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FHT-1 • 5

FHT-1 CIRCUIT DESCRIPTION

Basic overview: The FHT-1 is a crystal controlled FM transmitter that uses a

varactor modulated crystal oscillator followed by a 9 times frequency

multiplier and power amplifier. Test points are built-in for easy alignment. The

“brains” of the unit is a Motorola microcontroller programmed to remember

your CW ID and to control transmit power and timing.

Detailed description: Transistor Q1 functions as a Colpitts crystal oscillator

whose frequency is determined by Y1 and varactor diode D1. Transistor Q2

functions as a buffer amplifier to isolate the crystal oscillator from other

portions of the circuit. The crystal oscillator frequency is multiplied by 3

(tripled) in transistor Q3. Frequency multipliers are nothing more than

amplifiers that produce lots of distortion! In this case we're interested in

having enough distortion so that the third harmonic is fairly strong. We "pick-

off" or filter the third harmonic with a band pass filter, comprised of L9,13 and

capacitors C28,21,22,16. This allows transistor Q4 to be driven only by the

third harmonic of the crystal frequency - in this case, around 48 MHz. Q4 is

another tripler, multiplying up the 48 MHz to 144 MHz. Inductors L5,11 and

capacitors C25,17,18,10 form a band pass filter for the three times output

frequency.

From here on out, we're working at the actual carrier frequency and use a

couple of transistors to amplify the signal up to a 4 to 6 watt level. Transistor

Q5 boosts the signal to the 250 mW range and Q6 then produces the full

power output. Impedances must be matched between stages to allow for

maximum power transfer, and that's the function of a couple of coils that are

hand wound. A low pass filter follows the final amplifier to limit out of band

signals (remember those multiplier stages?). Modulation is accomplished by

varying the capacitance of varactor diode, D1. This varying capacitance shifts

the frequency of the crystal ever so slightly causing a frequency shift, which

is FM or Frequency Modulation. And yes - this frequency shift does get

multiplied as it travels through the multiplier stages. The signal used to vary

the varactor diode is our desired audio modulation. There is no need to

control the voltage to any other transistors since they all operate class "C". A

class C amplifier draws no current unless it is driven, so there is no need to

switch the later stages on and off.

The microcontroller U2 contains the programming to control the transmitter’s

functions . When in programming mode, the chip is enabled to remember the

CW ID that is “sent” to it via the Paddles input. The four DIP switches can be

set to run one of sixteen different programs controlling on and off times and

power levels, including one user-programmable mode.

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Ramsey Electronics HR40 Bedienerhandbuch Seite 5