Electronics > Beginners
Does this circuit even make sense?
hermitengineer:
--- Quote from: atmfjstc on October 27, 2019, 01:53:04 pm ---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.
--- End quote ---
But wait... don't all circuits involving the capacitor necessarily include the transistor? And we're back to: Doesn't a 2N3904 (BJT) only allow current flow in ICE direction? At least, the datasheet doesn't even bother to show current flow in the opposite direction.
--- Quote from: atmfjstc on October 27, 2019, 01:53:04 pm ---If you have an oscilloscope, it would be nice to probe SYNC and the cap to see exactly what is happening.
--- End quote ---
Oh, now you want real-world measurements? >:D
SYNC (magenta) vs Q207/C261 junction (yellow)
SYNC vs FC pin
SYNC vs FC pin with C261 completely removed from the circuit
Basically, I see no difference with C261 present or not.
capt bullshot:
--- Quote from: hermitengineer on October 27, 2019, 07:31:42 pm ---But wait... don't all circuits involving the capacitor necessarily include the transistor? And we're back to: Doesn't a 2N3904 (BJT) only allow current flow in ICE direction? At least, the datasheet doesn't even bother to show current flow in the opposite direction.
--- End quote ---
A bipolar transistor can be operated with C and E swapped. Parameters like hfe get worse then, but works to some extent. So in a circuit like this, the transistor is driven into saturation by plenty base current, the C is a near short to E for small C currents. If C polarity gets negative in respect to E, the transistor operates in "reverse" mode, and for small enough currents, C is still a near short to E.
So effectively, the transistor connects the capacitor to the respective node if the base is driven, or opens the connection if the base is not driven. A very simple and cheap analog switch. Works as long as the collector current is small enough, this kind of circuit is sometimes seen as a mute switch for audio in / out.
hermitengineer:
--- Quote from: capt bullshot on October 27, 2019, 09:58:24 pm ---
--- Quote from: hermitengineer on October 27, 2019, 07:31:42 pm ---But wait... don't all circuits involving the capacitor necessarily include the transistor? And we're back to: Doesn't a 2N3904 (BJT) only allow current flow in ICE direction? At least, the datasheet doesn't even bother to show current flow in the opposite direction.
--- End quote ---
A bipolar transistor can be operated with C and E swapped. Parameters like hfe get worse then, but works to some extent. So in a circuit like this, the transistor is driven into saturation by plenty base current, the C is a near short to E for small C currents. If C polarity gets negative in respect to E, the transistor operates in "reverse" mode, and for small enough currents, C is still a near short to E.
So effectively, the transistor connects the capacitor to the respective node if the base is driven, or opens the connection if the base is not driven. A very simple and cheap analog switch. Works as long as the collector current is small enough, this kind of circuit is sometimes seen as a mute switch for audio in / out.
--- End quote ---
Interesting. This is one thing my semiconductors teacher never mentioned. Maybe I should do some breadboard experiments to get a feel for it.
Questions to experiment on: In a reverse current flow, is it still VBE that determines the current flow? Or is it VBC? What's the order of gain difference for reverse flow?
BJTs are usually drawn as equal size N regions (for NPN), but as I recall, the collector has a much larger N region. I guess that's why gain is so much smaller?
Okay, I played with a breadboard with the following circuit:
Press the top button and the LED lights, but goes out immediately. Press the left button while the top button is pressed, and the LED goes out until the capacitor charges. Release the left button, then release the top button. LED goes out. Then press left button and LED lights up until capacitor discharges.
Demonstration of reverse-active mode successful.
But is this what they call reverse-active mode? Because it says that reverse-active mode requires negative VBE bias. But the simplified circuit still has forward VBE here?
Of course more specific to the logic board in question: Why does removing the capacitor entirely make no difference?
Obvious answer: Q207 is dead?
hermitengineer:
The part of the diagram before the 74ls624 is two buffers driven by a couple of flip-flops. Guessing purely by what I know SECAM needs, I referred to this as the color burst strobe. But after modeling it in a state diagram, that's apparently not it.
Instead, each flip-flop is clocked by a different source: Z201B by the output of the 74LS624, and Z201A by a 4.45MHz resonant circuit. When this block is enabled at all, both flop-flops will go high on their respective clock cycles, and when both are high they are (almost) immediately reset simultaneously. The Q output of the first mixes with the Q output of the second to drive the range voltage, so this has the effect of pulsing the output, which the capacitor(s) will help to smooth. Because the pull-up resistor is 220K, the quiescent voltage is only around 1V with the buffers detached, so the combination of buffers can easily pull it towards 0V, 2.5V, or 5V (but 5V is only for a propagation delay of time), and 2.5V seems to be the most common.
So, this seems to be a PLL circuit. Or am I off the mark?
capt bullshot:
--- Quote from: hermitengineer on October 29, 2019, 08:51:16 pm ---So, this seems to be a PLL circuit. Or am I off the mark?
--- End quote ---
Agreed. Looks like the phase detector (that appears kind of similar to the edge based phase detector in a 4046) locks the '624 mid frequency to the crystal frequency using the RNG input, while the SECAM FM is done through the FC input. Q207 / C249 change the loop time constant (I guess it's slow while the visible content of TV line is sent, and fast while or near the sync pulse to allow for whatever burst).
According to the datasheet, nboth RNG and FC have influence on the VCO output frequency, so it looks like the voltage at RNG sets the center frequency (to the crystal frequency) and FC controls the FM deviation.
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