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Mini-Circuits T1-1T
Because the 4:1 transformers tried so far (TT4-1A, T4-1, ADTT4-1, FT37-43) never fulfilled all requirements with the same configuration some 1:1 transformers have been tested too. The original reason to operate the switches at 200 Ohm probably has been to minimize the Ron losses of the switches. W4ZCB has reported trying 1:1 transformers in order to improve the IP3 with the argument that half the voltage will likely decrease the IMD. IP3 did not noticeably improve in his tests, but conversion loss was not affected much too!
The following table shows the results with the T1-1T transformers:
| FSA3157 + T1-1T + 74AC04 |
Vdd = 7.0V, Vbias = 1.9V |
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 |
5,44 |
132 |
128 |
64,0 |
|
|
|
37,9 |
|
|
1,9 |
37,9 |
| 80 |
1,4 |
5,32 |
133 |
131 |
68,0 |
|
42,7 |
44,0 |
44,7 |
46,0 |
46,7 |
1,9 |
44,7 |
| 40 |
2,3 |
5,28 |
133 |
130 |
70,0 |
|
|
|
47,9 |
|
|
1,9 |
47,9 |
| 30 |
3,4 |
5,24 |
133 |
129 |
66,0 |
|
|
|
46,5 |
|
|
1,9 |
46,5 |
| 20 |
3,4 |
5,28 |
133 |
129 |
62,0 |
|
|
|
45,2 |
|
|
1,9 |
45,2 |
| 17 |
3,0 |
5,28 |
133 |
129 |
68,0 |
|
|
|
44,3 |
|
|
1,9 |
44,3 |
| 15 |
3,9 |
5,40 |
132 |
128 |
71,0 |
-2,5 |
|
|
42,1 |
|
|
1,9 |
42,1 |
| 12 |
3,7 |
5,48 |
132 |
128 |
47,0 |
|
|
|
41,8 |
|
|
1,9 |
41,8 |
| 10 |
2,3 |
5,56 |
132 |
129 |
41,0 |
|
|
|
40,8 |
|
|
1,9 |
40,8 |
| 6 |
|
5,68 |
132 |
|
57,0 |
|
|
|
40,7 |
|
|
1,9 |
40,7 |
| 4 |
|
6,00 |
126 |
|
27,2 |
|
|
|
37,1 |
|
|
1,9 |
37,1 |
Conversion loss is 0,3dB worse than with other transformers, somewhere in between TT4-1A and T4-1 and worsening on higher frequencies.
Average spur level with the T1-1T is excellent at -2,5dB. Note that this applies to a mixer adjusted for best RF-IF isolation, so nothing special was done to minimize the spur level.
RF-IF isolation is really excellent between -70dB and -80dB on most bands, if band specific adjustments are made. Only 6M and 4M are limited to -60dB. The levels in the table are for the best isolation on 15M. Adjusting the mark/space ratio to those excellent levels is very sensitive and difficult to do with a 20 turn trimpot.
The T1-1T combination had the best average bias point at 1.9V and this did not change much per band. Also the 1.9V itself was not very critical, just a mild dip. I would call this a very docile mixer much in contrast with the T4-1!
Except for 160M the T1-1T mixer actually does +40dBm or better on all the HF bands including 6M. This was a very promising start with the 1:1 transformer experiment indeed!
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