Slow Scan Television Experiments

[Harry Dalek]

The values of the components and the output frequencies listed within the 4046 diagram above agree fairly well with the Philips datasheet.

As it shows a 9-12V supply it's a HEF4046 (Philips) or a CD4046 (Motorola & Texas), so your examples should be OK.

I say 'fairly well' as there is an immense spread amongst these devices. This is down to the Silicon process used to fabricate the chips, it's more suited to logic circuits rather than linear ones - which is what the VCO is - linear. For the same reason these devices also have a chronic sensitivity to the supply voltage, it must be regulated otherwise the frequency will be all over the place. Note that pin 15 has a 7.5V Zener for precisely this reason. The HC version doesn't have or need this, a third phase comparator output is present instead.

But if you've got the unijunction version going I would continue with it.

Steve A.

Thanks Steve its good to know the 4046 i have will work that will be my experimental go a VCO ,i like to get the hang of these things to see how they should work .

Well i did end up dropping by Jaycar store today and got the ujt its in the circuit now and i know that now works it has a better or wider frequency range than the bc547 circuit version from the 73 magazine its pretty much just a little over or lower than the needed range ,This unijuction oscillator frequency range is about 600hz to over 10khz .

I will do a full test on the rest of the circuit when i am off work later this week .

I am putting together a simple tv bar generator for testing the sstv VCO circuits before i start playing with a head amp with it .

[Harry Dalek]

Working on a staircase grey bar generator for testing my sstv voltage controlled oscillator ,it will give me an idea whats needed for the head amp to work with both versions made so far . .

I was going to just use the 4060 to do both clock and stair case generator but ended up adding a 4017 so the 4060 is just an adjustable clock and the other for the bar generator .

Using this circuit but only as an idea mines a little changed ,the resistors i found which change the pulse voltages do need adjusting from the schematic but its is easy to adjust any out of whack with a resistor wheel and solder the correct value in .

I will post results show and tell

[Harry Dalek]

Theres i look at the stair case generator i will try and turn into a gray bar sstv test card picture .

The 4017 can do 10 steps but the voltage steps become narrow which i suppose make the black to grey to white look less dramatic i think would rather fewer bars any case i am going to leave it as for now to see ,at the moment theres a few bars in the dark grey as i am still using all the counts.

i think i need to ad the sync to the 4017 reset in this situation then the video then to the vco circuit and see what happens.

[Harry Dalek]

Successful today on the SSTV test patten generator and the unijuction transistor VCO .

Started here with no sync just to see its so slow not like your going to miss it and it worked with the mechanical glow drum sstv ,i will ad sync to stop the drifting but.

I tried it first on the bipolar 71 magazine circuit i could just see it was working as the thing needs 12 volt voltage swing to do the same thing the UJT will do with just the voltage out put from the 4017 waveform so the gray scale was very narrow towards the white but good to see and showed i was getting there .

Went back to the UJT circuit and did some testing i can see now playing with it working how much of the circuit could be simplified, so i will have one more go at the 4046 pll.

The evolution of the tests are below the successful one.

I tried in circuit just powering the UJT not powering the 741s if i could get any results and it works as in the bottom schematic ,not sure about the 2 diodes but as is it works on its own.

[Steve Anderson]

Slow to post - been 'rural' for a few days, no Internet, no phone - lovely!

Yep, the bars near black will seem compressed/squashed due to the approx. 0.7V Vbe drop of the output transistor, and this varies to some degree with emitter current.. A better solution would be to use a single-supply op-amp, a section of a LM358 (dual) or a LM 324 (quad) in a unity-gain configuration.

The two diodes in the original circuit acted as a simple white limiter, as you've re-drawn them they'll probably stop the thing from working, simply remove them.

Steve A.

[Harry Dalek]

Slow to post - been 'rural' for a few days, no Internet, no phone - lovely!

A phone with no Wi-Fi no connection is a bit of a pain back to the dark ages

Yep, the bars near black will seem compressed/squashed due to the approx. 0.7V Vbe drop of the output transistor, and this varies to some degree with emitter current.. A better solution would be to use a single-supply op-amp, a section of a LM358 (dual) or a LM 324 (quad) in a unity-gain configuration.

I was looking at using TLO72 or 71s i had handy might have a 324 have to check my junk box ,so you feel replacing the 741s with the above op amps in a same levels out as input level.

I also had another look at the 4046 VCO circuit as is i posted a few posts back with that 10nf cap and resistance giving around 2100hz bit short of whats needed but thats ok to test for now , its as i said not an instant recovery with a test voltage its as i said before sluggish to recover ,i need to change the cap and resistance for the same frequency range to see

The two diodes in the original circuit acted as a simple white limiter, as you've re-drawn them they'll probably stop the thing from working, simply remove them.

Steve A.

I will give that a go and see i put the diodes in as the 10k pot adjusts the oscillator as well as the frequency pot ,the ujt voltage controlled oscillator works well.

I will be interested to see what my head amp does hooked up to it and testing reflecting shades to the light sensor as i can see the results on the frequency meter ......in this case SSTV is so much easier .

I will work more on it as i can later this week .

[gary]

[
A phone with no Wi-Fi no connection is a bit of a pain back to the dark ages

All he has to do is go to the local McDonald's...

(Even I wish I hadn't written that... )

[Steve Anderson]

Just to be clear - there was no phone coverage at all, no signals. No WiFi, no G3/4. The village I stayed in has a population of around 150-200, served by two landline phones only, one in the general store the other I'm not sure. It recently (2011) got grid 220V, prior to that it was by local generators which were only run during darkness. Tractive farming power is still generally by Buffalo. Rural it certainly is.

Nearest 711, McDonalds etc. is 120km away.

Anyway, back to the subject at hand...

Steve A.

[Steve Anderson]

I was surprised by the comment r.e. the sluggish response of the 4046 VCO output frequency to a change on its control input. So I quickly breadboarded a HC4046 and a HC4040 as per the circuit below.

The two waveforms shown below show the VCO response to a step change in VCO control voltage. As I suspected it's instant. The VCO control input simply controls a pair of switched current sources/mirrors internally, there is nothing inside the chip to slow response time down except at frequency extremes. Although here the frequencies are higher than 'direct' generated SSTV I can't see any reason for a problem at lower frequencies.

This was tried using HC components, if I still have some HEF-style versions I'll repeat the exercise, at lower frequencies too.

Steve A.

[Steve Anderson]

Well, the HEF4000 series components performed exactly the same, supply raised to +12V and frequency dropped to around SSTV subcarrier values. Circuit and waveforms below.

Luckily I had some of these things hidden away.

Steve A.

[Harry Dalek]

I was surprised by the comment r.e. the sluggish response of the 4046 VCO output frequency to a change on its control input. So I quickly breadboarded a HC4046 and a HC4040 as per the circuit below.

The two waveforms shown below show the VCO response to a step change in VCO control voltage. As I suspected it's instant. The VCO control input simply controls a pair of switched current sources/mirrors internally, there is nothing inside the chip to slow response time down except at frequency extremes. Although here the frequencies are higher than 'direct' generated SSTV I can't see any reason for a problem at lower frequencies.

This was tried using HC components, if I still have some HEF-style versions I'll repeat the exercise, at lower frequencies too.

Steve A.

I am looking at your circuit to see what i am doing differently on the ic i can see i have not grounded pin,3 and the resistors between power supply and the VCO input.

I should of been more clear on the results i was getting it was instant change but i am getting a very slow change back to the start frequency when i disconnected a test voltage to the vco input which i wasn't sure about seemed some thing was up , it should of been instant as i saw in the bipolar transistor circuit and UJT .
So far i like the UJT oscillator
I will try again and see if it still does not work show the result .
I am also looking at The 565 Pll not sure how old these things are but i had two so i will have a look at this ic as well.

[Steve Anderson]

Harry, those two circuits above are not really modulators, really they're just to see if it really was slow to respond. As you can see it isn't.

Now when you say, "I am getting a very slow change back to the start frequency when i disconnected a test voltage to the vco input." Are just leaving the VCO input floating? Not connected to anything when you remove the test voltage? If so, it will be slow. The input impedance of these devices is very high, really hign. The input pin/socket/board trace have capacitance, this with the giga-Ohm input resistance forms a time constant which can be up to seconds long. This perhaps is what you were seeing.

The two resistors between 0V and the supply at the VCO input (above) are there just to set a centre-point for this experiment, in conjuction with the resistor from the counter it swings the control voltage between 1/3 and 2/3 of the supply.

I'm sure you've seen warnings like, "Never leave CMOS/MOSFET inputs floating." - that applies to linear circuits too. You could have fallen prey to this.

Pin 3 doesn't really matter - but it is an input - so I grounded it. Pin 14 is internally biased so no need to think about it.

Steve A.

A quick check of the Philips datasheet for the HEF4046 rates the DC input resistance of the VCO control input at 10^6 M, i.e. one Terra Ohm! Even the other maufacturers give it a figure of several hundred meg-ohm.

[Harry Dalek]

Harry, those two circuits above are not really modulators, really they're just to see if it really was slow to respond. As you can see it isn't.

Yes Steve this is a good thing as i have looked at my go again.

Now when you say, "I am getting a very slow change back to the start frequency when i disconnected a test voltage to the vco input."

Yes that was the result ,perhaps 3 seconds i could view the change back......... but not any more !

Are just leaving the VCO input floating? Not connected to anything when you remove the test voltage? If so, it will be slow. The input impedance of these devices is very high, really hign. The input pin/socket/board trace have capacitance, this with the giga-Ohm input resistance forms a time constant which can be up to seconds long. This perhaps is what you were seeing.

Yes i have to hang my head in shame ! i was That would be it Steve !

I just copied the 4046 vco i posted a while back its a bit of a hit and miss or a leap of faith with copying off the internet but what you mentioned did not come to mind as it didn't matter with the other VCO's just didn't think of it.

The two resistors between 0V and the supply at the VCO input (above) are there just to set a centre-point for this experiment, in conjuction with the resistor from the counter it swings the control voltage between 1/3 and 2/3 of the supply.

Well heres the good news once i grounded pin 3 also and i added the 2 100k resistors to the vco input that did the trick Ya for Steve ! instant change back to start frequency .

I also made the 565 but the bad news is nothing out of that ic at all yet i checked for mistakes i am not sure but i am looking at the round can ic and thinking the connections in the pin outs are looking at it from the top down . Oh well either the ic is not working or i have wired it wrong way around more than likely the later my luck.

I'm sure you've seen warnings like, "Never leave CMOS/MOSFET inputs floating." - that applies to linear circuits too. You could have fallen prey to this.

Steve A.

[/quote]

Another shameful head hanging Steve it does ring a bell but i didn't and don't understand that ic as to what to ground or not when used like this i can just copy and hope

[Steve Anderson]

Well at least the root cause of this oddity has been found. It has also confirmed that the frequency change is instant - well as fast as these devices can manage. I assumed this was the case but until today never checked that it was so.

Metal can chips etc.. Valve/tube pins are numbered clockwise when looking at the pins, the bottom of the device. This was carried over into multi-lead semis whether they're arranged in a circle or a DIP package. So when they show a top-view the pin numbering goes anti-clockwise. The mechanical drawings usually at the end of a datasheet mostly show the pin/lead view of metal can semis, clockwise numbering - easy to get confused!

Steve A.

[Steve Anderson]

Continuing... I was wondering yesterday if it's easier to approach this in a different manner. VCOs with their supply and temperature headaches, then the square wave needs turning into a sinewave which is usually done by heavy-handed filtering. The chip count was climbing too.

As this is such a low frequency application, using direct digital synthesis (DDS) might be possible with a simple micro. It is..

This is just the start, I have yet to work out the modulation method, but it looks promising. Below is the simple R2R D-A I used on the port output of the micro followed by an equally simple filter. The waveforms show a 1kHz sinewave output, pre and post-filter. The spectrum display shows the 2nd harmonic over 50db down, not bad for such a simple arrangement...and the filter at 2kHz isn't doing anything, it only cuts in above 5kHz.

This may just come down to two chips, a 14-20-pin micro and a 8-pin dual op-amp.

The frequency can be adjusted in 3.3mHz steps (0.0033Hz).

More to come.

Steve A.