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W0XIPosted - 7 July 2013 17:34  Show Profile  Email Poster  Edit Message
HEY, THOUGHT WHILE I'M ON HERE THAT I'LL POST. SCRATCHING AROUND ON PAPER TO COME UP WITH AN AM SIGNAL GENERATOR. MY BK PRECISION SIG GEN HAS RF AND DOWN TO AUDIO WITH SQ WV AND SINE OUTPUT AND ADJUSTABLE VOLUME. HOWEVER, IT DOES NOT HAVE A BUILT IN AM GENERATOR.

I DID A GOOGLE ON THE NET AND NOTE THAT THERE ARE SOME HOBBY ATTEMPTS THERE BUT FIND THAT THESE (THEY DID NOT KNOW) OR IT WAS INADVERTENT IN THAT THEY MULTIPLED THE CARRIER WITH THE MODULATION BUT DID NOT ADD BACK THE CARRIER. TRUE AM IS GENERATED WITH THE FOLLOWING A(1+m8(audio sign)* carrier signal but then one must also add back the carrier. Otherwise you just get DSB-SC (double sideband suppressed).

LOOKS LIKE THE NE602 OR SA602 WOULD WORK FINE, USING MY RF GEN TO DRIVE THE OSC INPUT OF THE 602 AND ADDING A 1 KHZ SINE AS THE MODULATION. I COULD THEN ADD A COPY OF THE OSC OUTPUT TO THOSE FOR TRUE AM GENERATOR.

MORE LATER GUYS. THANKS FOR LISTENING.

UNCLE PHIL W0XI

gzimmerPosted - 7 July 2013 23:45  Show Profile  Email Poster  Edit Message
> I COULD THEN ADD A COPY OF THE OSC OUTPUT TO THOSE FOR TRUE AM GENERATOR.

The NE602 is a double balanced mixer. When it is perfectly balanced you get out DSB (eg no carrier), but if you purposefully unbalance it slightly you get out true AM.

In practice, most AM modulators use a simple unbalanced mixer so there is plenty of carrier out, no need to add it in later.

If you like, there is a DC term in the calculation which adds in the carrier for you.

...........Zim


Edited by - gzimmer on 7/8/2013 1:47:53 AM

W0XIPosted - 9 July 2013 19:50  Show Profile  Email Poster  Edit Message
Hi Zim, long time since chatting. Yes, I'll try the bit of off-off-balance you suggest but want to do it the generic way too with add back. It is easy to do with todays hi freq op amps to just add the two, given I can get the carrier off the 602 if using it's internal osc.

By the way, have you every tried to build your own coherer?

Uncle Phil, W0XI

gzimmerPosted - 10 July 2013 3:21  Show Profile  Email Poster  Edit Message
Hi Phil,

> but want to do it the generic way too with add back...

But that's NOT the generic way.

Why would you carefully adjust the balance to get DSB, then muck up the balance by adding the carrier back in again?

Much simpler to set the balance to get the correct carrier level in the first place.

And don't forget that the phase of the added carrier is critical.

> By the way, have you every tried to build your own coherer?

Built one using a Triac running on AC with a bell in series. The bell dings when it detects RF and the AC makes it self restoring. Makes a great lightning detector.

........... Zim

gzimmerPosted - 15 July 2013 1:19  Show Profile  Email Poster  Edit Message
A few people have asked for more info:

The maths assumes a perfect multiplier, hence "Audio x Carrier = DSB output".

In hardware terms a perfect multiplier is a "four quadrant multiplier". This is a mixer with two differential (balanced) inputs, eg each input can handle dual polarity signals
(two x dual polarities = four combinations, hence "four quadrant").
If either input has zero applied, there is zero output. Four quadrant multipliers are expensive and usually have poor HF response, so are almost never used at RF.

More common is a "Two Quadrant" multiplier. An example is the NE602. One input is differential and can handle dual polarity signals, while the other input is single ended. This combination can only cover two quadrants, but is commonly used where a Double Balanced mixer (DBM) is required.
As a modulator it will output DSB, however if the bias is offset to give 50% carrier it will give true AM out.

The most common RF mixer is a "Single Quadrant" Mixer. An example would be a simple dual-gate FET with the Local Oscillator fed into one gate and the RF signal fed in the other.
When used as a Modulator, the FET is biased so that with no audio input the RF output will be at 50%. It can only output AM, it cannot be "balanced" to give DSB.

For a simple unbalanced modulator the maths is: Carrier x Audio + DC = DSB + Carrier
so in practical terms there is no need to "add in" the carrier, it would be completely redundant.

....... Zim

Edited by - gzimmer on 7/15/2013 1:34:37 AM

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