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Miniature FM transmitters

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Miniature FM transmitters

Post by crackle » Sun Jan 05, 2014 3:02 pm

I came across these videos on youtube today, has anyone tried these circuits.

FM transmitter.png
FM transmitter.png (8.61 KiB) Viewed 2964 times

and this one

FM transmitter 2.png


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Re: Miniature FM transmitters

Post by Marconi_MPT4 » Sun Jan 05, 2014 4:04 pm

Hi Mike,
For a cheap'n'cheerful circuit these work after a fashion. This circuit and variants of it has been around at least since the 70's and most of the students on the R&TV course built their own versions of them with parts to hand. None of the components are critical and my version used the ever-useful BC108 ! They can suffer from excessive drift though, and not unreasonably is worse at higher supply voltages. I tried one at 15 volts achieving a distance of at least 100 yards for a minute or so.. :aad

Best results are obtained with a 3 to 5 volt supply and some transistors tend to have lower drift characteristics. One lunchtime managed to transmit from the workshop inside a metal framed and partially clad factory unit to the car park outside with a Sony walkman providing audio source. Shame about the technicians choice of music! :qq1

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Re: Miniature FM transmitters

Post by Michael Watterson » Sun Jan 05, 2014 4:27 pm

They are all poor. I used to make these 10 to 30 years ago. Now I buy a cheap ready made one with synthesiser & stereo coder under £5.

Many cheaper ones only take one AAA cell, yet IC needs 2.75V min. So I use the 5V mini-jack socket (there is a 5V to 3.3V LDO regulator on board).

One resistor + 2 ceramic caps and 3 wires to an inline 3.5mm jack allows a plug in RDS encoder locked to 19kHz pilot.

Two basic kinds:
1) Simple chip with pre-programmed channels and slide switch to select.
2) Deluxe chip 76MHz to 108MHz 100kHz steps via I2C, also with CPU (drives LCD direct) and LCD on PCB and buttons for channel up/down.

They are in "pound" type shops and also from Hong Kong. The Hong Kong are #2 at local cheapest #1 price inc postage.

The more deluxe kinds are up to £75 and use the SAME synth/stereo encoder/modulator chip as #2 usually £5 to £8 above. Waste of money to have a built in FM receiver to find empty channel or an iThing Apple port.

The #1 model is same quality signal as #2 but limited to 6 channels instead of any channel.

Put a ferrite clamps on 3.5mm jack cable and power cable to set the dipole size as the "aerial" is coupled to 3.5mm or 2.5mm stereo in Jack!

PSU needs to be 50/100Hz and RF clean. I've had to add extra PSU filter on output of some 5V phone SMPSU plug-top PSUs. The medium size Nokia coax power jack fits most of these as do most USB to HDD PSU jack cables.

I power mine from laptop USB with off the shelf adaptor and then WinAmp to the FM Radio sets in the house.

An unmodified CE marked model is legal. The home brew ones are not legal and without a wideband spectrum analyser you have no idea what interference there is.

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Re: Miniature FM transmitters

Post by Michael Watterson » Sun Jan 05, 2014 6:21 pm

Murphyv310 wrote:
One resistor + 2 ceramic caps and 3 wires to an inline 3.5mm jack allows a plug in RDS encoder locked to 19kHz pilot.

I've asked before for some info on this but you didn't reply, any chance of some diagrams please.

Sorry. I'll dig out photos.

Basically all the chips have a 19kHz pilot out pin. One 1nF cap, this is Reference for the RDS encoder PLL lock in. The datasheets for all the chips on line.
The VCO tank circuit has varicap and coil. Varicap fed from pin which is Encoder output via resistor. Add 220k resistor to varicap close to it. Plus decoupler 100pF and coupler 1nF, this is fed from RDS Encoder out.

So 3 caps.

I used chip parts on one and discrete on another.

I got my UK Amateur licence in 1972 so I could legally build and use 144MHz to 146MHz transmitters.

I've no doubt some of the "iTrip" models sold are poor and some may have fake CE marks. But they are cheap, crystal controlled and more than one channel. I'd use my time to build something interesting not available with a CE mark for less than £6. You'd have difficulty retail buying the parts for VHF Synth with stereo encoder and 5V to 3.3V regulator.

I have built MANY professional VCOs part of synthesisers from 1983 to 2008 for the day job. Final RF Frequencies 2MHz to 10.6GHz. I've got plenty of good circuits. They are useful for L.O. in specialist receivers.

I'll look for the photos.

Small cheap RDS encoder you can plug into PC serial port. There are "plug-ins" for winamp so it outputs the text of "now playing" to the serial port or TCP/IP port. Writing a small program to reformat is easy. Actually We did one on a PIC with USB and Serial and plugged our PIC into PC USB port to take WinAmp text AND control Winamp. One PIC serial port controlled the RDS encoder. An off the shelf 433MHz modules TX on keypad and 433MHz RX allows remote control of WinAmp/PC.

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Re: Miniature FM transmitters

Post by Michael Watterson » Sun Jan 05, 2014 7:20 pm

The Remote control:

It has FM Radio chip, Stereo decoder, 8 pin DIL headphone amp, RDS Decoder and PIC with USB to control LCD and a 433MHz OOK/ASK off the shelf transmitter module. There is also an IR LED and IR 38KHz 3 pin receiver module.

media-handset.jpg (41.8 KiB) Viewed 2903 times

The Base unit can use the custom RDS messages to acknowledge the commands.

As the PC /Winamp is remote controlled over 433MHz, the messages have FEC using CRC and are encrypted. The encryption keys are automatically exchanged at short range using IR link.

The handset is also a Universal Learning Remote for IR controlled gear in same room. Known code tables can be downloaded via RDS or IR from PC via Base.

The Base has USB to connect to PC/laptop. It has 4 stereo audio in, one for PC /laptop sound out and 3 for other things like DTT set-box, DAB Tuner, Satellite Set box. It has IR out to remote control these.

The handset selects which audio in to use. PC/Laptop/WinAmp is remote controlled via USB which also provides the "now playing" text.

Front panel has GUI for Setup. There is 2.5mm jack for IR LED, but also IR LED and IR 38kHz detector on front.
Inside is
1) Off the shelf VHF_FM Stereo transmitter €5 type. The channel switch removed and PIC selects channel.
2) Off the shelf Preprogrammed PIC as RDS Encoder. Not complicated enough to need a PCB. The Data comes from PIC via TTL serial. The modulated 58KHz data out via simple resistor based DAC from PIC parallel pins. Software based 19kHz pilot lock in PIC code.
3) Off the shelf 433MHz RX module (bought as pair with TX in handset)
4) USB socket, crystal and PIC on veroboard. A few caps and couple resistors. The PIC wires direct to USB socket.
5) LCD module, IR LED, 3 pin 38kHz IR Detector (same as in VHS) buttons

Missing is 4 way CMOS switch (one IC) and audio inputs. For test only one stereo audio in.

FM Transmitter:

Based on #1 type. The A7303 chip. The next level up is I2C controlled ANY frequency from 76 to 108.

The two squiggly white points are 19KHz out (needs DC block cap) for sync and the varicap (needs series 220K resistor + DC block capacitor + parallel VHF decoupler cap for 57kHz RDS (or other subcarriers) in.

I found from data sheet if that I unsoldered 6 way channel switch and cut /wired tracks then D C B A (4 connections) parallel data from PIC (or a 16 way 4 bit rotary switch) allows all 16 channels the chip can do rather than only 6.

If I was doing commercial version I'd use the I2C version of chip. The commands are simple and using JAL for programming PIC the I2C is simple.

You need suitable test gear to set the correct RDS carrier level drive (a pot on the RDS encoder "DAC").
Last edited by Michael Watterson on Sun Jan 05, 2014 7:47 pm, edited 1 time in total.

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Re: Miniature FM transmitters

Post by Michael Watterson » Sun Jan 05, 2014 7:39 pm

So when I am building the Home Remote & Media Streaming to Rule them all, why would I waste time with an inferior FM transmitter when in the LOCAL shop for €6 with Stereo and multiple crystal stable channels?

BTW JAL has libraries to read SDIO slot (SD card) and even PATA IDE FAT!. The PIC USB is slave, it can't read USB Stick. A $5 Stereo DAC chip has MP3 decoder option, so integrated SD card playback of MP3 possible too.

But the idea was a mix of Satellite (pre programmed times), Internet Radio & MP3 from old PC/Laptop etc. All to be listened to on ANY VHF-FM Radio, with a living Room Remote wirelessly (even outside workshop) allowing edit of the automated Radio station.

My only standalone RDS set was the car Radio. So my son (who did parts at my suggestion for his final year project, he wrote the RDS decoder* and 433 MHz Encryption of handset for his FYP).

So I sat in Car and he fiddled with the mix of Off the shelf & custom SW to get the WinAmp text on RDS!

Then I added the Philips RDS receiver Chip to data socket on my FT817ND portable Communications (the Data port has no audio filtering, the raw discriminator on WBFM). His code on a PIC (basis of handset) then decoded the 1190bps data from Philips RDS chip and displayed on the LCD module!

We then tried driving a Stereo decoder & the RDS receiver from the Cheap FM Receiver IC in €2 FM radios. That was tricky and easily swamped by interference from our PCs/Laptops and PIC :(

fm-radio-tda7088-receiver.png (6.17 KiB) Viewed 2900 times

The RDS receiver IC connects in parallel to the Stereo Decoder input marked Mpx

The Philips RDS Receiver gives 1190bps raw RDS encoded real time data from the 57KHz subcarrier. The actual text needs decoded. The RDS is madly complicated as it has all kinds of info and FEC so you know about data errors etc.

There is User defined data :)

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Re: Miniature FM transmitters

Post by Michael Watterson » Sun Jan 05, 2014 7:51 pm

The 7088 isn't a good choice!

It's clever though. Uses the AFC/PLL to reduce the FM deviation (tracking filter PLL FM decoder) so only 70KHz approx IF works. The standard capacitor values need to be reduced a little I think to have reliable 57kHz subcarrier. It does work OK using standard values for 38kHz mpx SC.

One easy to used RDS Decoder (PIC, code and LCD needed, I can supply code) is Philips SAA6579

You need 4.332 MHz or 8.664 MHz crystal. A PAL or other similar TV crystal will not work.
I got a load cheap from Gollidge.

Source of cheap RDS Encoders

This is preprogrammed PIC I used (cheapest solution then PIRA32 MCU). Easily built on vero. ... oduct_line
You DO need a 4.332 MHz crystal for this too! Every thing else needed you'd have.
They now have a cheaper pre-programmed PIC.

If you are busy or less experienced with PIC best to buy a complete kit.

Note these people also [used to] sell stuff quite illegal in Ireland & UK, even if you have RSL because any transmitter under RSL has to be ETSI approved. [Edit: the dodgy stuff seems gone]

I can probably spare one or two 4.332 Crystals if anyone is mad keen to DIY an RDS encoder or decoder without a kit.

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Re: Miniature FM transmitters

Post by Michael Watterson » Sun Jan 05, 2014 8:29 pm

Seems I was using TDA7330 as the RDS receiver though, not SAA6579
Makes no difference to FM Receiver or to the PIC based decoder and LCD display.

With TDA7330 or SAA6579 and PIC and LCD you can add RDS display to any FM Radio as long as IF is OK.


I have code for PIC + LCD.

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Re: Miniature FM transmitters

Post by crackle » Sun Jan 05, 2014 10:13 pm

I probably wont bother with the youtube one, and stick with my "legal"one.

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Re: Miniature FM transmitters

Post by Terrykc » Sun Jan 05, 2014 10:44 pm

Marconi_MPT4 wrote:... They can suffer from excessive drift though ...

To true, Rich, especially if cobbled together on a bit of tag strip ...

Also they are very sensitive to capacity effects on the aerial.

The best one I ever built was this one:

FM_Tx.png (9.62 KiB) Viewed 2867 times

I've drawn it from memory but I think it's right ...

It appeared in Wireless World around about 1965 as the RF modulator in a Stereo Multiplex test generator.The modulation uses a varicap which has a DC bias to keep it on the most linear part of its characteristic but the important difference, compared with any other design I've seen, is that the inductor is configured as part of a Pi network which is then intended to feed the output socket via the 6dB pad.

I built this on a hand drawn PCB I etched, using nail varnish for the resist. The original intention was to mount it in a 2oz tobacco tin, but that never happened!

When it was finished I just connected it to a PP9 and off it went - a very powerful carrier that filled the workshop and, as we later found, the entire shop area which was in front of the workshop. No aerial was ever connected to it and as for stability, it didn't matter what you did with it - dangle it in mid air on its connecting wires or lay it down on the bench, it didn't budge! In fact, the only way to detune it was to bring a finger in very close proximity to the coil!

One excellent use for it was when we were inundated with several sets with intermittent FM faults, all of which had to be soak tested. FM radio 50 years ago was a pain! The Morning Story, Women's Hour, Mrs Dale's Diary (or was it Waggoners Walk by then?) - all rather tedious and boring to us youngsters, so we relayed Radio Caroline on FM instead!

My copy of the relevant issue of WW disappeared many years ago but if someone can trace the original, complete with the component list, I wouldn't mind a copy ...

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Re: Miniature FM transmitters

Post by Michael Watterson » Mon Jan 06, 2014 12:04 am

The Minicircuits VCO modules actually have two varicaps. One is tuning and the other is the output low pass filter.

My simple VCOs (Clapp Osc I think) for testing synth chips didn't have the tuned filter:


A = High VHF
B = Hyperband/Low UHF
C = 1GHz to 2GHz! The "inductor"/coil is a 1/3rd thickness vero strip on left edge.
D = Minicircuits 850MHz band module. Rather more expensive

The vero modules have a copper earth plane glued to other side.
Veroboard is really good for SMT if cleaned and degreased.

These actually work plugged into breadboard. The edges are SIL mini- header strips that can fit in IC sockets, not the fat square pins.
These all work as FM modulators with DC tuning, or as the VCO of a synthesiser for Receiver or Transmitter.

This is Dielectric + brass screw + varicap 9.75GHz VCO (not my design, but I was modifying the board)

When in the case varicap voltage is measured via access screw hole and the brass screw above the puck adjusted to put varicap at ideal voltage range when locked.

The L.O. feeds a prescaler for Synth lock and two mixers (one for about 10.2GHz receive and one for 10.570GHz transmit) for the RX IF (about 480MHz) and TX 2nd IF (about 820 MHz). A separate mixer converts outgoing 1st if 20MHz to 820 using an 800MHz VC0.

I did have a photo and circuit someplace of the inside of Minicircuit VCO. Any VCO is potentially an FM transmitter unfortunately.

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