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#1
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45 MHz IF breakthrough
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!! 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. |
#2
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45 MHz IF breakthrough
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. -- Linux 2.6.15 Remove 'X' to reply by e-mail. Yes, my username really is: nospam |
#3
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45 MHz IF breakthrough
Unless you have to move the IF osc around, why not use a crystal?
With a 5 KHz PLL reference, you might have some sidebands from this. What is the cutoff of the loop filter? R 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!! 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. |
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