Post subject: Unwanted Modulation - Nasty Stuff! Posted:
Fri Jun 12, 2009 10:59 am
Joined: Mon Feb 09,
2009 12:26 pm
I'm sure that at some time(s) in the
past and if not then definately in the future you lucky RF people will
experience unwanted modulation on your transmitter carrier / receiver
local oscillator. This nasty issue has reared it head (again) on a recent
design I am working on so it seems appropriate to bang in a post on
this subject. Maybe there is a design gem or two out there in one of
your brains that may help me and others out now and in the future?
My particular nasty unwanted modulation is due to a cellular
1. VCO Isolation is
not the cure all
Adding a cascode amplifier stage(s) between
VCO output and final PA is always cited as a good idea. However, you
may still find a boat load of modulation on your output signal even
with these additional amplifiers in place. In practice I have found
that although cascodes and other amplifier stages between a VCO and
the antenna do provide lots of isolation you can still end up in a world
of unwanted modulation pain. I have found in the past that I have got
rid of FM modulation due to VCO pulling to end up with AM modulation
due to variation in bias on my buffer and ampifier stages caused by
the AM modulated interference signal (GSM is one such animal). If you
can run amplifiers with a reasonable bias current the problem is eased
somewhat. However, in practice it is often the case that you want to
run with just a sniff of bias to get decent amplifier efficiency. In
this case the AM on your transistor junctions reaps havoc!
SAW it off
It's a no brainer but if you can slap a decent filter
on the output this will gnaw off a great deal of the out of band signal
energy. Of course it doesnt help if the QRM signal is in band.
3. Isolators - fantastic
If you have the room an isolator
or two is definately a must have on the output of the transmitter. I
think there are now some chip parts around TOKO or Murata I think?
5. Synthesizer loop bandwidth
If your running a synthesized
frequency source, you can dramatically improve unwanted VCO modulation
by simply running with a wide loop bandwidth. Of course this has trade-offs
elsewhere by worth considering if your spectrum is a comb of unwanted
6. Avoid running your oscillator at F.
your oscillator at frequeny F when you know that this is likely to have
a large interference source near or in band is not a good idea. You
should consider running your oscillator at a harmonic or sub-harmonic
of F and then dividing or multiplying to get your wanted frequency F.
The divider version of this is quite common on a few COTs radio IC's.
That's a good start I think!
Your thoughts on this subject
would be much appreciated.
My VCO is definately getting pulled around.
Post subject: Re: Unwanted Modulation - Nasty Stuff!Posted:
Sun Jun 14, 2009 9:44 pm
Joined: Mon Jun
27, 2005 2:02 pm
is a great and challenging design issue.
From my own experience,
most of the unwanted modulation problems were solved by improving the
filtering of the VCO supply voltage. A careful selection of capacitors
and ferrite beads is a first step.
In addition, there is a well-known
circuit implemented by an NPN transistor with a voltage divider in its
base that drops the supply voltage to the the voltage required by the
VCO (This requires a higher voltage source for the VCO). This circuit
has shown a significant improvement on the VCO's supply.
issues are careful PCB layout and shielding.
stage, this is of course helpful but the buffers need to have a considerable
S21-S12. There are specific amplifier models for this purpose.