Vectronics VEC-1002K - Manual (Page 28)

VEC-1002K Owner's Manual 2 Meter FM Receiver Kit
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THEORY OF OPERATION AND SPECIFICATIONS
Technical Circuit Description:
The VEC-1002K FM is a sensitive voltage-tunable dual-conversion NBFM
receiver that includes an effective squelch circuit and gated AF-amplifier IC for
speaker operation. Incoming signals are filtered through a bandpass filter at
L1/L2 to reduce out-of-band interference, then boosted by low-noise
preamplifier Q1. Q1 is series-matched to the input of U1.
U1 is a Motorola device that performs nearly all receiver functions for the radio.
The first LO (local oscillator) is voltage-tuned by a temperature-compensated
varactor diode built into the receiver chip. Signals are converted in the receiver's
DBM 1st-mixer stage down to 10.7 MHz, the radio's 1st IF. Here, they are fed
through a roofing filter (FL1) to reduce in-band interference. The second LO,
crystal-controlled at 10.245 MHz, drives the 2nd mixer for converting signals to
455 KHz. FL2 establishes the radio's message-channel bandwidth prior to 100-
dB of signal amplification and limiting in the 2nd IF. Signals are demodulated
by quadrature detection, and recovered audio signals are preamplified to line
level by U1's output stage. Audio is then sent to the receiver's volume control
and to AF amplifier U2.
In addition to processing FM signals, U1 also measures signal strength through
log-amp RSSI circuitry. RSSI output is sent to a built-in op-amp comparator,
where signal strength is compared to a reference level set by the radio's squelch
control. The comparator then generates a logic signal to operate the "mute" pin
of audio amplifier U2 for squelch action.
Audio is supplied to U2 via the radio's volume control. Here, audio signals are
contoured for frequency response and amplified to speaker-level. The output of
U2 is fed to a speaker/phone jack.
Specifications:
Tuning Range ..................................145.1 - 147.6 MHz (approximate)
Sensitivity.........................................5 uV typical for useful audio recovery
Selectivity........................................20-kHz message-channel bandwidth
AF Output........................................60 mW into 8-ohm load
Current Drain...................................15 mA squelched, 40 mA on voice peaks
Power Source...................................9 Volt flatpack battery
PCB Size .........................................3.300" x 3.000"