I am unable to achieve IMD products at -30 dBc or better at any power level above 2 watts. I have searched this board and do not find IMD measurements. What Is the consensus on acceptable IMD levels and the power at which those levels have been achieved?

WA8TOD

Warren,

The -30dbc for IMD of third order is good.? Whats no so good is only 2W.

It was actually hard to find that data for a lot of the QRP/low power radios.
For many it doesn't exist.??

I never got as far as to measure that for uBitx.? I did it for the old bitx20
(2006 version) and it was not at all good at 4W and -20dbc.? Reworking
the amp? for better stability as that design oscillated when bias hit 60ma
improved matters to -25dbc at 5W.?

A lot of the newer 100W radios out there are barely -26dbc.
Nominally its measured at max stated output power or 10W for ubitx,
and anything better than -26dbc is as good as a lot of radios.

I suspect on 20M and up that will be limited by available power without
hitting 1DB compression point.? Once you hit compression all bets are off.

Allison

Allison

I may have needed to define my terms better. It was not -30 dBc but -24 dB referenced to one of the two tones used for testing. That is a figure of merit that I normally use to set drive levels and I consider it the maximum acceptable although, as you point out, there is no spec for most modern equipment and no FCC level other than "must not cause interference outside the signal bandwidth". Modern equipment like the Icom 7300 uses a general specification of -50 dBc for all unwanted emissions and this presumably includes IMD products outside the filter bandpass.

Maybe another way of looking at it: Testing on 80 meters, through a very sharp bandpass filter that limits harmonics and spurs to -60 dBc or better, I set the Drive level to produce +37 dBm on CW. I then put a single tone at -43 dBV into the audio input to get +30 dBm output on SSB. I then increased the tone input in 1 dB increments until I achieved 1 dB of compression. That happened at an audio level of -34 dBV, 9 dB higher than the level to make one watt, and the RF output with that input was +38 dBm. An increase of 9 dB of audio produced an increase of 8 dB in RF output; 1 dB of compression.

I then added a second tone at the same level and measured the difference between one of the tones and the first adjacent product: -15 dB. Opposite sideband products were less than 30 dB down from the primary tone level.

Operation of my radio at the 1 dB compression point results in unacceptable (to me) IMD as well as unacceptable (to everyone) splatter in the opposite sideband and outside the filter sideband on the desired sideband. I could not achieve my desire for -24 dB for the first IMD product with any more than -41 dBV of audio drive which produced an RF output of 1.6 watts.

WA8TOD

Your results are disappointing as I'd suspected they would be.

The amplifier chain seems ot struggle to make power and the quality
of the power is poor.

What also is suspect is the two IF stage 12 and 45Mhz have excess
gain and the mixers are likely getting over driven.? How much excess
depending on how you count it overall at least 16db.? I don't believe
that helps except to make up for low gain in the transmit chain.

Allison

I am finished. This is simply not worth the effort. The difference between this radio and my FT-817 is about $350. The difference between the uBitx and my HobbyPC RS-HFIQ is $100. Both are worth the increment in order to assure:

1) FCC/ITU compliant transmissions

2) Excellent IMD and in-band spurious response

3) Adequate audio output

4) AGC

5) ALC

Not to mention FM, 6m, 2m, 70 cm for the Yaesu.

I completely understand the attraction of making up your own box, modifying the code, and other customizations. But there is simply too much to be added to the basic uBitx as delivered in order to make it a compliant, user-friendly, neighbor-friendly rig. I maintain it is not possible to make compliant transmissions from this board without the knowledge and equipment to modify it for compliance and a properly equipped lab to set it up and monitor it.

My board is going on the shelf marked "Nice try....... not worth the effort", along with a complete set of filter relays, failed output filter attempts, octal to decimal decoders to drive the filters, 45 MHz SAW filters, and a stack of test data. It was fun. It is done.

WA8TOD

Hi,Warren, I will always appreciate the data that you generated!

Nicer radios, are certainly nicer radios! Some people like Honda FIT , some prefer a high end BMW!

For the purposes of coaxing local club members into learning HF and some radio, some building techniques, & electronics, this is perfect!

For HF ALE monitoring, it even beats the heck out of my ICOM 718’s. For winning a contest? Nah!!

Thanks for all your input

Gordon

Hello Warren Allgyer,

I want to thank you for all the effort you have put into the uBITX spur/harmonic problem definition and the excellent data presented. I realize your project is shelved ....but, would you be interested in trying out an 8 relay switched LPF board with 4 plug in LPFs using the parts off the uBITX board. You ran an experiment like that earlier with a perf. board. I have boards (75mm x 62mm) coming in soon and can supply the relays and misc parts (no charge).

73 Kees K5BCQ

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Mr. Kees, I am interested if you make this up as a consumer product like your AGC board. ?Please let me know when you get the kit and pricing figures out. ?Will you provide the ferrite cores? ?I can do the winding.

Hi Kees

I would be honored to test and record data, publish if you wish, any filters you would be willing to send. I would of course return them after testing is complete. If you would like to proceed please contact me privately for shipping info.?

Thank you Kees.?

WA8TOD

Allen and David,

Right now this is just a test to see what the results look like and "if there is a pony in the room". We may just provide the Gerber files, the LPF Relay boards, A complete LPF upgrade kit (not sure I can handle the potential demand after the AGC/Click activity), or just give it to someone else to run with ...?

73 Kees K5BCQ

Warren, you did quantitative tests that surprise me to a certain extent. Surely the uBitx is not an IC-7300. Even my SWL ears find the IC-7300 transmitted audio one of the best ever. But as I continously monitor my transmitted audio with an IQ SDR receiver I have not found the TX audio that terrible. Mine may sound tinny, the BFO is almost 1khz far from the passband but the opposite sideband seems to me quite well rejected. Does the audio sound distorted? Not really, reports were fine. I imagine that the IMD products may give more problems within the filter passband rather than outside. Maybe it would be worth to test the IMD with digital modes like PSK and see the reports by others.

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Iz

The primary problem caused by high IMD is splatter outside the bandpass of the sideband filter. This shows up on the opposite sideband and on the high (audio) frequency side of the desired sideband. We have all heard this when an overmodulated signal is parked 3 KHz away and we get "static crashes" in the 3 KHz that we are listening to. On a waterfall is shows up as a broadening of the base of the signal.

I don't think you will find the IMD products affecting the intelligibility of the audio even at the very high -12 dBc levels I measured. What did happen is the transmitted noise floor on the opposite sideband and outside the filter rose 20-30 dB over the the single tone level. If there is interest I can replicate that test and show what it looks like on the spectrum analyzer.

WA8TOD

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David,

?Will low-pass filters not correct the spurs generated as well as harmonics? Thanks for sharing your wisdom.

No.? Often the spurs are below the cutoff of a low pass frequency , they are low in frequency so they get to pass.
They are their to suppress harmonic and spurs that occur above the cutoff frequency.? Important but half the
job in some cases.

Good looking filter.

Allison

Iz oos,

Its rare that distortion is heard on frequency.

Is the guy 3khz up or down from you that would like to not hear you and you sputter and splatter.
So with the good radio listen to your audio then flip its sideband and listen for stuff likely unintelligable
and then move up or down and listen.??

Did this onece on 6M (deserted part of the band) to prove a point tto someone that?
felt it was great ausio even though he had the knobs set to 11 on the scale of 10.
My commercial 6M rig if I run the audio up the image on the SA gets wide yet no one
on frequency hears much other than, its louder.? If I add over driving the power amp
its the same!? However at the extreme tested on the air one time The band width
at 15db down was 60khz and yet the guy maybe 10 miles away was sounds great
and he even knows my voice from club meetings.

Good audio reports are very misleading about all the stuff their receiver is filtering out
but the other poor guy trying to share the band is not enjoying it.

Allison

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David,

You must have missed the last three months.... and a pair of very long threads!? ;)

To clean that up yes, band pass filters are the option or some very fancy combination of
high pass and low pass which results in the same pass band as a band pass.

Generally the in band spurs happen to be extremely weak and as such not a worry
other than being annoying on receive.

Allison