EVAL-ADRF6820 Datasheet, PDF (4/20 Pages) Analog Devices – On-board USB for SPI control

UG-652
LO INPUT/OUTPUT
The ADRF6820 offers two alternatives for generating the
differential local oscillator (LO) input signal:
• Generate the signal externally via a high frequency, low
phase noise LO signal.
• Generate the signal internally via the on-chip Fractional-N
synthesizer.
In either case, the differential LO signal can be routed off chip
to SMA connectors labeled LOOP and LOON for the purpose
of daisy-chaining multiple ICs requiring LO signals.
For an internal LO configuration using the on-chip Fractional-N
synthesizer, apply a low phase noise reference signal to the
REF_IN connector. The PLL reference input can support a wide
frequency range because the divide or multiplication block can
be used to increase or decrease the reference frequency to the
desired value before the signal is passed to the phase frequency
detector (PFD). The integrated synthesizer enables continuous
LO coverage from 356.25 MHz to 2850 MHz.
For optimum performance using an external LO source, the LO
input pins (LOIN− and LOIN+) should be driven differentially. The
wide input range of the external LO inputs span from 350 MHz
to 6000 MHz. Unless an ac-coupled balun/transformer is used
to generate the differential LO, the inputs must be ac-coupled.
The input impedance of the differential LO signals is 50 Ω.
QUADRATURE I/Q OUTPUTS
On the evaluation board, the quadrature I/Q outputs are ter-
minated with 200 Ω differential. This is achieved by using a
1:1 transformer and an L-impedance matching network. The
1:1 transformer, TC1-1-13M+ from Mini-Circuits, is a very
wide bandwidth (4.5 MHz to 3000 MHz) transformer that allows a
flat pass-band response of up to 1 GHz. Using a wide bandwidth
transformer is necessary to accurately evaluate the wideband
pass-band response of the ADRF6820 without the effects of the
EVAL-ADRF6820 User Guide
transformer/balun roll-off. However, to terminate the ADRF6820
with 200 Ω and also match the output impedance of the
transformer with 50 Ω, an L network is placed between the
I/Q outputs and the transformer. This introduces some power
loss because the L network acts as a voltage divider (see Figure 3).
The power loss due to the L network is approximately 17 dB.
C64
BBIN ^0.1µF
R37
86.6Ω
R33
0Ω
DNI
R41
32.4Ω
R49
0Ω
DNI _{6 T6 1}
BBIN1 1 BBIN
5432
C63
BBIP ^0.1µF
R38
86.6Ω
R34
0Ω
DNI
R42
32.4Ω
R45
0Ω
R46
0Ω
PRI
SEC
4
3
TC1-1-13M+
R50
R68
0Ω
0Ω
DNI
DNI
JOHNSON142-0701-851
R52
0Ω
BBIP1 1
BBIP
5432
JOHNSON142-0701-851
Figure 3. I/Q Output Matching Network
To avoid the power loss from the L network while achieving the
proper impedance matching, a 4:1 impedance transformer, such
as the TC4-1W+ from Mini-Circuits can be used in place of the
TC1-1-13M+. This 4:1 impedance transformer allows the primary
side of the transformer to be terminated with 200 Ω and the
secondary side to be terminated with 50 Ω. The L network then
becomes unnecessary and can be shorted out by replacing R35,
R36, R37, and R38 with 0 Ω while R31, R32, R33, R34, R39, R40,
R41, and R42 can all be DNI (do not install). Attention needs to
be paid to the bandwidth of the transformer because impedance
transformation transformers tend to have narrower bandwidths.
The TC4-1W+ has a listed bandwidth of 3 MHz to 800 MHz;
however, the 1 dB bandwidth is limited to 100 MHz. As a result,
the roll-off of the transformer may distort the pass-band flatness of
the ADRF6820, depending on which bandwidth configuration
is used for the ADRF6820.
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