How do I check an RF attenuator?

[nenea dani]

Hello colleagues,
  I recondition a Systron Donner 762-1 ancient Spectrum Analyzer. I started with the Midwest Microwave Model 1058 attenuator inside. It contains a drum with 7 plastic coaxial  tubes with contacts at the extremities that connects for each stage the input to the output. Tubes can not be opened because they are cast in the drum. I want to find out if they are functional. For 0db between input and ground is O.C. Between the exit and the ground is O.C.  Between the ends of the tube is S.C.  For 10db between input or output and ground are 58Ohm and between ends 51.9Ohm. For 20, 30, 40, 50, 60db between input or output and ground are 48,8 - 58Ohm and between ends 88,4-98,6 Ohm. Could be these coaxial attenuating π type?  How do I use the oscilloscope to check this attenuator? Should I connect the oscilloscope in 50Ohm mode and use a RF cable with SMA sockets? Or do I use it with the normal probe? I'm obviously trying to enter a radio signal at the entrance.

[IconicPCB]

Ah ..
I think I have one of those...spec ans... I would need to check the model number....

Mine has a problem with crystal filter ...its going to be a real problem to source parts i reckon.

[xrunner]

How do I use the oscilloscope to check this attenuator? Should I connect the oscilloscope in 50Ohm mode and use a RF cable with SMA sockets? Or do I use it with the normal probe? I'm obviously trying to enter a radio signal at the entrance.

Sure you can check it with a scope provided the rf is well below the scope's bandwidth, and it's fed into 50 ohms. Say you wanted to work with 20 MHz, just use a good 50 ohm coax and send the signal into the attenuator, and then connect it to the scope terminated in 50 ohms. If the attenuator is set to 3 dB then the power into the scope 50 ohm load is going to be 1/2 the input power. You can calculate the voltage you should see on the scope or the scope measurement settings can display it for you for verification. I've done it before and it does work very well.

[wolfp]

Imho the first checks of an attenuator can be made by measuring the resistances (with DC) as you already did. The next step would be to use a network-analyser.

[nenea dani]

    I used a  ADF5355  set on 80Mhz ,  Rigol DS2072 hacked  set on 50ohm  and two RG316 cables with SMA connectors and an SMA / N adapter. I used a conversion table Vpp to dbm / 50ohm. It would seem like the ADF5355 provides in my case 4dbm in 50ohm. For 0, 10,20 30, 40db attenuation I have established by comparison that the measured values are very close to 10db for each step. For 50 and 60db attenuation, it seems to be between 4 and 6db / step instead of 10db. I do not know what confidence I have in measurements at 1-2 mVpp at 80Mhz for last steps.  I am pleased, although the mechanical switch does not work very well on the contacts and needs a little fit on the last steps. Thank you all .

[Wolfgang]

Hello colleagues,
  I recondition a Systron Donner 762-1 ancient Spectrum Analyzer. I started with the Midwest Microwave Model 1058 attenuator inside. It contains a drum with 7 plastic coaxial  tubes with contacts at the extremities that connects for each stage the input to the output. Tubes can not be opened because they are cast in the drum. I want to find out if they are functional. For 0db between input and ground is O.C. Between the exit and the ground is O.C.  Between the ends of the tube is S.C.  For 10db between input or output and ground are 58Ohm and between ends 51.9Ohm. For 20, 30, 40, 50, 60db between input or output and ground are 48,8 - 58Ohm and between ends 88,4-98,6 Ohm. Could be these coaxial attenuating π type?  How do I use the oscilloscope to check this attenuator? Should I connect the oscilloscope in 50Ohm mode and use a RF cable with SMA sockets? Or do I use it with the normal probe? I'm obviously trying to enter a radio signal at the entrance.

Hi,

for a basic test you just need a voltage source and a multimeter. If you really want to evauluate RF performance, you need a VNA bceause most scopes do go up far enough in frequency.

[nenea dani]

I have a VNA with one port , replica by EU1KY .   How  am I doing? I have to couple the attenuator to pure 50 then O.C. then S.C.?   I am thinking of plugging an RF signal at the input and outputting a block with AD8317 and I see by comparison.

[Wolfgang]

In principle thats correct. But the dynamic range and accuracy will be poor (if you are measuring attenuators with more than, say 20dB).
Just for S21, what about a spectrum analyzer with tracking gen, a la DSA815TG or something similar ?

[IconicPCB]

doing a spot frequency check is OK for a first degree approximation.
You need to do a swept frequency measurement across each range and across full frequency specification

[nenea dani]

   For the moment I fixed the attenuator in his place in the analyzer. It seems that 0,10, 20 and 30 dB attenuation are consistent with the indications on the screen . This is a  CRT with 10db/div. The device is not precise anymore and now I'm trying to understand how it works and especially how it should be set in the absence of another analyzer. It's a good exercise for the very beginning in the world of spectrum analyzers. The device in question does not have a reference or calibrator, so it measures only comparatively or better said indicatively. In the coming days I will disassemble it for new checks and then I will do the VNA test for attenuator .

[David Hess]

For functional checks an oscilloscope works fine however dynamic range is limited so the attenuator sections needs to be checked individually which may not be possible with a rotary attenuator.  I use my fast reference level pulse generator to drive the attenuator and then check the attenuation and transient response on the oscilloscope.

[TheUnnamedNewbie]

Also keep in mind that the VSRW of your measuring equipment will impact the performance. You are measuring source + attenuator + load, and if the source and load (your equipment) are worse than the attenuator (which might happen faster than you expect with decent attenuators at anything past a few MHz) you don't learn much.

[nenea dani]

   I just received a 0-31db attenuator with the PE4306 and  OLED display from the SV1AFN. By comparison, I'll see what's going on. As I have already studied the manual of the device from which the attenuator comes, it appears to be in T although not explicit and not in π  as originally I thought. Anyway, it does not change things.  Worse is the SMA output jack that connects to it has often been removed out by my predecessors and does not look good. This jack is part of a handyform cable and does not bend too much. I will replace it with RG142 because the device must reach theoretically at least 12.4Ghz.

  I did some tests with a 5.6dB attenuator manufactured by me: 158Ohm, 34Ohm(series), 158Ohm. I used a VNA with a single port and connected the 5.6dB attenuator in the 50 Ohm, O, S tests at 10Mhz. For 50Ohm with the attenuator connected or directly in the load there is almost no difference. S11 is in these cases -27dB. I used some  cables and SMA adapters. For the other two situations, there are differences as follows. Obviously for absolute R, but these are the same as in DC measurement. S11 appears differently with the attenuator connected versus directly connected situation(without  ATT).  In principle, the attenuator creates an adaptation or a kind of good transfer in these two situations (ATT + SC, ATT + OC). VSWR is less than 2. It is clear for me that a VNA with two ports is required.