La la la radio AM modulator - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: May 23, 2025, 01:08:50 pm
content: Found a double sided test PCB for 14 pins DIL, from the TTL era, when most TTLs were 14 pins, with pins +5V/GND at pins 14 and 7.Both sides of the PCB have the same traces, one side is for Vcc, the other for GND. All holes are for 14 pins IDs, except there is one place with only 4 holes, for one of those 4 pins quartz oscillators can/brick the size of a 14 DIL IC. That reminded me of an unfinished project of a TTL based AM modulator. Maybe all those parts sitting for decades doing nothing inside various storage boxes will make more sense if soldered together.That gave an itch to solder something this weekend.Digital part was easy:- a 4.9152 MHz oscillator - that frequency was not a design choice, it's whatever happened to find through the scrap boxes. It's the frequency usually used to generate the baud rates for RS232 serial ports, from the times when the serial port was made with discreet TTL ICs.- a divider by 3 (not by 2) with a single D-type FF and a tuned LC, a trick seen in a PDF by Charles Wenzel- the second D-type FF was used normally, as a divider by 2, to get a symmetrical square wave at 819.2kHz, which is only 2.2% away from 819kHz (one of the MW channels)This is a 12Vpp square wave carrier, from an 7406 open collector fed with 12V.That was easy. And it worked from the first try.Instant gratification!Didn't solder anything yet, this is the breadboard, with a 4 pins osc on 4.9152MHz, and a 74LS74 dual D-type FF as a divider by 6.First FF of the 74LS74 is used with a trick, an LC tuned makes it to divide by 3 instead of 2, like this:From "Unusual dividers idea from a PDF" by Charles WenzelThe above picture is a capture from the linked PDF. My input frequency is 4.9152MHz, not 100MHz, and my desired divider factor is 3, not 10 as in the picture.I've tried various L for the best lock around the desired frequency: 100uH, 150uH, and 220uH. Best locking range for my frequency 4.9152MHz, and for my divider factor 3, was with L=150uH and C=100pF (it was still dividing by 3 for any input frequency between 3.9 and 6.3MHz, tried that with a signal generator instead of the 4 pins TTL fixed frequency DALE 4.9152MHz osc).(side note, measured The RLfor these coils, and it was about 3\$\Omega\$ no matter the L value)The other FF is dividing by 2, so a total 4.9152MHz divided by 6 = 819.2kHz, which is pretty close (2.2%) from 819kHz, one of the broadcasting MW frequencies.So, have a square wave carrier, now what?I'm thinking of these 2 options for AM modulation:- a transistor with a SRF and the Vcc average driven by the audio signal, or- use the open collector gates 74N06 in parallel, as a power amplifier, and at the same time as an AM modulator.In either of these, the audio voltage is modulating the voltage of the open collector transistors from the buffer TTL gates.Or maybe some other kind of AM modulator?Then, how do I connect a filter, and an antenna?

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 23, 2025, 01:25:37 pm
content: That pcb confuses me a lot, are you supposed to turn every other IC 180°?No wait, the spacing is also not the same everywhere... I guess it makes more sense when holding it in your hand.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 23, 2025, 01:38:14 pm
content: Added the not visible traces from the other side, all the apparently disconnected pads are connected on the other side of the PCB, like shown for the second IC. Also, in a similar manner with the Vcc thick traces, there is a similar GND thick trace on the other side.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 23, 2025, 01:44:25 pm
content: Now it makes senseSomehow my mind tried to fill it much denser.