http://www.timewave.com/Acrobat/ANC_4TW8x11a.pdf
The schematic is on the last page. Has anyone here worked with a
similar circuit? I'll explain what little I do understand so you can
build on this.
My questions:
1. What is the Q1 transistor for? Is Q1 an amplifier or just a
buffer? At DC, I notice that it's fed by a supply voltage just under
12V (only 20 ohms and some inductance away from 12V). The CR1 and CR2
diodes, of course, are there to prevent overloading from unexpected
large signals. Capacitors C3 and C2 couple the RF to ground. I
assume that R3, R4, and R8 play a role in determining the gain. (I
can't remember enough of FETs to show the exact relationship.) The RF
output must be the voltage across T1 and is not connected to ground.
The inductance provides RF impedance between Q1 and the power supply.
2. What does transformer T1 do? I notice that the not-quite-12V DC
(I'll call it 11.8V DC) is connected to two of the T1 windings. So if
the input of T1 is .01V RF, it extends from 11.79V DC to 11.80V DC.
Then the output would extend from 11.80V DC to 11.81V DC. Until I get
an answer from Timewave, any idea what the turns ratio and inductance
values might be? Does it play a role in the noise phase adjuster? Do
I need to treat this as a current transformer rather than a voltage
transformer?
3. How does the noise phase shifter work? How does noise phase
shifter work? Adjusting the value of potentiometer R6 is supposed to
change the frequency response. The phase shift should vary between
about -pi/2 and +pi/2 depending on how the pot is adjusted, but I'm
not getting anywhere in the simulations. The FREQ RANGE switch should
control where the phase shift equals 0, but I can't make any sense of
this, either. Unfortunately, information on the transformers is not
provided. But even assuming 1:1, 10:1, and 1:10 turns ratios on
FT50-43 toroids isn't helping. I must be thinking about the circuit
in the wrong manner. The adjustment for the noise gain seems
straightforward enough to me - the pot controls a simple voltage
divider.
4. What is the Q2 transistor for? It's also fed by a supply voltage
just under 12V (only 10 ohms and 2 inductors away from the 12V power
supply). Capacitor C8 seems to be like capacitor C2 - coupling
capacitor. I guess R9 and R10 play a role in setting the gain, like
R8, R3, and R4 in Q1.
5. What does transformer T2 do? The strange thing is that the DC
power supply is connected to the OUTPUT winding. I'm lost.
6. How does the noise gain work? Is the R12 pot a simple voltage
divider? I notice that C11 lies between R12 and Q3, so it has no DC
connection to it. I also notice that part of this pot is in parallel
to R37.
7. What does Q3 do? I don't remember my BJT transistor theory very
well either. The collector is 2 inductors away from the 12V power
supply and is thus connected at DC but isolated at RF. C12 is a
coupling capacitor, so R16 plays a DC role. R13, R14, R15, and R16
determine the gain.
8. How exactly does T3 work? Again, there is a DC supply voltage on
the output side.
9. What does Zener diode CR12 do? Do it, CR4, and CR3 play a role in
dealing with unexpected large RF signals, or am I off base here?
I don't yet understand the T/R switch at the bottom left of the page,
but I'll save that for another post.
Jason Hsu, AG4DG
usenet AAAAAATTTTTTTTT jasonhsu.com
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