Midland Radio SP-300 service manual RF Circuits

MAXON

SP-300 HAND HELD

Mute (squelch) Circuit

The mute circuit which is controlled by the output of IC409 (micro) Pin 77 (386EN) is connected to Q34 via R122 which mutes the LM386IC on the RF circuits. Pin 77 also controls IC402B which mutes the audio path to the RF circuits.

TX Audio And Filter Circuits

The TX audio from the internal mic or external mic is fed into IC411 (Pin 3). The TX audio is output on Pin 4 (IC411) and into the high pass filter (IC410), where sub-audible voice products are removed. The TX audio output from IC410 is fed into IC404D & C, with associated parts form a mic amplifier and limiter. The output from Pin 8 IC404C is fed to RV402 (TX Modulation Level Adjust) and fed into IC404A & B to form a 3k low pass filter. The output of IC404A (Pin 1) is then fed into the Audio Mixer Circuit.

Analog Ground Supply

IC406D supplies (2.0V) to operational amplifiers circuits.

Audio Mixer And Inverter

IC405A is an audio mixer where audio and sub-audible tones are combined. RV401 is used as a balance control. IC405B is an inverter.

Monitor

The unit will contain a switch mounted on the PTT assembly for monitor function. This will be enabled or disabled by programming software.

RF CIRCUITS

Transmitter

The transmitter is comprised of:

1.Buffer

2.P.A. Module

3.Low Pass Filter

4.Antenna Switch

5.A.P.C. Circuits

Buffer

VCO output level is -6 dBm and amplified to +10dBm (UHF), +6dBm (VHF). The buffer consists of Q16 and Q17 for isolation and gain.

P.A. Module

The P.A. Module consists of Q501, Q502 and Q503. Three stage amplifier Q501 amplifies the TX signal from +10 dBm to 100 mW. Q502 is amplified to 1.0W. Q503 amplifies to 5W and then matched to 50 OHM using the L.C. network, thereby reducing the harmonics by -30 dB.

Low Pass Filter

L7, L8, L11, C72, C73, C74 and C75 are the 7th order Che- byshev low pass filter. Unwanted harmonic are reduced by -70 dBc.

Antenna Switch

When transmitting, the diodes D5 and D6 are forward biased enabling the RF signal passage to the antenna. D6 is shorted to ground inhibiting the RF signal to the front end. In receive the diodes D5 and D6 are reversed biased passing the signal from the antenna through L13 and C83 to the front end with- out signal loss.

Automatic Power Control (APC) Circuits

The APC circuit containing the variable resistors RV1, RV3, IC3 and Transistors Q18,Q19,Q21 and Q22. The current from the collector of Q503 is detected at R109, it is con- verted to voltage at IC3A and compared with the reference voltage at IC3B. In transmit (+5V) is applied to the input of IC3B via the potential divider R45 and R46. The differential signal at the output of IC3B is passed to Q21 and Q22, This voltage controls collector voltage of Q502 that produces a constant power output. RV1 is used to adjust the high power and RV3 is used to adjust the low power.

Page -10- May 2001