FM transmitter circuit - Page 1
Forum section: electronics / beginners

author: yalect
author_rank: Regular Contributor
timestamp: April 19, 2020, 05:22:23 pm
content: Hi,many circuits that uses this oscillator as FM transmitterI would like to ask you what to consider about the compenets or relationship to get a good frequency value or stable and what about C1 and C2 and VC1 rules?Regards

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 19, 2020, 05:40:25 pm
content: I built a similar circuit about 20 years ago as a spy bug for my son to fiddle with. It transmitted on standard FM broadcast band.L=0.1uHC2=4.7pFC1=0.01uFVC= 4-40pF (variable capacitor)L1 made with 8-10 turns of 22 gauge magnet wire on a 1/4" diameter (air core)

author: yalect
author_rank: Regular Contributor
timestamp: April 20, 2020, 05:51:23 am
content: thank youno, I meant about calculations and rules, the frequency of this circuit is based onLC(tank circuit) and other capacitor, if I increase C1 to for example to 1uf, could that effects on the frequency?How to get a good frequency value or stable,C1andC2andVC1andLrules?

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 20, 2020, 10:52:29 am
content: That is a common base Colpitts oscillator. Look it up on wiki. C1 should not effect oscillation frequency since the base should look like ac ground at oscillation frequency.

author: Zero999
author_rank: Super Contributor
timestamp: April 20, 2020, 08:35:10 pm
content: Quote from: yalect on April 20, 2020, 05:51:23 amIt's not possible to achieve decent frequency stability, with such a simple circuit. It will vary depending on the supply voltage and the load. As the battery discharges, the frequency will go down, as the capacitance of the transistor's junction will increase. If the loading on the antenna changes, due to it being obscured or left in in open space, the frequency will also change.The sound quality is also pretty poor, as it lacks pre-emphasis. To reduce the noise, radio stations boost the higher frequency parts of the audio signal, which are then attenuated by the receiver. Because this circuit doesn't boost the high frequencies, but consumer radios still attenuate them, it will sound muffled.

author: yalect
author_rank: Regular Contributor
timestamp: April 21, 2020, 06:05:34 am
content: thank youabout the frequency how to determine or calculate its frequency, it's notLCfrequency only?if I increaseC1as I said to 1uf for example , it will effect oscillation frequency?may we talke aboutC1,C2,VC1,Lcalculation rules and value in shortdetails?Greeting

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2020, 08:15:28 am
content: Quote from: yalect on April 21, 2020, 06:05:34 amNo, as mentioned above:Quote from: Wimberleytech on April 20, 2020, 10:52:29 amQuoteC2 provides positive feedback to the emitter which is input to the common base amplifier. It can't be too big, because it will dampen the oscillations. It just needs to be big enough to provide some feedback. In the 100MHz region a few pF will do.Use the following formula to calculate the frequency:F = 1/(2π√(LC))WhereF is the frequency.L = L1 in HenrysC = VC1 in FaradsF = the Frequency in Hz.If you want better frequency stability you need to build a more complicated circuit.