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

about VCO output imdedance and Buffer - 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.

Post subject: about VCO output imdedance and Buffer Posted: Mon Jul 30, 2007 5:25 am


Joined: Sun Jul 29, 2007 10:30 pm
Posts: 1
i have the question about VCO output impedance...

1. do not we really know VCO output impedance??

2. in order to get enough output voltage swing, we use buffer in VCO output port.
as i know, because we do not the output imdepance of VCO, the output swing should be check for the enough power for Mixer input power requirement by using buffer...

3. Actually...what is the exact usage of the buffer connected to VCO output port...moreover is there any method to check output impedance of VCO?

4. After buffer, the output impedance is usally 50 ohm..is that correct???
plz tell me anything wrong and things i asked...

i hope i can get ur answer..


Post subject: Posted: Mon Jul 30, 2007 9:51 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Tommy,

1. The output impedance can be characterized by a parameter called pulling, which defines how much the frequency will change at a given load (Usually that gives -12dB return loss at all phases). I never saw that an output impedance of the VCO is measured directly.

2. The buffer is used for better isolation and to increase the output power of the VCO in order to be able to drive the LO port of the Mixer.

3. The buffer is an amplifier of which output impedance should be matched to 50 ohm. A good practice is to choose a buffer which is capable to an output power of at least 3dB more than the LO power and to insert a pad between the LO port and the buffer output to set the correct output power and to improve the matching between the LO port and the buffer output.

4. Yes, the buffer's impedance should be identical to the system characteristic impedance, which is usually 50 ohm.


Post subject: Posted: Tue Jul 31, 2007 9:46 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Tommy
To add a little to IR's point no.2:
In Analyser-speak, the s-parameter S12 of a buffer amplifier is usually very low, and can be made even lower if negative feedback is used. Thus any reflected power from a load on the buffer does not find its way, out of phase, to the VCO and result in frequency being pulled off nominal, or erratic starting of the VCO when power is applied.


Post subject: Posted: Wed Aug 01, 2007 10:12 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
You might find this helps.

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

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