SNR

[C2]

what is meant by SNR? I don't mean signal to noise ratio...

I have this GPS receiver that interfaces with some software. It lists the satellites and indicates SNR. For example, SNR: 35.

35 what? dB? I think it's dB, but I'm not sure.

OK, on another note; my other receiver has a noise floor spec. of -167 dBm. What is the minimum power level I'd be able to detect? Would it have to be 3 dB above the noise? I have seen most receivers either specify 10 dB SNR (or S+N/N) or 12 dB SINAD (10log(SND)/ND), or something to that effect

 

[Hamin' X]

It does indeed, stand for Signal-to-Noise-Ratio. A GPS signal that is received with less than a 6 SNR is to be considered unreliable for good accuracy.

Rich

 

[C2]

OK, well it was interesting that I was using a passive 8-turn heliax antenna and the SNR for a couple sats was around 28 in the clear and then I bought a 50dB gain active antenna and I get more sats, but the SNR is still no more than 45. The antenna was a big improvement so that is good and worth the $.

I was also interested in understanding more about receivers. I wish I had better receivers to help me see what's going on.

 

[Hamin' X]

An active antenna is really nothing more than adding a preamp. Preamps amplify noise, as well as signal, so it is not surprising that your SNR did not rise as much as you expected. I maintain several repeaters at dense communications sites and preamps do more harm than good, most times. With a very well tuned 20db GAsFET preamp that is designed to maintain linearity under overload conditions and pass cavities, you can only expect to gain 2-4 dbm for 12 db SINAD. Overall noise floor must be considered, before using preamps.

Rich

 

[C2]

Well, this antenna is a Gilsson active antenna. It worked great because indoors I was not getting anything, but with this I lock to about 7 sats. Unfortunately, the antenna it'self has actual loss, like about -4dBi. I make a wild guess that the Heliax I built has a gain of about 5dBi.

The reason I was asking about SNR is because my spec an's noise floor is around -100 dBm, which is really poor performance for looking at weak signal stuff. I'm guessing the sats come in at around -140 dBm on my Heliax, and the actual noise level is probably around -160 (as a "ya long" kind of approach). So I was going to put in a bias tee to supply the active antenna and to protect the spec an (which don't like DC none too much). But, if the SNR is only comming in around 45 dB, then I still don't have enough gain to get that signal about the instrument's noise floor. However, I understand your point about the LNA increasing the noise level.

I guess I'll just have to try it out.

 

[ss8541]

you have to remember that you will never get all the noise gone. after you reach a certain input level to the rxer the snr will not change even when a stronger signal is applied. you reach a saturation point.

you state that before the preamp was added you had a snr of 28db (the signal was 630 times stronger than the noise). after the active antenna was added the snr rose to 45db (the signal now is 31,600 times stronger than the noise). when you look at it this way, that active antenna is doing one hell of a job for the snr. it made a 17db jump (50 times increase) in the snr.

 

[C2]

Yeah, just musing. Wish I had a better spec. an. I've tried similar testing at work, but the steel reinforced concrete is not good for the sat. reception.

 

[Toll_Free]

Yeah, just musing. Wish I had a better spec. an. I've tried similar testing at work, but the steel reinforced concrete is not good for the sat. reception.

Maybe a microwatt sig gen? At microwaves, a steel desk makes a dandy antenna range You know, like the old GI type desks that the market was flooded with surplus in the 90s?

DANDY WiFi type antenna test ranges

Anywho..... I'm following this thread as well. Living among the redwoods, I find it a bit hard to get my Sirius radio at times. Thinking of moving to a parabolic, but...


--Toll_Free

 

[C2]

but yeah, are those geostationary sats? I know the GPS sats are not.

That's one reason I liked playing with microwave, you can have a lot of fun with small signal across the room, or yard. I'm glad I kept my C-Band dish too. The last thing I set it up for was 23cm ATV using a double bi-quad with lips I'm thinking about 30dB of gain, and I have the PA at the feedpoint. Nothing like hitting a repeater 50 miles away with 80mW. It helps that I can see it though.