FM Transmitter help!

Does anyone else find it unusual that C1 is only 470 pF?

I imagine it makes the 9V battery look like a fairly low impedance at the oscillator frequency, bit I'd normally expect to see a higher value capacitor here.

 

Normally you have a capacitor in parallel with a battery (especially small ones), not to smooth the DC per se, but to provide an alternate low impedance source for short term transient loads.

 

"The Art of Electronics" is a great book, but to describe it as something for a beginner is a bit of a stretch.

It's great for someone who has a real interest in electronics and who wants a practical understanding without all the math that goes along with it.

It's also not cheap.

But if you're interested, and you can browse through it before you buy it, I would encourage you to have a look at it.

 

It all depends on how you define beginner.

If you understand the description of the FM transmitter, then you may have enough background for The Art Of Electronics. Like I said, find a copy and leaf through it.

I think Google will show you some pages

 

you shouldnt ignore the caps in the cct either side of the regulator. They are there for a reason ..... apart from steve's valid comments, the other reason there are different values of decoupling caps on PSU rails are for covering a range of freqs.

get into the habit of using them !!

This is really important in say where that regulator is feeding a RF circuit where you need not only to decouple low freq ripple on the DC rail but also any much higher freq that appears on the DC rail at what the circuit is operating at.

In the RF work that i do its not uncommon to have 3 or 4 wide spread values of caps decoupling a DC rail. For example you mite have a 10uF a 0.1uF and a .001uF cap side by side from the DC rail to ground.

without it you can get crazy things happening with say VCO's operating at UHF and SHF freqs where instability on the DC rail translates to instability on the output freq of the VCO.


Dave

 

I'm glad you brought that up. I was thinking along those lines when I was crafting one of my replies but thought I'd gloss over it.

In addition to having multiple capacitors of different values, they (especially the smaller valued ones) can also often be duplicated and spread throughout the circuit (especially where there are relatively long power rails).

The same techniques are also often seen in logic and power supply (especially smps) circuits.

At low frequencies the internal resistance of power sources tends to be an issue. As the frequency rises, the issue shifts to being one that is more dominated by (stray) inductance.

 

Mine cost the equivalent of Rs 6750 ($AUD150) so I guess one of us got a better deal.

Having said that, for the amount of value I've got from it, I would probably have paid double that.

But I bought a few books on my recent trip to India too.

 

Simply put .... The tank cct is part of the oscillator and its freq of operation.

modulation from the simple 1 transistor microphone amplifier, Q1, is being applied to the base of Q2 via C3. With no modulation present the bias voltage on the base of Q2 is stable, producing a steady transmitted carrier at the osc. freq. But when modulation is applied to the base of Q2 the bias is varied and this varies the freq of the osc. producing an Frequency Modulated signal.

The oscillator resembles a colpitts oscillator google that for more info.

cheers
Dave