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RFFC5071A/2A

DS140110

7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical

support, contact RFMD at (+1) 336-678-5570 or customerservice@rfmd.com.

External Reference

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

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

able 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 2mA, 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 30MHz up to 6000MHz. 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 15mA and 45mA 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 cir-

cuitry 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 -2dB (not including balun losses) is achieved with 100

 differen-

tial 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

0.5nH 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 2k

 resistance in parallel with some capacitance, approxi-

mately 1pF 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 50Ω and 500Ω, typically 200Ω. 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

0.5nH on each pin) becomes more significant. Above about 4500MHz, it is beneficial to lower the output load to 50

 to mini-

mize the effect of the ouput capacitance. The following diagram is a simple model of the mixer output:

RFFC507xA

Mixer Input

0.5nH

0.5nH

Rin

Typ 85



0.5pF

RFFC507xA

Mixer Output

0.5nH

0.5nH

1K



1pF

1K

