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Old 29th Sep 2018, 4:20 pm   #21
ms660
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Default Re: FM to AM converter - variation on a theme!

What decouples R4 in the PCB layout....?

Lawrence.
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Old 29th Sep 2018, 5:13 pm   #22
David G4EBT
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Default Re: FM to AM converter - variation on a theme!

Thanks for spotting that Lawrence - C1 (100 uF) on the layout doesn't conform to the circuit, which shows it from R4 (the 470R resistor to drop the voltage to the modulator section), to ground. On the layout, C1 is across the 9V input. I'll alter the layout accordingly and repost it.
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Old 29th Sep 2018, 5:18 pm   #23
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Default Re: FM to AM converter - variation on a theme!

Might be worth de-coupling for RF as well as per the original article schematic (Guy Fernando)

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Old 30th Sep 2018, 8:45 am   #24
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Default Re: FM to AM converter - variation on a theme!

I am glad to see people interested in this. I have been continuing to experiment with the idea and also tried a variation on the mini mod 2 circuit. Attached is a beta version of the circuit, it's more or less a cross between the original mini mod and the later FM/AM converter.

This version seems to work very well to. It has a little better output stage especially if you use the transformer version. I did find that it appears to introduce a slight hum - even when running fro battery. Not quite worked out why yet - having said that it's not particularly notable.

I'm definitely going to use the PCB layout and thanks.

Derek
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Old 30th Sep 2018, 10:54 am   #25
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Default Re: FM to AM converter - variation on a theme!

Quote:
Originally Posted by ms660 View Post
Might be worth de-coupling for RF as well as per the original article schematic (Guy Fernando)
Thanks for your advice Lawrence.

I've amended the layout to reinstate what was 'C1' on the original Guy Fernando converter - 100nF, and have repositioned the 100F cap on the layout so that it conforms to the circuit. I've renamed the 100F cap 'C8'. I also stated in my earlier post that the PCB masks were flipped, but in fact they weren't. In any event, I've updated the masks to incorporate the addition of C1 and repositioning of the tracks.

Pic 1 is the updated circuit reinstating what was 'C1' on Guy's circuit.
Pic 2 is the updated component overlay.
Pic 3 is the updated PCB artwork, which is also now correctly 'flipped'.

I'm intending to mount the PCB on spacers using the same mounting screws and positions as does the FM module, and I won't know the actual dimensions for sure until the module arrives, (from a UK supplier) Hence, (assuming that there aren't still any errors!) though the track layout will stay the same, the actual length of the PCB will depend on the position of the mounting holes. The holes shown on th board are therefore just an indication - they're not accurately positioned.

Hope that's of interest.
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Old 30th Sep 2018, 11:18 am   #26
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Default Re: FM to AM converter - variation on a theme!

Re: Post 26, sorry to do this but in the original the 100nF was across the supply that directly feeds the Osc, the Osc in your circuit is only de-coupled by an electrolytic, electrolytics are not the best de-couplers for RF, a suitable ceramic capacitor would be best placed across the electrolytic in my view.

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Old 30th Sep 2018, 1:09 pm   #27
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Default Re: FM to AM converter - variation on a theme!

The easiest thing would be to swap C1 and C8 'round.

The other thing that I'm wondering about is L1 and L2 being parallel and next to each other. Isn't there a risk that they'd form an ersatz transformer and interfere with each other?
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Old 30th Sep 2018, 1:15 pm   #28
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Default Re: FM to AM converter - variation on a theme!

Quote:
Originally Posted by ms660 View Post
Re: Post 26, sorry to do this but in the original the 100nF was across the supply that directly feeds the Osc, the Osc in your circuit is only de-coupled by an electrolytic, electrolytics are not the best de-couplers for RF, a suitable ceramic capacitor would be best placed across the electrolytic in my view.
No need to apologise Lawrence.

It will be evident that my practical skills and abilities have always far exceed my rather basic technical skills, so any advice is always welcome.

I have a history going back decades, of making 'workmanlike' homebrew projects which look well turned out and nice on a shelf, but don't work! Hence, there's no point in my building something which either contains redundant components which serve no useful purpose, or conversely, degrading the performance by leaving out components which would improve the performance. That's especially so when posting on a forum such as this, where other members might wish to replicate the project, as was so with the Guy Fernando Converter.

I simply used the information that Derek ('coopzone') provided in his earlier post outlining what he did to adapt the transmitter part of Guy's original design. You'll note that Derek added the 100F electrolytic to reduce hum from his 'wall wart'. In the updated circuit above, I just reinstated the 100nF cap (C1) to where it was in Guy's circuit.

Are you saying that C1 would be better positioned in parallel with C8 - the 100F cap, and if so, in 100nF a suitable value?

to quote Derek's post:

I built everything to the right of R1 470k (including the resistor; I later changed the value to a 100k as the volume was a bit low)

Next I permanently connected R2 1k to ground (-ve line from battery), so as to enable the transmitter all the time.

Next I changed the values of C4/C5 to 150pf to change the frequency, as the original values just happened to be very close to a local station. You may not need to do this step.

Next I added a 100uf cap and a 470ohm resister across the +ve/-ve ( for the cap) and the resistor in series withe the +ve. This reduces the voltage to the circuit and reduces the ripple (and hum) caused by the low quality plug-in-the-wall PSU I used. This power supply was 9v unregulated and was also used to power the Bluetooth/radio/mp3 module.

End quote.

Thanks once more for your interest and input.

Dribble 'Confused, of Cottingham'
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Old 30th Sep 2018, 1:20 pm   #29
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Default Re: FM to AM converter - variation on a theme!

It's not critical, and the oscillator will probably be fine without it, but it's a good belt and braces approach to decouple the power rails with a 10nF-100nF capacitor as well as an electrolytic. This is because electrolytics tend not to work very well at RF frequencies.

You can overthink these things. This is a very simple circuit and all it needs to do is oscillate. It's difficult to imagine anything that would stop this happening unless there was an actual wiring error or faulty component.
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Old 30th Sep 2018, 5:32 pm   #30
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Default Re: FM to AM converter - variation on a theme!

Quote:
Originally Posted by ThePillenwerfer View Post
The easiest thing would be to swap C1 and C8 'round.

The other thing that I'm wondering about is L1 and L2 being parallel and next to each other. Isn't there a risk that they'd form an ersatz transformer and interfere with each other?
Thanks for reading the thread Joe, and for your comments.

The modulator part is exactly the same layout as the PCB I developed for the Guy Fernando Summer BVWS Bulletin FM/AM converter of which I've built 20 or so, which seem to be very stable and works really well. Many more will most likely have been built to Guy's strip-board layout and others on PCBs, with no reports of problems, so the proximity of the two little chokes seems not to have been problematical.

That project used the remarkable RDA5807M FM module and Guy's programmed PIC.

The only difference with this one is that Derek, ('coopzone'), used just the modulator section of Guy's design, which he built on strip-board and fed it from what is perhaps confusingly described on e-bay as a "Wireless Bluetooth MP3 WMA Decoder Board Audio Module USB TF Radio For Car". Basically an FM scanning radio with a digital display, a separate remote for changing stations, and allows for other inputs too.

The units are available for under a fiver from China but to save the delay, I bought it from a UK supplier for a couple of sovs more.

Clearly a drop shipper because payment went to 义乌市雅锐电子商务有限公司 !!

Should be here midweek, so as and when, I'll maybe knock up a board and give it a go.

Just doing it out of interest really - I'm more than happy with Guy's design.

Guy's design was powered from 2AA cells, hence, no need for the electrolytic. Derek included one here as this unit is powered via a wall-wart, which might introduce hum.

As Paul says, the aim is to keep things simple, consistent with acceptable performance.
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Old 30th Sep 2018, 6:21 pm   #31
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Default Re: FM to AM converter - variation on a theme!

You know far more about RF than I do, it's a Dark Art to me. I was wondering as we're always told to mount the cores of power and output transformers perpendicular to each other.
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Old 17th Oct 2018, 12:05 pm   #32
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Default Re: FM to AM converter - variation on a theme!

Quote:
Originally Posted by coopzone View Post

2 GND wires, one -ve power the other audio ground to the -ve per on the PSU where in connected to the board I made.

1 +ve (9-12volt required) to the PSU +ve direct from the transformer. So the same power supply is used for both the transmitter and the module.

Left+Right audio connected to R1 as Audio input to the transmitter.

Thats it. The module is so simple it does not even come with a connection diagram, it's printed on the PCB!
Derek
My module has just arrived from a UK seller. Ordered on 30 Sept, was despatched to Courier firm same day, delivery stated as 4 Oct. Actually took till today to arrive. Ironically, I'd ordered something else but from China on 30 Sept, which also arrived today. Not the fault of the supplier. No dramas - not in a hurry for it - I've been furtling with a min-mod meantime, which just needs boxing up.

As Derek said, the unit comes with no instructions, and I'm glad he said: "they're on the PCB" as indeed they are, but underneath the PCB, where initially I never thought to look! One of the connections is for an antenna and if Derek happens to read this, perhaps he can comment on what he did about that. With the Carl Fernando FM/AM converter design which uses the RDA5807RDM module, it just uses 15" of wire for both the FM & AM antennas.

Based on helpful feedback in this thread from forum members about the PCB layout I'd designed for the unit, I've updated the layout to incorporate the suggestions, and have attached the updated layout and PCB masks. However, I've not yet built it so I'm not suggesting that anyone uses the circuit or layout - I've just attached it for interest. Basically, it just uses the digital FM module in place of the RDA5807 FM module and Guy's programmed PIC, 'bolted on' to Guy's MW AM modulator circuit, which is tried and tested and works well.

However, whereas Guy's design runs off 2 x AA cells, this is mains powered via a 9V 'wall wart', hence the 100uF smoothing cap on the power supply input, with a100nF parallel cap. I've incorporated a diode to safeguard against inadvertent reverse polarity, but haven't added a Voltage regulator as I don't think the supply voltage is that critical, but I notice that the PCB power input does state '5V' whereas Derek says '9 - 12V required'. (The unit I have looks identical to his). That said, it has what looks live a 5V regulator on board marked '78M05.

I hope this update is of interest.
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Old 17th Oct 2018, 12:21 pm   #33
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Default Re: FM to AM converter - variation on a theme!

There's no de-coupling downstream of R4.

Lawrence.
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Old 17th Oct 2018, 12:46 pm   #34
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Default Re: FM to AM converter - variation on a theme!

Thanks for reading the post and for your prompt response Lawrence.

The circuit is attached - what are your suggestions?

Basically, I'm intending to power the unit from a traditional transformer/rectifier/capacitor wall-wart rather than a modern switched mode supply, and trying to keep it as simply as possible, though have already added C8 with C1 in parallel, though another cap won't complicate life too much! I don't want to build it and find it doesn't perform well, but that said, with this - as with the original design and the Mini-Mod - the original design concept was to keep things as simply as possible and easily reproducible.

All good clean fun!
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Old 17th Oct 2018, 1:07 pm   #35
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Default Re: FM to AM converter - variation on a theme!

It might well work ok without R4 being de-coupled, but to me it's not good RF/AF practice, screaming abdabs in a radio, a good chunk of the replies will say check the de-couplers or the supply impedance.

If it were me I would decouple the downstream side of R4 with a 100uF in parallel with a 100nF ceramic, that's between the junction of R4 and L2 and the -ve rail. The original design had the +ve feed decoupled with a 100nF for RF, any AF component on that line would have in effect been decoupled by the low impedance (at AF) of the supply (the battery) By introducing the 470 ohm dropper the impedance of the +ve supply to the osc has effectively been increased, in such cases it's normal practice to de-couple, in other words provide a low impedance path for the frequencies concerned.

Lawrence.

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Old 17th Oct 2018, 1:39 pm   #36
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Default Re: FM to AM converter - variation on a theme!

The antenna connection is exactly what it seems. You can attach anything to it from a few inches of wire to a full scale dipole. The radio tuner is quite sensitive.

These things are usually sold for use in a car with a 12V power supply, which goes straight into a 5V regulator. I use mine with a spare unregulated wall wart that supplies about 16V off load and it's been fine. I would imagine there would be problems trying to use it with a 5V supply unless the regulator was bypassed.
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Old 17th Oct 2018, 6:27 pm   #37
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Default Re: FM to AM converter - variation on a theme!

I would imagine that this would be a better de-coupling arrangement. I doubt the that MP3 player would be upset as that should have it's own arrangements.
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Old 17th Oct 2018, 7:58 pm   #38
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Default Re: FM to AM converter - variation on a theme!

Thanks everyone for your interest and responses.

Joe's suggestion has some appeal as it only involves a minor track alteration as shown below.

As to the antenna, I guess I'll do the same as with the Guy Fernando converter - just two short lengths of wire for FM & AM.

I just wondered what Derek did on his arrangement.
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Old 17th Oct 2018, 8:17 pm   #39
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Default Re: FM to AM converter - variation on a theme!

It's a bit like Post#22 and 27 etc revisited.

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Old 18th Oct 2018, 2:26 pm   #40
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Default Re: FM to AM converter - variation on a theme!

Is something like this what you're suggesting Lawrence?

100F on the DC input with a 0.1F in parallel, and likewise, after R4 (referred to as C9&C10).

Thanks.

Still at the doodling stage!
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