Completely newbie planning to build EM theremin

Some weeks ago i saw this thread and i wonder if a Yaege FC-1 is suitable for testing my project too. I'm searching for somesthing cheap; even a DIY frequency counter would be very fun!

IMO, the Yaege FC-1  is a neat little meter for its price - one needs to buy the adaptor and 'scope lead discussed on the thread you referenced, but I dont believe you could make a frequency counter of similar quality for even double the price.

The other place to look is at used bench test kit on Ebay or similar - one can pick up bargains.. If you want a frequency counter for general purpose use you do not need the high frequency (> 2GHz) the FC-1 is capable of, and the ability to count lower frequencies with better accuracy can also be useful .. 1Hz to 100MHz is more than most people are ever likely to need if not dealing with high frequency RF or fast digital sysyems.. Even 1Hz to 10MHz is fine for almost all applications, certainly for anything an analogue theremin will ever throw at you!

But the FC-1 is small, cheap and reliable - IMO it is worth getting.

Fred.

"But the FC-1 is small, cheap and reliable - IMO it is worth getting."  - FredM

Uh oh, baby's maybe going to need one of those!  (I don't know why because my scope does a pretty good low res job of frequency.)

This LC meter is fairly invaluable though, wish I'd had it long ago.  Not the most analytical (works off of resonance with a fixed L or C rather than at a fixed frequency, self resonance effects are slightly worked into the result, and it doesn't tell you anything else but the frequency of resonance) but it's pretty sensitive and useful, particularly if you are considering winding your own coils.

I should just say that my statement "and reliable " is not to be taken as any kind of warranty - its been reliable for me so far - But I dont use it much.. I mostly use my Philips 6661 80MHz bench counter, which IS certainly reliable - Its been used for probably tens of thousands of hours over a couple of decades, never given one moments trouble, and its calibration hasnt drifted one iota (been checked against ovened crystal recently) .. I keep expecting it to pack up, but even its red-LED display is bright.

When it comes to test equipment, the old stuff from HP and Philips and Telequipment and Tecktronic are real hard to beat.

Hi there!

I've been doing some research about C13 and C15 and their mission in the schematics because their capacitance changed dramatically from EM to EW.

The most important question is about their material:
- C13 is listed as 1µF tantalum in EM but 22 µF in EW.
- C15 is listed as 0,01µF polyester in EM but 6800 pF in EW.
Do you think I should still use tantalum and polyester for them?

I've been told that capacitors like C13 are very important because they connect +12V to ground. Is it true?

PD: I'm going to start building it soon. Wish me luck!

Thank you :D

I've been told that capacitors like C13 are very important because they connect +12V to ground. Is it true?

Yes, it is true.. FOR AC!  If you look up capacitive reactance, you will discover that the reactance (equivalent to resistance at AC) of a capacitor goes down as frequency goes up - So a big capacitor at high frequencies becomes a low resistance, and connects THE AC present on the 12V rail, to ground.

Alas, real capacitors are imperfect.. they also have resistance and inductance in series with the capacitance.. Inductive reactance goes up as frequency increases ;-(

So one needs a capacitor with low inductance and low resistance..

Tantalum was good - but became difficult to source and expensive, probably the major reason for the design change.

Fred

That's interesting! So now i think i can get rid of the expensive tantalum capacitor i had for C13.

I'm quite sure i can replace it with a non-polarised capacitor (ceramic). Am i right?

That tantilum is a nice capacitor!

But yes, you dont need it - get a good 22u aluminium electrolytic, and a good ceramic of about  470nF in parallel with it.

This is my usual method - a good small ceramic to bypass high frequencies, amd a larger electrolytic for LF / smoothing.

Bypass capacitors have several functions - they couple HF AC on the supplies and therebye reduce this - and in the case of the theremin, they provide a ground connection for the tank inductor and capacitor.. They must be MUCH larger than the combined capacitances of all tank capacitors, and they also provide a 'reservour' of charge for anything which takes current 'gulps'

Get the capacitors, leave some space on you board to add a couple more - you cannot add too many, you can only have too few! - The trade-off is usually (tantilums being an exception) that the bigger the capacitance, the higher the ESR / ESL .. Combining different types of capacitor in parrallel is a way to get the best of most worlds.

Fred.

ok - done some thinking about the capacitors with reference to the EM.. and comparing specs on tantilum vs aluminium vs ceramic..

Tantilum has actually quite a high ESR and ESL compared to ceramic - so in terms of coupling the HF signals, a cheap 22uF electrolytic like this combined with a multi-layer (bog standard) 470n ceramic should do fine.

But its probably best to go with the components specified for the EW.

Hi, i have a quick question. Hazel asks some posts above about the polarization of capacitors C10, C11, C29 and C30. Fred, you say that a non-polarized ceramic capacitor will do. However, if i do use a tantalum capacitor, how will i place them on the board?

Thanks!