Thankyou Jerry for trying and fairly successfully explaining what Ashhar is doing by using the ACTUAL frequencies used in the uBITX. This is VERY important to this discussion. Any other explanation using completely different frequencies is re-inventing a completely different wheel in my opinion, very confusing. Hopefully discussions about this will continue to use Ashhars frequency plan, thus eliminating much of the confusion.? Thanks again for not being confusing.
On Wed, Mar 7, 2018 at 12:57 PM, Jerry Gaffke via Groups.Io
<jgaffke@...> wrote:
I hate sitting through videos.?
The math involved is addition and subtraction,? no need to solve any differential equations.
The 12mhz crystal filter has a 3dB passband of around 11.996 to 11.998 mhz, something like that.
The BFO on clk0 should be roughly 500hz away from the passband, so either 11.9955 or 11.9985 mhz.
Will vary from rig to rig, crystals get sorted by frequency so each rig has a matched set of crystals for the filter.
Original plan had been to switch the BFO between high and low side when selecting USB vs LSB.
But the 11.9985 mhz BFO had harmonics that beat with the 16mhz harmonics from the Nano processor,
so Farhan elected to freeze the BFO at 11.9955 mhz.? Instead moves the second oscillator
between 33 and 57 mhz when selecting between USB and LSB.
The VFO is always 45mhz higher than the operating frequency.
Here's an old discussion of how it worked on the Bitx40, using very round numbers to make it easy to follow:
? ??/g/BITX20/message/24724
Jerry, KE7ER
?
On Wed, Mar 7, 2018 at 12:24 pm, Michael Shreeve wrote:
And first of all, this uBITX works perfectly. It sounds great. Its very sensitive and has good tone quality. Literature says this :?"To invert the sideband between USB and LSB, the second oscillator is switched between 33 MHz and 57 MHz. " . If usb matches with 33 and lsb matches with 57 which it does at the site here : (clearly on the diagram where it clearly says 33 usb, 57 lsb, then its time to do the math ? So I checked very clearly the radio in the next video. Block diagram is here. Ashhar says we should do the math. Now, if I can figure out how to do that math, its another video lol.
Michael Shreeve