NBTV/SSTV-625 Up Converter MkIII

Where it all started as far as most are concerned and saw heavy use from the 60s through to the 80s. Colour and Hi-res modes have unfortunately pushed this system into the backwaters of SSTV. Time to resurrect interest in this simple analogue system.

Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Thu Dec 26, 2019 1:32 am

What do you think of the Iout of the DAC08? Would it be better to supply there a fixed +6 volt? That is one transistor and one (emitter) resistor extra. Or would you rather connect it directly to +12 in stead of ground? I don't know the internal circuit. If the internal base is at a voltage low enough, it would not be a problem.

I remember that long, long ago, I used a DAC08 in a reversed direction. I forgot how and why, but I was satisfied with the result. If you know the internal circuit, you can sometimes do more with a circuit than where it was designed for.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Thu Dec 26, 2019 1:25 pm

Attached is the pdf for the DAC0800, the DA-08 is virtually identical. On page 4 (Figure 4} is an overview of the internals of the chip. The current source transistors and the resistive ladder are all close to the -ve supply. The constant current from each transistor is routed to one of the two outputs as set by the code on the D0-D7 logic inputs. This provides the complimentary current outputs. It's also the manner in which the wide compliance of output voltage/current is achieved.

If not using one of the outputs as here, the unused/unwanted current is dumped into a more positive load, here it's ground/0V. The rest is just voltage/logic conversion to drive the current-steering switches. Other datasheets provide the complete internal circuit, all resistors, transistors etc..but that's a bit too much to just get an understanding of the internal operation.

It's quite a clever bit of design and fabrication considering its age. I believe it was developed by Philips...the other manufacturers were simply second-sourcing the device. Often military bodies demand critical components are second-sourced. The Intel/AMD relationship is an example, if one company goes bust there's always the other. For hush-hush applications the public may not be privy to the second-sourcing arrangement.

Steve A.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Fri Dec 27, 2019 1:11 am

Steve, I think the DAC08 was designed by Signetics. Yes, Philips bought Signetics, but they were kept more or less separated. I found a databook from 1985 of Philips/Signetics that contains the DAC08 with lots of application circuits.

But it is also not impossible that Analog Devices was the first developper of the DAC08, and that Signetics copied the circuit.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Dec 27, 2019 1:12 pm

From this link it's inferred that the DAC-08 was from 1974...

http://smithsonianchips.si.edu/augarten/i42.htm

If that is the case that makes it something like 45 years young! Not many valves/tubes had a production run that long, there are exceptions, say the ECC83/12AX7, still in production, but in quantities of far less than 1% of the 50s & 60s...and often from dubious sources...yet still hideously overpriced...

If you click the Index on that page there's a whole lot of other chips most of us older guys have forgotten about, sometimes with good reason...and the younger have never heard of...and good luck in getting hold of any...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Dec 27, 2019 6:54 pm

Enough of that...this is the D-A chip used in the MkI up-converter, I think the logo is Signetics and the date code is the 19th week of 1984. So that chip is 35 years old!

I don't recall where I got that batch from, but it was within the last five years I would say...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Dec 27, 2019 7:21 pm

Also, some time back Harry sent me six of these chips, not heard of a chip maker with that name, but the cars we're all aware of...untried as yet...sorry Harry...I'll give them a blast soon...

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Fri Dec 27, 2019 9:12 pm

Oh never thought i would lay eyes on those again ... :wink:
They away's worked well for me a few times ,you can tell from just the pins they are well made .
If you are using them in the project that is good for me i still have a fair few .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Sat Dec 28, 2019 9:00 am

The "Fat S" is indeed Signetics. and as far as I know Philips bought Signetics in 1975. So that was one year after the DAC08 was designed and introduced. The difficult part is that the name "DAC08" does not sound a bell for me. In the Philips-Signetics databook of 1985 the DAC08 is the only circuit with a type number like that. Many uA... LM... NE... numbers and of course TBA... and TDA... numbers. The latter I recognize as typically Philips types. The uA, LM and NE types might be then Signetics coding.

I alsi have read that Signetics made a digital family of DTL circuits. I know that Philips was also a "believer" of DTL. When I can at Philips in 1971, there were plenty of DTL logic IC's, while the TTL circuits were introduced later. It is not impossible that the DTL circuits came originally from Signetics, years before Philips bought that company. The DTL circuits had a coding of FCH... for gates and FCJ... for flipflops and so on. Then came TTL, which was coded as FJH... for gate circuits and FJJ for flipflop circuits. There was no number coincidence with the 7400 numbers. It was the same mess as with the American coding of tubes (12AX7) and the European (Philips) coding (ECC83) for the same tubes.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sat Dec 28, 2019 12:10 pm

Klaas Robers wrote:...It was the same mess as with the American coding of tubes (12AX7) and the European (Philips) coding (ECC83) for the same tubes.

I think it's part of the human condition that forever we will make life difficult for ourselves!

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Jan 01, 2020 5:12 pm

First off - Happy New Year everyone! ...and a New Decade!

Some more 'adjustments' to the MkIII up-converter...

Note:- IC101 processor is 3.6V maximum, no 5V required in this whole device at all.

Steve A.
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SSTV-625 MkIII Main 1 v3-Model.gif
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Wed Jan 01, 2020 6:10 pm

Happy New year Steve and every one too.
Good to see your still hard at work on the MK3
When done do put up a parts list might have to order some ics i don't have.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Thu Jan 02, 2020 1:37 pm

Thanks Harry,

OK, I'll do a parts list when nearing completion. I intend to produce a full pdf version with all the final drawings, photos, screen-shots and descriptive text in one document. The parts-list will be an appendix.

I don't think there'll be any hard-to-find parts except the pre-programmed micros, I'll do those at cost and send them on unless you can program them yourself. Also the DAC-08/DAC0800 if you find it difficult to obtain. In your case Harry, you say you still have some Tesla examples, so I'll definitely give a couple of those you sent me a try.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Thu Jan 02, 2020 8:49 pm

I wish you all a happy new year 2020 and I am following the circuitional developments with great care. I am abslotutely going to build this circuit.

Steve what is the sensitivity of the PIC SSTV input-pin? Is it worthfull to circuit a limiting amplifier before it? In my analogue monitor I start with a circuit for FM IF amplification on 10,7 MHz. That is preceded with the two diodes, so the input has a sensitivity of 0,3 mV to 30 V. That is nice, I never have to bother about the level of the input signal, it is always high enough.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Jan 03, 2020 5:58 pm

Klaas, the FM SSTV signal feeds the inverting input of a comparator biased at 1.2V by R101 & R102, tolerance there is the 3.3V supply and the resistances mentioned. The non-inverting input of the comparator is fed from an internal 1.2V Fixed Voltage Reference of +/-50mV across the supply voltage range of +1.8 to +3.6V and temperature from -40 to +85 Celcius.

Unfortunately this device doesn't have the selectable 65mV hysteresis on the inputs a very few others do have. So I guess I'll find out in time whether an external squaring circuit is required. I'm hoping not. If it is, it is still possible to provide some hysteresis with a little external circuitry (a resistor or two) - I may do anyway, it can't harm. The comparator output polarity can be selected to provide the positive feedback required for the hysteresis function. (An internal XOR gate).

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Sat Jan 04, 2020 7:00 pm

A couple of minor changes to the comparator input, hysteresis added and the reference voltage is now half of the supply voltage (+1.65V). It's not worth posting the circuit again just for that. That reference is ratiometric with the supply voltage.

My next dilemma is how to convert the zero-crossing info into a fixed quantity of samples (128 or 256) per line in software. I have an idea, but converting that into code is not going to be easy...

There could be between 165 and 253 zero-crossings in 55ms, in the active video, and 12 in the 5ms sync period. IF the modulated frequencies are spot-on. So some allowance needs to be considered for those that are not. Also line timing, whether US or EU line-rates and the timing accuracy.

It may be worth making a modulator that can be deliberately off-frequency and timing...and throw in some noise too?

Steve A.
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