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74LSxx series chips can make small AM modulated transmitters?

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neazoi

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hello, I have found some links on the net that use 74ls or other gates chips to produce FM modulated signals and their harmonics can be useful. Can the same be performed for AM modulation (video signal). I.e. is there any way I could make a simple video transmitter out of a gates IC? I am thinking of using the harmonics.
 

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I assume you mean RS-170 and not the new HD format video. If you are referring to HD, then no. Standard RS-170 would be difficult as frequency multiplying the sidebands would mess up the 3.58 sideband relationship to the carrier. However, on a straight frequency relationship, using the logic chips at carrier followed by a simple ring modulator will get you to DSB video and while DSB video screws up the folded sideband frequency passband (i.e. an excess of higher frequencies), diode ring modulators were what we used in camera chains (Sarkes Tarzian Broadcast Div.) On color, differential gain (amplitude distortion of the sub carrier with luminance) and differential phase (phase shift of the color subcarrier as a function of luminance) appear quickly and make the video look strange with color shifts as a function of picture brightness. Remember that the sync tips are at 100% of carrier and "white" is a small percentage of carrier for B & W and saturated high brightness colors need some room. Theoretically the use of a sawtooth at carrier summer with the video and a ultra-fast comparator should get you symmetric phase modulation at carrier frequency but I don't know of a fast enough comparator. Maybe stacked ECL 10K drivers would work, but the negative supply is a pain. I would suggest using the 74HC or 74HCT as the LS is very slow in comparison.
 
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neazoi

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I need to make myself more clear. This is all about a standard composite video signal with no audio sub-carrier. Also this is all about a B&W video signal, meaning no color information.
Maybe is this easier to reproduce now?
 

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WA9VEZ

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I need to make myself more clear. This is all about a standard composite video signal with no audio sub-carrier. Also this is all about a B&W video signal, meaning no color information.
Maybe is this easier to reproduce now?
Under that subset, then using a pair of PNP fast transistors, emitters tied together, base of one to the logic/RF pulses and the other to the video with sync tips at zero, (probably need "base stopper" resistors)
, collectors to Vcc, will work as a fast and nasty video modulator. This was the old "non additive mixer". . . Use low capacity (Cob < 5 pfd) high ft > 300 mhz devices. (The 2N4917 comes to mind) A simple diode clamp (we liked germanium OMC28's) will keep the sync tips at ground.
The fancy alternative would be a schottkey diode limiter out of the logic instead of the transistor, but that gets into temperature drift issues.
By using a NPN emitter follower after the NAM, it keeps the temperature tracking issues fairly close. It will take some RF linear amplification if you need appreciable power other that a few microwatts.
However, this is not a "logic" solution to an analog modulator, it is the analog use of logic chips like we used to do with 914 Fairchild RTL chips.

---------- Post added at 10:55 ---------- Previous post was at 10:10 ----------

Under that subset, then using a pair of PNP fast transistors, emitters tied together, base of one to the logic/RF pulses and the other to the video with sync tips at zero, (probably need "base stopper" resistors)
, collectors to Vcc, will work as a fast and nasty video modulator. This was the old "non additive mixer". . . Use low capacity (Cob < 5 pfd) high ft > 300 mhz devices. (The 2N4917 comes to mind) A simple diode clamp (we liked germanium OMC28's) will keep the sync tips at ground.
The fancy alternative would be a schottkey diode limiter out of the logic instead of the transistor, but that gets into temperature drift issues.
By using a NPN emitter follower after the NAM, it keeps the temperature tracking issues fairly close. It will take some RF linear amplification if you need appreciable power other that a few microwatts.
However, this is not a "logic" solution to an analog modulator, it is the analog use of logic chips like we used to do with 914 Fairchild RTL chips.
Sometimes I get focused on the wrong rabbit hole.

There are very inexpensive chips (LM1889N historical exemplary) that produce very good quality (5% DP/DG) video at RF with color and audio. Using LSTTL to make video is akin to designing a wheel with flat surfaces. The following doesn't include the vestigal sideband filter required for flat receiver response but the circuit is pretty simple to reproduce:

Tv Transmitter with lm1889 for 500 meters, audio modulator and video of DVD, Camera, video cassette or computer

I bought my daughter an entire AC mains powered RF modulator for $5 this past Christmas. . .
 
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neazoi

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Hm... this integrated solution seems a little bit too complex yet, compared to the elektor's video modulator in that page Sinclair computer
I was thinking of a single ttl chip alternative, so the other solution/experiment you propose seems more attractive to try. Just find a ttl chip that has an internal pair of transistors like you proposed and try this...

---------- Post added at 17:40 ---------- Previous post was at 17:37 ----------

What you think about these.......................

Digital IC Transmits Both FM and AM Simultaneously - Transmitter Circuit - Zimbio

I think this oscillator is ok.
Maybe work as AM , if you modify output stage as modulator by controlling supply voltage. (Osc. stage must have stabil supply. )

FM Transmitter Schematic Circuit Diagram (Using Logic Gates) | schematicwiring.com

Regards KAK
I was referring on these circuits at the beginning of this page, but these are all FM modulated.

They won't work as AM modulators. The oscillator is inside the chip, as well as the buffers and this chip has only one VCC for all gates.

I am wondering if varying the VCC of the chip, will be able to modulate it? Or the gates outputs are not affected by the varying vcc? (I think this is the case)
 

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Did you consider switching an audio signal using CD4066 for example? This needs, after the switch, a bandpass filter around the carrier though.

Kerim

I wrote you this, though I know you look for a more direct way to generate AM... sorry
 
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neazoi

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well, maybe this is helpfull, maybe not. These are the internal schematics of the 74ls05 http://focus.ti.com/lit/ds/symlink/sn7405.pdf
Is is really impossible to AM modulate this thing by varying the VCC or by other techniques?
Especially the 7405 which does not have an input diode.
The only problem I see is the limited vcc 4.5-5.5v
 
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