UWB Subharmonic Mixers
3.2 Theory of a Diode Mixer
3.3.2 Circuit Realization and Measurements
Fig. 3.4 and Fig. 3.5 show photograph and configuration of the proposed UWB mixer. The circuit is fabricated on RO4003 substrate with a dielectric constant of 3.58 and thickness of 20mil. The RF and IF frequency are chosen to be 3-13GHz and 60MHz, respectively.
The total circuit consists of a UWB RF/LO directional coupler, two diodes, a low-pass filter, and a high-pass filer. UWB RF/LO directional coupler utilizes the multisection direction coupler discussed in chapter 2. LPF and HPF are realized by a simple conductor and capacitor.
Figure 3.4: Photograph of the proposed UWB subharmonic quadrature-IF mixer
RF/LO hybrid
Ground pad
LPF
HPF
Jump wire
IF hybrid RF
IF LO
IF
IF RF
Figure 3.5: UWB subharmonic quadrature-IF mixer circuit configuration 50
NCTU-CM MIC LAB
RF
7
f = GHz
IF
2
LO RFf = f − f
RF
8
f = GHz
20° 90°
Figure 3.6: Time domain wave form of quadrature-IF signal
Applying sinusoidal waves into RF and LO port, a time-domain voltage wave-form can be observed by an oscilloscope. Fig. 3.6 is the measured voltage wavewave-form of proposed mixer under the condition of local power 10dBm, RF frequency 8GHz.
Fig. 3.6 shows that the amplitude deviation is almost zero at this frequency and phase difference between two output ports is 90 degree.
Fig. 3.7 shows the conversion loss of the quadrature-IF mixer. The conversion loss is lower than 15dB when local power is larger than 10dBm. The RF and IF frequency are given at 8GHz and 60MHz, respectively.
Local Power (dBm)
0 2 4 6 8 10 12 14 16
Conversion Loss (dB)
10 15 20 25 30 35 40
RF Frequency (GHz)
2 4 6 8 10 12 14
Isolation (dB)
12 14 16 18 20 22 24 26
3.7 3.18
Figure 3.7: Conversion loss of quadrature-IF mixer
Fig. 3.8 and Fig. 3.9 show the I/Q amplitude deviation and quadrature phase deviation of the quadrature-IF mixer. The amplitude deviation is less than ±3dB when RF frequency is from 3-13GHz and IF is fixed at 60MHz. The local source power is set around 10dBm.
3.4 The Proposed UWB Subharmonic Image Re-jection Mixer
3.4.1 Introduction
The proposed topology of UWB subharmonic image rejection mixer (IRM) is shown in Fig. 3.10. This image rejection mixer is same as the quadrature-IF mixer dis-cussed in the previous section except an additional IF hybrid. We utilize the same method to analyze the proposed mixer.
RF Frequency (GHz)
2 4 6 8 10 12 14
Quadrature Phase Deviation (dB)
-80
RF Frequency (GHz)
2 4 6 8 10 12 14
I/Q Amplitude Deviation (dB)
-3
Figure 3.8: I/Q amplitude deviation of quadrature-IF mixer
RF Frequency (GHz)
2 4 6 8 10 12 14
Quadrature Phase Deviation (dB)
-80
RF Frequency (GHz)
2 4 6 8 10 12 14
I/Q Amplitude Deviation (dB)
-3
Figure 3.9: Quadrature phase deviation of quadrature-IF mixer
53
RF/LO 90º Hybrid
A
B
LPFLPF HPFHPF
IF 90º Hybrid RF
LO
IF1 IF2
ID1
VRF,A
VRF,B
ID2,RF
ID1,RF
VLO,A
VLO,B
ID2.LO
ID1,LO ID2
ID2'
V I
t
1/fLO
gLO(t) VLO(t) (a)
(b)
Figure 3.10: (a) Topology of UWB subharmonic IRM (b) Diode current configura-tion
54
After ID1 and ID20 going through the IF directional coupler, outputs IF1 and IF2 can be expressed as
IF 1 = ID1+ (−j)ID20 = ID1+ (j)m+1(−j)nID1 = (1 + (j)m+1(−j)n)ID1 (3.10) IF 2 = −jID1+ ID20 = −jID1− (j)m(−j)nID1 = −(j + (j)m(−j)n)ID1 (3.11)
We now see what signal will going out when harmonic index when (m,n) equals (1,-2)
IF 1 = (1 + (j)2(−j)−2)ID1 = (1 + (−1)(−1))ID1 = 2ID1 (3.12) IF 2 = −(j + (j)1(−j)−2)ID1 = −(j + j(−1))ID1 = 0 (3.13)
this means that no signal component will show on IF2 when fIF = fRF − 2fLO. When (m,n) equals (-1,2)
IF 1 = (1 + (j)0(−j)2)ID1 = (1 + 1(−1))ID1 = 0 (3.14) IF 2 = −(j + (j)−1(−j)2)ID1= −(j + (−j)(−1))ID1= −2jID1 (3.15)
2fLO − fRF only appears on IF2. If the wanted signal is fIF = fRF − 2fLO, then 2fLO − fRF is the image signal. Therefore, the output signal will appear on IF1 and the image signal is totally eliminated by this proposed subharmonic IRM. As the same results, if fIF = 2fLO − fRF is the wanted signal, the image signal is eliminated and the output signal will appear on IF2.
The fundamental mixing signal appears when (m,n) equals (1,-1) or (-1,1). For
(m,n) equals (1,-1)
IF 1 = (1 + (j)2(−j)−1)ID1= (1 + (−1)j)ID1= (1 − j)ID1 (3.16) IF 2 = −(j + (j)1(−j)−1)ID1 = −(j + j(j))ID1 = (1 − j)ID1 (3.17)
and (m,n) equals (-1,1)
IF 1 = (1 + (j)0(−j)1)ID1 = (1 − j)ID1 (3.18) IF 2 = −(j + (j)−1(−j)1)ID1 = −(j + (−j)(−j))ID1 = (1 − j)ID1 (3.19)
The above two equations show that not only the fundamental mixing signal but also the image signal reach output ports.
3.4.2 Circuit Realization and Measurements
Photograph and configuration of the proposed UWB subharmonic image rejection mixer are shown in Fig. 3.11 and Fig. 3.12. MA-COM JHS-115, which is a surface mount quadrature hybrid with bandwidth of 40-80MHz, is used as the IF quadrature hybrid of the proposed mixer. The total circuit consists of a UWB RF/LO directional coupler, two diodes, a low-pass filter, a high-pass filer, and an IF quadrature hybrid.
Fig. 3.13 shows the conversion loss versus local power of the image rejection mixer. The conversion loss is lower than 15dB when local power is larger than 8dBm. Fig. 3.14 is the graph of RF to IF conversion loss versus RF frequency, conversion loss of image signal is also shown in this figure. The conversion loss of RF to IF is lower than 20dB when RF frequency is from 3-13GHz, totally covered the entire UWB bandwidth.
Figure 3.11: Photograph of the proposed UWB subharmonic IRM
RF/LO hybrid
Figure 3.12: UWB subharmonic IRM circuit configurationIF Frequency (MHz)
0 20 40 60 80 100 120
Image Rejection (dB)
0
LO Power (dBm)
0 2 4 6 8 10 12 14 16
Conversion Loss (dB)
10
IF Frequency (MHz)
0 20 40 60 80 100 120
Conversion Loss (dB)
10
Figure 3.13: Conversion loss versus local power of IRM
RF frequency (GHz)
2 4 6 8 10 12 14
Image rejection (dB)
0 5 10 15 20 25
RF Frequency (GHz)
2 4 6 8 10 12 14
Conversion Loss (dB)
10 15 20 25 30 35 40 45
IF Image Col 1 vs Col 17
Mean conversion loss = 16.5dB
Figure 3.14: Conversion loss versus RF frequency of IRM
Image rejection ratio is shown in Fig. 3.15. Most of the frequencies in UWB bandwidth have the image rejection ration larger than 15dB, which is a basic spec-ification of a wideband image rejection mixer.
Fig. 3.16 shows the IF bandwidth of the mixer. Conversion loss is measured under the following conditions. LO frequency is fixed at 4GHz and varying RF frequency from 8.01GHz to 8.11GHz. The mixer can have a better performance from low IF frequency of about 40MHz to 80MHz. Fig. 3.17 is the image rejection ratio versus IF frequency and Fig. 3.18 is the isolation of LO to RF versus RF frequency.
59
RF frequency (GHz)
2 4 6 8 10 12 14
Image rejection (dB)
0
RF Frequency (GHz)
2 4 6 8 10 12 14
Conversion Loss (dB)
10
Figure 3.15: Image rejection ratio versus RF frequency of IRM
IF Frequency (MHz)
0 20 40 60 80 100 120
Image Rejection (dB)
0
Conversion Loss (dB)
10
IF Frequency (MHz)
0 20 40 60 80 100 120
Conversion Loss (dB)
10
Figure 3.16: Conversion loss versus IF frequency of IRM
60
IF Frequency (MHz)
0 20 40 60 80 100 120
Image Rejection (dB)
0
LO Power (dBm)
0 2 4 6 8 10 12 14 16
Conversion Loss (dB)
10
IF Frequency (MHz)
0 20 40 60 80 100 120
Conversion Loss (dB)
10
Figure 3.17: Image rejection ratio versus IF frequency of IRM
Local Power (dBm)
0 2 4 6 8 10 12 14 16
Conversion Loss (dB)
10
RF Frequency (GHz)
2 4 6 8 10 12 14
Figure 3.18: Isolation of LO to RF versus RF frequency of IRM
Chapter 4
Conclusion
In this thesis, a five-section 3-dB quadrature hybrid realized by conventional PCB process with Rogers RO4003 has been successfully demonstrated. The pro-posed modified VIP couplers have solved all three of the problems that a multi-section 3-dB quadrature hybrid has always encountered, including an extremely tight-coupled center section, equalizing modal phase velocities, and minimizing the discontinuity effect between each section. VIP couplers with compensated stubs have shown that return loss and isolation can be effectively improved. The proposed directional coupler has shown a near-constant coupling over an ultra-wideband fre-quency bandwidth of 1.5-13.5GHz, return loss and isolation are better than 13dB over the entire bandwidth.
We have demonstrated an UWB subharmonic quadrature-IF mixer with con-version loss better than 15dB when RF frequency is from 3-13GHz. The minimum layout area and fewest numbers of elements are the benefits of this mixer. I/Q amplitude deviation has shown fewer than 3dB in the bandwidth.
An UWB subharmonic image rejection mixer has also demonstrated in this thesis. The mixer, when connected with an IF directional coupler, exhibits 15-18dB conversion loss and 10-22dB of image rejection. IF bandwidth can cover the frequency range about 40-80MHz due to the limitation of the IF directional coupler.
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