Confused by signal levels at input to MMIC - Page 1
Forum section: electronics / rf-microwave

author: perdrix
author_rank: Frequent Contributor
timestamp: March 16, 2018, 03:58:53 pm
content: I'm looking at a PCB that has a 4 lead component (circular pill with 4 leads at right angles) marked with the code A03. The A03 code leads me to believe that this may be an MSA-0385.When I apply what is nominally a 1.1Vpp, 10MHz sine (50 ohm impedance) signal to the input side of the input blocking cap with no power applied to the circuit, I measure a voltage with my 'scope of 1Vpp (so far, not too bad).Now bit that's confusing the heck out of me. When I apply power to the circuit, the signal level I measure drops immediately to 0.4Vpp? Should I expect this behaviour?ThanksDavid

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 16, 2018, 04:16:09 pm
content: Perhaps you could add some pictures of the boards? this might make it easier to understand what it is you are referring to.

author: metrologist
author_rank: Super Contributor
timestamp: March 16, 2018, 04:30:12 pm
content: wouldn't an unpowered mmic circuit reflect the energy?

author: perdrix
author_rank: Frequent Contributor
timestamp: March 16, 2018, 06:04:44 pm
content: Here's the piccie: The part in question is U100 to the RHS ...Dave

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 16, 2018, 07:20:37 pm
content: Not a solution but when I look for A03 I find MSA-0386, not MSA-0385.http://www.farnell.com/datasheets/7254.pdf

author: rf+tech
author_rank: Frequent Contributor
timestamp: March 17, 2018, 08:54:33 pm
content: A03 is Minicircuits MAR-3+See marking chart in attached file.RF+ Tech

author: perdrix
author_rank: Frequent Contributor
timestamp: March 17, 2018, 10:45:00 pm
content: No its not - SMD charts are incorrect.

author: rf+tech
author_rank: Frequent Contributor
timestamp: March 18, 2018, 02:23:31 am
content: Sorry to disagree. If you take the time to compare datasheets, it becomes obvious MSA-0386 is a second-source equivalent to MAR-3+Neither is a manufacturer's data sheet an "SMD Chart"It's actually smart for a second-source manufacturer to use the same marking code - to enable correct part identification, irrespective of who actually manufactured the part in question. That is to your advantage.RF+ Tech

author: perdrix
author_rank: Frequent Contributor
timestamp: March 18, 2018, 03:52:31 am
content: OK thanks for the correction, but back to the main issue please ...ThanksDave

author: Bud
author_rank: Super Contributor
timestamp: March 18, 2018, 04:46:37 am
content: You did not say where you measure.

author: perdrix
author_rank: Frequent Contributor
timestamp: March 18, 2018, 09:39:19 am
content: At the left side of the blocking capacitor (i.e. capacitor is between signal source and MMIC). External signal source is 2.2V into O/C, 1.1V into 50R, drops to 0.4V when power is applied to DUT ?Dave

author: eb4fbz
author_rank: Regular Contributor
timestamp: March 18, 2018, 11:03:14 am
content: MMIC input impedance changes when power supply is applied, it usually lowers, so your signal generator output is loaded and voltage is reduced.

author: chrisl
author_rank: Regular Contributor
timestamp: March 18, 2018, 01:27:59 pm
content: I assume the 1.1Vpp is measured with a 50 ohm termination.As others have mentioned MMIC amp input Z will change when power is applied but 0.4V seems a bit too low.What do you measure at the output? Do you get the expected gain?A picture of the setup showing how the input is being injected (also how the probe is attached) may help... Even thought the input is at only 10MHz the amp has a plenty of gain up to 2GHz.An oscillation at a higher freq can cause what you see.Also try to lower the input power. I think 1.1Vpp is about 5 dBm of power. Depends how the amp is biased, the amp has a P1dB of 10 dBm with the gain of 12 dB or so.The amp may be in a fully saturated mode when the power is applied.A close up angled side view picture of the amp may help to identity the amp (MSA-0386 vs MSA-0385). It could well be the Minicircuits amp as well since HP was where they used get their parts in old days.Minicircuits got the parts that not met HP spec (but still good) and sold them as MAR series amps.

author: perdrix
author_rank: Frequent Contributor
timestamp: March 18, 2018, 03:43:05 pm
content: Yes, I too read 1.1Vppas just under +5dBm - I can try a lower input signal of say 500mVpp== -3dBm which given the 12dB gain and P1dBof 10dBm might be less problematic. Yes I guess it's possible it could be oscillating or saturated, but the output from the following gain block1looks OK at about +23dBm. I'll have to try probing the ouput of the first stage directly. Here's a close up of the first gain stage:Nothing sophisticated for the signal injection a nasty old maplin BNC to a pair of grabbers with the centre core to the blocking cap, and the outer to ground.1. I cannot identify the MMIC used in the 2nd stage as it's just a gold coloured four legged MMIC with no marking AFAICT. - Here's its portrait:CheersDave

author: ta2gc.engin
author_rank: Newbie
timestamp: May 08, 2025, 05:06:04 am
content: @perdrix hello I just saw your message, maybe it can be useful to someone. unnamed chip, old HP Agilent or Avago MMIC MSA-0520 200 mil BeO, good luck..