Noise floor - Page 1
Forum section: electronics / rf-microwave

author: mayor
author_rank: Regular Contributor
timestamp: April 30, 2019, 12:59:18 am
content: Hi,I just took VNA measurements of a board I made to tune a PCB antenna. I get a good match (< 2:1 swr for 40ish MHz bw) around the target frequency (which is at ~1.12:1). In fact, comparing RSSI for my board vs the TI CC1350 launchpad, I am measuring a ~10 dBm improvement. So I figure: "great, that's properly tuned!".But then, I check the noise floor. My board's is at ~-90dBm, whereas TI's is at around -110dBm. Overall, link budget for my board is much inferior.I'm not experienced enough in RF to understand why that is, much less to fix it. Pointers appreciated!

author: langwadt
author_rank: Super Contributor
timestamp: April 30, 2019, 01:04:48 am
content: Quote from: mayor on April 30, 2019, 12:59:18 amnoise match vs. power match ?

author: mayor
author_rank: Regular Contributor
timestamp: April 30, 2019, 01:09:49 am
content: Quote from: langwadt on April 30, 2019, 01:04:48 amSorry, not sure I understand your question.

author: langwadt
author_rank: Super Contributor
timestamp: April 30, 2019, 01:52:23 am
content: Quote from: mayor on April 30, 2019, 01:09:49 amnot really a questions, more that a "perfect" match might not result in the best noise figure and that might bewhat you are seeing

author: KE5FX
author_rank: Super Contributor
timestamp: April 30, 2019, 02:04:20 am
content: Remember that a 49.9 ohm resistor will give you a near-perfect VSWR.VSWR is one of the least important properties of an antenna, but it seems to get ALL of the press coverage. It's just one of several boxes that need to be checked. Focus on other aspects of your design, maybe post some photos.Is it really "noise" that you're seeing? Or interference from another part of the circuit?

author: langwadt
author_rank: Super Contributor
timestamp: April 30, 2019, 02:12:28 am
content: Quote from: KE5FX on April 30, 2019, 02:04:20 amremember some colleges working on a Bluetooth board some +15years ago, everything was tested and worked great,until powered it with a battery like it was intended, then the sensitivity was sometimes bad. Turned out to be an LDOthat oscillated but only at certain input voltages

author: mayor
author_rank: Regular Contributor
timestamp: April 30, 2019, 10:07:07 am
content: Quote from: KE5FX on April 30, 2019, 02:04:20 amHi- good point on the noise, but I don't think that's it, though. A different matching network showed around -110 to -120 dBm (mind you RSSI wasn't as good during a transmission, hence this new matching attempt).What are other important properties to look out for which a hobbyist such as myself can try? I can't use an anechoic chamber, can't measure directivity, etc, etc. I don't have a spectrum analyzer, but have a Pluto SDR (which proved useful to measure frequency offset!). Very much looking forward to learning more!Following is a pic of my boards, which are based on the CC1352 Launchpad, with major differences being the use of two antennas, ground size and an IPC balun. In the present case, I'm talking about the sub 1-GHz antenna. Note that the antennas strictly follow TI reference designs.

author: mayor
author_rank: Regular Contributor
timestamp: April 30, 2019, 10:08:46 am
content: Quote from: langwadt on April 30, 2019, 01:52:23 amAh! So that is a possibility? Is it common for a poorer match to result in greater link budget?

author: ejeffrey
author_rank: Super Contributor
timestamp: April 30, 2019, 04:06:21 pm
content: Yes.An amplifier will have an input impedance and a noise impedance and they can be different. Matching to the input impedance gives maximum power transfer and gain, matching to the noise impedance gives lowest noise figure. They won't necessarily be the same. What is going on "under the hood" is that the amplifier can have correlated noise. If so, engineering a deliberate reflection from the antenna/matching network can cause the noise to partially cancel itself (at least in the bandwidth of interest).

author: ejeffrey
author_rank: Super Contributor
timestamp: April 30, 2019, 07:46:43 pm
content: The low frequency equivalent of this is obvious in opamps or simple transistor amplifiers. The input impedance of a non-inverting opamp circuit is effectively infinite but the current and voltage noise density ratio gives a noise resistance. You get the best noise figure by matching the sensor resistance to the noise resistance.

author: KE5FX
author_rank: Super Contributor
timestamp: April 30, 2019, 09:17:21 pm
content: Quote from: mayor on April 30, 2019, 10:08:46 amYes, but the effects of noise match are typically seen in the single-digit dB range (or less). This is mostly a concern when developing high-performance microwave LNAs for satellite or other work where most of the engineering and hardware budget has already gone into antennas, feeds, and transmission lines. That's not what's happening here.Sounds like something relatively basic is going wrong, but not being familiar with these boards I don't have a good idea what. The TI customer forum might be a good place to ask, since their app engineers may have encountered the same question from other users.Edit: one fairly-obvious hint you've probably already thought of is to keep this stuff well away from computers and cell phones when testing.

author: mayor
author_rank: Regular Contributor
timestamp: May 01, 2019, 08:37:41 am
content: Hi,on the TI forum, the thread ended on one person's conclusion that it was due to an antenna mismatch. An earlier revision of the board had micro connectors which allowed me to test an external antenna (on which mine is based), and its noise floor was good (ref:http://www.ti.com/lit/an/swra416/swra416.pdf). Could ground size be the issue?What's not clear to me now is what to do next to confirm that. I _think_ my match is good, I've followed manufacturers' literature on matching with a VNA.Pre-match and post-match charts attached. Ideal match is at 930 MHz, but still OK at 915. That's due to not having exact values for match components.I wouldn't mind getting suggestions on what to do next...!

author: mayor
author_rank: Regular Contributor
timestamp: May 01, 2019, 11:54:32 pm
content: Here's a measurement update that may tell you guys more than it does me. Looks like my board is sensitive to noise when the bandwidth filter is set to a higher value.As before, any help or insight you can provide is appreciated!

author: radiolistener
author_rank: Super Contributor
timestamp: May 02, 2019, 06:44:50 am
content: bandwidth and noise level are coupled together. If you're cut some part of spectrum (with filter) you're also cut some noise power and noise floor will be better. This is also known as process gain for ADC, where quantization noise is spreaded across full ADC bandwidth. When you apply filter for ADC output, you will get smaller level of quantization noise.This is because wide bandwidth leads to catch more noise power. Narrow bandwidth leads to catch less noise power.And this is why you're need to specify measurement bandwidth for noise floor level.

author: veryevil
author_rank: Unknown
timestamp: May 02, 2019, 05:27:02 pm
content: Are you reading the rssi straight from the cc1350? If it's anything like the CC1125 which I know well you will need to calibrate that value with a known input level.

author: mayor
author_rank: Regular Contributor
timestamp: May 03, 2019, 12:00:36 am
content: Quote from: veryevil on May 02, 2019, 05:27:02 pmWell, I'm using SmartRF Studio. My baseline is a CC1350 (or CC1352) LP. I measure my boards the same way, without any "calibration". I am unaware of this-- can you point me in the right direction? At this stage, I'm kind of lost as to the next steps.

author: veryevil
author_rank: Unknown
timestamp: May 03, 2019, 07:44:10 am
content: Hi, sorry had a quick look and the CC11350 is completely different to the CC1125 so I have no idea about the calibration routine. it might be at least useful as a delta check to set up your pluto board as a constant carrier at low power and then try your board against the ref design at the same location / distance. This would at least show the RSSI difference between the two boards with the same power signal?

author: mayor
author_rank: Regular Contributor
timestamp: May 03, 2019, 08:56:48 am
content: Quote from: veryevil on May 03, 2019, 07:44:10 amThanks for checking. I will try the Pluto as you suggest (I've so far only used it in RX). I have been doing this test with yet another TI board transmitting CW. If nothing else, it will be good to confirm my current findings.