Hi
@rogs
Today i got to play with different frequency crystals, my new oscilloscope with FFT, and my custom inductor, and MKH 50pf capsule.
When you refer to highest Q, what do you mean? Is it the shape of the RF signal curve in a typical sense like with Eq Q, attached image? Or is it just the voltage of the RF signal oscillator puts out?
When i look at FFT on the oscilloscope i get similar shapes (Q) with several crystals ranging from 4Mhz to 15Mhz as long as i adjust the inductors for max output. I do get some variation in voltage it puts out. I'm doing this for the first time, so just trying to figure out if i'm doing it right.
Also when i adjust for maximum sensitivity, with some of the inductors, with very low signal coming from the headphones exciting the capsule i can clearly hear some harmonics appearing, and im sure it's not the rest of the circuit distorting as it's quite low level. Could it be it's oscillator signal folding back to audible range? I can also see on the FFT harmonics above the crystal main frequency.
This project is so fascinating to me.
In this context I'm using the 'Q' factor to describes the 'goodness' of the resonant circuitry around the inductor assembly..... A high 'Q' inductor configuration can effectively supply 'noise free voltage gain' (sort of!
)
Actually, Wikipedia describes 'Q' much better than I can .... See here:
Q factor - Wikipedia
The capsule movements we're talking about here provide
tiny changes in capaitance, so we need all the help we can get, to amplify those signals (without adding too much noise if possible ).
The whole AMX inductive assembly -- with the primary of the second transformer connected
across the bridge - is not easy to analyse mathematically.
There are too many unknown variables to permit a sensible model to be set up. Abbey did some simulations back in 2019 (around post #257 of this thread as I recall) but he concluded that the unknowns made his model less than ideal. As I recall, what it did show is that under certain conditions -- with the inductors tuned for maximum 'Q' within the whole assembly -- there were some pretty high gains available from the inductor assembly.
Really sharp boosts (and therefore gain) at resonant frequencies.
So the RF output of the oscillator itself remains constant. What does change is the signal level presented across the bridge when T1 is 'tuned'. The 'Q' of the tuned inductor can create in significant gain of the RF carrier.
T2 has a fixed capacitor across its secondary, That is chosen to optimise the tuning of T2 in conjunction with the chosen oscillator frequency, and the value of the inductor ( This calc can be useful :
Resonant Frequency Calculator )
What makes things difficult is when those 2 inductors are connected as we have them here..... It all gets very complex, mathematically.
What happens is that you
can get a very good result, signal quality and noise wise.
But there are
lots of variables --- capsule capacitance, oscillator frequency and amplitude, inductor values, bridge detuning etc etc -- that make it very difficult for the simple hobbyist (like me!
) be able to make definitive comments on what works best.
The Spectrum inductors were specified around 7MHz. They seem to work well at 8NhHz, even 10MHz with high 'Q' result still possible.
Above that frequency, I found the RF voltage available from the resulting lower 'Q' of the inductors started to drop off a bit. Not 'drop off a cliff', but get slightly less 'good'
)
The further you get from the specified frequency for this type of indutor assembly, the lower the 'Q' -- and the lower the amplitude of the RF signals derived from the same amplitude source.
Regarding your point on RF 'foldback' --- I've not come across anythng like that?..
The time contstants used on the source and drain of the JFET are in the audio range.
Way below the RF carrier frequency? ... I have to confess I have no ideas on that one, at present....
Now were starting to try out different inductor values, I suspect we'll open the flood gates to all sorts of possible variables. Some of them may prduce really good results --- other won't! .
I've made around 30 different version of this mic circuit over the last 4 + years -- and I'm still finding surprises. There is one problem I find makes things more difficult -- not being a graduate engineer!
One other aspect that makes formal calculations even more difficult is that tuning the inductors for best results can only be done with the headbasket on. The RF field - although quite small - is affected by the metalwork of the headbasket.
Just one more thing that makes this project 'experimental'..... I imagine it could of course all be calculated 'theoretically' - but I'll bet it never is!
