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Fairchild FST3125
The FST3125 (SOIC-14 package) contains 4 SPST switches with a separate control line each. All 4 switches are needed to implement the H-Mode mixer. The PCB layout of the FST3125 with the 3 TT4-1A transformers is less compact than the layout of the SPDT type mixers. Especially the connection between the outer transformers is considerably longer. The FST3125 requires an 74AC74 divide by two squarer as it does need both Q and !Q.
The following table summarizes the measurements with the FST3125:
| FST3125 + TT4-1A + 74AC74 |
Vdd = 7.0V, Vbias = 3.5V |
Best bias |
| Band |
BPF |
mixer |
MDS -dBm |
IF rej |
spur |
mixer IIP3 (dBm) @ 20KHz spacing |
Bias |
IIP3 |
| IL |
CL |
- |
+ |
|
avg |
-6 |
-3 |
0 |
+3 |
+5 |
|
0 |
| -dB |
-dB |
BPF |
BPF |
-dB |
dB |
dBm |
dBm |
dBm |
dBm |
dBm |
V |
dBm |
| 160 |
3,3 |
5,08 |
133 |
129 |
52,8 |
|
|
|
41,9 |
|
|
3,5 |
41,9 |
| 80 |
1,4 |
5,04 |
133 |
131 |
52,8 |
|
44,7 |
44,2 |
45,2 |
46,5 |
47,7 |
3,5 |
45,2 |
| 40 |
2,3 |
5,04 |
133 |
130 |
53,2 |
|
|
|
43,7 |
|
|
3,5 |
43,7 |
| 30 |
3,4 |
5,00 |
133 |
129 |
53,6 |
|
|
|
41,5 |
|
|
3,5 |
41,5 |
| 20 |
3,4 |
5,00 |
133 |
129 |
54,0 |
|
|
|
40,4 |
|
|
3,5 |
40,4 |
| 17 |
3,0 |
4,96 |
133 |
128 |
52,4 |
|
|
|
39,3 |
|
|
3,5 |
39,3 |
| 15 |
3,9 |
4,96 |
133 |
127 |
52,8 |
4,3 |
|
|
39,1 |
|
|
3,5 |
39,1 |
| 12 |
3,7 |
4,92 |
133 |
129 |
49,6 |
|
|
|
37,3 |
|
|
3,5 |
37,3 |
| 10 |
2,3 |
5,00 |
133 |
125 |
48,4 |
|
|
|
36,5 |
|
|
3,5 |
36,5 |
| 6 |
|
5,40 |
125 |
|
28,0 |
|
|
|
34,7 |
|
|
3,5 |
34,7 |
The FST3125 is tested at 7V Vdd with bias at the mid point. There is no other point were the IMD dipped better. There is no steep dip. FST3125 is rather insensitive with respect to the bias point.
I am not able to measure at -6dBm reliably as the 3rd IMD products are too close to the noise floor.
Again degrading IP3 with frequency and peaking IP3 on 80M and 40M.
160M performance seems to suffer from the transformers limitations.
The spur reduction on 15M is fairly good, but slightly less than with the FSAV332.
Unlike the mixers with the SPDT type switches, the 9MHz IF rejection is very good at below -50dB fairly constant on most HF bands!
MDS level on 6M and RF-IF isolation are very poor indeed. High spur levels were observed on 6M.
The FST3125 really performs very much like the FSAV332, although the FSAV332 is slightly superior.
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