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Does this circuit even make sense?

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Mechatrommer:
range what? freq range or amplitude range? do i have to read all those text to understand what is what, and then there's FC, another freq select what? the table only shows their input voltage range.. its my miserable day...

drussell:

--- Quote from: Mechatrommer on October 27, 2019, 08:28:22 am ---range what? freq range or amplitude range? do i have to read all those text to understand what is what, and then there's FC, another freq select what? the table only shows their input voltage range.. its my miserable day...
--- End quote ---

Huh?  You're complaining that you can't understand the datasheet without reading it?   :-//

The first paragraph (at least of the 1980s TI datasheet) talks about the basic features of the chip, like having Frequency Control and Frequency Range voltage controlled inputs.  These basic features and attributes are then referred to multiple times throughout the datasheet.

Do you expect to just intuit the functionality of the chip through osmosis by waving your hand over the datasheet or something?     ;)

hermitengineer:

--- Quote from: Mechatrommer on October 27, 2019, 08:28:22 am ---range what? freq range or amplitude range? do i have to read all those text to understand what is what, and then there's FC, another freq select what? the table only shows their input voltage range.. its my miserable day...

--- End quote ---
For that much, you'd start by referring to figure 3.  It gives you a frequency graph with 3 plots for FC=5-RNG volts, so that you can choose your extremes and choose a capacitor to base it on.  Looking at it from the analyst side, use the external capacitor value to find the frequency ranges.  This is 56pF+, depending on the setting of the capacitor, so it's going to be towards the left side of the graph, in the MHz range (as expected).

Figure 1 gives you a 50pF reading with FC on the horizontal axis and plots for RNG=0V to RNG=5V, so you can see what the expected freqs are at that setting.  Figure 2 is for 15pF.  Figures 4, 5, and 6 don't apply to this particular chip.

SECAM blue starts at 4.25MHz and drops to 3.9MHz at bluest.  SECAM red starts at 4.406MHz and moves up to 4.756MHz at reddest.    So naturally the VCO needs to be in that range.

The second capacitor is because there's supposed to be a bandstop filter centered at 4.286MHz, right between the two colors but closer to blue.

So anyway, the 74ls624 should be somewhere in the 3.9-4.75MHz orbit.



--- Quote from: ThickPhilM on October 27, 2019, 08:17:21 am ---So it looks like the transistor is being used to switch the 4u7 cap in and out of circuit to alter the frequency range of the VCO.

--- End quote ---
But does a 2N3904 provide an adequate discharge route for this?  If not, then the first charge of the capacitor is also its last.  That's why I wonder if it's the wrong part.


--- Quote from: ThickPhilM on October 27, 2019, 08:17:21 am ---Given that the transistor gate connects back to the VSECAM connection and SYN (synchronisation?) pin on the (CPU?) IC it looks like a region selection for PAL/SECAM maybe.[/color][/b]

--- End quote ---
It's SECAM only, but SECAM is quite complicated in its color sync patterns.  I can't say I fully understand it myself yet, but from what I do understand:

There's a time in the vertical blanking period where 9 color sync pulses are sent out.  They alternate as red and blue, with the last one identifying what the first line represents.  This is one way that SECAM syncs up the colors.  A second way is to provide a smaller pulse on each horizontal sync line.  I suspect that the synchronous counter IC is part of setting up the former.

The pattern of a color sync pulse is to begin at the innermost frequency and sweep out to the outermost in 15us (red) or 20us (blue).  During the blanking interval, the sweep stays at the outermost until that line is over.  Because of the bandstop filter, the amplitude begins low and increases with the frequency sweep, creating what people have termed "green bottles" on the scope trace.

SECAM also uses a four-field interlace instead of the usual two, but I am still struggling to understand that part.  But it probably explains why this circuit is so complex.

Anyway, further reading on what I've been able to find here:

https://radios-tv.co.uk/Pembers/World-TV-Standards/Colour-Standards.html#SECAM
Also, ITU-R  BT.470-6

Mechatrommer:

--- Quote from: drussell on October 27, 2019, 10:49:20 am ---
--- Quote from: Mechatrommer on October 27, 2019, 08:28:22 am ---range what? freq range or amplitude range? do i have to read all those text to understand what is what, and then there's FC, another freq select what? the table only shows their input voltage range.. its my miserable day...
--- End quote ---
Huh?  You're complaining that you can't understand the datasheet without reading it?   :-//

--- End quote ---
maybe i'm used to microchip/atmel way of explaining things... table describing each pins is what is unavail in that VCO DS so i browsed the text looking for RNG, nowhere, how can i relate "freq range" as RNG? it might be FC might it not? anyway it put the guess to the reader.


--- Quote from: hermitengineer on October 27, 2019, 11:19:07 am ---For that much, you'd start by referring to figure 3.  It gives you a frequency graph with 3 plots for FC=5-RNG volts, so that you can choose your extremes and choose a capacitor to base it on.

--- End quote ---
its not clear either. ok looking at figure 1 and 2 explained it in somewhat acceptable manner. so i guess your "doesnt make sense" circuit is to control how slow or how fast the freq ramping up or down. or maybe its somesort of diy PLL or feedback control.

atmfjstc:

--- Quote from: hermitengineer on October 27, 2019, 11:19:07 am ---But does a 2N3904 provide an adequate discharge route for this?  If not, then the first charge of the capacitor is also its last.  That's why I wonder if it's the wrong part.

--- End quote ---

Looking at the broader circuit now, it is clear that there are many other components who can discharge that capacitor while it is in circuit, e.g. the Z209 buffers. So the transistor doesn't need to discharge the cap, it just takes it into and out of the circuit. While it's not in circuit, it will likely retain whatever voltage it had before the transistor cut off, and slowly discharge it through leakage.

If you have an oscilloscope, it would be nice to probe SYNC and the cap to see exactly what is happening.

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