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Messages - Albert H

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I've done a little more to the U-LULU.  I tried a few other FETs with varying degrees of success.  I've included my own design of power supply sequencing (based on a bit of simple CMOS logic).  With some work, I got efficiency up to around 92% with tuning that's open enough to allow it to go anywhere from 6.25 - 6.45 MHz without much change in power output.  The carrier cleanliness is also now up to commercial standards, since I decided to extend the output LPF by an extra section.  Just for the sake of amusement (and with a nod towards possible automatic mod depth adjustment) I introduced envelope feedback to the modulator.  The audio distortion that I previously had on the unmodified version has completely disappeared (it was previously in the order of 9% at peak mod).  Measurements now show the distortion to be well below 0.2%, and I've been able to add some audio response shaping to make it sound a bit "nicer" (according to my wife!).

I also built a couple of these things for MW (1.2 - 1.4 MHz) with really good results.  They run virtually cold, provide really good harmonic cleanliness, and have really good sounding modulation. 

We've made a few of the early 6.3 MHz versions and they've been snapped up by our Eastern European friends (as predicted).  Perhaps the English commercial constructor earlier in this thread has upset potential Eastern European clients, because we've had a lot of expressions of interest from over there - for both versions (SW and MW).  The enhanced version - with automatic modulation level control (a three-band audio processor), a built-in MP3 player and a "Universal Input" SMPSU to power it up looks like it should be particularly popular, and I may well make the details available on here for everyone to use.  We have also shipped a couple to South America, and received good reports. 

It was quite fun to hear one of my hand-built early prototypes - being run in southern Spain - up here at home in the Benelux at New Year!  That one produces ~26 W carrier / ~102 W PEP, and uses one of those "ProgRock" synthesiser modules.

We don't intend these to ever be a commercial product, but we're in touch with many clandestine SW operators, and we'll be sure to supply a few of them to interested parties.

In closing, I'd like to thank CDS for his efforts in improving the basic LULU.  The original was a pretty clever idea despite its flaws, and the CDS improvements made it into a great little transmitter.  There's a whole lot more that can be done to these little beasts, and - when I find the time - I'll be doing some additional design work. 

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Any ideas on something still available from China in 2023?

I bought a couple of these: HW-576 mono amplifiers from Aliexpress.  They cost around £5 each, and are supposed to be "100 W"....  With a 24V supply, the positive speaker terminal sits at 12V DC without audio applied, and when I lashed one up to a thrown together "U-LULU" circuit (though using a PLL synthesiser rather than a crystal), I got round 24 Watts carrier and could fully modulate it without anything getting hot!  The 3116 IC on the amplifier board has a small heatsink on it, and I expected it to roast, but was pleasantly surprised at how well it survived the abuse!

I added extra filtering to the output of the amplifier, since I really didn't want spurs either side of the carrier - offset by the amplifier switching frequency!  My next iteration of the design will continue to use this audio module (I don't think that I could do much better), a programmable oscillator module (one of the really cheap ones programmable with an Arduino), and a PCB that's off at JLC at the moment.  The high efficiency and small number of cheap parts required suggest that it might be worth extending this design for higher power, adding some simple audio processing and possibly a little MP3 player board that works with either SD cards or USB sticks.  I can see lots of these being shipped to Eastern Europe in the 48m and 41m bands!

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The RF Workbench / Re: Variable frequency oscillator design for HF
« on: October 24, 2022, 0141 UTC »
You'll never get a VFO to be as stable as a crystal or a PLL synthesiser.  With a lot of care and effort, and careful component selection, you can get a VFO reasonably stable, but if the temperature changes , or the relative humidity changes, the VFO frequency will change, or even drift!  If you insist on making the rig "frequency agile", another approach to oscillator stability is the "Huff & Puff" frequency correction circuits.  This is actually a great approach, as long as you have a handy freqency counter!  I've experimented with various FLL (frequency-Locked Loops) over the years, and have got a couple of reliable designs that will certainly keep a transmitter in place for the duration of a broadcast, and they only use a handful of components.....

Many years ago, I experimented with various diode types, trying to find cheap parts that would provide a reasonably wide range of capacitance as the bias voltage was varied.  I found that a reverse-biased green LED was just about as good as an expensive "real" varicap diode!

The "minimal" circuits published in the RSGB's Magazine "Radcom"  and in "Sprat" magazine by Hans Summers are excellent and I can recommend them.  The version I build uses a cheap computer crystal for the reference, and allows tuning of the oscillator in 10 Hz steps.  Once the frequency is selected, the rig will stay where it's put for many hours, irrespective of temperature changes, barometric pressure or moon phase......

If anyone is interested in trying out the "Huff & Puff" approach to frequency control, I'll put some circuits up on here.  I also (sometimes) incorporate a simple PIC-based frequency counter, and I'll also be happy to provide that circuit.

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The RF Workbench / Re: Variable frequency oscillator design for HF
« on: October 18, 2022, 2206 UTC »
I often use a simple CMOS PLL, using the 4060 for the reference oscillator (with a standard crystal, usually 4MHz) and reference divider, a 4040 as a preset divider (programmed with diodes), and a 4046 for the phase comparator, loop filter and Voltage Controlled Oscillator.  The 4046 gives a squarewave output, and the mark / space ratio is somewhat proportional to the frequency, so if I want a really accurate 1 : 1 mark / space ratio, I generate twice the output frequency, and then use a 4013 or 74HC74 bistable to guarantee the waveshape.

The whole CMOS PLL fits on a tiny PCB (really tiny if you go for the Surface Mount versions), uses a standard crystal (I frequently use 4 MHz because I have a boxful of them!) and is easy to set up.  It costs just a couple of £ in parts, and will generate any frequency I want. 

There's an even simpler version for mediumwave - a 40103 preset divider, 4060, a 4046 and a 4013.  The crystal is a 4608 kHz (baud rate generator crystal) which is cheaply available from Farnell / Newark and many other suppliers.  This gives 9kHz steps (for Europe.  If I want 10 kHz steps, I have to use a 5120kHz rock.

My current cheap MW rig uses the 4-chip synthesiser, driving an IRL-series FET (logic level input), and is modulated by a really simple series modulator, using a TDA2040 IC as a "power op-amp".  This basic design is capable of ~20 Watts carrier / 80 Watts peak, and sounds really good on the air. The most expensive parts are the heatsink and the mains power supply.   It's not super-efficient, but it's simple and cheap! 

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The RF Workbench / Re: Stretchyman 40 W TX Reliability Modifications
« on: October 18, 2022, 1831 UTC »
CharlieDS : Thanks for the moulded inductor suggestion..  It's really speeded up construction, and goes some way to saving my arthritic fingers!  You're right about them being nicely lossy, and they've made my PWM modulator filter both smaller and better.  My PWM circuit goes at 75kHz (it was a convenient frequency for several reasons), and the filter is now trivially simple.

I don't entirely agree about the ClassE site info - granted he's still using 25-year-old FETs, and his resistance to PCBs is rather silly, but some of the info about voltage rating specifications, and the provided equations are useful.

I'm wedded to the idea of using proper gate driver ICs for the FET drive, and have experimented with opto-isolation of the drive, which has proved to be useful in keeping the sections of the transmitter well isolated.  I am still using a very simple CMOS PLL for frequency generation, despite the availability of the pre-programmed oscillator modules.  Frequency agility (over a few hundred kilohertz) is important to the people who want to use these rigs.  I also want to keep them as "low cost" as is feasible - I don't want the loss of a rig to the "Authorities" to be financially significant.

I've also been looking at "low level" modulation - gate mod in the case of the FET experiments.  Obviously the audio quality suffers somewhat, so I've been playing with methods of "Envelope Feedbck" (like I did on my screen-grid modulated medium wave rigs back in the 70s!). The results are encouraging.

Another approach I've been experimenting with is the Cheirix "Outphasing" approach, using very high efficiency PAs, and a semi-digital approach to generation of the phase-modulated carriers.  Getting the PLL VCOs to swing linearly enough in frequency/phase has proved to be tricky, and I don't like the way that the antenna loading can affect the modulation, but it looks like a way of gererating a hell of a lot of modulated power at relatively little cost.  Having experimented extensively with "Ampliphase" rigs back in the 70s, this looks like an approach that could be made to work....

It's great to be retired from the Rodent Race these days - plenty of time to get to the bench and experiment!

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The RF Workbench / Re: Stretchyman 40 W TX Reliability Modifications
« on: October 06, 2022, 0208 UTC »
I got really annoyed trying to get true frequency accuracy and stability for Medium Wave from the three or four DDS modules I tried....  I gave up, and built a PLL that's locked to the 198kHz BBC Radio 4 transmitter.  The 198 kHz rig is run from the National Physical Laboratory's frequency standard source, so the Medium Wave boxen I built were rather more accurate than I could show on my frequency counter!

Obviously, there are standard frequency transmitters all over the world - I went for 198 kHz because the rigs were for the UK and Netherlands, and it's easy to derive frequencies in 9 kHz steps from that source with the minimum number of CMOS logic ICs!  Incidentally, I didn't mind adding proper gate driver ICs, rather than trying to cane a logic IC for the drive, and generated twice the frequency I wanted, then used a final bistable to guarantee an absolutely 1:1 M/S ratio.

The advice given on Steve Cloutier's "ClassEradio" website about component ratings is pretty useful, and has helped me avoid the fireworks displays that others have had!  For higher powers, I've been using ex-telecom SMPSUs for high current 48V supplies, and PWM modulators for efficiency.  It's got to the stage that a 200W carrier / 800W peak rig can be built for pretty cheap these days.  The expensive parts it seems are the damned toroid cores and the mechanical stuff.

Now.... back to designing a little (80W PEP) PWM rig for as little cost as possible.....

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The RF Workbench / Re: Another helpfull site for making RF
« on: November 10, 2018, 1229 UTC »
I've experimented with a couple of the cheap Chinese DDS boards, and have had endless problems programming them - either with PICs or Atmel chips.  Most of the software that's around the 'net is either written for DDS ICs I've never seen, or just doesn't work!  I'd be very interested in your programmer software and details of the "cheap" DDS board you use.

The CARDINAL programmable crystal oscillators look like a good, cheap solution.  I wonder if I can get some "blanks" and programme them myself....?

I still favour my cheap 74HC CMOS approach, though.  The parts are dirt cheap, widely available, and if you're really anal about frequency accuracy, you can phase lock your PLL reference to a "standard" like the BBC 198kHz (accurate to 14 digits!) or other reference frequency station.  Incidentally, I use the 198kHz off-air to control the reference oscillator of my frequency counter, so when it reads 62XX.000000kHz, I know that the rig's bang on frequency!

Incidentally, I've been playing around with a Lulu of a rig on the bench, and 10 Watts carrier / 40 Watts peak is now pretty easy with a "car battery" supply.  On a quick test into an inverted-V dipole here in the southern Netherlands, I got reports from Switzerland, Italy, Germany, the UK, Ireland and Spain.  Signal strengths were described as "fading a bit" and pretty weak, but generally audible, and achieved S9 in Switzerland on peaks!  I'm quite astonished at the coverage attainable by such a simple (and cheap) set-up.  I used to just do Medium Wave (often through a ¼-wave "tower-block sloper"), and got some great daytime results on pretty low power, but now I'm hooked on SW and will definitely put on something reasonably powerful over Christmas.  I had some big FETs arrive in the post this morning, and my target is 250W carrier / 1kW peak - just to prove that it can be done cheaply. 

I'm considering ways to modulate the 250W beast, and PWM looks favourite, but I'm tempted to go for Outphasing - I have an idea for a modulator that generates square waves at 135° to each other (derived from different outputs of a ÷8 CMOS IC).  These will provide the references for a couple of 4046 VCOs, which will be frequency modulated in phase opposition to each other by injecting the audio into the loop filters.....  This makes the PAs relatively simple - it's just the output bridge that gets tricky.  I've built outphased rigs before, but only ever with pairs of 807s or 813s in the outputs!

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The RF Workbench / Re: Another helpfull site for making RF
« on: November 10, 2018, 0117 UTC »
Hi Stretchyman

I've been using the '240 as a driver for IRFL FETs for years.  I usually use a CMOS PLL (74HC4046 for the phase comparator and the VCO, 74HC4060 for the reference, and diode-programmed 74HC4040s for the pre-set divider) and a standard crystal (usually 4 MHz because they're cheap) and can generate any frequency from <2MHz up to around 18 MHz in 5 or 10 kHz steps.  The synthesiser costs around a Fiver to make and avoids the need for expensively cut crystals (I got fed up with being asked what I wanted the "unusual" frequency crystals for!).  I've also found that it's a good idea to run the VCO at 2f, and use a 74HC74 to give an accurately 1 : 1 squarewave into the '240 buffer. 

I ran out of '240s and used a 4049 as the driver on medium wave.

I'm still messing around with various ways of modulating my rigs - for lower power (<10 Watts carrier), I've been using the TDA2040 as a "power op-amp" through a choke on to the drain of the output FETs.  I have a simple feedback compressor before the '2040, and have carefully chosen the feedback and input capacitors to shape the frequency response.  There's also a simple clipper circuit to handle the overshoots from the compressor to prevent splattery overmod.  There are even a couple of blinking LEDs to show you when the mod pot is about right - red for peaks, yellow for loud and green for OK.

The final issue is a simple and effective antenna for 6.2XX MHz and a simple matcher.  The rig itself is pretty much a "throwaway" item - if it gets removed by the authorities, there'll be another to replace it the next weekend!

I'll put the details of the synthesiser up here for everyone to use.  It saves a lot of headaches!

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The RF Workbench / Re: Another helpfull site for making RF
« on: November 07, 2018, 1213 UTC »
Cool, QRP!  That's a new site for me.  I see they have the Steve Quest transmitter there.  I feared that one was lost to the ages.  Thanks!

The site is (mostly) worthless.  It's just a collection of various diagrams from all over the 'net, many with errors so they won't work as drawn, and most of the higher frequency designs will be layout-critical, and there are no layouts provided.  Whoever put that waste of bits on the 'net doesn't have a clue about electronics - he's just collected stuff and put it on "his" blog.

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FM Free Radio / Re: Isn't it rather easy to get caught?
« on: August 30, 2013, 2035 UTC »
Given the way FM in the 88-108 Mhz band propagates, wouldn't it be very easy for the radio authorities to zero in on an FM pirate?

You'd be surprised how difficult it can be!  Over here in Europe, we NEVER broadcast from home - it's easy to put a transmitter, car battery, MP3 player and an aerial up a high tree on a hill and get plenty of coverage with just a few tens of Watts.  Many of the land-based FM pirates over here use UHF, microwave or even wireless ethernet links from studios to transmitters.  The transmitters are usually at the top of a housing "project" - easy access with standard Fire Brigade keys!  Some of the stations in major cities use hundreds or even thousands of Watts.  Many are technically indistinguishable from the "legal" stations.

Be bold!  Don't mess around with two or three Watts.  Get 40 or 80 going, and get some sensible coverage.  Get your modulation quality and deviation right - audio limiting is essential.  Make certain that your transmitter is "clean" - no spurs or harmonics - that precludes the use of ANY of the BA1404, BH1415 or BH1416-based junk.  Don't buy the Chinese crap off Ebay either - they just cause massive interference!  If you interfere with other services, you DESERVE to be caught and heavily prosecuted, so make sure you don't!

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