Thoughts on this AM Transmitter

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Albert H
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Re: Thoughts on this AM Transmitter

Post by Albert H » Thu Jan 18, 2018 1:05 am

I'm converting from scribbles in the notebook to something that can be understood! Please wait a little while.
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Re: Thoughts on this AM Transmitter

Post by davemartin » Sat Jan 20, 2018 12:14 pm

There has been some talk about the Mosquito TX elsewhere recently and this is what the makers say about the 50% mod, "The modulation stage can provide 0 to 110% modulation and has been optimised for 50% modulation for the Netherlands."
So it can be turned up.

Dave

Albert H
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Re: Thoughts on this AM Transmitter

Post by Albert H » Sun Jan 21, 2018 2:55 am

Dave
The "Mosquito" is designed for match into a shortened aerial. It's supposed to be used for the Dutch restricted MW service - 1 Watt pep to go around the block.

As you know, 5 Watts carrier, 20 Watts peak into a good aerial and earth can go for miles. Unfortunately, the folks on here don't seem to understand the concept of AM, and don't appreciate that amplifying an AM signal isn't trivial - they don't understand that modulation linearity really matters. They're used to Class C amplifiers for FM.....

Your "Corsair" is probably the minimum rig that's practical. I've seen versions of this used to cover fairly large areas. I've even seen one used on Long Wave!

My approach uses the three-chip CMOS PLL to generate the carrier - a standard crystal and three CMOS chips cost a lot less than a custom-cut rock for your frequency of choice. I then use a couple of small bipolar transistors to drive a couple of cheap FETs in a Class E output stage. With a simple linear modulator - using a TDA2040 through a choke - to the PA, I get 10 Watts carrier and 40 (+) Watts peak. There's a lot of heat from the modulator, of course, since the rig can never really be more than about 33% efficient. However, it's cheap, simple, and gives great sounding modulation. It's also really cheap! You can also apply envelope feedback (to the input of the 2030) to guarantee real linearity.

For more power, I've gone over to PDM (Pulse Duration Modulation for the beginners). With a high efficiency Class E PA driven by a PDM modulator, I get efficiencies around 90%. Using four cheap FETs, 100 Watts carrier and 400 Watts peak is easy. The only expensive parts are the mains transformer and the high voltage capacitors for the output match and lowpass filter. The PDM modulator uses a NE555 generating a triangular wave form (at about 75kHz to keep the DC filter components small) and an LM311 comparator to modulate it. The squarewaves are then used to drive a couple of cheap switching FETs to give some current, and then through a filter to get rid of the switching off the variable DC. I use a quad op-amp for the audio lowpass (at about 7kHz) and for envelope-derived negative feedback.

There are some interesting rumours about the lack of interest in enforcing medium wave broadcasts, so if you don't cause interference, you'd probably get away with it for a long time!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

Gigahertz
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Re: Thoughts on this AM Transmitter

Post by Gigahertz » Sun Jan 21, 2018 3:55 pm

In the description the antenna to the Mosquito is either 3 Metre Loaded Vertical Antenna (or suitable alternative 50 Ohm)

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Re: Thoughts on this AM Transmitter

Post by thewisepranker » Sun Jan 21, 2018 10:37 pm

You are not using PDM. PDM is very different from PWM, which is probably what you are using, especially if you are crudely generating it using a 555.

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Re: Thoughts on this AM Transmitter

Post by MiXiN » Sun Jan 21, 2018 11:44 pm

I have the manual & schematic for the Mosquito 1W AM Transmitter in PDF format here, but I'm unsure how to host it on this forum - so if anyone wants it, please send me a PM with your details & I'll get it eMailed over.

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Re: Thoughts on this AM Transmitter

Post by Albert H » Tue Jan 23, 2018 2:57 am

thewisepranker wrote:
Sun Jan 21, 2018 10:37 pm
You are not using PDM. PDM is very different from PWM, which is probably what you are using, especially if you are crudely generating it using a 555.
It's not just a 555! It's true PDM, not PWM. I could never get PWM rigs properly clean. PDM - by comparison - is entirely clean. The only problem I'm having with the little PDM rig at the moment is the "floating ground" output stage. I went this way in an effort to avoid RF ground issues and to minimise modulation hum. This was meant to be an introduction to MW transmitters for anyone who wanted to give it a try.

The big PDM rigs work well, are entirely reproducible, and are relatively cheap to make. Their RF output is the cleanest of ANY medium wave rig I've ever seen. I'm rather concerned about making hundreds or even thousands of Watts of MW available to the masses! Even a hundred Watts into a piece of wire for an aerial can give rise to lethal RF voltages....
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by thewisepranker » Tue Jan 23, 2018 10:46 am

Fair enough. You'd expect PDM (pulse density modulation) to be more spectrally pure and "fundamental" because the on and off times are constant except for when there are more than two consecutive "off" cycles, which aren't even there anyway.

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Re: Thoughts on this AM Transmitter

Post by davemartin » Tue Jan 23, 2018 6:48 pm

Albert H wrote:
Sun Jan 21, 2018 2:55 am
Dave
My approach uses the three-chip CMOS PLL to generate the carrier - a standard crystal and three CMOS chips cost a lot less than a custom-cut rock for your frequency of choice.
Yes that is my approach also. I have recently been in touch with a small Chinese operation that knocks out all sorts of stuff for literally pennies. They reckon they can knock up the 3 cmos PLL for £1 post free for full size chip version and around £1.20 for a SMD version which will be the size of a FT240 xtal not including the Dip switches. They are looking at potential interest and if they think they could sell 250+ then no cost to me.

Dave

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Re: Thoughts on this AM Transmitter

Post by davemartin » Tue Jan 23, 2018 7:30 pm

Albert I have just been provided with the schematic for the Mosquito, it's nothing new in fact change three transistors in the PA and it is identical to what I have been building for years. It is however made to match directly into 50 ohms unlike some of the FCC part 15 TXs which are designed for anything but.
I'll post the schematic later.

Dave

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Re: Thoughts on this AM Transmitter

Post by davemartin » Wed Jan 24, 2018 7:33 am

Schematic for Mosquito is attached, component values are missing though.
mosquito TX.png
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Re: Thoughts on this AM Transmitter

Post by MiXiN » Wed Jan 24, 2018 2:46 pm

So, by looking at the schematic, will this Mosquito do 1 full Watt or is it just exaggerated claims do you think?

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Re: Thoughts on this AM Transmitter

Post by Albert H » Thu Jan 25, 2018 2:32 am

Three 2N7000s in parallel should do about a Watt, but that's probably PEP - 250mW carrier, 1 Watt PEP!

The pair of bipolar transistors driving the gates of the FETs could actually drive much bigger FETs for more power. I used a similar topology in a Class D PA that I used to use quite widely. The big trick is to be able to discharge the Gate to Source capacitance of the FETs

I used to build an "out-phased" medium wave rig that used a digital sinewave source feeding a couple of comparators which comprised the switching modulators, working on phase and anti-phase audio. The switched squarewaves were then used to drive a pair of Class D PAs, which had their outputs combined by a couple of resonant coils. The resultant signal went through a lowpass filter, and I had lots of lovely, clean medium wave. The earliest versions delivered around 140 Watts PEP and 35 Watts carrier and the bigger ones would do 125 Watts carrier, 500 Watts PEP.

The amazing thing about all the AM FET designs is that they cost next to nothing to build! The expensive parts are the heatsinks and the mains transformers. The 500W PEP out-phased beast only cost about £150 to build, and was simple enough to throw together in an evening. The ones we built in pretty boxes with meters on the front were sold for £2500+ and we couldn't make enough of them! There are a few of them being used in the 'States for night time (reduced power) transmitters, and quite a lot of them in Eastern Europe. We sold the rights to a firm in Greece, since we wanted to concentrate on other products (including a DAB solution for small, local services), so we haven't made any of those for about 10 years. The Greeks are shipping a smaller version to small town in Africa and South America.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by davemartin » Sun Jan 28, 2018 3:42 pm

This is another PLL AM TX circuit, not a lot of power from the Netherlands. They don't include a circuit but a scan of the top and bottom of the PCB then flipped and overlayed tells you all you need to duplicate it. It use a fairly novel way of producing AM.
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Albert H
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Re: Thoughts on this AM Transmitter

Post by Albert H » Tue Jan 30, 2018 1:41 am

I know the guy who designed this. It's a sort of PWM, using a 4053 CMOS switch and using the audio to alter the point at which the 4053 chops the squarewave carrier. There's a resonant filter after the 4053 modulator, and the resultant signal is pretty clean. It's capable of 100% mod (just), and delivers a few 10s of milliwatts. I built something like this on Veroboard some years ago, and with a 3m bit of wire for an aerial, a 15V supply, and a sensible choice of frequency, it gave me a good signal throughout my property. Maximum usable range was just about the end of the garden from my study - about 25m in the open, and a bit less through the walls of the house.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: Thoughts on this AM Transmitter

Post by hailstorm » Fri Feb 02, 2018 5:01 pm

Has anybody bought the Mosquito 1 Watt and if so are there any reviews yet?

Gigahertz
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Re: Thoughts on this AM Transmitter

Post by Gigahertz » Fri Feb 02, 2018 8:32 pm

Ordered a Mosquito. Just in process of looking at antenna to fit in the garden!

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Re: Thoughts on this AM Transmitter

Post by Albert H » Sat Feb 03, 2018 1:00 am

If you're going for the higher frequency end of the band, a long, rectangular loop can be very effective. The vertical part of the loop nearest to the house has to be coax, with the outer connected to the rest of the loop. The inner conductor then provides the feed - using the coax as a (sort of) linear transformer. The loop can be resonated on to your frequency of choice.

I used one of these (based on a Practical Wireless article called "Looping over the Garden"), and with 5 Watts carrier, 20 Watts peak, got more than enough daytime coverage to annoy the authorities!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

Gigahertz
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Re: Thoughts on this AM Transmitter

Post by Gigahertz » Sat Feb 03, 2018 7:07 am

Thanks Albert,

Lucky I have one of these to help, goes down as far as 300KHz
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Re: Thoughts on this AM Transmitter

Post by Gigahertz » Sat Feb 03, 2018 7:17 pm

So.....The Mosquito Transmitter has arrived!

Quickly grabbing the unit & PSU I added VSWR meter (range of meter was 1.8MHz to 525Mhz) and 50Ohm dummy load. Powered up the unit with MP3 it will peak around 500mW. Once I get some free time I intend to get some sort of antenna built!

This is not a review or a FULL test! just quickly thrown together!

Does sound good though!

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