Thursday, October 31, 2024

Characterizing an Advantest R3361B Spectrum Analyzer Input with a miniVNA in the 1-180MHz range

As per my knowledge of VNAs this should be a very easy REFLECTION measurement.

According to the results I get, ...it is not.

I know: the setup itself is questionable due to the differences between Golia (the R3361B) and David (the miniVNA). But this is what I have in hand and it is an experiment done for the purpose of learning something, not to make actual measurements.

Equipment:

  • ADVANTEST R3361B Spectrum Analyser;

  • vanilla miniVNA (no Pro, no lite, no tiny, ...nothing);

  • VNA-J software versione 3.4.8;

  • BNC adaptors from China (!);

All VNA calibrations are made for an OPEN over a 1-180MHz range with 6k points.

To make a prove of work I first calibrated the miniVNA for REFLECTION with a M2M BNC adapter followed by a F2F adapter. The calibration plan is expected to be located at the free side of that last adapter (far left end in the picture).

Cal plane for 50ohms load
Cal plane for 50ohms load
 

Then a 50 Ohms load was connected….

miniVNA with a 50ohms load connected
miniVNA with a 50ohms load connected

 

 

 

 

 

 

Scanning the range I’ve got exactly what I expected:

50ohms VNA plot
50ohms VNA plot

 

 

 

 

 

 

What is worth noting here is the constant behavior over the entire range (forgetting the spikes at the far ends of the range).

 

 

 

I then replaced the F2F BNC adapter with a BNC2F_M adapter and re-calibrated the VNA:

BNC replaced by F
BNC replaced by F


 

 

 

 

 

 

 then I connected the whole thing to the SA:

miniVNA connected to SA
miniVNA connected to SA

 

 

 

 

 

 

 

 

 

 

 

 

and here are the plots i've got from the VNA with different ATTENUATOR values:

VNA plot with SA input ATT = 0 dB

VNA plot with SA input ATT = 0 dB


VNA plot with SA input ATT = 10 dB
VNA plot with SA input ATT = 10 dB

VNA plot with SA input ATT = 20 dB
VNA plot with SA input ATT = 20 dB

 

 

 

 

 

 

 

 

 

 

The 0db and 10db look very ugly. How is that possible that the SA fronend is so bad??

I posted the question to the eevblog forum and....

Quote from an eevblog forum fellow (credits to EggertEnjoyer123):

The problem then probably is that you have a mixer with RF, LO, and IF inside the spectrum analyzer, and the miniVNA is looking at the RF port. If the IF has a bandpass filter after it, then you will see a bad match if the mixed frequency is outside the passband. The signal that gets mixed down will get reflected back at the mixer from the IF filter, and it will get converted back up to the same RF frequency. So you end up getting power reflected back at the VNA, which shows up as bad return loss. The jumps are because the LO and RF sweep at different rates so it will appear to jump around.
Try setting your spectrum analyzer to 100 MHz or something, zero span mode.

 

He has guessed correctly about the SA frontend topology:

R3361B frontend and IF block diagram
R3361B frontend and IF block diagram

 

 

 

 

 

 

 

So I’ve set the SA to 90MHz and 0.SPAN and the results are surprising:


VNA plot with SA input ATT=0dB & 0.SPAN

VNA plot with SA input ATT=0dB & 0.SPAN


 

 

 

 

 

 

 

 

 

 

VNA plot with SA input ATT=10dB & 0.SPAN

VNA plot with SA input ATT=10dB & 0.SPAN


 

 

 

 

 

 

 

 

 

 

VNA plot with SA input ATT=20dB & 0.SPAN

VNA plot with SA input ATT=20dB & 0.SPAN



 

 

 

 

 

 

 

 

 

I posted this because it was an interesting experience, very useful to get how often you have to learn from good fellows.

Ciao.
Emanuele.

Wednesday, April 17, 2024

Kenwood TS-430s: SSB RX with very low AF volume and S-meter stuck at S-0.


 Repair Brief – Kenwood TS-430s – bad SSB RX.

 

DISCLAIMER: the following instructions are provided only as a report of my hobby experience. In no way I am responsible of any damage you or your device can suffer by following this procedure. Lethal voltages are present in the device!! Qualified personnel only! You know what you are doing and you are proceeding at your own risk!

 

Symptoms (RX only):

  • In USB and LSB low volume and no S-meter deviation.

  • Strong (local) SSB station are demodulated OK and can be heard with very high volume setting. IF-Shift and RIT working OK as well as NOTCH.

  • AM and CW are OK.

 

Findings:

  • In USB and LSB, the ACG line (gate of Q13) stuck at 2.5V, no matter of signal strength.

  • Q17 (what is it useful for?!) voltages: 2.6V all terminals as per schematics.

  • SSB line goes high when USB or LSB are selected (as expected).

  • On AM S-meter is well calibrated for S-0 and S-9.

  • CAR signal present @305mV RMS in USB, CW (8.8314KHz) and LSB (8.8284KHz).
    It disappears in AM.

Signals at IF board connector 27 seem not to be OK. it seems the SSB filter is never selected, leaving the signal path open.

 

The fault:

The SSR (SSB Receive - connector 27, pin 6) signal is driven HIGH in USB or LSB. That's ok.

It is then routed to the NAR/WIDE front panel switch through the IF board connector 27. It should come back as SSW (connector 27, pin 4) with the switch in the WIDE position, or SSN (connector 27, pin 2) with the switch in the NAR position. 

 

S18 and IF connector 27

 

Neither SSW nor SSN go high regardless of the switch position: the switch is not working or the switch board is not properly connected.

S18 found not working.

Restoring S18 solved the problem.

Tuesday, January 30, 2024

TOWER 5 Transistors Walkie talkies from the 60s - The ones that ignited my radio/electronics hobby.

Tale of my first transmission : 27.125MHz - CB Channel 14.

When I was young (late sixties, first seventies) I got a couple of black TOWER walkie talkies.
They were shiny and looked like rocket science objects.

My actual units

It wasn't long before I realized they weren't usable: communications with a friend were difficult, if not impossible, even between rooms.
On the other hand, however, one evening, I heard voices coming out of one of the radios. They were people who chatted about this and that using esoteric language...as well as coded words (CQ, QTH, QRZ, YL, 88, Roger, K, ...).
I tried to make myself heard, but no one could listen to me.
Suddenly, while I was desperately trying to make myself heard, (are you listening? Hello, ready, are you listening?...) one of those voices answered me. He greeted me, welcomed me and ... explained to me that I had a "super regenerative" radio in my hands and that it could simultaneously listen to many frequencies but only transmit on a single one. That's explained me why I was hearing many voices but I had difficulty make me listen.
After a few days I met that person: he was an OM with a shack that looked like a spaceship to me.
BINGO. Since then I have been CB for years and have become passionate about radio and electronics.
Eventually I also got an ham license.