Daniels MT-3 Manuale di istruzioni - Pagina 8

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Theory of Operation

RF AMP

The output from the ﬁve pole helical ﬁlter is

coupled through impedance matching network

L4 and C9 to the base of RF ampliﬁer Q2. The

RF ampliﬁer stage is supplied by the regulated

+9.5 Vdc line and typically draws 8 mA of

collector current. Transistor Q2 provides high

gain, typically 20 dB in the VHF band and 16 dB

in the UHF band, and low noise ﬁgure typically

2.5 dB in the both bands, for increased receiver

sensitivity. Transistor Q1 actively regulates

the bias to RF transistor Q2. The output signal

is coupled to an image rejection ﬁlter via

impedance matching network C11 and L2 (VHF)

or L3 (UHF).

IMAGE R E J EC T ION FILTE R

The ampliﬁed RF signal is followed by a 7 pole

Chebyshev lowpass or highpass image rejection

ﬁlter which further rejects the image frequency.

The image ﬁlter topology (either lowpass or

highpass) depends on if the Front End is using

lowside or highside local oscillator injection. The

lowpass image ﬁlter consists of capacitors C16

to C19 and inductors L8 to L10. The highpass

image ﬁlter consists of capacitors C12 to C15

and inductors L5 to L7. Jumpers JU1 and JU2

are used to select the appropriate ﬁlter for the

desired receive frequency band. The table below

summarizes the receive frequency, whether

lowside or highside injection is used, and which

ﬁlter is installed.

MT-3 Front End Image Filter Selection

Frequency Band

132 to 150 MHz

150 to 174 MHz

406 to 430 MHz

450 to 470 MHz

470 to 490 MHz

490 to 512 MHz

Note: There is an error in the silkscreen on PCB v1.0. JU1 A & B designations are reversed.

MT-3 Low Current Front End Instruction Manual

IM11-FE3

Local Oscillator Injection

Highside: RF + 21.4 MHz

Lowside: RF - 21.4 MHz

Highside: RF + 21.4 MHz

Lowside: RF - 21.4 MHz

Printed Copy Not Controlled

L O C A L O S C I L L ATO R I N P U T

The MT-3 Front End local oscillator input port

(50 Ω) connects directly to a double balanced

mixer (M1). Any one of Daniels Electronics MT-3

Synthesizer modules or MT-3 Crystal Controlled

modules provide a suitable RF local oscillator

source. The local oscillator input level should

never exceed +7 dBm (5 mW into 50 Ω)

D O U B L E B A LA N C E D M I X E R

Double Balanced Mixer, M1, down converts

the received signal to 21.4 MHz. The mixer has

a maximum conversion loss of 7.5 dB and its

maximum local oscillator drive level is +7 dBm.

Image Filter

Lowpass: JU1 and JU2 - B side installed

Highpass: JU1 and JU2 - A side installed

Lowpass: JU1 and JU2 - B side installed

Highpass: JU1 and JU2 - A side installed

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