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?

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

?

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

?

There is an excellent and easy to remember rule of thumb on sideband inversion.? Explained here:

On Thursday, December 15, 2016 9:52 PM, "Ken Chase chase8043@... [BITX20]" <BITX20@...> wrote:

?

Hi John

I had a chance to try some testing with sideband inversion.

BFO = 11.9986 MHZ and VFO = 4.8486 MHZ this gives me a starting frequency of 7.15 MHZ(RF = BFO - VFO). Set the encoder to increase frequency in CW direction. But this to increase VFO or decreasing RF. So I switched it so RF goes up in CW direction.

I usually listen in on a net at 7.163 MHZ. To tune in the voice I had to increase RF. So I find that LSB is on the wrong side of the (non-existent) carrier.

Is this the sideband inversion?

73

Ken VA3ABN

On Thu, Dec 8, 2016 at 11:56 PM, iam74@... [BITX20] <BITX20@...> wrote:

?

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

?

Not really.

I would guess the confusion arises because there are two different inversions happening. The first is what you experienced with the DDS -- the frequency span is the same but TUNING is inverted. That is mostly tradition as we ordinarily count going up in frequency as we turn the dial clockwise. In the case of the BITX40, if the VFO tunes from 4.7 MHz to 5 MHz, then the RF tunes from 7.3 MHz to 7.0 MHz. So to get a "proper" tuning we actually have to reverse or "invert" the VFO signal. That is what you have actually described in your rig.

The second type is much more subtle. If one thinks of sidebands, the USB extends lower audio frequencies from the carrier outward UP in frequency, and the LSB does the reverse, i.e., the higher audio frequencies are DOWN in frequency. Merely shifting the USB to the LSB area will invert the audio; now the upper audio frequencies are nearer the carrier and the lower ones further away. That is not the way we hear things.

How then does one decode USB on LSB side (or vice versa)? Clearly some other mechanism has to happen which also inverts the effect of the LSB into one of an USB (ibid). Then the phases will be correct and the original audio will reappear. This is not a simple problem.

Most of the traditional 9 MHz IF and 5 MHz VFO rigs (the 20/60m rigs) did not have this problem. Generally if the filter were a lattice filter, they used both a USB and LSB crystal and adjusted the BFO frequency to either sideband; That is, they use dual BFO frequencies. They also used (as does Farhan in his 20/40 rig) dual bandpass filters to accommodate each of the different sidebands. This practice continues; some BITX rigs use the same mechanism even though they do not use lattice filters (they generally use Cohn filters).

With the advent of the sharp bandpass crystal filter rigs (such as the BITX) this did not work any more and there is ONE BFO frequency. Note that the modulating frequency on transmit is the IF. The received modulated frequency is the RF. The VFO is passive and merely adds or subtracts enough to produce either an RF output or IF input. So where can sideband inversion of the second type take place? In the mixer. Note that the IF is the actual modulated RF signal on transmit.

? "The resulting upper and lower sidebands each contain both of the modulating frequency components. The upper-sideband components consist of the sum of each modulating frequency plus the carrier frequency, and no inversion takes place. The lower-sideband components consist of the carrier frequency minus each of the modulating frequencies, and they become inverted. Inversion occurs in any frequency-translating process when the mixing frequency is higher than the signal frequency and the difference products are selected in the output circuit. This principle can be used for sideband switching in both transmitters and receivers, since by this means an upper-sideband signal is converted to a lower-sideband signal or vice versa."

Source:SSB - QRZ Israel HAM radio portal.htm

Since our mixing frequency is the VFO? (5 MHz), and the output is the RF (7 MHz), no inversion of the second kind takes place. But there is another product (VFO + IF) where it does because the IF is greater than the VFO (-IF + VFO). This product is suppressed by the final BPF. Consider that BPF. It is critical. It keeps out non-40m frequencies from the receiver, and also suppresses non-40m signals from the transmitter, including out-going inversion products.

I know this is confusing. I wonder all the time if I have gotten it right. The math is somewhat complex. Hope this helps anyway.


john
AD5YE


---In BITX20@..., <chase8043@...> wrote :

Hi John

I had a chance to try some testing with sideband inversion.

BFO = 11.9986 MHZ and VFO = 4.8486 MHZ this gives me a starting frequency of 7.15 MHZ(RF = BFO - VFO). Set the encoder to increase frequency in CW direction. But this to increase VFO or decreasing RF. So I switched it so RF goes up in CW direction.

I usually listen in on a net at 7.163 MHZ. To tune in the voice I had to increase RF. So I find that LSB is on the wrong side of the (non-existent) carrier.

Is this the sideband inversion?

73

Ken VA3ABN

On Thu, Dec 8, 2016 at 11:56 PM, iam74@... [BITX20] <BITX20@...> wrote:

?

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

?

Thank you, Ken. Glad it helped.

You know, you can get USB and LSB through the same Cohn filter. Remember that the filter is a passband filter about 3 kHz wide with an upper and lower skirt. For proper phasing, the LSB has to reflect off the lower skirt and the USB off the upper skirt. But the filter itself is really a passive object.

The trick is in adjusting the BFO frequency so that the filter thinks it is getting one or the other of the sidebands. If the incoming IF is just below the center frequency of the filter, it will pass it as LSB, and if the IF is just above the center frequency, it will pass it as USB.

So, since one generates both sidebands at the BFO mixer, merely changing the BFO frequency enough is all it takes to generate one or the other. Ha! Then if one changes the VFO to reflect which sideband is coming through the filter, one can have a dual sideband rig!

Say you wanted 20m and 40m. Then the 40m rig would use a VFO to generate ~7 MHz with the IF BELOW the center frequency of the filter, and a VFO to generate ~14 MHz with the IF ABOVE the center frequency. It is not so easy with analog VFO's; probably you would need two separate ones. But it is easy with a DDS.

Setting the BFO frequency is a matter of loading the crystal. Remember that we have to move the frequency only a few KHz to get either sideband. That is done by diode selection of each tuning (or diode selection of 2 crystals). We are generating both sidebands all the time. The particular IF we choose just selects which to send to the passive filter. Since the IF amplifier is wideband, it can easily handle a few KHz difference in frequency.

One would need to provide separate BPF's and LPF's for each band, of course -- but it is not too hard a thing to do.

There are some caveats to heed. The parallel resonant frequency is not too far up the USB and reaching that can have strange effects; so one must be careful in tuning around the USB. Else, it works. Alternatively, one can choose a different IF with a different, somewhat higher (by a few KHz) frequency crystal. Also remember that the center frequency is probably NOT that marked on the crystals. It is the measured serial frequency of the filter. In the case of the BITX40, that can be anything around 12 MHz. Say your IF is 11.996600 MHz. Then the center of the filter passband is probably 1.5-2.5 KHz above that. Exactly what is a measurement.

CW is also possible by allowing the BFO carrier to get through. It is less effective, however, since the passband is 2-3 KHz wide, and CW should have one about 700 Hz wide. But it can be done. One merely adds a third crystal to the BFO and allows the carrier through for the center of the passband. Some people have gotten around the wide passband problem by using varicaps to adjust the filter capacitance. But that introduces impedance problems and more extensive changing of circuits at times...

73
john
AD5YE

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

John

You did an excellent job on explaining this subject. It's something I will be investigating more. Once in a while something comes along that I don't understand and I don't give up till I do.

People like yourself are a credit to the hobby and I appreciate all your work here.

73 Ken VA3ABN

?

Bravo, Ken. You have taken on a huge chunk of RF Engineering, to say the least.

john
AD5YE

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

Hi John

Tonight I gave a small presentation on side band inversion to the homebrew group I belong to. A couple were aware of the?'phenomenon', but most found it very interesting. I used this to help in explaining.

Thanks again for your help, very much appreciated.

73

Ken VA3ABN

?

Ken:?? I think?sideband inversion is a lot simpler than that and can easily be explained without resort to "negative frequencies."
Just consider an LSB signal coming in at 7.2 MHz.? It will be spread out from about 7.1997 down to? 7.197 Mhz.? 7.1997 corresponds to the 300Hz audio, 7.197 corresponds to the 3000Hz audio.
Assume your IF and crystal filter ate at or around 10 MHz.
You have two options for the VFO:? 2.8 MHz?or 17.2 MHz.
With 2.8 MHz you take the sum product out of the mixer.?7.1997+2.800=9999.7 MHz?? 7.197+2.800=9997? That's an LSB signal at 10 Mz. NO INVERSION
With 17.2 MHz you take the difference: 17.2-7.1997+10.003MHz????? 17.2-7.197+10.003 MHz..? That is a USB signal at 10 MHz? SIDEBAND INVERSION!
It helps to draw this all out.? See attached.?
This proves the rule:? Sideband inversion only takes place when you SUBTRACT the signal carrying the modulation FROM the signal without the modulation (the VFO or LO).
Please let me know what you think about this explanation.?
73? Bill N2CQR

On Wednesday, December 21, 2016 10:31 PM, "Ken Chase chase8043@... [BITX20]" <BITX20@...> wrote:


? Hi John
Tonight I gave a small presentation on side band inversion to the homebrew group I belong to. A couple were aware of the?'phenomenon', but most found it very interesting. I used this to help in explaining.


Thanks again for your help, very much appreciated.
73
Ken VA3ABN
On Mon, Dec 19, 2016 at 2:11 AM, iam74@... [BITX20] <BITX20@...> wrote:

? Thank you, Ken. Glad it helped.

You know, you can get USB and LSB through the same Cohn filter. Remember that the filter is a passband filter about 3 kHz wide with an upper and lower skirt. For proper phasing, the LSB has to reflect off the lower skirt and the USB off the upper skirt. But the filter itself is really a passive object.

The trick is in adjusting the BFO frequency so that the filter thinks it is getting one or the other of the sidebands. If the incoming IF is just below the center frequency of the filter, it will pass it as LSB, and if the IF is just above the center frequency, it will pass it as USB.

So, since one generates both sidebands at the BFO mixer, merely changing the BFO frequency enough is all it takes to generate one or the other. Ha! Then if one changes the VFO to reflect which sideband is coming through the filter, one can have a dual sideband rig!

Say you wanted 20m and 40m. Then the 40m rig would use a VFO to generate ~7 MHz with the IF BELOW the center frequency of the filter, and a VFO to generate ~14 MHz with the IF ABOVE the center frequency. It is not so easy with analog VFO's; probably you would need two separate ones. But it is easy with a DDS.

Setting the BFO frequency is a matter of loading the crystal. Remember that we have to move the frequency only a few KHz to get either sideband. That is done by diode selection of each tuning (or diode selection of 2 crystals). We are generating both sidebands all the time. The particular IF we choose just selects which to send to the passive filter. Since the IF amplifier is wideband, it can easily handle a few KHz difference in frequency.

One would need to provide separate BPF's and LPF's for each band, of course -- but it is not too hard a thing to do.

There are some caveats to heed. The parallel resonant frequency is not too far up the USB and reaching that can have strange effects; so one must be careful in tuning around the USB. Else, it works. Alternatively, one can choose a different IF with a different, somewhat higher (by a few KHz) frequency crystal. Also remember that the center frequency is probably NOT that marked on the crystals. It is the measured serial frequency of the filter. In the case of the BITX40, that can be anything around 12 MHz. Say your IF is 11.996600 MHz. Then the center of the filter passband is probably 1.5-2.5 KHz above that. Exactly what is a measurement.

CW is also possible by allowing the BFO carrier to get through. It is less effective, however, since the passband is 2-3 KHz wide, and CW should have one about 700 Hz wide. But it can be done. One merely adds a third crystal to the BFO and allows the carrier through for the center of the passband. Some people have gotten around the wide passband problem by using varicaps to adjust the filter capacitance. But that introduces impedance problems and more extensive changing of circuits at times...

73
john
AD5YE

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

John
You did an excellent job on explaining this subject. It's something I will be investigating more. Once in a while something comes along that I don't understand and I don't give up till I do.
People like yourself are a credit to the hobby and I appreciate all your work here.
73 Ken VA3ABN

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