From: Phil Hobbs
Subject: Re: 1GHz image filter
Date: Wed, 27 Nov 2002 16:19:19 -0500
Organization: IBM T. J. Watson Research Center
NNTP-Posting-Date: 27 Nov 2002 21:19:21 GMT
X-Mailer: Mozilla 4.61 [en] (OS/2; U)
Leif H wrote:
> I have made a downconverter which converts the band 950-1050 MHz down to
> 0-100MHz. I need a iamge rejection filter for this. Trying to make a
> Butterworth, Chebyshev I end up with inducrtors less than 1nH. These are
> difficult to realise with discrete components. What can I do? One solution
> is to buy a a discrete filter with SMAs, but I want to make it on the PCB.
> Any ideas?
This is easy, if you don't mind reflecting the image back into the
mixer. Your image frequency is going to be 850-950 MHz, or 1050-1150
MHz if you're using a high-side LO. Mini-Circuits sells cheap lowpass
filters for this sort of job. (You're building a 100 MHz lowpass
filter, not a 1 GHz band reject, I trust.)
For extra image rejection, you can add a coaxial trap.
Put an open-circuited piece of miniature coax across the mixer output,
and trim it to length with sharp diagonal cutters until the image goes
away. If you haven't got a good way of measuring the image strength,
just cut it to a quarter wavelength (electrical length) at the centre of
your image band. You've only got a 10% bandwidth at the image, so a
simple single stub should work fine, and you can use one at each band
edge if that isn't good enough.
It won't affect your IF significantly (being basically a small
capacitance at that frequency), and if it bothers you you can put an
inductor in shunt with it.
If your image rejection requirements are modest (say 20 or 30 dB), you
can probably just use the trap by itself. On the other hand, if you're
worried about the increased spurs due to reflecting the image back into
the mixer, you'll need to absorb it instead. You can use a simple
diplexer or (if you don't mind a bit of dynamic range loss) a 10 dB pad
on the mixer output.