Re: Multiplex decoder nearly rewired.



Jon Yaeger wrote:

> in article 1131928841.537937.234860@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, Bob at
> wa2ise@xxxxxxxxxxxxx wrote on 11/13/05 7:40 PM:
>
> > "I have an FM stereo miniature transmitter based on the BA1404 chip
> > running
> > off a 1.40 V supply, ( which is the right voltage btw ).
> >
> >
> > There is also some hum on both stereo and mono, 100Hz, and I don't know
> > where that's coming from, but it is in the signal from the ratio
> > detector,
> > so it isn't from the MPX decoder. Rails are clean as can be.
> >
> > If i switch off the FM signal gene and allow it to "run on" from the
> > store of energy
> > in the caps in its PS then there is no cessation of the hum
> >
> > Ah, HUM, the Royal Pain in the Arse when you cannot
> > find any reason for it to exist.
> >
> > I can't hear any hum in the background of other stations, but then
> > that's difficult
> > to hear with a signal going on. "
> >
> > Maybe the hum is in the BA1404 chip circuit? Try listening to it with
> > another good receiver and see if the hum is present or not. Hum is a
> > big problem with local homebuilt AM "micro" transmitters, usually it's
> > the RF signal paths being switched from one path to another thru
> > rectifier diodes in power supplies, causing undesired amplitude
> > modulation as seen by a receiver.. FM should be less sensitive to
> > this, but it may still "leak" thru and be heard at the levels you are
> > talking about. FM receivers do a good job of rejecting AM, but some
> > still can get thru. Like the 50db down you are getting. The fact
> > that you still have hum after shutting the receiver's power supply down
> > but before the filter caps discharge, and that you can't hear it on
> > radio stations, would make me think it's the BA1404 chip and or
> > residual AM from the rectifier diodes of the BA1404 chip power supply.
> > Try passing that power supply's DC output wires thru a ferrite ring
> > with a few turns bifilar wound to block off the RF from flowing down
> > those wires to the power supplie's rectifier diodes. That may
> > eliminate the undesired AM and the hum.
> >
>
> This isn't an "answer" and it isn't very helpful, but the BA1404 is what it
> is . . . an inexpensive, mediocre solution designed to run on a single AA
> cell. The circuit has a lot of deficiencies (e.g. frequency drift with
> temperature).

Yes, I am aware of its deficiencies, but today when i carefully compared the
recieved
signal of my new decoder and a fairly noise free Audio Reflex tuner using chips,

I concluded the AR is only 8dB better.
The AR makes 0.15Vrms output with a certain level of modulation on one channel,
and the decoder of mine makes 2.3Vrms, so when i amplified the AR signal
up to match the tube version, then removed the modulation on the test signal,
it was then I found the AR was quieter. I could live with the tube decoder,
but I am a perfectionist, and like low noise if its possible.

>
>
> There are a lot of considerations when using that device. Layout and
> shielding are important. I'd try shielding the circuit as best as possible.

It seems to be OK; the BA1404 is on a board with an RF buffer amp on its output
with filtering for harmonics, and is in a steel metal box. The box is inside a
slightly larger box
which has two modulation oscillators which give 5 different F for each channel.


>
> A much better candidate for the task is the ROHM FM transmitter series, the
> BH14xx. These feature a PLL frequency synthesizer, lower distortion, etc.
>
> I designed and built a portable stereo FM transmitter based upon the
> manufacturer's data, which was very comprehensive. It used SMT parts, and I
> needed a microscope to prototype it. LAYOUT WAS CRITICAL; I heard all kinds
> of spurious crap until it was done correctly. I got a former RF engineer
> from Scientific Atlanta to help me with that.

But for basic set up and channel sep adjustment, the BA1404 will do.

At the end of the day if better signals come from the broadcast stations then i
should hear it better
once the basic technical set up of the receiver has been established properly.


>
>
> I'd be happy to forward the schematic to anyone interested. I built it and
> it does work . . .
>
> Much

I followed a link from this site to extract the information below,
http://members.tripod.com/~transmitters/index.htm

Patrick Turner.

The copied notes re various chips....


20th Oct '99
My look at the NJM2035 Stereo Multiplexer chip by New Japan Radio
( To be read in conjunction with my "Stereo for Dummies" page )

The Rohm BA1404 tried to do too much on one chip, what with a Stereo
Multiplexer, Pilot Generation Circuitry, RF Oscillator and RF Buffer all on
board.

On top of that, Rohm's Spec Sheets did not show the basic filters on the input,
pilot and MPX that we all know are totally necessary. Low-end Kit manufacturers
did not bother to do a bit of basic design homework themselves. They decided to
do away with such "niceties" and just rake in the money. Most BA1404 kits are
exactly a copy of the Rohm application note with maybe an extra RF stage for
little more power.

BA1404 kits started a rash of FAQ's, newsletters, FTP sites. They all
concentrated on how to increase power or make it a bit more frequency-stable.
Nobody decided to get to the root of the Audio problems - Add the filters.
Wavemach came close by using only the Stereo Generator of the BA1404, adding a
Compressor/Limiter to prevent clipping/aliasing and ignoring the RF parts of the
BA1404.

New Japan Radio's NJM2035 (Specs PDF here) seems to be the nearest contender to
the market addressed by the BA1404. But even though the BA1404 is not being
produced any more for a long time, Low-end Kit manufacturers did not start
making a NJM2035 kit. I think the reason is obvious : The NJM2035 Spec ***
does not have a suggested PCB while the BA1404 one did ;)

OK, that's enough venom for a day. Let's get technical again.

Can I just unplug my BA1404 and put in a NJM2035 ??????

No, you cannot. The NJM2035 does not have an RF Oscillator and RF Buffer stage.
Maybe that is wise as now you have no chance of leakage of the MPX signal to the
RF and vice versa. That was one of the internal design flaws of the BA1404.

Is the NJM2035 any good ?????

The NJM2035 is just a Stereo Multiplexor chip using TDM chopping at 38KHz. It
does not use oversampling techniques.

The NJM2035's basic electronics seem to give a channel separation of about 35-40
dB, out of which the suggested MPX filter makes you loose about 5-10 dB in the
3-10 KHz range. Pro equipment would aim at 55dB separation.

The NJM2035's distortion figures are about 0.4% -0.6% in the modulation range
you will want to use it. Pro equipment would aim for distortion much below 0.1%

The PDF claims Signal to Noise figures of around 67 dB. That's quite fair
indeed.

The NJM2035 is made by a respectable company with many other high end Audio
processing chips like DNR, Surround Sound, Digital Equalisers.

So all in all, quite a nice chip for the low end. It will definitely outperform
the BA1404 and might even sound similar to the cheaper 38 KHz TDM switching kits
out there ( But for 20% the cost).

Of course, you should not even remotely consider it for serious broadcasting. It
is a toy, but a much better toy than the BA1404 and maybe some of the other
simpler kits out there. I would say as a rule of thumb, If all you can afford is
1-7W of RF Power, the rest of your Audio equipment would be such that the
NJM2035 would be ideal.

Mods

Don't know if I should call them Mods, as there are no NJM2035 kits out there to
begin with. Anyway, here are some engineering notes -

A) The PDF uses hard to obtain values for the MPX filter. Using SPICE, it's a
matter of minutes to redesign the MPX filter to original specifications, using
more easily available parts. ( A little tad better actually, but what's 0.1 dB
amongst friends)

The Goals are that the new filter should remove harmonics centered around 114
KHz ( Third harmonic of 38 KHz) atleast as good as existing circuit. Also, the
phase shift should be lower/same as existing circuit.

B) The original PDF shows a pre-emphasis stage with very slight roll-off of the
pre-emphasis curve at the high end. We would have loved the pre-emphasis to stop
emphasising signals above 15 KHz. Infact, it would be nice if there was a
brickwall that stopped signals after 15 KHz altogether. It is also nice to notch
out the Audio signals around 19KHz as they interfere with the Pilot Tone and
create beats.




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