SMD FM transmitter circuit

Let’s construct a low-power FM transmitter using surface-mount devices (SMD) that will be received with a standard FM radio. Soldering surface mounted devices is not so hard and actually is quite easy. There are many designs for small FM transmitters but they have some problems. First, you need an audio amplifier to get enough modulation. Second, the antenna is attached directly to the collector. Third, the coil L must be wound by hand and adjusted by stretching. It all ads with a weak signal that tends to drift in frequency. In contrastm the transmitter schematic we present here eliminates some of those problems, using varactor diode for tuning and modulation, givind great sensitivity without an audio amplifier.



FM Transmitter – How it works
The figure below shows the schematic of the transmitter which consists of two stages: an oscillator and an output amplifier. Modulation is from an electret microphone but you can use a low power audio source.
Oscillator stage
Transistor Q1 is a Colpitts oscillator where the frequency is determined by the parallel resonant circuit formed by inductor L, varactor V1 and capacitors C7 and C8. Q1 is a common-collector amplifier where the power gain counts. V1 is actually a dual varactor that eliminate the possibility of forward conduction at the sinewave peaks.
The frequency of oscillation is set by adjusting the DC voltage on V1 with potentiometer R2. R4 and C3 form a low-pass filter to prevent RF from feeding back onto the DC.
Capacitors C7 and C8 form an AC voltage divider to provide feedback at the emitter of Q1 to sustain oscillation. A necessary condition for oscillation to start is for the radio (C7+C8)/C7 to be sufficiently bigger than 1.

SMD transmitter circuit schematic

 

 

 Frequency Modulation
Modulation is done by superimposing an audio signal from the electret mic onto the DC bias applied to V1. R3 and C1 form a low-pass filter to prevent RF from feeding back to the mic. R3, R4 and R2 form a votage divider for the audio.
Transmitter output stage
The output of the oscillator is fed through C9 to the Q2 emitter-follower. The output of Q2 drives the antenna through C11. The Q2 emitter-follower it ensures that the oscillator is not loaded down by the impedance of the antenna and it provides power gain to drive the antenna.
SMD Transmitter layout
The figure below shows the layout of the PCB and it uses surface-mounted devices like resistors and capacitors (non-polar devices). All the caps are size 0805 and all resistors are size 1206. use through-hole components for Q1, Q2, IC1 and V1. You can use an SOT-89 device for IC1 and an SOT-23 device for V1. Use MPSH10 or a transistor equivalent.

 

 

 The inductor
A coil would consist of two or three turns of wire but for this schematic we will use an inductor with loops of copper on the PCB. Such flat spiral inductor are common at these frequencies.
One formula for flat spiral inductors is:

where
L = inductance in uH
r = radius of coil (outer radius + inner radius divided by 2 ) inches
N = number of turns
d = depth of coil (outer radius minus inner radius) inches

Tuning range
While commercial FM band goes from about 88 MHz to 108 MHz, the L and C values used in this design allow tuning up to 100 MHz.

Transmitter testing
You will need a portable FM radio and an assistant. First, find an empty spot on the FM dial and set your radio about 30 feet away (9 meters). The radio’s volume control should not be set too high to prevend feedback. Next, power-up your transmitter and talk to yourself as you adjust the frequency with the trim-pot. When your assintant hears you, your transmitter is tuned. You might have to adjust the radio’s tuner slightly for best reception.
Have fun with it but remember that using the transmitter as a bugging device may not be legal in your country. To use the circuit as a wireless microphone, increase the value of R3. The transmitter range is about 100 feet (30 meters) inside a building.

 

Easy way to test your Transistor!

Testing a Transistor

1. Set your meter to "diode" mode.

2. Connect the red lead to the base of the transistor and the black lead to the emitter. If it's an NPN transistor, the meter should show the forward voltage drop of the transistor, usually a value between about 0.5 and 1 volt. With a PNP transistor, the meter should read "OL," indicating an open circuit. An open circuit means that there's no electrical connection.

3. Switch the black test lead to the collector and leave the red lead connected to the base. The readings should be identical to the previous test.

4. Reverse the test leads so that the red lead is connected to the collector and the black lead is connected to the base of the transistor. The readings should be the reverse of the previous test.

5. Connect the red test lead to the emitter of the transistor, with the black test lead still connected to the base. If it's an NPN transistor, the meter should read that it's an open circuit. If you're testing a PNP transistor, the meter should show the forward voltage drop.

6. Connect one of the test leads to the collector of the transistor and the other to the emitter. It doesn't matter which lead you connect to which. The meter should indicate an open circuit.

How consumer-electronics makers are faring

A look at how selected makers of phones and other consumer-electronic gadgets are faring:

Jan. 19: Microsoft Corp. says it shipped 8.2 million Xbox 360 video game consoles in the latest quarter, compared with 6.3 million a year earlier.

Jan. 24: Apple Inc. says it sold 37 million iPhones in the latest quarter, double the figure of the previous quarter and more than twice as many as it sold in the holiday quarter of 2010. The iPhone 4S launched during the quarter, and Sprint Nextel Corp. joined as an iPhone carrier in the U.S. Apple shipped 15.4 million iPads in the quarter, again more than doubling sales from a year earlier. Apple sold 5.2 million Macs during the quarter, up 26 percent from a year earlier. The company sold 21 percent fewer iPods, at 15.4 million.

Jan. 26: Nokia Corp. says it sold 19.6 million smartphones in the quarter, down from 28 million a year earlier. That includes more than 1 million Lumia devices running Microsoft's Windows software. The Lumia 800 and Lumia 710 hit stores in Europe and Asia in November, while T-Mobile started offering the 710 in the U.S. in January.

Nintendo Co. says worldwide sales of the 3DS handheld totaled 11.43 million in the last nine months of 2011. The company is now forecasting sales of 14 million machines for the fiscal year through March 2012, down from an earlier 16 million, despite an August price cut. Nintendo sold 9 million Wii machines during the nine-month period. It is now expecting to sell 10 million Wii machines in the year ending March, down from an initial estimate of 13 million, which was revised lower to 12 million in July. The company did not provide quarterly breakdowns.

Motorola Mobility Holdings Inc. says it shipped 10.5 million mobile devices in the quarter, including 5.3 million smartphones and 200,000 tablet computers. A year earlier, the company shipped 11.3 million devices, including 4.9 million smartphones.

Jan. 27: Samsung Electronics Co., maker of the Galaxy line of phones and tablet computers, doesn't disclose the number of smartphones it sells, but it says shipments rose by about 30 percent in the latest quarter, compared with the third quarter. No comparison to the previous year was given.

Feb. 2: Sony Corp. says it sold 6.5 million PlayStation 3 units, on par with the 6.3 million in the 2010 holiday quarter. Sales of the PlayStation Portable declined to 2.4 million, from 3.6 million a year earlier.

Feb. 6: HTC Corp. says several newly introduced 4G and other high-end smartphones disappointed as competition grew more intense from Apple and Samsung. It predicts its revenue will drop further in the current quarter, but will return to normal after that with the introduction of several new models. HTC doesn't disclose the number of devices it sold, citing competitive reasons.

Feb. 9: Lenovo Group Ltd. says it shipped 14 million PCs, up from 10.2 million a year earlier.

Wednesday: Garmin Ltd. says it sold 6.1 million devices during the quarter, roughly the same as the year before. Higher volumes in outdoor, fitness and marine devices helped offset declines in auto navigation devices.

Coming up:

March 29: Research in Motion Ltd.

USIRI E

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Two things involves

Two things