Topics

Searching for IMD

Warren Allgyer <allgyer@...>
 

The generally accepted limit for SSB intermodulation products is a minimum of 24 dB between the lowest of the twin tones and the highest of either the third order (products immediately adjacent to the twin tones) or fifth order (products next over above and below the third order) products. My uBitx shows -12 dB at 3.6 MHz. Conditions are: 30 mVrms audio input and RV1 set for 5 watts RF output through a 4 MHz LPF. My board has the onboard PA filters removed and strapped and has the additional 45 MHz filter with 12:1 output impedance transformer in place of R27.

In order to better understand where the IMD is being introduced in the radio I started all the way back at the balanced modulator output and measured IMD at thoughtfully provided test points up through the driver output. In general, once IMD products are introduced at early stages they tend to only get worse as the signal progresses down the chain. The key to fixing it is finding the root cause as early in the process as possible.

TP17 is the output of the balanced modulator and the 12 MHz SSB filter. IMD products here were below the noise floor of my measurement configuration and I did not chase them down into the mud because they are at least 40 dB down and do not pop up through the floor.

TP16 is the output of the first bi-directional amplifier and IMD at this point measured -35 dB, already much too high and and indication of non-linearity in the amplifier that must be addressed.

TP14 is the output of the onboard 45 MHz filter following the 2nd mixer. This actually shows a slight improvement but probably within measurement error at -37 dB. This measurement pretty much exonerates the 2nd mixer as a significant contributor to the IMD issue.

TP16 is the output of the second bi-directional amp and again there is a serious deterioration in IMD with the amp adding 11 dB to the problem.

TP1 is the output of the 2nd transmit mixer (labelled 1st mixer in the text) and is the first time we see a signal at air frequency of 3.6 MHz. The mixer added 5 dB of IMD to the total.... much too much and probably indicative of low injection levels as has been stated in the past. On the other hand it is not the primary culprit by far.

TP3 is the output of the first pre-driver and of RV1 and it adds a little over 1 dB of IMD. The total IMD at this point is 5 dB less than the acceptable amount and it is only beyond this point that we are able to control power levels with RV1 which would normally be the adjustment point for controlling PA IMD. In other words we are starting out with an unacceptable signal from the low level stages and only now getting to where IMD is normally introduced. A contemporary radio would show normally show IMD levels at -45 dB or better at this point.

From this point forward, at the five watt level, the combination of pre driver, driver, and PA added 5 dB of IMD. This amount would be perfectly acceptable in most radios starting out with clean drive and would allow the total power to be  increased by RV1 adjustment to significantly higher levels.

The IMD problem is rooted first in non-linearities in both bi-directional amps and then in both mixers.

WA8TOD


Kees T
 

What about buffering the three S15351 outputs on the Raduino board with three 74LVC1G04GW's as suggested earlier by Jerry and Allison (and add ferrite choke power filters) ?

73 Kees K5BCQ

Warren Allgyer <allgyer@...>
 

You can do that but the first mixer is adding no IMD and the second only 5 dB. Between the two bidirectional amps there is at least 15 and perhaps 20 dB of IMD added. The amplifiers are the primary culprit and the radio will never pass muster until they are linearized.

WA8TOD

 

Warren,

Thank you for that precise explanation, I now understand where the IMD is coming from. So at this point do you recommend the I/O transformers on the additional 45Mhz filter replacing R27? If so, can you detail which cores to use and the exact winding instructions. I think all of us greatly appreciate all you and others are doing to get to the bottom of all this.

Joel
N6ALT

Jerry Gaffke
 

Warren,

Excellent information!
Thanks!

Jerry

Gordon Gibby <ggibby@...>
 

Very helpful information, thank you!  


On Sep 12, 2018, at 11:00, Warren Allgyer <allgyer@...> wrote:

The generally accepted limit for SSB intermodulation products is a minimum of 24 dB between the lowest of the twin tones and the highest of either the third order (products immediately adjacent to the twin tones) or fifth order (products next over above and below the third order) products. My uBitx shows -12 dB at 3.6 MHz. Conditions are: 30 mVrms audio input and RV1 set for 5 watts RF output through a 4 MHz LPF. My board has the onboard PA filters removed and strapped and has the additional 45 MHz filter with 12:1 output impedance transformer in place of R27.

In order to better understand where the IMD is being introduced in the radio I started all the way back at the balanced modulator output and measured IMD at thoughtfully provided test points up through the driver output. In general, once IMD products are introduced at early stages they tend to only get worse as the signal progresses down the chain. The key to fixing it is finding the root cause as early in the process as possible.

TP17 is the output of the balanced modulator and the 12 MHz SSB filter. IMD products here were below the noise floor of my measurement configuration and I did not chase them down into the mud because they are at least 40 dB down and do not pop up through the floor.

TP16 is the output of the first bi-directional amplifier and IMD at this point measured -35 dB, already much too high and and indication of non-linearity in the amplifier that must be addressed.

TP14 is the output of the onboard 45 MHz filter following the 2nd mixer. This actually shows a slight improvement but probably within measurement error at -37 dB. This measurement pretty much exonerates the 2nd mixer as a significant contributor to the IMD issue.

TP16 is the output of the second bi-directional amp and again there is a serious deterioration in IMD with the amp adding 11 dB to the problem.

TP1 is the output of the 2nd transmit mixer (labelled 1st mixer in the text) and is the first time we see a signal at air frequency of 3.6 MHz. The mixer added 5 dB of IMD to the total.... much too much and probably indicative of low injection levels as has been stated in the past. On the other hand it is not the primary culprit by far.

TP3 is the output of the first pre-driver and of RV1 and it adds a little over 1 dB of IMD. The total IMD at this point is 5 dB less than the acceptable amount and it is only beyond this point that we are able to control power levels with RV1 which would normally be the adjustment point for controlling PA IMD. In other words we are starting out with an unacceptable signal from the low level stages and only now getting to where IMD is normally introduced. A contemporary radio would show normally show IMD levels at -45 dB or better at this point.

From this point forward, at the five watt level, the combination of pre driver, driver, and PA added 5 dB of IMD. This amount would be perfectly acceptable in most radios starting out with clean drive and would allow the total power to be  increased by RV1 adjustment to significantly higher levels.

The IMD problem is rooted first in non-linearities in both bi-directional amps and then in both mixers.

WA8TOD


<Screen Shot 2018_09_12 at 10.57.54 AM.png>

Gordon Gibby <ggibby@...>
 

Possibly the gain  of the transistors at 45 MHz is insufficient, and thus there is much less effective negative feedback.  

Better transistors might allow the negative feedback to reduce the IMD.   

Cheers,

Gordon



On Sep 12, 2018, at 11:00, Warren Allgyer <allgyer@...> wrote:

The generally accepted limit for SSB intermodulation products is a minimum of 24 dB between the lowest of the twin tones and the highest of either the third order (products immediately adjacent to the twin tones) or fifth order (products next over above and below the third order) products. My uBitx shows -12 dB at 3.6 MHz. Conditions are: 30 mVrms audio input and RV1 set for 5 watts RF output through a 4 MHz LPF. My board has the onboard PA filters removed and strapped and has the additional 45 MHz filter with 12:1 output impedance transformer in place of R27.

In order to better understand where the IMD is being introduced in the radio I started all the way back at the balanced modulator output and measured IMD at thoughtfully provided test points up through the driver output. In general, once IMD products are introduced at early stages they tend to only get worse as the signal progresses down the chain. The key to fixing it is finding the root cause as early in the process as possible.

TP17 is the output of the balanced modulator and the 12 MHz SSB filter. IMD products here were below the noise floor of my measurement configuration and I did not chase them down into the mud because they are at least 40 dB down and do not pop up through the floor.

TP16 is the output of the first bi-directional amplifier and IMD at this point measured -35 dB, already much too high and and indication of non-linearity in the amplifier that must be addressed.

TP14 is the output of the onboard 45 MHz filter following the 2nd mixer. This actually shows a slight improvement but probably within measurement error at -37 dB. This measurement pretty much exonerates the 2nd mixer as a significant contributor to the IMD issue.

TP16 is the output of the second bi-directional amp and again there is a serious deterioration in IMD with the amp adding 11 dB to the problem.

TP1 is the output of the 2nd transmit mixer (labelled 1st mixer in the text) and is the first time we see a signal at air frequency of 3.6 MHz. The mixer added 5 dB of IMD to the total.... much too much and probably indicative of low injection levels as has been stated in the past. On the other hand it is not the primary culprit by far.

TP3 is the output of the first pre-driver and of RV1 and it adds a little over 1 dB of IMD. The total IMD at this point is 5 dB less than the acceptable amount and it is only beyond this point that we are able to control power levels with RV1 which would normally be the adjustment point for controlling PA IMD. In other words we are starting out with an unacceptable signal from the low level stages and only now getting to where IMD is normally introduced. A contemporary radio would show normally show IMD levels at -45 dB or better at this point.

From this point forward, at the five watt level, the combination of pre driver, driver, and PA added 5 dB of IMD. This amount would be perfectly acceptable in most radios starting out with clean drive and would allow the total power to be  increased by RV1 adjustment to significantly higher levels.

The IMD problem is rooted first in non-linearities in both bi-directional amps and then in both mixers.

WA8TOD


<Screen Shot 2018_09_12 at 10.57.54 AM.png>

iz oos
 

Indeed it shows some mess on the SSB mode. It is not a K3. I am curious how the MFJ SSB hf transceivers, like the 9420 which have a powerful average power, behave on this respect (IMD). And also how ham transceivers behave with external speech boxes.


Il 12/set/2018 17:37, "Gordon Gibby" <ggibby@...> ha scritto:
Very helpful information, thank you!  


On Sep 12, 2018, at 11:00, Warren Allgyer <allgyer@...> wrote:

The generally accepted limit for SSB intermodulation products is a minimum of 24 dB between the lowest of the twin tones and the highest of either the third order (products immediately adjacent to the twin tones) or fifth order (products next over above and below the third order) products. My uBitx shows -12 dB at 3.6 MHz. Conditions are: 30 mVrms audio input and RV1 set for 5 watts RF output through a 4 MHz LPF. My board has the onboard PA filters removed and strapped and has the additional 45 MHz filter with 12:1 output impedance transformer in place of R27.

In order to better understand where the IMD is being introduced in the radio I started all the way back at the balanced modulator output and measured IMD at thoughtfully provided test points up through the driver output. In general, once IMD products are introduced at early stages they tend to only get worse as the signal progresses down the chain. The key to fixing it is finding the root cause as early in the process as possible.

TP17 is the output of the balanced modulator and the 12 MHz SSB filter. IMD products here were below the noise floor of my measurement configuration and I did not chase them down into the mud because they are at least 40 dB down and do not pop up through the floor.

TP16 is the output of the first bi-directional amplifier and IMD at this point measured -35 dB, already much too high and and indication of non-linearity in the amplifier that must be addressed.

TP14 is the output of the onboard 45 MHz filter following the 2nd mixer. This actually shows a slight improvement but probably within measurement error at -37 dB. This measurement pretty much exonerates the 2nd mixer as a significant contributor to the IMD issue.

TP16 is the output of the second bi-directional amp and again there is a serious deterioration in IMD with the amp adding 11 dB to the problem.

TP1 is the output of the 2nd transmit mixer (labelled 1st mixer in the text) and is the first time we see a signal at air frequency of 3.6 MHz. The mixer added 5 dB of IMD to the total.... much too much and probably indicative of low injection levels as has been stated in the past. On the other hand it is not the primary culprit by far.

TP3 is the output of the first pre-driver and of RV1 and it adds a little over 1 dB of IMD. The total IMD at this point is 5 dB less than the acceptable amount and it is only beyond this point that we are able to control power levels with RV1 which would normally be the adjustment point for controlling PA IMD. In other words we are starting out with an unacceptable signal from the low level stages and only now getting to where IMD is normally introduced. A contemporary radio would show normally show IMD levels at -45 dB or better at this point.

From this point forward, at the five watt level, the combination of pre driver, driver, and PA added 5 dB of IMD. This amount would be perfectly acceptable in most radios starting out with clean drive and would allow the total power to be  increased by RV1 adjustment to significantly higher levels.

The IMD problem is rooted first in non-linearities in both bi-directional amps and then in both mixers.

WA8TOD


<Screen Shot 2018_09_12 at 10.57.54 AM.png>

Warren Allgyer <allgyer@...>
 

Joel

Here is what I did. I did not try with a second transformer because the results were good enough. After the IMD measurements I would be even more concerned about adding a second transformer because the Q22 emitter follower is operating at an impedance of about 400 ohms and, as such, is one of the major contributors to the IMD issue. Adding a low impedance transformer to this source is likely to make that problem worse. But I have not tested it so I cannot be sure.

WA8TOD

Warren Allgyer <allgyer@...>
 

WHOOPS!!!!

Forgot a critical component. You need a capacitor in the low impedance output of the transformer.

WA8TOD

Jerry Gaffke
 

Warren did us a service by showing that IMD on the uBitx is worse than it should be
and where that is coming from. 
I agree it should be fixed.

However,  some IMD and some residual carrier are relatively minor sins
compared to strong harmonics and out-of-band spurs.

Looking at Warren's screen shots, the two highest peaks are 1khz apart,
and in a perfect rig are the only peaks that would be in the display.
We see one fairly strong peak 1khz beyond on each side,  and some lesser peaks
20dB down from those strong unwanted peaks.
As shown, that signal is taking up less bandwidth than an AM phone signal. 
At QRP power levels, some will consider this acceptable.

Warren's screen shots only show as far as the D1,D2 mixer,
not shown is how much worse this gets after going through the power amp.
He has reported that "On 40 meters the best IMD achievable was 12 dB"
That was likely with RV1 gain set quite low, not much power out.
But if those lesser peaks remain 20dB below the strongest unwanted peaks,
then the uBitx likely remains within the acceptable bandwidth of an AM phone signal.

For now, evaluate your rig carefully for splatter, especially if goosing the mike gain. 
Acknowledge reports that your signal is splattering into adjacent channels,
and perhaps report such events to the forum.
Do not use an external high power amp with the uBitx, keep it at 5 or 10 Watts.

We should all find the time to fix the transmit LPF's somehow, perhaps just
adding an external filter in line with the antenna.
Also do the simple 45khz filter fix for spurs once a preferred mod is arrived at.
With that, it appears this rig may be ok wherever AM phone is legal. 
Especially when operating at QRP levels.

Jerry, KE7ER


On Wed, Sep 12, 2018 at 08:59 AM, iz oos wrote:

ndeed it shows some mess on the SSB mode. It is not a K3. I am curious how the MFJ SSB hf transceivers, like the 9420 which have a powerful average power, behave on this respect (IMD). And also how ham transceivers behave with external speech boxes.

 

Warren Allgyer <allgyer@...>
 

Reasonable people can disagree. At -12 dB IMD distortion products become audible within the signal itself. 

IMD is one component of the Total Harmonic Distortion spec that we see for our audio equipment. Generally it is accepted that 3% THD is the maximum acceptable for any of the components in the consumer electronics world. In the professional world we want to be below 1%. Above that and the distortion becomes noticeable to the consumer ear. 

3% THD corresponds to -30 dB IMD. 12% IMD is nearly 100 times that amount. 

There is is nothing illegal about putting distorted audio in the air. But I would not do it. 

WA8TOD

 

Thanks Warren,

That is exactly what I was looking for. It just so happens I have all the parts and will make the mod asap and wait to see what else will be recommended.

Joel
N6ALT

Jerry Gaffke
 

Distorted audio is easy enough for us to be aware of,
even if we cannot all measure it accurately.  
Spurs and harmonics are more apt to slip by unnoticed,
and can wreak havoc outside the ham bands.

Most of the reports in the forum about bad audio seem to come down to
a BFO not placed correctly with respect to the 12mhz filter,
or a poorly shaped 12mhz filter passband. 
Perhaps some are due to IMD, and we simply have not been aware of that aspect. 
But many here report good sounding audio.

In most cases, distortion due to IMD will go down with lower audio levels into the mike.
This will not help if the issue has to do with the 12mhz filter shape or BFO placement.

These levels of IMD should not be ignored.
I'm hoping there will be a v5 with much improved IMD specs.
We may eventually come to some easy partial fixes on v3 and v4,
but a complete fix may require a significant redesign.

Jerry, KE7ER


On Wed, Sep 12, 2018 at 10:22 AM, Warren Allgyer wrote:
There is is nothing illegal about putting distorted audio in the air. But I would not do it. 

Kees T
 

Since homebuilt diode DBMs are quite "touchy" what about using a better matched diode ring quad mixer like the HSMS-282R. 

Maybe this should be in the "spurs" thread

73 Kees K5BCQ

ajparent1/kb1gmx
 

Kees,

The mixers are already using monolithic diode pairs.  Going to diode quads might help balance
but I can say replacing the mixers with packaged Tuf-1 or SBL-1 made not one bit of difference.

However having an amp that by 20M can barely apply 150mw(clean) to the finals and is
struggling to do that because its prior stages are also gain impaired has a lot to do with that.
So then people people harder to get the 10W they "paid for" which is mostly IMD and splatter.

Allison

Nick VK4PP
 

How about replacing the 2N3904 in the Bi-di amp with BFR106? as with Q60 pre-driver?
Would that help in making them more linear?

73 Nick VK4PP

Kees T
 

Allison,

If the old reliable standby SBL-1 mixers didn't make any difference (thanks for testing), I would guess the HSMS-282R would not do any better, ......although I've had luck with the HP "die matched" pairs/quads before.

73 Kees K5BCQ

ajparent1/kb1gmx
 

Kees,

With what HFsignals used it would left to the transformer symmetry.

The testing was part of the what int he world is going on after spurs were spotted.


Allison

Glenn
 

Perhaps it's possible to improve the Bi-Amps?   The article by Wes Hayward and Bob Kopski shows a slightly different biasing arrangement and feedback in the first stage compared to that used in uBITX. As shown, gain is 15dB and flat to 100MHz within 1dB.

More conventional resistive voltage divider is used and the feedback is AC only. Two extra parts are used. The article doesn't mention IMD though specifically, as a performance target or measure it.

glenn