Homepage - RF Cafe
Webmaster: Kirt Blattenberger | KB3UON | Sitemap | ©1996-2014
Menu below is just a small sample of what is here!
 
Custom Search
More than 10,000 searchable pages indexed.
•−•  ••−•    −•−•  •−  ••−•  •
RF Cafe Morse Code >Hear It<

-119dBc! - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


darcyrandall2004
Post subject: -119dBc! Posted: Mon May 19, 2008 8:47 am

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
Hello Its me again,

I was reading an application note that stated that the adjacent channel rejection of a typical pocsag pager was 65dB at +/-25kHz

This places the following restrictions on the phase noise of my paging transmitter:
Assumming a margin of 10dB, the phase noise must be less than or equal to -65dB - 10log(25kHz/1Hz) - 10dB = -119dBc/Hz at 25kHz from the Carrier!

For a PLL, My understanding is that the Output phase noise follows that of the crystal ref for frequencies less than the loop bandwidth and follows that of the VCO for frequencies greater than the loop bandwidth.

Because the channel spacing is 25kHz, I am going to choose a loop bandiwdth << 25kHz/10 to avoid problems with reference spurs. Therefore at 25kHz offset from the carrier, I can assume phase noise output is as a result of the VCO.

Assume I require a VCO that outputs from 440MHz-470MHz. I believe even a state of the art VCO would have trouble achieving -119dBc at 25kHz.

How then are the big boys producing paging transmitters with PLL's that meet these requirements?

Can someone point me in the right direction?
All I can think of is perhaps they:
1. Use multiple small bandwidth VCO's with low phase noise and switch in the right one for the correct frequency.
2. Maybe I can design a low phase noise frequency synth for a lower frequency and then use a mixer with another low noise oscillator to achieve the correct frequency.
3. Perhaps by using a number of PLL's I could assign the last PLL a large channel width, and a large loop bandwidth thereby having the phase noise output still follow the phase noise of a crystal reference.

Do you have any suggestions?
Which approach is typically the easiest and the most common?

Your help is very much appreciated thankyou.

_________________
Regards, Darcy Randall, Perth, Western Australia


Top

yendori
Post subject: Posted: Tue May 20, 2008 11:43 am

General


Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
Is the additional margin really needed? It's my understanding that the phase noise spec @ 35KHz offset is -105dBc/Hz.

I think you can find a VCO to meet this repeatably.

Good Luck,

Rod


Top

darcyrandall2004
Post subject: Posted: Sat May 24, 2008 4:26 am

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
If I use two PLL's I can absolutely annihilate the 119dBc requirement.

The first loop has an extremely small bandwidth, hence phase noise in this loop is predominantly a result of the 120MHz Crystal VCO.
The second loop has an extremely large bandwidth, hence phase noise in this loop is attributed to the low noise output from the first loop.



I simulated phase noise using typical values and estimated contribution due to the power supplies.



The problem is either I cant calculate the divider values to achieve 25kHz channel spacings or it is just not possible.

Does anyone have any suggestions?

_________________
Regards, Darcy Randall, Perth, Western Australia


Top

darcyrandall2004
Post subject: Posted: Sun May 25, 2008 7:57 am

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
I solved the choice of divider values problem.

If I use the following parameters I just might be able to achieve the -119dBc requirement.

N1=17600
R1=1023
N2=240
R2=2
Xtal Vco ref 2=3.666666MHz
Xtal Vco ref 1=213125Hz

This sets my loop 1 channel bandwidth to 208.3Hz.

Any suggestions?

_________________
Regards, Darcy Randall, Perth, Western Australia


Top

RFDave
Post subject: Posted: Mon May 26, 2008 11:05 am

Captain


Joined: Sat Apr 22, 2006 11:14 pm
Posts: 10
Hi:

I've got a couple of comments-

1-10 dB of margin is pretty big, you might want to relax that.
2-Take a look at off the shelf VCO's. I just took a look at Synergy Microwave,
and they have parts that look like they are close to -115 dBc/Hz phase noise
There are quite a few VCO vendors, you might want to look at the ad's here
on rfcafe.
3-You might want to look at packaged VCO/PLL combinations as well.
4-I'd look at Fractional-N PLLs. You lock time with a 200 Hz BW is going to be pretty slow. I'm not sure if a paging transmitter will have the same constraints, but a paging receiver has some pretty stringent lock time requirements to improve battery life.

Dave


Top

darcyrandall2004
Post subject: Posted: Mon May 26, 2008 10:43 pm

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
Thanks Dave,

Ive just been looking into fractional synthesizers and I believe they will simply the design.

Cheers

_________________
Regards, Darcy Randall, Perth, Western Australia


Top

RFDave
Post subject: Modulation?Posted: Wed Jun 04, 2008 10:18 pm

Captain


Joined: Sat Apr 22, 2006 11:14 pm
Posts: 10
How are you planning on doing your modulation for this?

Dave


Top

darcyrandall2004
Post subject: Posted: Mon Jun 09, 2008 6:48 pm

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
I was going to use a technique called two point modulation where I inject the modulation both on the reference oscillator and the VCO at the same time. I read that this will remove any restrictions the loop bandiwdth places on the baud rate I wish to use.

_________________
Regards, Darcy Randall, Perth, Western Australia




Posted  11/12/2012
A Disruptive Web Presence

Custom Search
Over 10,000 pages indexed! (none duped or pirated)

Read About RF Cafe
Webmaster: Kirt Blattenberger
    KB3UON

RF Cafe Software

RF Cascade Workbook
RF Cascade Workbook is a very extensive system cascaded component Excel workbook that includes the standard Gain, NF, IP2, IP3, Psat calculations, input & output VSWR, noise BW, min/max tolerance, DC power cauculations, graphing of all RF parameters, and has a graphical block diagram tool. An extensive User's Guide is also included. - Only $35.
RF system analysis including
frequency conversion & filters

RF & EE Symbols Word
RF Stencils for Visio

Product & Service Directory
Personally Selected Manufacturers
RF Cafe T-Shirts & Mugs

RF Cafe Software

Calculator Workbook
RF Workbench
Smith Chart™ for Visio
Smith Chart™ for Excel
Your RF Cafe Progenitor & Webmaster
Click here to read about RF CafeKirt Blattenberger... single-handedly redefining what an
                                 engineering website should be.

View the YouTube RF Cafe Intro Video Carpe Diem! (Seize the Day!)

5CCG (5th MOB): My USAF radar shop

Airplanes and Rockets: My personal hobby website

Equine Kingdom: My daughter Sally's horse riding website