Does it matter at what end of the cable I install the attenuator? - Page 1
Forum section: electronics / rf-microwave

author: Miti
author_rank: Super Contributor
timestamp: April 30, 2020, 02:57:53 am
content: Imagine an automated test system for some RF products. The output is from multiple PAs, each at about +32dbm, and I have to combine these outputs into one cable to a spectrum analyzer. Does it matter if the attenuators are at one end or the other end of a coax? For example:1. PA -> 60cm coax -> 20db/20W att -> 20db directional coupler -> 4 ways combiner - 10db att -> SA2. PA -> 20db/20W att -> 60cm coax -> 20db directional coupler -> 4 ways combiner - 10db att -> SAEach option is times 4. All components are good quality 50 Ohm. Do you expect any difference between the two options? Frequency is around 3.5GHz.

author: OwO
author_rank: Super Contributor
timestamp: April 30, 2020, 03:45:17 am
content: The attenuator will present better return loss than the coax cable (-25dB vs -15dB) unless you go for very ex🅱️ensive cables, so it's a matter of whether you want the PA to see a better match or the directional coupler.

author: Miti
author_rank: Super Contributor
timestamp: April 30, 2020, 12:41:44 pm
content: Thanks OwO!It makes sense and I suppose is better to see better return loss at higher power than at lower power. At least that's what I make out of the designer's recommendation to place the attenuator at the PA.

author: Miti
author_rank: Super Contributor
timestamp: May 01, 2020, 11:45:35 am
content: I did a little experiment yesterday. I connected my IFR2025 to the HP 8594E with a 50cm SMA cable and a 10 db attenuator at one end then the other end of the cable. There’s almost no difference, it is only 0.02 db less with the attenuator at the SA side at +13dbm, the maximum I can get from the siggen.

author: OwO
author_rank: Super Contributor
timestamp: May 01, 2020, 05:06:35 pm
content: Transmission measurements will be exactly the same, because you get the same amount of mismatch loss regardless of where the mismatch is. You want to check the impedance looking into the cable => attenuator combination, and compare that to attenuator => cable.

author: Miti
author_rank: Super Contributor
timestamp: May 01, 2020, 11:50:20 pm
content: Yes but my question was if I would see any difference in level between the two options that I presented above. I had a discussion with the customer the other day and he said that the readings on the SA are higher with the attenuator towards the PA compared to the other end. I asked why and he couldn't explain. The SA measurements are level and channel power, that's it.

author: OwO
author_rank: Super Contributor
timestamp: May 02, 2020, 04:48:07 am
content: If the signal source is well matched, no, since mismatch loss is well defined. If the source is not 50ohms, then mismatch can create loss or gain. For example if the PA and the load both happen to be 25ohms at a frequency, it will deliver more power than into a 50ohm load. Therefore to minimize influence of the PA output impedance on the delivered power, put the attenuator at the PA side.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 02, 2020, 10:42:55 am
content: Quote from: Miti on May 01, 2020, 11:45:35 amas long as the coax is properly made 50ohm Zc and terminated/looking at 50 ohm at the other end, you can safely assume/imagine its a 50 ohm resistor DC to daylight.