Hello everyone, I am dealing with a switch circuit using the hmc624lc4 mmic. The schematic is shown as follows, so it is the printed circuit board.
The rf board is RO4350B with a height of 0.254mm for the delectric material, so the 50 Ohm character impedance is for 0.58mm for microstrip width.
The control part for providing the -5V/0V control logic is working well, so it is with the -5V power for the MMIC.
The mmic is soldered with the smt processing, so there is little error for soldering, and I can see the back pad of the center of the mmic is soldered with enough Tin.
Since the total loss is terrible, I splitted a pcb using the edge soldering sma connetor to measure the loss of the front half and the back half microstrip.
Front half for microstrip
Back half for microstrip
So the insertion loss for the microstrip equals the front back half output plusing the back half output and minusing the test cable loss.
The table data is achieved by a signal generator for input power with -20dBm and measuring with a spectrum analyzer for output for the same cable in twice for the measuing.
The instrument is OK with no error.
|
Freq.（GHz） |
Input power(dBm) |
Front Half output (dBm) |
Back Half Output(dBm) |
Cable Loss（dB） |
Insertion Loss (dB) |
|
5 |
-20 |
-24 |
-21.7 |
1.3 |
3.1 |
|
6 |
-20 |
-22.3 |
-22.9 |
1.7 |
1.8 |
|
8 |
-20 |
-23.6 |
-22.9 |
1.9 |
2.7 |
|
10 |
-20 |
-23.4 |
-22.7 |
2.3 |
1.5 |
|
12 |
-20 |
-24.2 |
-25.5 |
3.8 |
2.1 |
|
14 |
-20 |
-26.8 |
-23.9 |
3 |
4.7 |
|
15 |
-20 |
-25.6 |
-24.8 |
3.3 |
3.8 |
|
18 |
-20 |
-27.5 |
-25.6 |
4.6 |
3.9 |
So I get a insertion loss for the microsrip about not more than 5 dB.
However, the total insertion loss is terrible with a measured value for 18dB, which means that the mmic loss is about 13dB, disobeying the 4dB loss in the datasheet.
In the last, can you tell me what is wrong with the circuit? Any suggestion is appreciated.
By the way, when measuring the total loss, I put some aluminum copper between the pcb backside and the metal box.
The capacitor has been replaced by some other ATC 0603 microwave capacitor, with only little influence, not more than 0.5dB.
