Another question I can't find ready answers to on google.
Intuiting is good, but perhaps you should put some numbers to it?
The math isn't so bad, it's just algebra and rational equations. No other functions, no sqrt (until you get into power, anyway), no exp or trig.
Cranking a big equation (say for >4 components in a non-trivial connection) is daunting, but at least in principle... if you know algebra, you know how to do it. Doing it successfully is just a matter of scale, not of missing knowledge.
Not that such a device exists, but imagine a load has a DC resistance R, and an impedance vs frequency that was flat above some f_c frequency and rolled off 20db/decade below f_c. (A signal would be attenuated less and less below f_c.) Then you put a capacitor in front of it with capacitance C such that the high pass filter formed between the capacitor and the load of 1/2*pi*R*C = f_c. Would the filter formed with C and R, interact with the impedance to produce a flat frequency response?
Actually, such a device does exist... put an R and L in parallel.
The slope below f_c is determined by the inductor (|Z| ~= w*L), and the flat "DCR" (EPR (equivalent parallel resistance), rather, since DCR won't have the same effect!*) is given by the resistor.
*We can, in fact, convert the circuit so that it has DCR (or ESR if you like) instead of EPR. But it's a poor model for most purposes, because the conversion depends on frequency squared, so the ESR isn't a constant value, but a function.
Knowing this, we can put a capacitor in series with the R||L. The impedance is:
Ztot = Z_C + Zeq
Zeq = Z_R || Z_L = j*w*R*L / (R + j*w*L)
so Ztot = 1 / (j*w*C) + j*w*R*L / (R + j*w*L)
Of course, using w = 2*pi*F for short (that's lowercase ω, except my keyboard doesn't have it, and the forum refuses to support it). Or if you prefer Laplace domain, s == j*w. j == sqrt(-1) (also 'i', but EEs use 'i' for current, so we use 'j' instead).
Now, your question was: will the impedance be constant? I think you will see that it won't, because there's no minus that can cancel out. Not that that's obvious, as written. (There is cancellation possible, because of the multiple j's floating around. But -- without getting too deep into this one equation -- this will only happen at one or two frequencies, which isn't "constant".)
Just looking at it geometrically -- as you started the question, after all -- suppose we had a component where the impedance is
constant at low frequency, falling above f_c. The left-to-right reverse of your start. Why, this would be a capacitor in parallel with a resistor, or Z = R / (j*w*C) / [R + 1 / (j*w*C)]. If we add this instead, we get something that's still not immediately obvious, but we
know, intuitively, that we should be able to equate something.
So, why not try to solve it?
Ztot = R1 / (j*w*C) / [R1 + 1 / (j*w*C)] + j*w*R2*L / (R2 + j*w*L)
(Note, I'm renumbering capacitor's R --> R1 and inductor's R --> R2.)
Let's clean up some of those denominators... yeh, right?
Ztot = R1 / (j*w*C) / [(j*R1*w*C + 1) / (j*w*C)] + j*w*R2*L / (R2 + j*w*L)
the j*w*C cancels giving
Ztot = R1 / (j*w*R1*C + 1) + j*w*R2*L / (R2 + j*w*L)
Common denominators,
Ztot = [R1*(R2 + j*w*L) + j*w*R2*L*(j*w*R1*C + 1)] / [(1 + j*w*R1*C)*(R2 + j*w*L)]
Now it might be tempting to continue simplifying and distribute and all, but this is a little better. Here's a trick: you want to try to factor it into factors of the form constants, constants*j*w, (1 + j*w/constant), (1 + j*w/constant - w^2 / constant^2), etc. Each factor of w is called a pole (if it's in the denominator) or zero (numerator). If we can find pairs of poles and zeroes that we can make equal by adjusting the constants, we can cancel them out. This is called pole-zero cancellation or compensation, and shows up in a number of places. (A good example is adding capacitors in parallel with resistors to compensate for capacitive loading in a voltage divider, as used in oscilloscope attenuators.)
Ztot = [R1*R2 + j*w*L*(R1 + R2) - w^2*R1*R2*L*C] / [(1 + j*w*R1*C)*(R2 + j*w*L)]
Shifting around the constants...
Ztot = R1*[1 + j*w*L*(R1 + R2) / (R1*R2) - w^2*L*C] / [(1 + j*w*R1*C)*(1 + j*w*L / R2)]
At this point, it might be worthwhile to insert some other variables. Anywhere we see L*C, we should be thinking: aha, resonant frequency! So we will use w_0 = 1 / sqrt(L*C) there. We might also combine (R1 + R2) / (R1*R2) = 1/Rpar, a nice observation. Also, when we have a quadratic in w, we can label the coefficients more specifically: (1 + w*zeta / w_0 + w^2 / w_0^2), where zeta is the damping ratio. This gets,
Ztot = R1*[1 + j*w*L / (Rpar*sqrt(L*C)*w_0) - w^2 / w_0^2] / [(1 + j*w*R1*C)*(1 + j*w*L / R2)]
It looks worse before it gets better. That middle term is looking useless, right? Hold on..
Ztot = R1*[1 + j*w*sqrt(L/C) / (Rpar*w_0) - w^2 / w_0^2] / [(1 + j*w*R1*C)*(1 + j*w*L / R2)]
Ztot = R1*[1 + j*w*zeta/w_0 - w^2 / w_0^2] / [(1 + j*w*R1*C)*(1 + j*w*L / R2)]
sqrt(L/C) / Rpar = zeta
This is important now! sqrt(L/C) is a characteristic: the resonant impedance of the LC circuit (which you'll notice is series resonant, except with resistors in parallel with each element, rather than one in series). The ratio of this against the equivalent damping resistance (i.e., the resistors act in parallel) is the damping ratio.
We're also in the final step, because we can factor this quadratic easily. We know that if zeta > 1, the network is overdamped, and will factor into two real parts. We can assign values of R1 and C to cancel the R2 and L parts, and achieve pole-zero cancellation. If zeta < 1, the network is underdamped, and will ring when excited by a transient (and will have an impedance trough at w_0, the ratio between valley and flat-band being zeta).
Doing the same for the denominator (and labeling the zetas and omega-naughts as 0 and 1) gives,
Ztot = R1*[1 + j*w*zeta/w_0 - w^2 / w_0^2] / [(1 + j*w*zeta_1 / w_1 - w^2 / w_1^2)]
w_0 = 1 / sqrt(L*C)
zeta_0 = Zc / Rpar
Rpar = R1*R2 / (R1 + R2)
Zc = sqrt(L/C)
w_1 = w_0 * sqrt(R2/R1)
zeta_1 = (Rav / Zc + Zc / Rav)
Rav = sqrt(R1*R2) [by the way, this is the geometric mean, hence 'av']
Now all we need to do is equate zeta_0 = zeta_1 and w_0 = w_1, and the poles and zeros cross out!
w_0 = w_0 * sqrt(R2/R1)
This is easy, just set R1 = R2!
Now that means R1 = R2 = Rav == R, just to simplify things a little, and Rpar = R / 2.
Zc / (R / 2) = (R / Zc + Zc / R)
2 * Zc = (R^2 / Zc + Zc)
Zc = R^2 / Zc
Zc^2 = R^2
Zc = +/- R
Noteworthy that the circuit will still be critically damped if we use a negative resistance (which can be synthesized with an op-amp). But for almost all purposes, we pick the positive root. So R = sqrt(L/C).
So, in conclusion, starting with an R||L circuit, we put in series with it, R1||C, where:
R1 = R
C = L / R^2
A lot of math, for such a painfully simple result, no? But this is how things go. There's a lot of things we can solve from those central equations, like how the impedance varies, or peaks (or troughs) with frequency, how wide the feature is (+/- 3dB points, say), what the voltage divider ratio is, what the impedance in the middle is, etc. All those options, and more, are essentially present in that equation, there for the solving; we just have to take a direction that results in our desired solution. So we should accept that the equation is much less simple, and that we will have some work to do, to find the one solution we're looking for.
Tim