MK484 single chip AM radio question

Yes, any old op amp will do....but, they all draw way too much power for my receiver. The MK484 only draws 300 microamps.

The TLl081 draws 2 ma .

I am thinking about using plain ole transistors, the receiver I am building doesn't need to be extremely sensitive.

I have a source for the MK484 at present, and I might just order some and try it out myself.

Regards,

Art

PS, did the vlf schematic show up on abse? If not, I'll gladly email it to you.

Just as an update, I did some more work on the vlf transmitter. I disconnected the coils, whcih allowed me to probe the ASIC output feeding each audio amp chip.

The very wide 17 Khz output peaks at 18.8 khz, but there are many many

60 hz peaks with 15 hz peaks on each side.

So, the 60 hz signal that many said must be power supply hum is clearly intentional, and is not a product of power supply filtering.

Ok Joerg, I'll email it.

Be on the lookout for it.

I am positive there is no intentional leakage or injection of 60 hz. It should be noted that rectified and filtered residual ripple would be 120 hertz. I put spectrum lab on the dc at the filter cap, and there was no sign of 120 hz or 60 hz.

There is no connection to any part of the ASIC to anything except dc power. I can't get info on the ASIC, but they were nice enough to include labels on the pins, so it is quite easy to figure out what's going on.

FYI, there are 4 coils, each large and air wound. The dc resistance of each coil is 4.5 ohms.

GL.

A

Joerg, I have photos of the inside and out, plus all the FCC data (FCC ID = KE3IF100). You can search the FCC data base and download all the documents OR I can send them to you. The files are big though, be warned.

I'll send them or will give you the URL for them.

Notice that the 5 volt supply for the ASIC has it's own filter and is derived from the 16 volt dc supply. I checked the 16 volt supply with spectrum lab, so it's pretty unlikely that the ASIC supply has large ripple on it.

It appears to me that hopping the thing up would be easy...either change R26, R24 and R18 to give the amps more voltage gain, or perhaps changing R30 to a lower value Which is in series with the power output pot (R33).

To get the range I want (300 feet) requires much more transmitter power, so I'd probably need a larger wall wart and larger heat sinks for the amp IC's.

I think this is a wide band FM signal, centered around 18.7 kilohertz. Which probably explains WHY the range is so short:>:

Regards,

A

Oh, forgot to say in the last message.....

I think they use FM because it rejects amplitude modulated noise, hence it provides good rejection for most of the beeps and whistles down there.

The wide band FM also restricts the range pretty severely as well, but it's simple, quick and dirty:>:

Regards,

A

Hello Albert,

I haven't but isn't this chip discontinued as well? Just FWIW, I would build LF stuff in discretes and plain jelly bean parts. After all, at these low frequencies any old opamp works well. If it is for series production that will make the purchasers much happier. If it is for a prototype you can still get parts a few years from now if something breaks.

Regards, Joerg

Hello Albert,

Not all of them. The LPV324 is under 10uA per amp. GBW is just 150kHz but for this app it should suffice. At 30-40 Cents a quad not a bad deal. Large gains I'd do with transistors anyway.

No, I couldn't. If you have a web site just post it there and a link here so all can see it. Else, yes, just email me.

Beats me why they chose 60Hz. Are you sure it isn't ripple from an unregulated power supply? Maybe intended ripple?

Regards, Joerg

Hello Albert,

Thanks, got it. I just hope others who read here can see your post on a.b.s.e.

They do feed non-regulated DC directly into the supplies of the audio chips. But considering their good PSRR you are right, this would not explain the rather high spectral level of 60Hz.

This must be a pretty nifty concept. I remember when we tried 3D localizing with such fields back at the university. It wasn't easy. The minute something metallic entered the area it distorted the field.

Probably the receiver has a 3D coil arrangement as well. Maybe it watches the field ratios to determine the distance from the transmitter. But that would still not explain the sidebands.

The only plausible explanation for desired sidebands I can think of is the creation of a unique yet easily detectable transmission "footprint" in order to dodge interference from man-made noise sources. Such as TV flyback transformers, PC power supplies, energy saver fluorescents and so on. They are usually all in the 15-30kHz range.

Regards, Joerg

Hello Albert,

A URL would be better. Then others can participate, too.

I think the amps are rated 20W. But there will also be FCC limits.

Yes, and sometimes that is necessary to stay within FCC limits. Like the dithering that is done to disperse the nasty peaks from processor clocks.

One issue with transmitters inside houses is the insulation. Ours has aluminum foil backed fiber in the walls, often one roll stapled to the next. That muffles RF pretty good.

Regards, Joerg

"Any old opamp" means a 741 and similar such as the 1458, and they can't even do well above 10kHz. So use something a lot better.

breaks.

The product is an Instafence (also called Instant Fence or wireless fence). The manual for the thing is available on petsafe.com, look for the PIF-300 product (which also includes the collar). It transmits a short range signal at 18.7 kilohertz using large air coils as antennas. It is designed to contain a dog within a 90 foot radius and to issue mild shocks if/when the pet attempts to leave the area. It does not work with a buried wire loop, it uses the transmitter to define the dogs play area.

For anyone interested in purchasing one, they are horribly expensive (list price $280) and they have a special battery for the collar, which is only used by petsafe, available at 6 dollars each. The batteries don't last long. The range is only 90 feet, so it doesn't allow the pet to move very much.

It appears to be grossly over-engineered as well, an ASIC feeds one each amplifier separately and the amps appear to run at very low power. No doubt a much cheaper transmitter could be made, this one is also physically large as well!

For anyone interested in the petsafe IF100 vlf transmitter, you can look on

for the manufacturers info-

Or, check out pictures, schematic and field strength documentation at:

Here are some notes based on my personal observations (before I returned the device).

SW2 is hardwired, and is not adjustable as shown in the schematic. For some reason, this switch was called 'mode' back in 1998 when petsafe submitted the documentation to the FCC. The schematic shows it as a switch though. When looking at the schematic, the position of the switch in the schematic is shown correctly.

Switching S3 to ground selects low power transmit.

The pot, R33 is the power control.

The ASIC is marked as follows: Supertex

500-029 204T770 0225

It is impossible to tell how many turns are on each loop (transmit antenna). But, each of the 4 loops has the same size wire and has a dc resistance of 4.5 ohms. Vdd for the ASIC is 4.85 volts.

I have spectral output for each of the output lines (X, Y, Z). The spectral display shows the same signal on each output although the same output signal on all 3 output lines, although there are different peak amplitudes (some of the amp stages are driven harder than others, but all have the same basic input signals).

There are 3 audio amp chips, they are each the LM 1875T. They can run

20 watts output, but the output power is much lower than 20 watts for each stage. This is based on the V+ being very low (16.8v) and the observation that the 1.8A wall wart (at 14v ac) runs very cool. Also, although the heat sinks for each stage are very small, they are barely warm to the touch.

If anyone wants to see the spectral output of the ASIC that feeds the amps, send me an emmail to:

K Y 1 K at pivot dott nett.

Enjoy,

A

Just noticed an error in my previous post. The amanfacturers website is petsafe.net, and is NOT petsafe.com.

Hello Watson,

Actually I did build my first DCF77 receiver with several 741. It's the output that peters out above 10kHz but only if you need a few volts of swing. In a receiver application these amps can go well above 100kHz, provided that you stay under 15dB or so per stage with your closed loop gain and signals are in the uV or mV range. It worked nicely. I did like the 709 more though but it was kind of expensive in the 70's.

Then I made another receiver with transistors only. With the good old hot rods like the AF126. There were two in each scrapped TV tuner, but very hard to pry out without damage. This receiver was better and much smaller. It also ran on a couple of battery cells with ease, something none of the old opamps could do.

Regards, Joerg

Hello Albert,

Somehow I still think training is worth more. Worked for our dogs, most of the time... And if they are after something that is really important to them (cat) no beep or shock is going to stop them.

If it is a 6V or 12V stick sometimes you can even buy them at Raleys/BelAir and at Longs Drugs. 90ft is not a lot, it would not cover most of the yards around here.

Judging from the 32kHz crystal hanging off of it, the ASIC could also be a uC. But likely not if it says Supertex on there. I believe the concept could be realized with a sub-$1 uC.

It doesn't seem to be too over-engineered. The LM1875 is quite cheap but I would probably not base a design on an audio chip. They can quickly become obsolete when the consumer industry migrates to some other chip.

Probably a whole lot of turns.

Another concern with a shock collar is water. I have seen reports about badly burned necks when a collar went into continuous zap. It wasn't from this system but for our dogs I wouldn't leave it on unattended.

Regards, Joerg

would

at

can't

the

loop

like

Yeah, like 15 dB is a V gain of 5 or 6. So it takes three 741s to get the gain of a single transistor(!)

Sometime back in the '70s, in some amps, I replaced a number of Germanium transistors with Si ones, and rebiased the stages. Worked okay for me.

Hello Watson,

If you needed a really hi-Z input like for the tuned loop antenna you had no choice. Germaniums in those days just wouldn't cut it and a JFET was only affordable to high society.

Then the discerning and financially strapped hobbyist of those days had to live with what's there. Scavenging AF126 from a tuner that was soldered shut took almost an hour and several items from the first-aid kit. Buying them would have cost me several weeks of allowance per piece. I lucked out, or at least I thought so, when a local electronics store had a special on uA741. Re-labeled, so you had to live with some discrepancies with respect to the data sheet.

I rarely did that because ye olde BC107 cost more ($1+) than the transistors scavenged from TV sets and radios.

Regards, Joerg

get

JFET

Well, that's what a tap on the inductor is for. And a couple megs at

100s of kHz is easy to do with a Germanium bootstrapped emitter follower. And you include the shield by connecting it to the emitter, so much of the shield's capacitance is nulled out.

had

electronics

That sounds familiar. Some of those old chips were not even marked because they were so far out of specs. Then these 'seconds' were sold to hobbyists at a high markup.

Well, I did my fair share of scavenging, too. Trouble was that the transistors were often from TV sets with house part numbers, like Zenith. So it was a wild guess for me of what I was scrounging. I had enough sense to know that a transistor from the IF was a lot better for Rf work than one from the audio amp. ;-)

Hello Watson,

Problem is that Germanium RF transistors in the 60's had no beta to speak of. So for a bootstrap scheme you needed several. Considering that it was an hour's job with cuts and bruises to pry two AF126 out of a tuner these things became just too precious for that. Often they were literally jammed into divider walls that were more than a millimeter thick.

When I was a kid I tried to buy one. They wanted the equivalent of two hours of hard work in a meat factory. No way.

In Germany they used mostly regular issue transistors. In cases where they didn't I placed them in bags and wrote down exactly which part of the TV they came from.

Regards, Joerg

at

emitter,

that

thick.

had

for

I remember when I was in the army, stationed in Wuerzburg in '68. Guys would give me broken radios and cheap cassette tape recorders, etc. I would scrounge parts out of them. But I didn't know squat about Euro parts; up to then everything I had done was with JEDEC "1N" and "2N" parts. So when I came across those AC or OC or whatever 'Euro' parts, I didn't have a clue what they were. It wasn't until the Web came into being that I realized how common they were in other parts of the world.

Ooh! Toobs! Check this out. Proves that wirewounds take a licking and keep on ticking! I'd like to see the Klystron that goes with that PS!

Hello Watson,

I bought a "Transistoren- und Kristalldiodentaschentabelle". They can really make long words over there. This book helped me to figure out what the AC and AF types could do. OC was short-lived but I still have some, including "micro transistors" for hearing aids.

When I was in the army we had one common locker in the quarters. This contained a tube radio that was never to be seen by the drill sergeant, unless we were sure he'd know nothing about electricity. Bare chassis, no housing, no fuse, stuff hanging out of it. But it played.

Wow. My last 1kW power resistor was made out of about 300 single carbon resistors in a large metal honey vat. Tractor oil cooled it. Until that stuff leaked...

Regards, Joerg