On Thu, 23 Mar 2006 07:50:41 +0000, John Wilkinson wrote:

Hi,
OK I have a 45MHz crystal filter for my first IF. The first LO ranges from
45-75MHz.
To test I am using a 45.455MHz LO, which is a pll design. When I connect
up the LO to the mixer, with no input signal I get a -80dBm level out of
my IF stage!!

-80 dBm at the IF output sounds pretty good. Are you using a
high gain, wide bandwidth IF amplifier? How good is the stopband
rejection of your IF filter? Two pole roofing filters are often
not much better than -20 to -25dB.

What kind of mixer are you using? A well balanced or double-balanced
mixer will help to keep the local oscillator signal out of the IF.

The PLL is clocked off 1MHz ref divided down to 5KHz. The 1MHz ref is a
TTL signal. Could this be the cause of the problem, and if so how do I get
around it?

Thanks,
John.

PLL oscillators can be very noisy, but it would take a very poor
design to produce strong spurious signals at +/- several hundred
kHz. Designing a PLL from scratch is not easy unless you have a
spectrum analyser or a good receiver that covers the VCO frequency
+/- a few MHz. Pay careful attention to grounding and screening,
especially around the VCO and loop filter. You must keep those
nasty TTL square waves away from the VCO. Make sure the DC supply
to the VCO and loop filter is very clean and stable.

73, Ed. EI9GQ.

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