SINGLE DIODE MIXER WITH RF TO IF PORT ISOLATION

Publishing Venue

Motorola

Related People

Abstract

Field testing of cellular phone base stations can require the use of a loopback circuit. This circuit couples a known transmitter output signal into a mixer that mixes the transmitter signal down to the receive band. This mixed down or looped back sig- nal is attenuated and injected into the receiver front end. There it is received, demodulated and com- pared with the known transmitted signal to deter- mine transceiver functionality. This can include received signal strength, and bit error rate measure- ments. All this can be done without interrupting ser- vice while the transceiver is still in the field. The RF (Radio Frequency) mixer required for a loop- back circuit has unique design requirements. This led to the development of a low cost single diode mixer that requires low LO (Local Oscillator) drive, and offers good RF to IF (Intermediate Frequency) isolation.

MOTOROLA Technical Developments

SINGLE DIODE MIXER WITH RF TO IF PORT ISOLATION

by Aaron W. Netsell and Bryan R. Rapala

BACKGROUND INFORMATION

  Field testing of cellular phone base stations can require the use of a loopback circuit. This circuit couples a known transmitter output signal into a mixer that mixes the transmitter signal down to the receive band. This mixed down or looped back sig- nal is attenuated and injected into the receiver front end. There it is received, demodulated and com- pared with the known transmitted signal to deter- mine transceiver functionality. This can include received signal strength, and bit error rate measure- ments. All this can be done without interrupting ser- vice while the transceiver is still in the field. The RF (Radio Frequency) mixer required for a loop- back circuit has unique design requirements. This led to the development of a low cost single diode mixer that requires low LO (Local Oscillator) drive, and offers good RF to IF (Intermediate Frequency) isolation.

PROBLEM DESCRIPTION

  Three design requirements made it difficult to find an existing RF mixer that was optimized for a loopback circuit. First, the mixer needed to be low cost. Adding a loopback circuit to a transceiver should not add significant cost to the radio. Existing mixers of the day were on the order of five or ten dollars. A design goal of less than two dollars was set for the mixer. Second the mixer would have to take an RF input frequency that was very close to its IF output frequency, and keep them isolated from each other as much as possible. Typical mixers have the LO frequency close to the RF frequency so again, there were no off-the-shelf mixers available. Finally, the mixer had a low LO drive requirement. Typical mixers require a minimum LO drive level of +7 dBm. Having an LO level that high presents spurious signals of the LO and its harmonics that might cause problems, particularly for the receiver. Further, a low LO drive reduces the power and cost

burden on the LO sythesizer. An LO drive level of 0 dBm was targeted.

PROBLEM SOLUTION

  The solution is the combination of a single ended mixer with a Wilkinson divider. The single diode provides the mixing with a minimum level of LO drive. The Wilkinson divider provides the RF to IF port isolation. See Figure 1 on the following

page.

  A slightly more detailed overview of circuit development follows. Both main components (the single ended mixer and the Wilkinson divider) of the mixer are well known. The single ended mixer uses one diode and is therefore low cost, and requires little LO drive. It uses...