This document discusses the development of a 6-24 GHz mixer in a novel chip-scale package using 0.25um enhancement mode PHEMT technology. Key advantages of the chip-scale package include eliminating die assembly steps, reducing parasitics, and enabling a thinner package for improved thermal dissipation. Measurement results show the uncapped mixer has a conversion loss of ~9dB up to 22GHz and an IIP3 of +19dBm, while the capped mixer has slightly higher loss and lower IIP3. Isolation measurements were over 35dB for LO-RF and 40dB for LO-IF. This represents the first reported chip-scale packaged mixer.
2. PCB or custom test fixture/contactor board for
testing.
• Estimated cost is $0.10/mm2
) even at mmW
frequencies.
A chip scale (GaAsCap) wafer is composed of a pair of
bonded GaAs wafers, in which all the I/O’s are routed through
Vias to the backside of the device wafer. The combination of
the gasket and the cap wafer provides an air cavity, structural
and protection for the device wafer. When the wafer is sawn
between the gaskets, it literally becomes thousands of
individually packaged parts. Photograph in fig. (2) is a
finished GaAsCap wafer.
Fig. 2 Photograph of a finished GaAsCap wafer.
Figure (3) illustrates a cross-sectional view of a GaAsCap
wafer. The base wafer is a standard processed GaAs wafer.
The backside vias serve as I/O and ground pads for the
package. The backside metal is plated thick enough to ensure
adequate coverage in the bottom of the vias without inhibiting
the use of standard solder pastes in assembly. The cap wafer
provides an air cavity, protection for the devices, and enough
structural support to allow thinning of the device wafer to 1.5
mils thickness.
Fig. 3 Cross-sectional view of a GaAsCap wafer.
IV. CIRCUIT DESIGN
The designed mixer is based on single balance design. S-D
connected FET has been used as diode. It has a LO balun that
feeds diode and mixes with RF/IF frequency to generate
desired IF/RF frequency as an down/up converter. A diplexer
has been used for RF/IF. Several Momentum simulations have
been run to optimize the performance of the LO balun. This
balun provides amplitude balance ±0.5dB and phase balance
±0.5o
from 5-25GHz. Use of 180o
hybrids ensures excellent L-
R isolation and eliminates or minimizes the need of a band
pass filter to filter out LO power at R-port. Design of LO
balun is based on Marchand technique. To reduce the chip
size the balun has been folded into rectangular spiral shape. In
addition to Balun entire layout has also been simulated using
ADS Momentum to take into account all coupling among
close proximity traces and all sorts of junction discontinuities.
The insertion loss of low pass section (IF) of diplexer is
<0.5dB from DC-3GHz and the insertion loss of high pass
section (RF) is <1dB from 5-25GHz. Such low loss diplexer
and excellent amplitude and phase matched Balun are key to
this mixer low conversion loss and very wide band
performance. Simplified schematic of Mixer is shown in fig.4.
Fig. 4 Simplified Schematic of Mixer.
The photograph of mixer chips developed is shown in fig.
(5a,b). The GaAsCap package footprint is 2mm x 2mm.
(a) (b)
Fig. 5 (a) Top View of Un-capped Package. (b) Bottom view of Un-
Capped/GaAs-Capped Package.
V. MEASURED PERFORMANCE
The Un-Capped and GaAs-capped mixers has been
soldered on high frequency Roger PCB board and
characterized in connectorized PCB media as shown in
fig.6(a,b,c). Measured performance of mixer includes all
losses such as connectors, PCB trace. This loss is of the order
of 0.2-1.5dB from DC-25GHz.
The measured mixers (UnCapped & GaAs-Capped)
performance is shown from fig.(7)- fig.(12) as an Up-
Converter. The down conversion performance is better or
similar to Up-conversion. The LO (frequency) = (RF-IF)/2
GHz and IF=2GHz.
Fig.(7) shows the measured uncapped mixer C.L. at
Plo=+14 to +20dB. It shows 8.5-10dB conversion loss from 5-
24GHz @ Plo=+16dBm.
Wafer Small area of a Wafer
Singulated
GaAsCap die
Cap wafer (GaAs)
Base
Backside Via
Gasket Gasket
Backside Via
Balun DiplexerLO RF
IF
GND
LO
IF
RF
239
Authorized licensed use limited to: Access Provided by Avago Technologies. Downloaded on July 14,2010 at 23:35:43 UTC from IEEE Xplore. Restrictions apply.
3. (a)
(b) (c)
Fig. 6 (a) Photograph of Un-Capped Pkg in a Test Fixture. (b) Top View of
Un-capped Package in fixture. (c) Top View of GaAs-capped Package in
fixture.
Fig. 7 Up-Conversion Loss of an Un-Capped mixer
Fig. 8 Up-Conversion Loss of an GaAs-Capped mixer
Fig.(8) shows C.L. of GaAs-Capped mixer at Plo=+14 to
+20dB. GaAs-Capped mixer has slightly higher loss than Un-
Capped mixer.
Fig.(9) & Fig.(10) shows the measured IIP3 of Un-Capped
& GaAs-Capped mixer in fixture. It can be seen from this
measurement that IIP3 fluctuates significantly with frequency.
This behavior attributes of RF/IF circuit impedance variation.
This impedance variation produces different load impedance
at harmonics termination of mixer. Also, the IIP3 of GaAs-
Capped mixer is lower than Un-Capped mixer. The reason for
GaAs-Capped mixer performance degradation compared to
Un-Capped mixer is due to GaAs-Cap lid close proximity
affect.
Fig. 9 Up-Conversion IIP3 of an Un-Capped mixer
Fig. 10 Up-Conversion IIP3 of an GaAs-Capped mixer
0
2
4
6
8
10
12
14
16
18
20
6 8 10 12 14 16 18 20 22 24
RF Freq. (GHz)
C.L.(dB)
LO=+14dBm
LO=+16dBm
LO=+18dBm
LO=+20dBm
GaAs-Capped
0
5
10
15
20
25
30
6 8 10 12 14 16 18 20 22 24
RF Freq. (GHz)
IIP3(dBm)
LO=+14dBm
LO=+16dBm
LO=+18dBm
LO=+20dBm
0
5
10
15
20
25
30
6 8 10 12 14 16 18 20 22 24
RF Freq. (GHz)
IIP3(dBm)
LO=+14dBm
LO=+16dBm
LO=+18dBm
LO=+20dBm
0
2
4
6
8
10
12
14
16
18
20
6 8 10 12 14 16 18 20 22 24
RFFreq. (GHz)
C.L.(dB)
LO=+14dBm
LO=+16dBm
LO=+18dBm
LO=+20dBm
Un-Capped
The Power leakage and Isolation behaviors of Un-Capped
& GaAs-Capped mixer are quite similar so they are shown as
a one mixer measurement in fig.(11) & (12).
240
Authorized licensed use limited to: Access Provided by Avago Technologies. Downloaded on July 14,2010 at 23:35:43 UTC from IEEE Xplore. Restrictions apply.
4. Fig. 11 L-R Isolation of an Un-Capped/GaAs-Capped mixer
30
35
40
45
50
55
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
LO Frequency (GHz)
L-IIsolation(dB)
Plo=+10
Plo=+12
Plo=+14
Plo=+16
Plo=+18
Plo=+20
Plo=+22
Fig. 12 L-I Isolation of an Un-Capped/GaAs-Capped mixer
VI. CONCLUSIONS
A low cost, broadband high performance mixer in a novel
package has been developed. This package fabrication technique
made it possible to integrate the package as a MMIC fab process
step and thus totally eliminated and die-to-package assembly
complexity, time and cost.
ACKNOWLEDGMENT
Authors are thankful to Avago Technologies Fab team for
fabrication of the designed mixer. Authors are grateful to Hue
B. Tran for doing all testing.
REFERENCES
[1] S. Kumar, M. Vice, H. Morkner, W. Lam, “Enhancement mode GaAs
PHEMT LNA with Linearity Control (IP3) and phased matched
Mitigated Bypass Switch and Differential Active Mixer,” 2003 IEEE
MTT-S Digest, pp. 1577-1580, June 2003.
[2] H. Morkner, S. Kumar, M. Vice, “ A 18-45GHz Double Balanced
Mixer with Integrated LO Amplifier and unique suspended Broadside
coupled Balun, IEEE GaAs Integrated Circuit Symposium 2003, 25th
Annual Technical Digest, USA, Nov. 2003, pp 1577-1580.
[3] Trantella C.J., “Ultra small MMIc Mixers for K and Ka band
Communications .” 2000 IEEE MTT-S digest pp 647-650.
20
25
30
35
40
45
50
55
60
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
LO Frequency (GHz)
L-RIsolation(dB)
Plo=+10
Plo=+12
Plo=+14
Plo=+16
Plo=+18
Plo=+20
Plo=+22
241
Authorized licensed use limited to: Access Provided by Avago Technologies. Downloaded on July 14,2010 at 23:35:43 UTC from IEEE Xplore. Restrictions apply.