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
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
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 telephone nearby.
1. VCO Isolation is not the cure
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!
2. 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 garbage!
6. Avoid running
your oscillator at F.
Running 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
Post subject: Re: Unwanted Modulation - Nasty Stuff!Posted: Sun Jun 14, 2009
Joined: Mon Jun 27, 2005 2:02 pm
First, this 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
Other issues are careful PCB layout and shielding.
Concerning buffer stage, this is
of course helpful but the buffers need to have a considerable S21-S12. There are specific amplifier models for