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Distribution of VCO - RF Cafe Forums

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gui076

Post subject: Distribution of VCO Posted: Mon Jun 16, 2008 4:40 am

Captain

Joined: Mon May 05, 2008 9:56 am

Posts: 15

Location: France

Hello,

i'm implementing a synthesizer with one or more VCO.

I will put some switch in order to choose between the VCOs and after i would like to provide VCO's signal to two Mixers.

I have seen some application with dual VCO buffer, is there other method ?

Thanks

Guillaume

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nubbage

Post subject: Posted: Wed Jun 18, 2008 3:33 am

General

Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Hi Guillaume

I have to split the VCO output of my synthesizer into a -10dBm signal for the pre-scaler and a direct signal at 0dBm into the VCO buffer followed by a mixer.

I just use a 10dB coaxial directional coupler on the VCO output.

If the buffer isolation into the mixer is inadequate, I also purchased a coaxial isolator. The VCO runs at 1.3GHz.

I only have the one VCO running. If I were to design for a second one to cover a wider range, I would use a coaxial transfer latching relay. This would ensure the idle VCO saw a matched load when de-selected.

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gui076

Post subject: Posted: Wed Jun 18, 2008 4:31 am

Captain

Joined: Mon May 05, 2008 9:56 am

Posts: 15

Location: France

Hi nubbage,

For my application it's to drive two mixers and they have 0 dBm LO input, so i thought a buffer will level the output to 0 dBm.

I have a wideband VCO 950-2050 and isolators wille be too narrow i think.

For the switch i have seen SP2T Absorbtive :

http://www.macom.com/DataSheets/SW65-0313.pdf

I didn't know "coaxial transfer latching relay".

Thank you for your help, i'm student in RF so i'm discovering a lot of thing

Guillaume

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FSomma

Post subject: Posted: Wed Jun 18, 2008 4:54 pm

Hi Guillaume

I think you can use the Tyco switch if you turn off the idle VCO with the same switch control signals .. In fact, I think it could be a good design practice to turn off the idle VCOs to ensure that you don´t have that tones around your circuit while the other VCO is active.

If your idea is to feed the dwn conv mixer and the up conv mixer with the same LO, you can use an splitter to feed them. If the isolation is critical (check the isolation balance of the RF-LO of both mixers and the splitter S32), you can think on changing the selected switch by an other one with no internal driver, and to boost up the LO for the mixers, you can use independent drivers on each output of the splitter to take advantage of their reverse isolation.

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gui076

Post subject: Posted: Wed Jun 18, 2008 5:22 pm

Captain

Joined: Mon May 05, 2008 9:56 am

Posts: 15

Location: France

Hi,

Yes the goal is to feed the up and down mixers.

So if i have well understood i should try this config :

VCO1 and VCO2 on SP2T followed by a splitter and one buffer on each channel.

Else there is a double VCO buffer from Maxim :

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2038/t/al

Thank you

Guillaume

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nubbage

Post subject: Posted: Mon Jun 23, 2008 6:54 am

General

Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Hi Guillaume

A latching coax relay is one that is magnetically biassed so when an impulse is given to the coil it switches over to the other state. Removing the coil drive leaves it in that state until another pulse is received. This avoids the need to drive current continuously into the coil for either state of the relay. I think mine is from MAComm/Tyco. The isolation of a transfer switch, a relay that transfers the unwanted signal to a load and the wanted signal to the output port, is typically greater than 60dB, so leakage should not be a problem.

I hope not to use the isolator I purchased. It was just for insurance "in case". The bandwidth is 20% of nominal centre. I think it was from Anzac.

I have some doubts about the wisdom of switching off the unwanted VCO, particularly if low phase noise and rapid switching time are important.

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FSomma

Post subject: Posted: Wed Jun 25, 2008 3:44 pm

Hi Guillaume,

Yes, the "VCO1 and VCO2 on SP2T followed by a splitter and one buffer on each channel" is one option.

But if you don´t need that much isolation, you can still think on the "VCO1 and VCO2 on SP2T followed by the buffer you where thinking on the first place and a splitter to OL-feed the mixers".

Regarding to switching off the idle VCO, yes, nubbage is right, you have to analyze your phase noise and rapid switching time requirements. I think I assumed too quickly that your application was a channel scanner (am I right?) where fast VCO switching is not that important... i mean, you can scarify that response time when you switch from band to band and save $$ to your design, coax relay are very expensive, Radiall has some SMD low power coax relays that you might like to take a view if you find it necessary.

Take care

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