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RF Technology T70/T150 Page 15

5 CIRCUIT DESCRIPTION 5.10 PTT and DC Remote Control

The outputs of U10a and U10c are connected to the frequency response shaping

networks C52, R133 (for 750µs pre-emphasis) and C61, R55 (for flat response). JP7

selects the pre-emphasized or flat response.

The audio signal is further amplified 100 times by U7d. U7d also provides the

symmetrical clipping required to limit the maximum deviation. The output level from

U7d is adjusted by RV1, the deviation adjustment, before being combined with the

tone audio signal in the summing amplifier U7a.

The composite audio from U7a is fed through the 3Khz low pass filter U7b. When the

links on JP23 are in their default state, the filtered audio is coupled to the TCXO

voltage tuning input and the modulation balance trimmer RV2. RV2, R99 and R98

attenuate the modulation signal before applying it to the VCO via varactor D3.

When a DMTX board option is required, Jumper JP23 allows the audio paths to be

re-routed. The DMTX board provides for an external digital modulation input signal.

When the two links on JP23 are positioned in the middle of the 6 pin header, the audio

from the exciter is passed to the DMTX board via pin 5 of JP15, where the signal is

conditioned and then returned from the DMTX board via pin 2 of JP15, and passed to

the two modulation points.

RV2 adjusts the level of the audio used to modulate the VCO. This primarily effects

the deviation of audio frequencies above 500 Hz. RV2 is used to balance the high and

low frequency deviation to obtain a flat frequency response relative to the desired

characteristic.

5.10 PTT and DC Remote Control

Two main PTT inputs are provided. The first, a direct logic level input, is connected

to pin 3 of the system connector. The transmitter can be keyed by applying a logic

low or ground on pin 3. Pin 3 connects to the PTT logic and microprocessor through

D10.

DC current loop control can be used for remote PTT operation. The current loop can

be configured by JP9, JP10 and JP11 for use with either a remote free switch or a

remote switched source.

Opto-isolator ISO1 is used to isolate the loop current signal from the transmitter PTT

logic. The loop current passes through the input of ISO1 and the output of ISO1

connects to the PTT logic.

A bridge, consisting of diodes D6, D8, D9 and D14 ensures correct operation

regardless of the current polarity. Q17 limits the current and D7 limits the voltage

input to ISO1. Any low voltage current source capable of providing 2 mA at 4 V or a

switching circuit with less than 4.8KΩ loop resistance can be used to switch the DC

loop.

The test PTT button on the front panel and the local microphone PTT button will also

key the transmitter. Both of these also mute the line audio input. The microphone line

also enables that audio input.

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RF Technology R150 Betriebsanleitung Seite 15