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about VCO output imdedance and Buffer - RF Cafe Forums

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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.


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