RFFC5071A/2A

WIDEBAND SYNTHESIZER/VCO WITH

INTEGRATED 6 GHz MIXER

Data Sheet Rev. D, August 22, 2019 | Subject to change without notice

8 of 27

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®

1pF

0.5nH

0.5nH

RFFC507xA

Mixer Output

1K

1K

External Reference

The RFFC5071A and RFFC5072A have been designed to use an external reference such as a TCXO. The typical input will be a

0.8 Vp-p clipped sine wave, which should be AC-coupled into the reference input. When the PLL is not in use, it may be desirable to turn

off the internal reference circuits, by setting the REFSTBY bit low, to minimize current draw while in standby mode.

On cold start, or if REFSTBY is programmed low, the reference circuits will need a warm-up period. This is set by the SU_WAIT bits.

This will allow the clock to be stable and immediately available when the ENBL bit is asserted high, allowing the PLL to assume

normal operation.

If the current consumption of the reference circuits in standby mode, typically 2 mA, is not critical, then the REFSTBY bit can be set high.

This allows the fastest startup and lock time after ENBL is taken high.

Wideband Mixer

The mixers are wideband, double-balanced Gilbert cells. They support RF/IF frequencies from 30 MHz up to 6000 MHz. Each mixer has

an input port and an output port that can be used for either IF or RF (in other words, for up- or down-conversion). The mixer current can

be programmed to between about 15 mA and 45 mA depending on linearity requirements. The majority of the mixer current is sourced

through the output pins via either a center-tapped balun or an RF choke in the external matching circuitry to the supply.

The RF mixer input and output ports are differential and require baluns and simple matching circuits optimized to the specific application

frequencies. A conversion gain of approximately -2 dB (not including balun losses) is achieved with 100

 differential input impedance,

and the outputs driving 200

 differential load impedance. Increasing the mixer output load increases the conversion gain.

The mixer has a broadband common gate input. The input impedance is dominated by the resistance set by the mixer 1/gm term, which

is inversely proportional to the mixer current setting. The resistance will be approximately 85

 at the default mixer current setting (100).

There is also some shunt capacitance at the mixer input, and the inductance of the bond wires (about 0.5 nH on each pin) to consider at

higher frequencies. The following diagram is a simple model of the mixer input impedance:

The mixer output is high impedance, consisting of approximately 2 k

 resistance in parallel with some capacitance, approximately 1 pF

dependent on PCB layout. The mixer output does not require a conjugate matching network. It is a constant current output which will

drive a real differential load of between 50

Ω and 500 Ω, typically 200 Ω. Since the mixer output is a constant current source, a higher

resistance load will give higher output voltage and gain. A shunt inductor can be used to resonate with the mixer output capacitance at

the frequency of interest. This inductor may not be required at lower frequencies where the impedance of the output capacitance is less

significant. At higher output frequencies the inductance of the bond wires (about 0.5 nH on each pin) becomes more significant. Above

about 4500 MHz, it is beneficial to lower the output load to 50

 to minimize the effect of the output capacitance. The following diagram

is a simple model of the mixer output:

0.5pF

Rin

Typ 85

0.5nH

0.5nH

RFFC507xA

Mixer Input