Browse Prior Art Database

SINGLE WIDE BAND VCO FOR RX AND TX

IP.com Disclosure Number: IPCOM000005937D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2001-Nov-16
Document File: 1 page(s) / 54K

Publishing Venue

Motorola

Related People

Swee Min Liew: AUTHOR [+2]

Abstract

In transceiver radios with high-side injection mixers utilizing a single local oscillator (LO) for receiving (RX) and transmitting (TX), to switch from RX to TX it is necessary to reduce the LO frequency. This can be accomplished by either increasing the inductance or capacitance of the tank circuit. Also, for FM operation it is necessary to provide means, generally a variable capacitance, to shift the frequency. The frequency deviation for FM is 5KHz, which requires a very small capacitance change, while the frequency shift from RX to TX may be more than 45MHz. At present, switch- ing from RX to TX is accomplished by increasing the inductance and a variable capacitor is used for modulation.

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MOTOROLA Technical Developments Volume 10 March 1990

SINGLE WIDE BAND VCO FOR RX AND TX

by Swee Min Liew and Samuel Wong

   In transceiver radios with high-side injection mixers utilizing a single local oscillator (LO) for receiving (RX) and transmitting (TX), to switch from RX to TX it is necessary to reduce the LO frequency. This can be accomplished by either increasing the inductance or capacitance of the tank circuit. Also, for FM operation it is necessary to provide means, generally a variable capacitance, to shift the frequency. The frequency deviation for FM is 5KHz, which requires a very small capacitance change, while the frequency shift from RX to TX may be more than 45MHz. At present, switch- ing from RX to TX is accomplished by increasing the inductance and a variable capacitor is used for modulation.

In the conventional VCO with a single oscillator circuit the modulation capacitor is connected to the RX/TX switch- ing circuit as shown in FIG. 1. Assuming high-side injection for the mixer, the following equations apply:

L, = Ln+ + LTxeq"

but

jwl~m" = ~WLTX - ~/WCS [in parallel with -j/WC mod]

It can be seen that Cm~d has an effect on LTXB~". This effect makes switching between RX and TX (which is already very complicated because LnmU is also frequency dependant) and contmlling the modulation flatness a very tedious job.

   The new VCO circuit has the configuration shown in FIG. 2. From FIG. 2 it can be seen that the LncssU does not include the Cmod term and...