Space Varmint
New Member
Hi, I'm building an HF receiver and I have almost completed the second IF strip. I will run it by cable RG-174U out of a shielded compartment to the a buffer amp and one more voltage amp and then the audio & AGC loop circuits.
Well, I have no pre-selector as of yet and most certainly plan to put one in. Here is my concern. I put the rf amp in a metal container along with the first mixer and 1st IF strip. The ist LO is isolated and comes from a PLL into the grounded metal compartment through coax RG-174U. I'm using a crystal filter that is very sharp and I see the selected signal come through very strongly but when sweeping the receiver with a signal generator I also see the IF frequency pass which is what is is supposed to do but also I see other frequencies pass through the crystal filter. I was scratching my head wondering where they came from. I'm sure some are harmonics and phase sidebands from the PLL but there are some odd balls getting through. Granted a good selectable pre-selector will get rid of most of them but I am concerned with ones that are close enough to be included in a band pass filter.
To make a long story short. Are there better ways to reduce these odd frequencies? I do see evidence that the LO is still getting into the rf amp and doing some mixing itself. Should I take out the rf amp and enclose it into it's own shieded compartment or will I still get some pick up through the power bus? I'm looking for optimum rf front end design both physical and electrical. I figure doing the rf amp right is a small price to pay for superior quality.
Any links or suggestions, lectures, whatever you got will be appreciated. Thanks
Well, I have no pre-selector as of yet and most certainly plan to put one in. Here is my concern. I put the rf amp in a metal container along with the first mixer and 1st IF strip. The ist LO is isolated and comes from a PLL into the grounded metal compartment through coax RG-174U. I'm using a crystal filter that is very sharp and I see the selected signal come through very strongly but when sweeping the receiver with a signal generator I also see the IF frequency pass which is what is is supposed to do but also I see other frequencies pass through the crystal filter. I was scratching my head wondering where they came from. I'm sure some are harmonics and phase sidebands from the PLL but there are some odd balls getting through. Granted a good selectable pre-selector will get rid of most of them but I am concerned with ones that are close enough to be included in a band pass filter.
To make a long story short. Are there better ways to reduce these odd frequencies? I do see evidence that the LO is still getting into the rf amp and doing some mixing itself. Should I take out the rf amp and enclose it into it's own shieded compartment or will I still get some pick up through the power bus? I'm looking for optimum rf front end design both physical and electrical. I figure doing the rf amp right is a small price to pay for superior quality.
Any links or suggestions, lectures, whatever you got will be appreciated. Thanks