Distribution of VCO - RF Cafe Forums
Post subject: Distribution of VCO Posted: Mon Jun 16,
2008 4:40 am
Joined: Mon May 05, 2008 9:56
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 ?
Post subject: Posted:
Wed Jun 18, 2008 3:33 am
Feb 17, 2006 12:07 pm
Location: London UK
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.
Post subject: Posted: Wed Jun 18, 2008 4:31 am
Joined: Mon May 05, 2008 9:56 am
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.
the switch i have seen SP2T Absorbtive :
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
Post subject: Posted: Wed Jun 18, 2008 4:54 pm
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
Posted: Wed Jun 18, 2008 5:22 pm
Mon May 05, 2008 9:56 am
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
Else there is a double VCO buffer from Maxim :
Post subject: Posted: Mon Jun 23, 2008 6:54 am
Joined: Fri Feb 17, 2006 12:07 pm
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
Post subject: Posted:
Wed Jun 25, 2008 3:44 pm
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
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