mjhjmtu

I have this tikzpicture/pgfplot (see below). I have a few questions about it.

1. How do you put the x-labels (10¹, 10² and so on) on top of the x-axis?


2. How do you point the y-axis downwards (the arrow below)?


3. How do you place an x-label at the green vertical (at 1591Hz)?


4. How can you put extra ticks on the y-axis without a label (eg. every unit a tick, only every third unit a label)?


5. With those extra ticks from former question, how do you place gridlines alongside of them?


6. Is it posssible to put a dot or a cross on the intersection red-green?


7. Can I also put an arrow pointing to that intersection point with a text label?


8. When I start the domain at 10 instead of 1, or when I use xmin=10, I get a white region between the y-axis and the plot/grid. How can you avoid this. I would like this plot starting with 10¹ at the y-axis.

Thanks very much in advance!

documentclassarticle
usepackagepgf,tikz
usepackagepgfplots
begindocument
usetikzlibrarycircuits.ee.IEC,graphs,patterns,calc,positioning,intersections,quotes,decorations.markings,angles,bending,arrows.meta,shapes
defdBmax0
defdBmin-12
defpi3.14159
defR1000
defC0.0000001
defFc1/(2*pi*R*C)
pgfplotssetwidth=14.5cm,height=6cm
begintikzpicture[scale=0.8]
 beginaxis[title=$Laagdoorlaatfilter-verzwakking$,
 axis lines=middle,
 ymin=dBmin,ymax=dBmax,
 ylabel=$A_v(dB)$,
 ylabel style=at=(ticklabel* cs:1),anchor=east,
 ytick=-21,-18,...,0,
 xmode=log,
 domain=1:100000,
 xlabel=$f(Hz)$,
 xlabel style=at=(ticklabel* cs:1),anchor=south,
 xtick style=above=1cm,
 xminorticks=true,
 xmajorticks=true,
 xminorgrids,
 xmajorgrids,
 grid=both,
 samples=100]
 addplot[no marks, blue] 20*log10(1/sqrt(1+(2*pi*R*C*x)^2));
% addplot+[no marks, green] 20*log10(1/sqrt(1+1/((2*pi*R*C*x)^2)));
 addplot+[no marks, red] -3;
 addplot+[no marks,green!50!black] coordinates(Fc,dBmax) (Fc,dBmin);
 endaxis
endtikzpicture
enddocument

edit:
like this (solved almost all your questions ... )?

(red lines indicate text borders)

documentclassarticle
usepackagesiunitx
usepackagepgfplots
pgfplotssetcompat=1.16
usetikzlibraryarrows.meta

begindocument
 begintikzpicture[pin edge=Latex-,black]
defdBmax0
defdBmin-12
defpi3.14159
defR1000
defC0.0000001
defFc1/(2*pi*R*C)
beginsemilogxaxis[ % <---
 width=linewidth,height=6cm, % <---
 title=Laagdoorlaatfilter-verzwakking,
 title style = at=(0.5,1.2),font=itshape, % <---
 axis x line=top, % <---
 axis y line=left, % <---
 ymin=dBmin, ymax=dBmax,
 ylabel=$A_v$ (sidecibel), % <--- 
 ylabel style=at=(-0.07,0.5), % <---
 ytick=dBmin,dBmin+3,...,dBmax, % <===
% ytick=dBmin,numexprdBmin+3relax,...,dBmax, % alternatively, 
 xlabel=$f$ (sihertz),
 xlabel style=at=(0.5,1.1),
%
 extra x tick style=% changes for x extra ticks % <===
 tick label style=font=footnotesize,text=green!50!black, % <===
 grid style=semithick,green, % <===
 ,
 extra x ticks=1591, % <=== 
 extra x tick labels=1591, % <===
%
 grid=both, 
 minor y tick num=2, % <===
 domain=10:100000, 
 samples=100,
 no marks]
 addplot[blue] 20*log10(1/sqrt(1+(2*pi*R*C*x)^2));
 addplot[red] -3;
 node[circle,fill, inner sep=1pt,
 pin=45:$f_c$] at (1591,-3) ; % <===
 endsemilogxaxis
endtikzpicture
enddocument

note:

• 
  tikz libraries should always be loaded in document preamble


• 
  pgfplots load tikz, so it is not necessary to load it (/again)


• better than scaling diagram is to prescribe its width, for example to be equal to text width