FM Transmitter circuit kit ham free project spy FM bug how do I
Cana Kit's CK222 Stereo FM Transmitter

19th July '99

The (in) famous BA1404
Around late 1980s, ROHM Corporation made a pretty interesting Chip, the BA1404. It was a complete Stereo FM Transmitter on a chip, with inbuilt Stereo Multiplexor, Pilot Tone generation circuitry, RF Oscillator and an RF Buffer. It costed only about 2$ and could run using as low as 1Volt !!

Obviously you would not expect the state of the art professional broadcast quality from a 2$ chip, nor would you expect it to have a tremendous range running on a single AA cell. Still the chip was fantastic in that it allowed you to transmit some kind of Stereo at the real low end. I guess the chip was aimed at hobbyists, FM microphones, Lower quality Car CD's that transmit to the car radio etc

As the chip allowed you to attempt to do what would have otherwise taken 50-75 $ of discrete components, many kit manufacturers immediately adopted the BA1404 at the heart of their kits. They made basic BA1404 kits straight out of the ROHM application notes, boosted its output power by using additional stages after it ( Ramsey FM 10, CPEQ, Cana Kits), they improved the internal Oscillator by adding PLL Control ( Ramsey FM 25) and even tried to add bells and whistles like limiters and compressors around it ( Wavemach FMS2). Some decided that it's Stereo Generation circuitry was OK but the RF parts could be improved, so they used their own RF Oscillators and Buffers ( Wavemach; also hinted by Crystal HF)

Many hobbyists detested the BA1404 when they tried to make it do what it was not supposed to do, making it the black chip of the FM Transmitter family. However, what I liked best about the BA1404 was the amount of excitement it created among the hobbyists. There were FAQ's and mod lists on BA1404 based kits that went up to 10 Editions !!! There were mailing lists and ftp sites arguing about the pros and cons of BA1404 kits.

Now ROHM has stopped making the BA1404, but there are enough going around in the market for the next many years. There is a Japanese improvement/replacement chip NJM2035 from New Japan Radio, but it just does not seem to have generated the amount of excitement in pirate circles that the BA1404 did. The BA1404 is gone, long live the BA1404 !!

BA1404 data sheet ( 150K Zip file containing Tif files) also here
( Notice that the chip accepts max Vcc of 3.6V, that Output frequency depends a lot on supply voltage, but is quite independent from Chip temperature in the 25-40deg C range.)
Ramsey FM25 Circuit Diagram
( Notice 3 forward biased diodes used as a "2.1V voltage regulator" to feed BA1404)
Crystal HF uses only the Stereo Generation circuitry

Cana kit CK 222

Mr. Nima Ahrari, GM of Cana Kits of Canada sent me a CK222 kit to review. It is a standard BA1404 kit, the only extra being an external RF broadband stage to boost the RF output a bit. Cana Kit claim 200mtr range in the open.

The PCB is very good quality and so are the components. However the PCB is single sided and does not have large ground areas. A Double sided PCB with large ground plane area is necessary for 100 MHz circuits. Everything you need is provided in the kit, including a neat battery holder. There is enough place to work around easily. The kit took me about an hour to make. It is a joy to assemble the kit. The Documentation describes how to make the kit very well, but does not mention any technical facts at all.

If one uses a monolithic IC to do all the work, one is limited in the amount of genius one can display using that chip. One is forced to do everything the way ROHM dictates. One interesting touch I liked is that Cana Kit use the constant voltage drop across the "On Air" LED to feed the BA1404. The BA1404 has to be fed by a regulated voltage because it accepts only 3.6V max and also because it's output frequency varies with supply voltage.

The kit pre-emphasizes the Audio inputs passively. Unfortunately parts are supplied only for 50µsec ( The European/ROW standard) with no mention of how to obtain 75µsec ( the USA standards)

I connected the kit to my Kenwood CD and tuned it to 90.40 MHz. Tuning was easy, The signal moved around by only 25 KHz initially till Thermal equilibrium was reached. I tried different antenna and found that could shift the operating frequency by 200 KHz. The Stereo light on my receiver came on immediately. So if you box the kit and don't touch/change the antenna, the operating frequency stability is good enough for hobbyist use, making PLL control a bit unnecessary. I found I could cover my whole house and atleast 150 mtrs out in the open.

The sound quality is good enough if you like loud Rock and Roll and are going to receive it on your small radio out on the porch while the kit's connected to your CD player indoors. It's even great as your Dorm station. Buy It !!!

But if you are the type that already owns 3000$ of Audio equipment and like Beethoven, you will find deficiencies in the sound immediately. I could always tell between my Stereo signal and the pro signals next to it, but I could not tell exactly why. So I pulled out my old "Sound Check" CD and the Yanni CD that I use when I am designing Loudspeakers.

A) The CK222 was much noisier to begin with. It had a noticeable hiss making it sound like an old cassette player playing an even older cassette.

It also had an additional very slight high frequency tone. To make matters worse, the music would modulate the high frequency tone. Not noticeable when you play rock, but when I played the Bass drum track, after each drum beat, the high frequency note would ring and then stabilise a second later. It was very slight but it was always there.

I really wonder if this slight high frequency note was caused by lack of a double sided board and large ground areas ? One does have valid frequencies upto 53 KHz and their unwanted harmonics running around, along with the 100MHz carrier, warranting a board designed to RF standards.

Also, if you look at the NJM2035 Specs PDF, they suggest two ways of mixing the MPX signal with the Pilot -

Simple method : Add them up passively, just making sure they are in phase
More complete method : Put Pilot through a bandpass filter, Put MPX through a low pass filter, then add them up actively.

As the BA1404 and the NJM2035 are using digital electronics to generate 19KHz pilot from the 38 KHz crystal, they have to output square waves at 19 KHz. The FM Stereo rules say that the Pilot should be a sine wave, not square as square wave would have harmonics. These harmonics would fall into the higher part of the MPX signal, deceiving the receiver into feeling that there was extraneous content in the original Audio channels.

Similarly, the switched MPX would have harmonics centered at 3x 38KHz and 5x 38KHz.

Most BA1404 kits chose the easy/cheap way out and did not filter the MPX or Pilot before summing them up. Maybe that's the reason for the deficiencies noted above ? Maybe the BA1404 was a decent enough little chip anyway, but nobody used it properly ? No use post-morteming the BA1404 as it's gone, but hopefully the kit manufacturers will do their NJM2035 kits properly.

Or could it have been simple clipping ? When you slightly overdrive a Mono signal, you get a slightly distorted output. The harmonics generated are harmonics, ie still slightly pleasing to the ear.

When you overdrive an MPX circuit, you are in for trouble big time. Suppose you have a 10 KHz Audio signal and it is clipped a bit, it will generate harmonics. The Third Harmonic will be at 30 KHz. This would fall right into the L-R part of the MPX signal, 8 KHz away from the suppressed carrier at 38KHz. The Decoder in the receiver will erroneously construe this as if there was an 8KHz signal originally present in the Audio. So Voila, a bit of clipping on a 10 KHZ stereo signal generates a small 8KHz signal in the receiver. That's not going to sound pleasant at all !! Stereo Generators do need Limiters because the slightest clipping gives rise to distortion that is not harmonic. That's not really a problem solely of the BA1404 or the CK222 as all Stereo Generators would freak upon clipping. Sound quality did improve a bit by reducing Modulation levels.

B) Channel separation is not so good. But horrors, I put in only one channel of Audio and ground the other channel's input. I found that the stuff that one channel was putting into the other channel was low level but really distorted bad. I was expecting a low level of signal to ooze through to the other channel, but not so distorted !!

C) I ran a 20Hz-20KHz sine wave sweep signal through. As expected the kit went berserk at the high end. I lost the "Stereo" and the "Tune" lamps on my receiver which began outputting odd sounds and beats. According to the way the FM standards were designed, one cannot transmit more than 15KHz as that would affect the 19 KHz pilot tone used to tell the receiver that the incoming signal is in stereo. Professional gear would have a "brick wall" filter to remove all content above 15 KHz before feeding it to the MPX. ( That's not as easy as it seems, you can write a book on Audio filters. The steeper the filter, the harder it gets to design them without other problems showing up)

D) Bass response was acceptable, but a bit lacking. I had to put my equaliser's bass buttons up a bit.

E) And of-course, the simple kit did not have a limiter / compressor so it never sounded as "loud" as the pro stations.

Bottom line : Great fun to broadcast around the house / yard / dorm. Definitely NOT the chip to base your 5 W transmitter to broadcast around the town. ( Forgive me, Yanni !)

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My review of the Broadcast Warehouse 1W PLL Exciter with LCD 08 Aug 1998
My review of the WaveMach FMS2 Stereo PLL Exciter 20 Dec 1998
My review of the Mighty MAX-1 FM Exciter 02 May 1999
Stereo for Dummies 24 Sept 1999
My look at the NJM2035 Stereo Multiplexer Chip by New Japan Radio 20 Oct 1999
Comments on the ROHM BH1416 Stereo Transmitter chip 26 Aug 2000

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