Thanks, Jack. I'm sure you're right. As for the carrier at 14010, I assume that was a real intruder, since I could hear it on all 3 radios. I would normally run a Dunestar bandpass filter in front of the second radio and SoftRock, so tomorrow, when this critter shows up again, I will switch it in and the signal will presumably disappear.
You motivated me to finally check and see whether the Z-10000U was producing some added gain - I had tried to set it to zero gain when building it, by the choice of R907. Measuring with and without the buffer amp with a 1 uV signal source, I was surprised to discover that it had an apparent gain of 18 dB (from Rocky). I was fooled once again by the resistor marking system. Instead of a 499-ohm (499R) resistor at R907, I had a 49.9 (49R9), which according to your gain table should produce around +15 dB. Close enough, considering my crude measurement tools. I changed the resistor, and it now shows about +6DB gain. I assume that I can continue to increase the value if I want to get closer to zero gain. Thanks again for your help. 73, Pete At 11:53 AM 6/5/2008, you wrote: >Pete: > >20 meter LO is at 18.73 MHz and division ratio is 4, so the real LO is at >4683 KHz. The 20 meter band is thus received via 3rd harmonic sampling, 3x >4683 KHz = 14.048 MHz > >There will be a direct response in the 4.68 MHz range, but that's not a >normal shortwave broadcast range. > >At 5th harmonic, 5x 4683 KHz, the reception frequency is 23.412 MHz, but I >don't think that frequency range is used by Radio China. > >Even order harmonics are suppressed considerably in the Softrock design, >but if the input signal is strong enough, you might still see some >response. The 2nd even harmonic is at 9366 KHz, which is in the shortwave >broadcast band area, and would be worth scanning for a match with what >you hear. > >If you tune around, you might also check around 6240 KHz. That would be >a truly odd response frequency, but it is 1/3rd the crystal frequency >and it is in an international shortwave frequency range used by Radio China. > >Fix will be a good bandpass filter on the 20 meter front end. > >Jack > > > > > >Pete Smith wrote: >> >>This morning, for the first time in a while, I was using Rocky with my 20M >>SoftRock. Imagine my surprise to see a large and varying hump around the >>local oscillator frequency. I set Rocky for USB and tuned to the LO >>frequency, and voila - the Chinese shortwave service, in English! >> >>With my ordinary receivers, I cannot hear this signal at all, so I presume >>it is some sort of mixing product. It is definitely coming in over my >>antennas, but aside from that I have no clue. Can anyone suggest what >>might be causing this? >> >>As a PS, I am also seeing an AM signal on 14010, broadcasting music, but >>that one looks normal on the bandscope, with a carrier and sidebands. I >>can hear it on both ordinary radios. I have not heard it identify so far. >> >>73, Pete N4ZR >
