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Shure U2 Hand-Held UHF Transmitter

Circuit Description 25B1022 (AG)2

Circuit Description

Audio Section

Audio enters L248, an inductor used as an rf choke. The signal is

ac-coupled through C201 into a 26 dB user-adjustable gain stage around

U201B. This gain stage is externally accessible to the user. C249, C250,

and C209 protect the preamplifier and bias circuits from rf interference.

R212 and R241 set up a half-supply bias, and R238 sets the ac input

impedance.

The amplified audio signal is then passed through a pre-emphasis

network before entering the compression stage. R230, R202, and C203

set up two corners for the pre-emphasis network. The pre-emphasis

boosts the high frequencies before transmission.

This network feeds an NE575 compandor, U203, which utilizes an

external amplifier U204B. The compandor performs 2:1 logarithmic com-

pression of the audio signal. Additionally, the pre-emphasis network

plays a role in setting the hinge point (0 dB gain) of the compandor.

Transistors Q211 and Q207, along with crystal Y202, form the tone

key oscillator circuit. This circuit provides a stable, continuous 32.768

kHz sine wave. Transistor Q201 buffers the tone key signal before it is

added to the audio signal.

The tone key signal is used in the receiver to provide audio output

only when the tone key signal is present in the transmitted signal; there-

fore, if the tone key or the transmitter is turned off, the receiver will be

muted. The tone key squelch eliminates receiver noise associated with

loss of a carrier. Q206 acts as a switch for toggling the tone key ON/

OFF. It is controlled by the microprocessor. R286 allows for the tone key

amplitude level to be set.

The tone key circuit is powered via U210B. This is a dc amplifier

used to multiply the 3V battery voltage up to 4.5 V (gain = 1.5). As the

battery voltage drops, the tone key supply voltage drops, decreasing

the amplitude of the oscillator. This is used to detect low battery in the

receiver. (This feature was eliminated for JB models.)

The tone key signal, along with the processed audio signal, is then

fed to a summing amplifier U204A. R231 and R234 set up a half-supply

bias. R207, at the output of the summing amplifier, is used to help pre-

vent spurious oscillations from the operational amplifier. After passing

ac-coupling capacitor C213, the signal is fed to the rf module.

Rf Section

Processed audio enters an internal potentiometer R227, which is

adjusted for 45 kHz deviation (100% modulation) with a -7.2 dBV 1 kHz

tone at the output of the front audio stage (pin 1 of U201). (On JB mod-

els, R227 is adjusted for 5 kHz deviation with a –67.2 dBV, 1 kHz tone

injected into the mic input.) The audio is then fed to the tuning voltage

pin of the voltage controlled oscillator (VCO) and modulates the carrier

directly. The use of a phase locked loop (PLL) frequency-synthesized

system eliminates the need for multiplier stages, resulting in a much

higher degree of spectral purity. The VCO is shielded to prevent interfer-

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Shure UHF MD Manuel de service Page 3

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