?

Think of it this way: VFO - IF = RF. Does this result in sideband inversion? (hint: -IF + VFO. It it positive or negative?) Why don't you try it and see what happens. If you get nothing but garble, then the answer is "no...it does not.

If the results are clear signals, then you must decide if the LSB is being inverted, or is it truly USB. Remember that the ordinary condition by convention is that the LSB on 20m is garble or non-existent. Therefore you must hear USB inverted to LSB or true USB for intelligibility. Be aware of AM; test your results through several different QSOs.

Actually, it doesn't matter which it is -- as long as communication takes place.

john
AD5YE

---In BITX20@..., wrote :

Opps. Not?18.998600 - 4.998600 = 14.0. Should be 25.998600 - 11.988600 = 14 or is this the wrong sideband?

73 Ken

?

Thanks for the input. All the technical aspects of the filters are a bit beyond me, but is it safe to say that the performance differences will not be noticable in my bitx?

Also, are the diode ring mixers wide band like the other mixers? If I were to modify my bitx for another band later on, could I use the same mixers?

On Mon, Dec 5, 2016 at 2:22 PM, bobledoux@... [BITX20] <BITX20@...> wrote:

?

Here is a comparison of mixers:

			? Comparative Measurements on Double Balanced Mixers By Nicolae Fieraru Document last updated: 28-Dec-2012 The following mixers have been checked a...
				Preview by Yahoo

?

bob-N7SUR

--

Mark Baldridge

marktbaldridge@...

608.561.3853

?

Here is a comparison of mixers:

			? Comparative Measurements on Double Balanced Mixers By Nicolae Fieraru Document last updated: 28-Dec-2012 The following mixers have been checked a...
				Preview by Yahoo

?

bob-N7SUR

?

I'm running this off one of the cheap Chinese 12V Li-Po battery packs I bought a few years ago.
I'll try it on 12V of alkaline cells tomorrow.

Thanks,

- John

On 12/8/2016 9:46 PM, Ashhar Farhan farhanbox@... [BITX20] wrote:

?

John,

there seems to be an strong 18 KHz moduation on the transmitted signal. i have a home-built spectrum analyzer that I have used to see the output spectrum. There was a strong spur in the output just below 5 MHz. It is well below 43 dbc that the FCC mandates. The other was the second harmonic at 14 MHz, again below the -43 dbc level.

I ran the scan again, just now, to be doubly sure. I set my resolution bandwidth to 1 KHz and ran the scan at 100 KHz span with a 2 tone, 50 mv audio injected into. I did notice a number of distortion products, but nothing 18 KHz away. The crystal filter took them al out.

The 18 KHz modulation could only be coming from the circuit after the crystal filter. try running this from clean battery to see if this persists. i would suspect, given the 18khz murmer, it is from an smps.

- f

On Fri, Dec 9, 2016 at 9:30 AM, John Sutter jds@... [BITX20] <BITX20@...> wrote:

?

I noticed that my BIT40v3 was well shy of the top of the band.
On the spectrum analyzer, I noticed two other peaks about 18KHz
on each side. They seem to be a bit higher than I'd expect,
but I'm a relative newbie on what's allowable.

I have a snapshot of the spectrum at:


First, is there a problem with my board or is that within limits?
I remember when I was setting up an SDR other signals were down by
47 or 48 dBm.

Second, does this mean I should adjust the trimmer to move the max
operating frequency to 7.3 MHz minus about 20 KHz?

I few minutes later I noticed that the other signals had pushed out about
6.5 KHz more:


The only change I've made to the RF side of things is replacing the
resistors
to reduce drift.

- John
W1JDS

?

I noticed that my BIT40v3 was well shy of the top of the band.
On the spectrum analyzer, I noticed two other peaks about 18KHz
on each side. They seem to be a bit higher than I'd expect,
but I'm a relative newbie on what's allowable.

I have a snapshot of the spectrum at:


First, is there a problem with my board or is that within limits?
I remember when I was setting up an SDR other signals were down by
47 or 48 dBm.

Second, does this mean I should adjust the trimmer to move the max
operating frequency to 7.3 MHz minus about 20 KHz?

I few minutes later I noticed that the other signals had pushed out about
6.5 KHz more:


The only change I've made to the RF side of things is replacing the
resistors
to reduce drift.

- John
W1JDS

?

P.S. T3 is the reference designator for the BFO transformer in the original BITX schematic.

john

---In BITX20@..., wrote :

There is no T3, either in the schematic nor in the outline diagram. It is an error in reference designators. If I were to guess, in the original on-a-napkin diagram, A.F. included two full mixers and changed the BFO mixer to the one shown, which is the original BITX configuration. Such things happen all the time. Hi.

john
AD5YE

---In BITX20@..., wrote :

can someone show me on a board overlay where t3 is?

on the bitx40v3 smd board?

?

Ah, yes. That is a slightly different problem...

The IF is determined by the VFO frequency +/- the BFO, not the nominal filter frequency. Note that you can get either out of the mixer and the function of the filter is to choose which one to pass. In the BITX40, that is 11.998.600 (roughly actual) - 4.998.600 = 7.000.000 (40m). This is filtered on 40m as LSB because of sideband inversion; the only trouble with it is that it will tune in reverse. (I am sure you have noticed this in the BITX40. It tunes "the right way" around 16 MHz with an analog VFO). In a real-world rig, one still has to adjust the BFO (or the IF offset) slightly to clarify things.

Note that by adding another VFO (which is a simple thing to do with a DDS), one can also get other bands: 11.998.600 + 2.002.200 = 14.000.000 (20m). On 20m, it would be direct USB, and the BFO would have to be adjusted to reflect that.

Eliamady recognized this on his board by providing 3 caps with diode switching in his BFO. One can tune one to LSB, one to USB, and one to CW (or digital if the bandwidth of the filter were good enough). The VFO was off-board, and could be anything one designs for it. Note that one is tuning for an IF at the actual slope or position in the passband, nothing else. One tunes below the center frequency for USB and vice-versa for LSB for IF+VFO. For IF-VFO, inversion takes place.

But, you say, isn't the 11.998.600 frequency below 12 MHz? Yes, it is. But sideband inversion takes place because the IF is more than the VFO! If it were the other way around, the result would be USB, and one would tune the signal to another slope in the filter to pass it. Remember that the actual received or transmitted signal is determined by the mixer output; it is there that the selection is made. Both sidebands are actually available in the chosen IF --- the filter decides which one is passed through. If the received signal is LSB (say) then the USB is gibberish and nothing is lost in the choice. But it also means that we can choose USB if that is where the real signal is...or DSB for that matter.

All the 20m BITX rigs I have heard about use an IF+VFO design.

It is very difficult to make a wide-ranging analog VFO, even one 1 MHz wide. The only widely-used practical one that has been devised uses an IF around 9 MHz; 9+5 = 14 and 9-5 = 4, hence a 20m/80m pairing. And that one covers only a portion of 80m. One covering several MHz is going to be a nightmare design. But a DDS provides a relatively simple solution...provided one can get the BFO right.


john
AD5YE?

---In BITX20@..., wrote :

Thanks John

I already have the 5351 running as a VFO and I may in the future also have the 5351 run the BFO.

In my sketch I assumed that the "BFO" value meant 12 MHZ, the IF. It should be the actual BFO frequency, in my case 11.9986 MHZ. This would explain why my readout was off slightly.

73 Ken

?

Ah, yes. That is a slightly different problem...

The IF is determined by the VFO frequency +/- the BFO, not the nominal filter frequency. Note that you can get either out of the mixer and the function of the filter is to choose which one to pass. In the BITX40, that is 11.998.600 (roughly actual) - 4.998.600 = 7.000.000 (40m). This is filtered on 40m as LSB because of sideband inversion; the only trouble with it is that it will tune in reverse. (I am sure you have noticed this in the BITX40. It tunes "the right way" around 16 MHz with an analog VFO). In a real-world rig, one still has to adjust the BFO (or the IF offset) slightly to clarify things.

Note that by adding another VFO (which is a simple thing to do with a DDS), one can also get other bands: 11.998.600 + 2.002.200 = 14.000.000 (20m). On 20m, it would be direct USB, and the BFO would have to be adjusted to reflect that.

Eliamady recognized this on his board by providing 3 caps with diode switching in his BFO. One can tune one to LSB, one to USB, and one to CW (or digital if the bandwidth of the filter were good enough). The VFO was off-board, and could be anything one designs for it. Note that one is tuning for an IF at the actual slope or position in the passband, nothing else. One tunes below the center frequency for USB and vice-versa for LSB for IF+VFO. For IF-VFO, inversion takes place.

But, you say, isn't the 11.998.600 frequency below 12 MHz? Yes, it is. But sideband inversion takes place because the IF is more than the VFO! If it were the other way around, the result would be USB, and one would tune the signal to another slope in the filter to pass it. Remember that the actual received or transmitted signal is determined by the mixer output; it is there that the selection is made. Both sidebands are actually available in the chosen IF --- the filter decides which one is passed through. If the received signal is LSB (say) then the USB is gibberish and nothing is lost in the choice. But it also means that we can choose USB if that is where the real signal is...or DSB for that matter.

All the 20m BITX rigs I have heard about use an IF+VFO design.

It is very difficult to make a wide-ranging analog VFO, even one 1 MHz wide. The only widely-used practical one that has been devised uses an IF around 9 MHz; 9+5 = 14 and 9-5 = 4, hence a 20m/80m pairing. And that one covers only a portion of 80m. One covering several MHz is going to be a nightmare design. But a DDS provides a relatively simple solution...provided one can get the BFO right.


john
AD5YE?

---In BITX20@..., wrote :

Thanks John

I already have the 5351 running as a VFO and I may in the future also have the 5351 run the BFO.

In my sketch I assumed that the "BFO" value meant 12 MHZ, the IF. It should be the actual BFO frequency, in my case 11.9986 MHZ. This would explain why my readout was off slightly.

73 Ken

?

well i got my dead bitx40 back to life but now i have very low audio output :)

any suggestions?

i can receive signals just the audio output to my headphones is very low?