It's quite common for noise sources in the house to go up the coax and come back in from the antenna.
Could be you need a good "common mode choke".
??
??
My totally uninformed opinion: ?I think the Si5351 noise everybody's talking about is likely hash from the processor driving it. ?And that if this less-than-optimal LPF is reducing the noise to where it's as good as the analog VFO, that's good enough. ?The ideal would be to clean up the noise somehow without band specific filters, then we're back to switching only two filters if we want a multiband rig. ?
The diode ring mixer wants to be driven hard. ?The ideal is for two of the diodes to turn on for 1/2 the LO cycle, the other two diodes to turn on for the other half of the cycle. ?Anything less, and changes in the other input signal (from the antenna when receiving) can disrupt the desired action of this commutating mixer. ?A square wave is actually preferred, and a perfectly clean sine wave from the LO will have a totally non-linear response from the diodes anyway since the current through a diode is exponential with voltage, not linear like a resistor.
Of course I may well have totally different opinions once I get a Bitx to play with.
Jerry
---In BITX20@..., <caulktel@...> wrote :
If your using the QRPlabs VFO for the BitX40, then the LPF you bought for 40 meters is optimized for 7-7.5MHz. You need a LPF for the actual VFO output which is 4.5-5MHz. The 7MHz one is probably helping to clean the output somewhat, but is not optimal.?
On Jan 5, 2017, at 9:53 PM,?dvoorhees@...?[BITX20] <BITX20@...> wrote:...I'm experiencing a high level local noise, but without the antenna it's very quiet.?
?
