1 (185)

Strona 46 z 46

Spy Circuits

100MHz rangę.

If you are building our transmitters, a Field Strength Meter is an essential piece of equipment.

USING THE FIELD STRENGTH METER

We are assuming the project works correctly and has been checked as per the "If it Doesn't Work Section." To check the output of an FM transmitter, place it on the work-bench with the antenna in a horizontal piane, a objects.

Switch it on and place the antenna of the Field Strength Meter about 20cm away, with both antennas in the s< Gradually tum the trimmer by moving the paper clip with your finger, while keeping away from the coil on the board until the maximum readout is detected on the LEDs.

The pointer will then give you the frequency at which the transmitter is operating.

As you move the FSM away, the LEDs will dim and as it is brought closer, morę LEDs will come on.

If you wish to compare one transmitter with another, simply put the second in exactly the same place on the b antenna at the same distance. You may have to re-tune the FSM to pick up the frequency; however you shou reading on the LEDs if both have the same output.

When working with detuned transmitters, you can use the scalę around the trimmer to give readings from 75!\ If you have a transmitter tuned to a band above 108MHz, the FSM will detect frequencies up to 140MHz. When using the FSM, it is important to keep your hands away from the board, especially the front end, as the body may affect the readings slightly.

QUESTIONS FOR ADVANCED CONSTRUCTORS:

1.    What is the purpose of the 47p in series with the 4 - 40p trimmer?

2.    Why is the coil etched on the PC board?

3.    What is the purpose of the 10Op and 1 k in the emitter of Q2?

4.    Why is Q3 biased fully ON?

5.    For the diodę pump, does the transistor or the 4k7 collector resistor charge the 100n?

ANS WE RS:

1.    The 47p in series with the trimmer adjusts the effective value of the trimmer to 3p5 - 20p. The easy way tc two equa!-value capacitors in series produces a value of half the smaller value. Thus 40p and 40p produces ^ value, the ratio is about 10:1 or 4p:47p. We use the same reasoning and see thatthe 47p will alter the 4p ver way to see things without using mathematics.

2.    The coil is etched or fixed on the board so that we can generate a scalę around the trimmer that will be th models.

3.    For Iow frequencies, (1 MHz etc) the 100p will have a high reactance and thus the gain of the stage will b< 4k7 to 1 k or about 5. Thus the stage will not amplify all the hash and noise of the Iow frequencies.

4.    Q4 is biased fully ON to further reduce the noise and hash produced by a self-biased stage and also to gf fuli voltage swing.

5.    The charging current for the diodę pump is supplied by the 4k7. The transistor merely pulls the 1 n Iow to c the lower diodę.

2011-12-08

mk:@MSITStore:F:\spy circuits.chm::/SpyCircuits-l.html