High Isolation Synthesizer Selection
Circuit for GSM, DCS1800, PCS1900
APN2016
The ability to switch between two synthesizers is a
standard requirement in GSM and PCS digital base
stations. Typically the isolation required between
synthesizers exceeds 90 dB. The most common design
solution is to use five commercial GaAs IC switches
[(1) SPDT, (4) SPST]. The benefit is that the isolation
specification can be met without elaborate circuit
manipulation. The downside is higher insertion loss, large
PCB area and increased cost. Skyworks has developed
a three switch solution utilizing high isolation SPDT
and SPST GaAs IC switches. Isolation values are >90 dB
@ 900, 1800 and 1900 MHz. The key is to lay out the PCB
so that DC to RF coupling, radiation and leakage effects
are minimized. The circuit is composed of one AS164,
AS186, AS196 SPDT and two AS165-59 SPST positive
control switches. It is non-reflective at the J2, J3 ports.
Synthesizer 1
~
J2
• CPW transmission lines and gaps are narrower thus
saving PCB area. For this design 0.019" line width 0.005"
gap yielded the best simulated and measured results.
The PCB could also be fabricated easily using these
values.
• Radiation and DC to RF coupling effects are minimized
because of the top ground layer. It acts as a termination
point for stray fields.
• Grounding under the GaAs IC and along the RF lines
is provided by plated thru-holes. This helps to further
isolate the individual switch paths.
• The voltage supply and control lines can be capacitively
bypassed due to the close proximity of the ground plane.
Hi Q 0402 multilayer capacitors worked well. Two values
of capacitor were chosen, 39 pF for 900 MHz and 18 pF
for 1800, 1900 MHz.
• Two layer PCB keeps RF and DC lines separated and
results in a further isolation improvement.
AS165-59
J1
The PCB material is low cost FR4, 0.028" thick, ER = 4.5.
The circuit was designed using two-layer groundedcoplanar-waveguide (CPW) as the transmission medium.
CPW has many advantages over microstrip when high
isolation is required.
AS164-80
AS186-302
AS196-307
J3
The assembly requires a fixed +5 V supply voltage (VS)
and four lines of voltage control (V1–V4) 0/+5 V.
Figure 2 shows isolation, return loss and insertion loss
measurements. The isolation measurements were made
without shielding the individual switches. An improvement
of ≈20 dB (>105 dB) can be achieved with proper
shielding.
~
AS165-59
Synthesizer 2
Figure 1. Block Diagram
Synthesizer Selection Circuit
Skyworks Solutions, Inc. [781] 376-3000 • Fax [781] 376-3100 • Email sales@skyworksinc.com • www.skyworksinc.com
Specifications subject to change without notice. 9/03A
1
High Isolation Synthesizer Selection Circuit for GSM, DCS1800, PCS1900
-70
-16
-18
-80
-20
-90
-22
-100
-24
-26
-110
-28
-30
-120
0.5
1
1.5
2
-5
Return Loss (dB)
-14
-1.2
-1.4
-10
-1.6
-15
-20
-1.8
-25
-2.0
-30
0.5
1
1.5
Frequency (GHz)
Frequency (GHz)
Synthesizer Switch
Isolation & Return Loss
Synthesizer Switch
Insertion Loss & Return Loss
2
Figure 2. Isolation, Return Loss, Insertion Loss Measurements
2
Skyworks Solutions, Inc. [781] 376-3000 • Fax [781] 376-3100 • Email sales@skyworksinc.com • www.skyworksinc.com
Specifications subject to change without notice. 9/03A
Insertion Loss (dB)
Isolation (dB)
-60
0
Output Return Loss (dB)
-10
-12
-50
APN2016