Do ferrites add to noise levels in RF circuits? - Page 1
Forum section: electronics / rf-microwave

author: Qmavam
author_rank: Regular Contributor
timestamp: April 01, 2021, 06:35:32 pm
content: Here's why I ask.Please go with my numbers for now, I do understand the difficulty of high ratio windings and the problem of parasitics.I have Dallas Lankford's 2 FET amp, supposedly a quiet amp. it has a high input impedance (350kΩ) and a low output impedance. (100ohms) If I have an antenna with a 350Ω output impedance and I could transform that up to 350k, that would be a voltage stepup of 31 times. That is 30db noiseless gain. Except for what noise is produced by the ferrite transformer.So do ferrites produce noise?

author: bob91343
author_rank: Super Contributor
timestamp: April 01, 2021, 06:42:43 pm
content: Since ferrites are magnetic materials, they are inherently nonlinear. So a sinusoidal voltage (or current) excitation results in a nonsinusoidal current (or voltage). In other words, they generate harmonics. So that is indeed noise.

author: Qmavam
author_rank: Regular Contributor
timestamp: April 01, 2021, 06:55:00 pm
content: Quote from: bob91343 on April 01, 2021, 06:42:43 pmI'll go with that, but I'd call it distortion, not noise. At what levels does the non linearity show? If my voltage swings from 1uV to 1V,i can see a lot of nonlinearity, but if it swings from 1uv to 30uv, wouldn't that limit the amount of nonlinearity and thus harmonics?

author: radiolistener
author_rank: Super Contributor
timestamp: April 01, 2021, 08:24:33 pm
content: RF transformer provide noise-free voltage gain. But actual transformer max impedance ratio is limited because of bandwidth and distortion limitation and practical limit is about 1:4 turns ratio (1:16 impedance ratio). Usually 1:2 turns ratio is used to keep wide bandwidth.If your circuit has high impedance input, there is a good sense to use RF transformer to obtain noise-free gain and improve noise figure.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2021, 11:43:57 pm
content: The core contributes core losses, but how much depends on how many turns you've put on it. The equivalent circuit is a resistance in parallel with your source. Presumably, you would want the core loss equivalent to be several times larger than your source, so that its contribution both in noise and attenuation (and inductive reactance, which will be dominant) is small.A >100kΩ impedance is not all that reasonable for a ferrite transformer, certainly not over any kind of bandwidth. If you are making a narrowband / tuned antenna, in a low (SW or below) band, it may be possible.A much better solution is to simply use a lower impedance amp in the first place, transforming as needed; this can be done wideband. The high impedance amplifier is simply a poor choice here.Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 02, 2021, 12:12:59 am
content: I think the only noise contribution is from the resistance of the windings. There are all kinds of clever ways to use transformers in feedback circuits to make amplifiers which have no noise from the commonly used resistive feedback network.

author: Qmavam
author_rank: Regular Contributor
timestamp: April 02, 2021, 12:57:06 am
content: Quote from: T3sl4co1l on April 01, 2021, 11:43:57 pm

author: Qmavam
author_rank: Regular Contributor
timestamp: April 02, 2021, 01:02:56 am
content: Quote from: David Hess on April 02, 2021, 12:12:59 amYes, I have two push/pull Norton noiseless feedback amplifiers I built in the 90s, again, using Dallas Lankford's schematic and layout.Used one in an antenna phaser.Mikek

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2021, 02:28:25 am
content: Why not just use a lower impedance amp with the same or better noise factor? That's really what you want, noise voltage is meaningless, noise power is the generalization.Not much trouble either way, AM BCB is very noisy and it doesn't take much antenna to get more atmospheric noise than amplifier noise.BTW, you can interleave quotes by ending a quote body with /quote (in brackets) and starting the next one with quote.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: April 02, 2021, 10:10:45 am
content: Quote from: T3sl4co1l on April 02, 2021, 02:28:25 amamplifier add noise, RF transformer provide noise-free voltage gain. The only cons is the bandwidth limitation of the transformer.For example, here is ADC with 1 kΩ differential input impedance, and three possible options with different RF transformer to improve noise figure. As you can see, the better noise figure is 1:4 turns ratio, which give NF=22.8 dB.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2021, 01:43:32 pm
content: Quote from: radiolistener on April 02, 2021, 10:10:45 amOP is seemingly set on using an amplifier regardless. Why not choose one that's already matched closer to the source, instead of trying to force it?Tim

author: cncjerry
author_rank: Unknown
timestamp: April 02, 2021, 04:51:06 pm
content: Isn't the transformer contribution minimal compared to even the variance in fet devices? I built several of those amps, Makis' version as well, and though i never tested the NF, they seemed to be about as good as the can get. You would have to.match the input and output anyway so if you aren't using a transformer, unless you can use air gap coils at high frequency, what would be the alternative matching network?

author: Qmavam
author_rank: Regular Contributor
timestamp: April 02, 2021, 06:01:22 pm
content: Quote from: cncjerry on April 02, 2021, 04:51:06 pmWhat amp? Are you talking about the Lankford 2 FET amp from the OP? I see nothing searching Makis.There are times when you don't have to do impedance matching. Impedance matching costs you 1/2 of your signal.

author: Bud
author_rank: Super Contributor
timestamp: April 03, 2021, 02:32:09 pm
content: Noone in their right mind would try matching 350K in RF frequency range.

author: radiolistener
author_rank: Super Contributor
timestamp: April 04, 2021, 05:40:48 pm
content: Quote from: Qmavam on April 01, 2021, 06:35:32 pmthermal noise of 350 kΩ input will be much higher than 350 Ω output and such impedance match circuit will have a lot of losses, limited bandwidth and suffers from RFI. In practical you can do matching 350 Ω with 5 kΩ.It's better to match 350 Ω output with 50 Ω input in order to reduce noise influence. But anyway thermal noise floor will be limited with 350 Ω output.

author: Co6aka
author_rank: Unknown
timestamp: April 10, 2021, 05:35:39 pm
content: Leif, SM5BSZ, covers this subject in several of his recent (past year or so) videos involving his interferometer project. He switched to air core inductors because ferrite was introducing nonlinearities. I don't recall which videos so one must dig...https://www.youtube.com/channel/UClmNy6VRE1YqOeBHwb3o0Yg/videos