LPAM Broadcast / Top Bander

[Zozo]

Recently started a new project for the handful of AMers on here. Basically a re-design of published designs from a many years ago from various sources all put together.

Typical series modulator with IRF510 final, and 4000 series logic for the PLL. Most AMERS will know this topology. Designed to cover the upper region of Medium Wave and 160mtrs for the Hams.

I hope to have the sample PCB's here before Xmas, but we're see how that goes. Undecided whether this will be open source project yet. But I plan to put many kits together available for sale.

LPAM BC.png

[Zozo]

Update, PCBs have finally arrived.

Kits will be likely be available from late January after testing and completion of all accompanying documentation.

MK1 PCB.jpeg

[SidB2]

That looks to be a good project Mr Zozo. I've built a couple of AM tx's in the past which have been based around the 10 watt IRF510 design, but they have been xtal controlled. As these are almost impossible to get now your PLL design looks like a good solution.

(apologies if this is a duplicate post)

[Zozo]

That looks to be a good project Mr Zozo. I've built a couple of AM tx's in the past which have been based around the 10 watt IRF510 design, but they have been xtal controlled. As these are almost impossible to get now your PLL design looks like a good solution.

(apologies if this is a duplicate post)

Thank you SidB2, I had to delay this project for a few weeks, but I've spent today assembling one of the boards.

It turns out I needed to make some changes to the IRF510 matching network. It's now the cooked-book design from the EMRFD manual.

The changes I've made required a couple of tracks to be cut, with one capacitor being dead-bug bodged on the bottom.

Anyhow I've reworked the PCB, and will order a new batch shortly. I will also upload the schematic and photo of the assembled board I put together earlier today.

[Zozo]

Here's the schematic of the board. it's capable of just over 1.5 Watts carrier, with around 6 -7 Watts PEP.

Schematic_AM MK2 .png

[Zozo]

Here's the assembled PCB. Updated version addressed the vacant capacitors regarding the modifications.

PCB ASM.jpg

[FMEnjoyer]

This was cool you know your stuff.

[Albert H]

I've got the Gerbers for a similar MW rig that uses a pair of IRF640s and can run tens of Watts of carrier (depending on the supply rail). The modulator we used was a simple series modulator, using a power Darlington transistor driven by an op-amp. The modulator side of the project grew a bit, to include a limiter, diode clipper and audio filter. It used just two quad op-amps and an analogue opto-coupler (just an LED and an LDR next to each other) - we used the LCR0202. A couple of the prototypes are in regular use in the Netherlands for LPAM stations.

[Zozo]

This is now a open source project, please find the Gerber, Manual and EasyEDA files attached. Enjoy

Manual

Cranefly-MK2-Manual.pdf.zip

EasyEDA

ZoZo-LPAM-MK2-EasyEDA.zip

Gerber

ZoZo-LPAM-MK2-Gerber.zip

[montaylor]

nice 1

[reverend]

Great work Zozo. Quick question for you... the TDA2030 that you use for the modulator can cope with supply voltages up to 36V. If the power supply voltage to that chip is increased, how much increase in power could be obtained? Would it be the IRF510 that would be the limiting factor? Could this be replaced by an IRF520? Or would the TDA2030 be the limiting factor.

Really nice design - just looking at how the output power might be increased from 1.5W carrier to something nearer 5W.

[Zozo]

Great work Zozo. Quick question for you... the TDA2030 that you use for the modulator can cope with supply voltages up to 36V. If the power supply voltage to that chip is increased, how much increase in power could be obtained? Would it be the IRF510 that would be the limiting factor? Could this be replaced by an IRF520? Or would the TDA2030 be the limiting factor.

Really nice design - just looking at how the output power might be increased from 1.5W carrier to something nearer 5W.

Hi rev and thank you.

The supply voltage to the modulator IC could be increased much higher to achieve that power level you mentioned. I would suggest a modification to omit resister R9 for a variable pot which biases the TDA2030 and will act as a "carrier set". I think you're be safe to try and experiment with any of the IRF5xx family of FETs in this design. However increasing the supply voltage will require the TDA2030 to be fitted to a much larger heatsink.

I've not tested this circuit with the TDA2030 being supplied with 32V, but you may experience some trade-off with "RF Power" vs "Modulation Depth", so it may not achieve all the way to 100% mod. Anything less than 90% is not ideal really.

[Albert H]

Practical experience with the TDA2040 or the LM1875 Show that you can run either IC from up to 36V supply, and get 100% modulation. You're right about the heatsinking requirement - as the power goes up, the heatsink requirement increases rapidly! My boards have the modulator IC and the FET(s) for the RF outright at the edge of the board, allowing the devices to be put into physical contact with the metal housing. The little 36V rigs used cheap knockoff diecast boxes, which provide plenty of heatsink area, are really robust, making them difficult to break, and make them largely waterproof for use outdoors.

A couple of other things I incorporated as standard on the board: I added audio filtering, to keep the broadcast bandwidth sane, and added a basic AGC / compressor so that it was easy to use almost any source for the audio - an MP3 player, a mobile phone, or pretty much whatever you want!

The AGC I favoured used an LCR0202 opto-coupler (it's a little plastic box with four terminals - LED at one side and LDR at the other), but more sophisticated circuits used the NE571.

[Albert H]

A quick and simple MW audio processor:

MW Processor.png

As you can see, filtering is important. The Limiting / Clipping function is done by means of a couple of transistors which are used (much like is often done with FETs) as voltage-variable resistors to ground. If the levels are wildly over the top, the transistors will also clip, shaving the excess off the signal, but adding a bit of harmonic distortion. The final filter helps to keep the audio bandwidth under control, but also helps to shave off the harmonic products from any clipping. Transistors were chosen as the attenuators because through-hole FETs are getting difficult to find, and the small amount of distortion incurred by use of the transistors isn't practically audible.

I've built these on Veroboard and on PCB. The layout isn't critical, but it's a good idea to include basic RF-filtering on the way in and out. I used simple filters with a couple of turns through ferrite beads, with 100pF capacitors either end to ground, and on the PCB version, I used 22µH moulded chokes (the value isn't critical) because I had a boxful of them!

The supply voltage should be well smoothed (and protected against RF), and I put 100nF capacitors from the IC power Pins to ground, as close as possible to the IC legs.

This basic version uses a dual (+/- 12V) supply, but I intend to re-design for a single rail - for ease of use from batteries on field sites!

In practical use, it's worth advancing the incoming audio level until the LED indicator just blinks on peaks. The modulation level into your rig is then firmly limited, but won't overshoot leading to nasty overmodulation distortion.

[stretchyman]

12W @ 24V or 3W @ 12V. Class E PA and the PWM modulator being class D. Fully programmable to any in band frequency. Fits in a shirt pocket size diecast case. Fairly tidy.

Str

[Premier-Carousel]

Well now that tickles my fancy Mr S. Is the class D amp bridged, or is just one channel employed?

S