开云体育I’m looking at the VFO (Raduino) output (not connected to mixer) and the 4th Harmonic is about 33 db down from the main bang. A low pass filter might be worth a try, but moving the VFO to 19 seems a better solution. Just my 2 cents… ? ? From: [email protected]
[mailto:[email protected]] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Tuesday, March 14, 2017 9:30 AM To: [email protected] Subject: Re: [BITX20] The harmonics are out to play ? Raj, As in post ??/g/BITX20/message/21997 Best bet is to move the VFO up to 19mhz, one of Allison's primary criticisms of the Bitx. ?Would be tough to get an analog VFO stable enough at 19mhz, though the Raduino could do it easily.? Transporting the VFO to the mixer as a sine wave might help a bit. ??A clean square wave doesn't have any even harmonics, only odd harmonics. ?I'd bet the fourth harmonic is likely due mostly to second order effects in the diode ring mixer, not due to what's coming out of ?the vfo, but I could be wrong. ?Worth trying. ?But no matter how perfect your sine wave is when going into that diode ring transformer, since the current through the diodes is an exponential function of the voltage across the diodes that current will be nothing like a sine wave. ?It is exactly there in the diode ring that the image is getting down converted to 12mhz.by the fourth harmonic of the vfo. ?A square wave is preferred around the diode mixers: ?/g/BITX20/message/22538 On the other hand, going to a sine wave on your 4.8mhz vfo would likely help on the birdie at 7.2mhz. ?There the 5'th harmonic of the vfo at 4.8*5=24, and that beats with the second harmonic of the bfo. ?With proper shielding around the BFO end of the filter and care taken to keep the 5'th harmonic of the vfo from radiating in from wires between your vfo and the Bitx40 mixer, the birdie should have nowhere to mix. > ?stick to a sine wave from the VFO or add a filter?between the VFO and mixer. Pretty much the same thing. ?A 4.8mhz bandpass filter on the VFO output would permit only a sinewave through to the diode ring mixer. ?The mixer is better off with a square wave. You must be trying to get a rise out of me. ?All favorite topics to set me off into a rant. ? ?;-) ? > Rahul: ?I feel that the mixer is being bit overdriven in transmit, this might result in some spurii.. ? Could be, I haven't looked. ?But that's a separate subject from Raj's images on receive. ? Jerry, KE7ER ? On Tue, Mar 14, 2017 at 05:34 am, Raj vu2zap wrote:
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