RFFC5061 Datasheet, PDF (7/25 Pages) RF Micro Devices – WIDEBAND SYNTHESIZER/VCO WITH INTEGRATED 6GHz MIXER

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about 2pF to 12pF. The PCB stray capacitance and oscillator input and output capacitance will also contribute to the crystal's
total load capacitance.
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 min-
imize current draw while in standby mode. On cold start, or if REFSTBY is programmed low, the reference circuits will need a
warm-up period. A crystal oscillator typically takes many milliseconds to settle. This time 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 4mA, 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:
0.5nH
RFFC506x
Mixer Input
0.5nH
0.5pF
Rin
Typ 85
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:
0.5nH
1K
1K
1pF
0.5nH
RFFC506x
Mixer Output
The RFFC5061 mixer layout and pin placement has been optimized for high mixer-to-mixer isolation of greater than 60dB. The
mixers can be set up to operate in half duplex mode (1 mixer active) or full duplex mode (both mixers active). This selection is
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