MC1496 Mixed technologies Pantry TX.

[Murphyv310]

Hi.
From the output of the toroid I get 100mv P-P. Into a 50 ohm load it drops considerably to about 5mv and distortion goes up. This is why I fed the output to a high impedance load AKA the 1J18B. Although this works perfectly I'm at a loss as to why it doesn't operate the way you have explained, I wonder if it's down to the veroboard? Somehow I doubt it though.

[ppppenguin]

I certainly wouldn't use stripboard for RF but I doubt it's the cause of your problems.

My usual prototyping technique is pinboard (like veroboard but without copper strips). First step is a good solid ground bus round the sides. Often with extra ground wires across the board. Components soldered to pins. Chips usually in sockets with decouplers directly underneath for logic. I've used this for video, with accurate performance up to 10MHz and decent results up to 30MHz. If I know I'm using higher frequencies then it's a ground plane. Either colander board which is like pinboard with a groundplane or ordinary PCB material, drilled as needed.

A thought. Are you driving the upper quad hard enough? You need a volt or so p-p of carrier. The undriven side of the quad must be biased to the same voltage as the driven side.

[Murphyv310]

Hi.
According to the data sheet I'm referencing off the carrier drive is 300mv P-P, I'm slightly over that figure.
I'm well aware of the limitations of Vero, I'd like to do this on a PCB but its one area I've never tried.

[Geordie McBoyne]

The MC1496 was used in the BBC digital 625-405 standards convertor as the video, sync and blanking mixer.
I believe it was the early metal can version.

Metal can LM1496: http://www.mit.edu/~6.301/LM1496.pdf

https://www.radiomuseum.org/tubes/tube_mc1496.html

Radiomuseum gives the intro date as 1972.

Geordie McBoyne.

[radioman]

I'd like to do this on a PCB but its one area I've never tried.

I'll make you a PCB for this project IF the design is complete and finished.
I used to make dozens of them for students when I worked in the college.
Any changes to a circuit generally means a new PCB design though... otherwise it can get 'messy'.

Andy

[Murphyv310]

Thanks Andy.
I've been still doing some small revisions and trying other suggestions. There has been a small increase in output with no detrimental effects on quality. Overall I'm very happy with the results and audio quality. Definitely the best pantry transmitter I've made.
Just a couple of things to try then I'll sign it off with another revised diagram.

[ppppenguin]

There are no electrical differences between the metal and plastic versions. The 1596 is an extended temperature range device but otherwise identical.

PCB not needed. Pinboard, colander board etc are fine. Obviously a PCB is useful if you plan to make several.

[Murphyv310]

Hi.
Three component changes give 50% more output from the chip. The gain setting resistor can be reduced even further to 150r from 390r, the emitter resistor on the oscillator can be dropped from 1k to 680r for more drive if needed, and the 820r from pin 8 that connects to the 470r & 8.2k can be reduced to 470r. This gives the output a boost of 50% overall. Without the 1J18B range has improved sufficiently to drive a good few sets within 5 to 10 meters.
My next idea is to rewind the toroid or simply reverse it to give a higher peak to peak drive to the 1J18B as at the moment its configured for giving a lower impedance output.
One for tomorrow though.

[ppppenguin]

If the top quad isn't driven hard the output will definitely suffer. Not sure what happens if you heavily overdrive it. The 1496 is not a true multiplier, the gain from the top quad to the output is very non-linear. So you really have to use it as a sort of switch. The input to the lower long tail pair is pretty linear, due to the resistor between the emitters. The 1495, based on the 1496, has a lineariser before the top quad which makes it into a true 4 quadrant multiplier.

[ppppenguin]

One thing that worries me about this TX is that there is no harmonic filtering. Need to check the output with a spectrum analyser or receiver tuned to likely harmonics.

With my background in video design, if the 1496 output alone isn't enough, I'd stick a wideband opamp on the output. The EL2020 was the archetype. That would easily deliver a few volts p-p into 75R. It's not the way an RF designer would do it, but I don't think of myself as an RF designer.