10kHz-40MHz AD9834 Scalar Network Analyzer

Test and upgrade

As some people know, I buy all the analyzers made in China that are compatible with DL4JAL software (because they will also work with SNASharp).
This way I can verify that it works and also write a SNASharp configuration file for that model.

I bought this analyzer available for a few months. It is indicated as being usable from 10kHz to 40MHz.

At the reception I made this high resolution photography , it is easier for me to zoom on an image to determine the schematic.

I quickly measured a quartz filter, and the results were catastrophic, I deduced that the analyzer did not implement correct impedance matching on his ports.

I then observed the output circuit of the tracking generator, and realized that there was a big design error. The output consists of an OPA695 operational amplifier (with feedback loop) followed by a 3dB attenuator. This configuration can only offer an output impedance around 18 Ohms, far from the expected 50 Ohms .

Output of tracking generator

I then observed the input circuit and there too there is a big mistake. We have a 3dB attenuator followed by the input of the logarithmic amplifier AD8307. The AD8307 offers an input impedance of 1100 Ohms, the consequence is that the input port offers an impedance of about 240 Ohms, we are also very far from the expected 50 Ohms.

Analyzer input

Fixing the issues

For the output it was quite simple, I removed the two 300R attenuator resistors and replaced the 18 Ohms series resistor by a 47 Ohms resistor, so we have an output that offers a ~50 Ohms broadband impedance.

Tracking generator output impedance fixing

For the input I had to do a little math. I wanted to change the minimum of components, I just replaced the first 300R resistor of the input attenuator by a 62 Ohms resistor. This offers almost 50 Ohms in broadband (49.4 Ohms measured)

Upgraded input, The resistor was directly soldered on the SMA socket because I damaged the PCB when unsoldering the 300R resistor.

Test and conclusions

Port matching is ok and the analyzer keeps 82dB of dynamic range on the low frequencies and 74dB on the high frequencies, which is very correct.

S21 with SNASharp Raw mode ( uncalibrated)

The linearity is also very good, here up to 60dB.

Linearity check
Mesure of a 4MHz SSB filter
Mesure of an 7.5MHz LPF

Once modified, this 20€ analyzer is perfectly usable for HF bands. My tests indicated that it could make measurements from 1kHz to 40MHz. The generator provides a fairly clean signal, the harmonics are at -48dBc (H2) and -50dBc (H3).

David, F4HTQ 2021/05/22

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