someone should moderate this group and stop all these posts about unrelated items..SDR Kits..I thought it was about Bitx40 Kits!!!

Please reread the post you complain about. SDR Kits is the name of the outfit that makes one of the **VFOs** being used experimentally on the BitX40. That does not mean the group has been hijacked by SDR enthusiasts. By the way, according to its name, this group isn't about the BitX40, either. That's okay with me, because the world really doesn't need more than one group for the BitX family of scratch-built, kit-built, or semi-built transceivers.

Instead of asking someone else to eliminate the posts you don't want to read, why not just use the delete key?

Todd K7TFC

This part (at least) is incorrect, I was taking the mental shortcut of assuming that a VFO negative offset would correspond with sidebands inverted at the IF.?

"When we use the 5mhz VFO from the Raduino, the received signal presented to the crystal filter is mirrored through the magic of that negative offset mentioned above, the highest frequency components of the incoming 7mhz signal are down in the 12mhz copy, the lowest frequency components are up in the 12mhz copy. ?So the upper and lower sidebands get swapped when we use a 5mhz VFO to receive 7mhz."

Assume we are listening to a 40m LSB signal with the suppressed carrier at 7.000 mhz transmitting a 1khz audio tone (illegal, but what the heck). ?That lower sideband will be at 6.999mhz. ?A 5.000mhz vfo mixes with that to put the suppressed carrier at 7.000+5.000=12.000mhz, and the lower sideband at 6.999+5.000=11.999 mhz. ?The sidebands did not get swapped.

A 19mhz vfo mixes with our 40m signal to put the suppressed carrier at 19-7=12.000mhz, and what was the lower sideband at 19-6.999=12.001mhz. ?The sidebands did get swapped.

Also, I doubt the crystal filter is centered at 12.000mhz, supect it is down around 11.997mhz. ?The BFO is apparently set at 11.985mhz on most rigs, and with no inversion of the sidebands when using a stock 5mhz vfo the lower sideband will wind up a bit down from the BFO.?

However, that previous post was correct in that operation with a 19mhz vfo will have the vfo going up in frequency as we tune up the band. ?And that with a 5mhz vfo we need to set up the vfo for a "negative offset" to have it read out our correct operating frequency, since as the vfo frequency increases the 40m frequency we receive will decrease.?

That's it for guessing, anything more will have to wait till a Bitx40 arrives here, hopefully in a couple weeks. ?Between now and then I'd best sign out of this forum and pull the plug on my internet connection so I can get my head buried into some other stuff. ?I've learned a lot and have enjoyed trying to reason through some of these puzzles. ?But plenty of other stuff to do here.

Jerry, ?KE7ER

?On Thu, Feb 9, 2017 at 08:39 am, Jerry Gaffke wrote:

You are correct, and the SDR Kits VFO can probably do this. ?I don't have one, but the SDR Kits VFO apparently doesn't know about negative offsets, so as you tune up from 7mhz, the SDR Kits display will show the frequency decreasing. ?That's fine if you are ok doing the subtraction in your head or on a hand calculator, but a bit inconvenient.

Example: ?

7.000mhz ssb is mixed with 5.000mhz vfo to create 12mhz IF

7.200mhz ssb is mixed with 4.800mhz vfo to create 12mhz IF

For the receive frequency to go up from 7.000 to 7.200, the VFO must go down from 5.000 to 4.800.

The SDR Kits VFO doesn't know how to subtract, so it will show a frequency that is decreasing.

At least, nobody here has yet figured out how to make it subtract.

Jerry, KE7ER

So the BFO has a fixed value of 11.9985 or 12..0015mhz, it is up to the user to whack at L5 and C103 to set the appropriate frequency.

Your BFO values work if using the Raduino with a VFO output freq of 5mhz. ?They get swapped if the VFO is at 19mhz.. I believe the SDR Kits VFO doesn't know about "negative offset", so the best way to receive a 7mhz signal is to have it emit a 19mhz VFO signal.

> ?vfo is what you see on the display,?

I assume the display is showing 7mhz. ?And that the Raduino VFO is actually sending out 5mhz. ?If you are talking about the SDR Kits VFO and some sketch for that beast, I have no idea what it is displaying.

> Your SDR kit has to generate the bfo value nothing more and you enter it on the BITX40 at the place of L4 (which has to be removed).

Your SDR kit is generating the VFO, ?not the BFO. ?The BFO is set by L5 and C103. ? But yes, the SDR kit is doing nothing more than injecting a 19mhz VFO signal into the DD connector near L4.

?

I think.

Jerry

开云体育

I may have screwed up. ?Looks like it needs to go up 3khz, not down as I previously claimed.

Sounds like the SDR Kits VFO assumes the VFO is always on the high side of the IF. ?So the signal from the SDR Kits VFO is 12+7=19mhz, where the Raduio is at 12-7=5mhz. ?In the 19mhz case, a rise in VFO frequency causes a rise in operating frequency: ?19.2-12=7.2mhz. ?In the 5mhz case, a rise in VFO frequency causes a fall in operating frequency: 12-5.2=6.8mhz. ?That's what Norm meant by "negative offset". ?Perhaps the SDR Kits VFO doesn't know about negative offsets and so is stuck with using a 19mhz on the Bitx0, in which case shifting the BFO frequency a few khz with a soldering iron is the best bet. ?

This paragraph is for those who really want to bottom out on the reasoning here: ?There's 4 variables: ?The operating freq (7mhz), the IF crystal filter passband center (lets assume it's 12.000mhz), the BFO freq (assume 11.998mhz), and the VFO freq (assume near 19 mhz, that's easier to think about than the 5mhz case). ?The IF crystal filter would be pretty hard to move, I doubt that's happening. ?That diode ring mixer up front combines the 7mhz received signal (where 7mhz is the freq of the carrier, if it had one) with 18.998mhz from the vfo, and gives the crystal filter a shifted down replica of the incoming SSB signal, the suppressed carrier of which is at 11.998mhz, exactly where the BFO is. ?The upper sideband of this 11.998mhz SSB signal is a little bit up in freq from the suppressed carrier, so the 2khz wide crystal filter only allows through the upper sideband. ?If we whack at the BFO to move it to 12.002mhz, and adjust the VFO up a bit to put the suppressed carrier of the SSB signal into the filter at 12.002mhz also, we will get the lower sideband coming through the filter. ?When we use the 5mhz VFO from the Raduino, the received signal presented to the crystal filter is mirrored through the magic of that negative offset mentioned above, the highest frequency components of the incoming 7mhz signal are down in the 12mhz copy, the lowest frequency components are up in the 12mhz copy. ?So the upper and lower sidebands get swapped when we use a 5mhz VFO to receive 7mhz. ?

I have not looked at the sketch. ?But that Raduino code has no way to change the BFO frequency from 11.998mhz to 12.002mhz (unless we do a hardware mod to the board to use a spare Si5351 channel to drive the BFO instead of the 12mhz crystal oscillator). ?So the only way Raduino code could possibly flip to the other sideband is by moving the Si5351 VFO from 5mhz to 19mhz. ?That code is not going to do anything when using the SDR Kits VFO.

Getting back to Marc, he said: ?"Jerry I can confirm the VFO ?output is ?at 4.8 to 5 MHz"

I can't imagine any way to flip sidebands other than to move either the BFO or the VFO (but moving both will get you back to where you started). ?Ok, could be one of those two possibilities, or demonic possession. ?Or me not quite having this figured out. ?He apparently didn't mess with the BFO. ?So my best guess remains that his SDR Kits VFO was sending out 19mhz. ?Probably not worth forcing the SDR Kits VFO to give a 5mhz signal if it doesn't know about negative offsets, because as the displayed VFO frequency rises, the frequency of the received 7mhz signal will be falling. ?Not very convenient.

Jerry,, KE7ER

Before hijacking this thread: ever looked at a Nextion display. Lot's of logic/drawings can be put in there, only need to send/receive values from the Arduino, minimal overhead.
And using the processor capacity of the lcd more and the arduino less. Only react on input, and it is touchscreen also so less buttons needed on the arduino/bitx40 side.