Do It Yourself

Test, problems, and solutions for the SMA to IPEX cable for ADALM-PLUTO second channel

Another important piece written by Gianni IW1EPY about the long-standing problem of the IPEX-SMA cables used for the second channel of the ADALM-PLUTO. Here is his tutorial:

A set of different SMA female to IPEX was purchased from China:

Test result for the Type 2 cable. Sorry, it’s not smooth due to the fast sweep, but the reality remains.

Question: Is the IPEX connector the problem?

Using two SMAs coming from type 1 and using the correct cable RG316 for that connector, with the center cable in the hole and the braid soldered.

Using an adapter for RG 1.13 with a good SMA connector

What about the IPEX?

Same cable length, same SMA with adapter for RG 1.13 and an IPEX IPEX junction.
See also the channel 2 Pluto test of one of those cables.

This means that with a good SMA connection and a couple of IPEX, it can work up to 16 GHz.
First, no blame to the cable producer for usage up to 6 GHz is assured, but for us, that we take out the last still of lemon juice is not enough.
We use the second Rx as a reference for the VNA up to 12 GHz, so we would like a flat response in this range.
What is wrong?

These types of SMA cannot function properly; the correct Type 2 has many discontinuities and impedance changes that limit its use to around 6 GHz. On the left, Type 1 is its incorrect usage, as shown by my test with the proper coax cable. Inserting the entire coax cable, including the braid, into the SMA hole and soldering it is not correct. What happens? The tin solder connects the braid only at the rear entrance of the SMA, leaving a ground tube unconnected, which can be seen in the picture.

This insulated ground tube is responsible for the notch in the range from 6 to 8 GHz

Solution

Find the SMA with the correct cable hole to enter with the insulated central coax cable and solder the braid to the outside of the SMA.

For RG 1.13, use a small-hole SMA but avoid using too much solder; connect all the braid tube to the inner tube of the SMA.

By now, the only solution to the problem is to purchase the IPEX cable and do the SMA connection.
Two good SMA-IPEX will allow the correct use of the second channel of ADALM-PLUTO up to 12 GHz.
Even without test systems is possible to use ADALM-PLUTO to make a comparison between cables to get the right one.

This is a bad cable with a large loss at 13,5 GHz

This is the long RG 1.13 coming from the couple already tested (a twin cable used to validate the IPEX connector)

This is an RG 316 precrimped with IPEX in China and a China SMA for this type of cable.

Due to the stiffness of the cable is a bit longer to accommodate a larger bend of the cable vs the RG 178, which allows a shorter path but a worse performance.

73 51 de IW1EPY

The Notch Problem

My friend Gianni IW1EPY discovered a tricky problem with the ADALM-PLUTO used above specifications. Here is his report:

This info is for ADALM-PLUTO users who experience this strange behavior.
During the metal boxing of an ADALM-PLUTO Rev C of my friend, I discovered a problem.

Connecting it to an LMX2595 to test the 5 harmonics.

Left a small box containing the LMX2595 board with the double coax transitions in WR42 to cancel the fundamental, and on the right, the Pluto under test.

A terrible notch appeared.

Without signal, Pluto noise is around -70, so the notch kills all around 22.3 GHz.
After the Pluto board investigation, bad SMA soldering was discovered.
The central pin was soldered on the pad, but a length of around 1.5 mm starting from the PCB edge was not soldered.
The missing soldering is mainly due to a non-planarity of the SMA vs the PCB.
The RF enters the PCB, not at the PB edge, and sees two lines, one of which is the correct path to the balun and a second line that extends for 1.5 mm to the edge of the board.
The second line acts as a stub with an open end, applying a short at the frequency of lambda-quarter resonation.
Resoldering the full length of the SMA central pin, the problem went out.

This is the normal behaviour of an ADALM-PLUTO without calibration, giving you a little more than 20 dB of dynamics in the 24 GHz band.

73, Gianni IW1EPY

SATSAGEN INTERFACE

Franck F1SSF has produced an interesting PCB that collects all functions and schematics of the USBDAALBFER interface project (Video).

With SATSAGEN and Franck’s PCB, you can:

Power a Noise Source for Noise Figure and Gain Analyzer measurements
Drive an ADF5355 PLL synthesizer as a SATSAGEN TX device
Connect AD831x log detectors to do power measurements up to 10 GHz
Enable external analog and digital input for spectrum analyzer triggers
Connect external SNAs thanks to a 10V pp ramp output synchronized with the SATSAGEN SNA/VNA scans
Connect an RF switch to enable real two-port VNA operations

Download the Gerber files here or contact Franck F1SSF, but his PCB availability is limited.

Download the Arduino sketch here to compile and upload it to the interface.

Follow Franck’s instruction steps to wire and make operational his SATSAGEN INTERFACE PCB:

Hi All, Some information here. This PCBA is the first realization:

Inserted components except Nano and resistors 0U:
- Use IBom:

Ibom Download

Power UP:
- Power supply on 12V IN connector
ADF5355 power supply
- Measure R10 pad = 6V, if OK solder R10=0U
Step Up 28V – Warning, add modify R19 as below picture
- Measure R23 pad, adjust P3 to obtain 28V max, If OK solder R23=0
Detector Step Up = ON:
- Measure divider R2/R3, you must have 3Vmax
+15V DAC
- Measure R13 pad = 15V, if OK solder R13=0U
Power OFF
Solder wires directly on serial chip Arduino (see schematic for pin numbers)
- Plug Arduino on support for removal easily, and solder wires DCD = TP1, RI=TP2
Check the values of the ADF5355 voltage dividers
- R12/R22. R16/R24. R18/R25
Now you can connect all peripherals
- Use connectors J6 , J11, J5, J7, J10
+3V ref
- Adjust P4 to have +3V on Arduino pin ref N°18
Configure Satsagen in tracking mode 0 to 6Ghz
- See on J3 voltage ramp from 0V to about 12V.

SW1 allows you to select the operating modes of the Arduino, depending on the use.

You can deport SW1, LED1, and LED3 on the front end with J11 and J9.

If you move the LEDs, then remove the SMD LEDs or if you leave them, then change the R6 and R22 to adapt the current. SW1 can stay on board.

J1 / J2 / J10 Footprints are BNC connectors, but you can use SMA connectors after cutting legs. You solder ground around directly around the body.

Because the ADF5355 consumes approximately 200mA, it is recommended not to exceed 12V power supply to reduce the dissipation of the 6V regulator. I added a small radiator with thermal glue.

If your RF switch HMC536 already contains 100U resistors on ports A and B under the shield, replace R34 and R35 with 0U.

73’s Franck F1SSF

Franck’s Satsagen Interface schematic Rev E-01

*MT3608* step-up 28 Volts section schematic with the R19 to add

450 MHz Band Pass Filter

Luigi IZ7PDX made this filter. It is a bandpass centered at 450 MHz that Luigi will use as an IF filter.

Luigi IZ7PDX’s centered bandpass filter at 450MHz front panel view.

The filter inside without the notch section activated (“parked” at 580 MHz).

Luigi characterized the filter using ADALM-PLUTO and SATSAGEN.

It’s the setup for the analysis of the filter. Note the attenuator placed on the RX side to improve impedance matching and preserve the RX stage of ADALM-PLUTO from possible damage.

The resulting response curve in a span of 200 MHz. The insertion loss is around 1.3 dB

It’s a scan performed with a 700MHz span.

Luigi has further improved the input and output impedance of the filter by acting on the coupling links and measuring with a vector antenna Analyzer:

It is scanning with a span of 200 MHz after improving the in/out impedance of the filter. The filter is “narrower” in terms of bandwidth than the previous scan before intervening on the coupling links.

Find Luigi IZ7PDX on:
- Youtube channel
- Site