km from New Zealand.
Travelling around in a motorhome I don’t have the luxury of a workshop. I am sure that those with access to more/better test equipment could come up with something with even better performance, or at least optimise some components that I have only guessed at. Have fun.