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Fairchild NC7SZ384
The NC7SZ384 (SOT23-5 package) contains only a single SPST switch. Therefore 4 units are needed to implement the H-Mode mixer. The PCB layout of the 4xNC7SZ384 with the 3 TT4-1A transformers is ultra compact because the switches can be placed very close to the associated transformers without compromising the distance between the outer transformers. The NC7SZ384 requires an 74AC74 divide by two squarer as it does need both Q and !Q.
The following table summarizes the measurements with the NC7SZ384:
| NC7SZ384 + TT4-1A + 74AC74 |
Vdd = 7.0V, Vbias = 2.5V |
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 |
5,04 |
132 |
129 |
45,6 |
|
|
|
41,4 |
|
|
3,0 |
41,4 |
| 80 |
1,4 |
5,00 |
133 |
131 |
47,2 |
|
44,2 |
45,5 |
47,0 |
49,7 |
52,2 |
2,5 |
47,0 |
| 40 |
2,3 |
5,00 |
133 |
130 |
49,6 |
|
|
|
47,2 |
|
|
2,5 |
47,2 |
| 30 |
3,4 |
4,96 |
133 |
129 |
44,8 |
|
|
|
44,0 |
|
|
2,5 |
44,0 |
| 20 |
3,4 |
4,96 |
133 |
129 |
41,2 |
|
|
|
42,7 |
|
|
2,4 |
42,7 |
| 17 |
3,0 |
4,96 |
133 |
129 |
41,2 |
|
|
|
39,8 |
|
|
2,5 |
39,8 |
| 15 |
3,9 |
4,96 |
133 |
128 |
41,6 |
7,7 |
|
|
40,9 |
|
|
2,5 |
40,9 |
| 12 |
3,7 |
4,92 |
133 |
128 |
37,2 |
|
|
|
38,8 |
|
|
3,0 |
38,8 |
| 10 |
2,3 |
4,96 |
133 |
128 |
33,2 |
|
|
|
40,5 |
|
|
2,5 |
40,8 |
| 6 |
|
5,48 |
130 |
|
43,2 |
|
|
|
38,7 |
|
|
3,0 |
42,0 |
The NC7SZ384 is tested at 7V Vdd with bias at 2,5V. 2.5V seems to be a good compromise for all bands. The IMD dip is most profound on 80M and 40M where IMD could be almost nulled-out. The optimum bias point is found to depend on the input termination of the mixer. A step attenuator set at 0dB before the mixer gives rather different results than no step attenuator at all.
I am not able to measure at -6dBm reliably as the 3rd IMD products are too close to the noise floor.
This mixer is keeping up good IP3 levels also on the higher bands!
160M performance seems to suffer from the transformers limitations.
The spur reduction on 15M is pretty bad.
Unlike the mixers with the SPDT type switches, the 9MHz IF rejection is rather good at below -40dB fairly constant on most bands!
MDS level on 6M and RF-IF isolation are rather good too!
IIP3 wise this mixer outperforms the FSAV332/FST3125 mixers. 4 separate (non-matched) packages that are adjacent on the reel don't seem to degrade performance much.
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