Flex 5000 - Its ability to produce positive peaks cleanly

[W1AEX]

Steve HX - I don't see that asymmetry with either of my plate modulated rigs, however, I do see it with the Flex. Watch the video and see what you can make of it. As a person with no formal electronics background, but lots of curiosity, I'm always interested in getting a better understanding. Duly noted regarding seeking a higher average rather than pursuing big positive peaks. That's what I see on every AM BCB signal that sounds good.

Al VTP - That's some great stuff on Jay's site. Your thoughts on the topic are exactly the same as what prompted me to take more than a casual look at my settings. I hear some great sounding SDR rigs on the air, and some that sound very grainy and gritty.

John JN - As noted to Al above, your point about receivers is exactly what I found interesting and why I choose to run settings that don't result in exaggerated positive peaks with my voice. Normally my scope deflects within the 110% to 125% range on voice peaks. Several of my receivers don't take kindly to much more than that.

Bruce KF1Z - In a recent chat I had with Dave W9AD he mentioned that in a conversation he had with Flex several years ago he suggested that they were missing the boat with the AM mode and its somewhat anemic modulation performance on AM. He said that much to his surprise they seemed to be interested and indicated that they would pursue it. At some point later in time the AM "Carrier Setting" showed up in Power SDR. I don't know what other things may have been done in PSDR or on the hardware side, but it would be interesting to compare the SDR-1000 to the 3000/5000 series. I did not use the built in Tone Generator because the way it is implemented makes it difficult to transition from just a carrier to a modulated signal. What I did instead was generate a 1000 cps sine wave with Audition and play it with the internal Power SDR wav player which has an incremental level control. A screenshot of a segment of the tone file is below.

[W8IXY]

A couple of things:

1.  Splatter on a "conventionally" modulated AM signal is caused when the carrier is "pinched off" on the negative peaks.  Think of it as tryng to transmit a squared off wave.  Remember a square wave consists of its fundamental frequency plus an 'infinite' number of odd harmonics.  If a high level plate modulated transmitter (which for our purposes of explanation is a "high level mixer", mixing the carrier with the audio {or other} modulating frequencies), and there is no very narrow filtering (in the PA output circuit), then you will have a theoretical infinite number of odd harmonically related "splatter" sidebands.  Since we usually modulate with a more complex waveform than a sine wave, we generate those additional distortion products at the PA stage and radiate them all over the place away from the carrier.  All the filtering of the input audio will not reduce the distortion components generated in the overmodulated stage.  As mentioned previously, this does NOT occur in a balanced modulator.

2.  AM broadcasters in the USA are required by FCC rules to limit positive modulation to an absolute limit of 125%.  However, that rule is frequently not paid attention to by some broadcasters.  And, as mentioned earlier, envelope distortion in typical consumer radios gets pretty bad sounding when the asymmetry is beyond 125%.

3.  Modern broadcast audio processors incorporate an "all pass" filter at their input stage to force the input waveform to be practically symmetrical.  Then, later in the processing scheme, another limiter circuit allows precise adjustment of asymmetry to be added back in, precisely controlled, to maximize any input waveform, voice or music, to peak at 125% (or any level the broadcaster chooses, depending on transmitter capabilities, etc.)  AM broadcasters who limit their negative peaks to  about 95% (simple diode detectors get very non linear at close to a 100% modulated negative peak), and allow their positive peaks to run up to 125%, typically gain ~2 db of "loudness" vs those who limit positive peaks to the same as negative peaks.

Natural asymmetry of most human voices taken into account will sound quite good through an audio/modulation system if the waveform is passed through essentially unaltered.  However, removing asymetry before processing, and adding it back in at the final stage of processing, results in AM broadcasters maintaining consistently loud, and yet clean audio.

73

Ted  W8IXY

[w3jn]

Whatever the case, Rob , you always sound fantastic here on the RF-590/Sherwood stink detector combo.

[KF1Z]

True Al....

But as discussed in another thread...., some people don't want to have to scrounge parts, and think about what they are doing.... which makes a kit more popular.
Even in a lot of cases, people PAY to have someone else put it together for them!

I bet hundreds of Softrocks were NOT assembled by the person who bought it...
( I have personally assembled more than 30 of them, in different flavors, and I know of at least 2 other people that have done a bunch)


I know other mixing schemes have been discussed....
But yesterday I used a NE602, with a Si570 based VFO, and managed to have a 100khz to 200mhz direct converting SDR reciever!

Sure, downconverted directly to audio (0 to 48khz useable ) "single ended", so no image rejection..

And obviously not very sensitive after about 10mhz... but it worked!
I did manage to hear the closest 2 meter repeater, but twas a tad noisy.  :-)
That's with the antenna tied directly to the NE602 input.

One could easily just use a crystal, in the osc circuit of the ne602, tie the input to the IF of the reciever... and tie the output of the ne602 to one channel of the soundcard.

That's less than $5 of parts
( and no toroids need to be wound, since you're connecting to the IF of the RX, and not an antenna)

OR, if you have a DC reciever hanging around the shack that covers the frequency of the IF you want to tap... you could diconnect the demodulator , product detector...whatever..
And tie the output of the DC mixer to your soundcard..

All kinds of interesting ways to be able to use SDR software for demodulating in any mode you want..

[KF1Z]

Bruce KF1Z - In a recent chat I had with Dave W9AD he mentioned that in a conversation he had with Flex several years ago he suggested that they were missing the boat with the AM mode and its somewhat anemic modulation performance on AM. He said that much to his surprise they seemed to be interested and indicated that they would pursue it. At some point later in time the AM "Carrier Setting" showed up in Power SDR. I don't know what other things they may have done on the hardware side, but it would be interesting to compare the SDR-1000 to the 3000/5000 series. I did not use the built in Tone Generator because the way it is implemented makes it difficult to transition from just a carrier to a modulated signal. What I did instead was generate a 1000 cps sine wave with Audition and play it with the internal Power SDR wav player which has an incremental level control. A screenshot of a segment of the tone file is below.

Ok on the tone generator scheme...

I'm going to see if I can duplicate what you're seeing on one of my rigs..
Granted, they are't flex... but it makes me wonder if some imbalance can create that effect.
As others said, seems like at 100% sine-wave modulation, both positive peaks and negative should be equal.
So if you hit 125% positive, you should see "wavelets" equalling 25% .

Curious indeed...

[K5UJ]

All of this is immaterial if the guy on the other end is running a diode detector, Rob.  Some receivers particularly 50s-60s Hallicrafters with ANL - start objectionable distortion at 70% modulation.  I seldom exceed 90% positive and, with the CRL audio chain, everyone tells me my audio is LOUD.  I think the key is more in the audio processing, than focusing on modulation percentage.

John, what CRL boxes do you run and with what rig?   Can vintage ham mod iron handle them?  I have a SEP400B and PMC450 but figured they might be too much for the old rigs, like Ranger and HT20. 

Rob

[W1AEX]

Whatever the case, Rob , you always sound fantastic here on the RF-590/Sherwood stink detector combo.

Thanks! That 390/Sherwood Synch Detector looks like a great combination. You've got nice toys John. By the way, you were very loud up here in CT the other day on the WFD net:

http://www.youtube.com/watch?v=c_KxqqoLbpI

[steve_qix]

Hi Rob, very interesting!

But, if you are indeed putting a pure sine wave into your transmitter - AND if you are not hitting 100% positive and negative at the same time, there is nonlinearity (or you are clipping off the negative peaks somehow).

The only true way to tell is with a triangle wave.  It is very hard for most transmitters to faithfully reproduce a triangle wave, and if the wave isn't perfectly straight up and down, you can immediately see where in the waveform the nonlinearity occurs.

An easy way to get a triangle wave is with an active integrator - and feed it with a symmetrical square wave.  You can whip this up with a 7400 or a 7404 as an oscillator (multivibrator) and something like an LM318 or similar wide-bandwidth op-amp as the integrator.

Of course, if you have a function generator, you're all set 

[WA3VJB]

Hey Rob, how come your screen in the upper left calls this an SSB QSO ?

Otherwise a nice-sounding recording.  I guess I'd have to put a piece of electrical tape over that part of the monitor if I ever bought a rig like you're using here.

Whatever the case, Rob , you always sound fantastic here on the RF-590/Sherwood stink detector combo.

Thanks! That 390/Sherwood Synch Detector looks like a great combination. You've got nice toys John. By the way, you were very loud up here in CT the other day on the WFD net:

http://www.youtube.com/watch?v=c_KxqqoLbpI

[WA3VJB]

I never cease to be amazed at Gary, WA1OXT's spectrum - he typically has the high end brickwalled at around 5 KC and it is SO evident.  Yet his "talk power" and quality is excellent.
Al

You got that right.  38,000 downloads of this video, and it's not even that good of a "take," with some receiver distortion evident.

http://www.youtube.com/watch?v=kbYKAPkS6lM

[w3jn]

John, what CRL boxes do you run and with what rig?   Can vintage ham mod iron handle them?  I have a SEP400B and PMC450 but figured they might be too much for the old rigs, like Ranger and HT20. 

Rob

I run the CRL PMC-400 and SEP-300 with the TMC GPT-750.  The 750 has a HB modulator with 2X813s and a BT-500 mod tranny.  Seems to handle the CRLs fine.  At my lake QTH I have a similar setup although the CRL models are different - the PMC is actually the AM stereo version.  I used that setup with a Valiant until I replaced it with a T-368.  No problems at all.

Remember the CRLs increase the density of the audio while keeping the peaks in range.  IMHO it's the peaks that kill mod trannies, not simply average power, so I'd have no qualms running the CRLs with vintage rigs.  I wouldn't run 'em full bore, though - they end up sounding like hell.

And Rob AEX, tnx fer the recording!  Sounds like I could use a few more highs.  I had cranked down the HF some time ago when I was bothering someone up band a bit and forgot to reset it.

[KF1Z]

Hey Rob, how come your screen in the upper left calls this an SSB QSO ?

Otherwise a nice-sounding recording.  I guess I'd have to put a piece of electrical tape over that part of the monitor if I ever bought a rig like you're using here.

Whatever the case, Rob , you always sound fantastic here on the RF-590/Sherwood stink detector combo.

Thanks! That 390/Sherwood Synch Detector looks like a great combination. You've got nice toys John. By the way, you were very loud up here in CT the other day on the WFD net:

http://www.youtube.com/watch?v=c_KxqqoLbpI

The only time it says "AM" is if you're on 3885  ...
But you can change that in on of the database files... at least in the older versions

[W1AEX]

Hey Rob, how come your screen in the upper left calls this an SSB QSO ?

Paul WA3VJB - Thanks for pointing that out Paul! I'm pretty sure it was a momentary glitch in Power SDR. When I hit the computer with a small hammer and rebooted it came up like the picture below. Should be fine now!

Ted W8IXY - Thanks for that explanation of how the broadcasters handle their audio processing and asymmetry. There are several stations in my area that sound very nice, and others that have very lifeless sounding audio. I guess they are at the mercy of the guys who run the racks in the studio!

[WA3VJB]

Much better, Rob.
I was about to ping the Flex people and say "QTF, over ?"

[W1AEX]

The only true way to tell is with a triangle wave.  It is very hard for most transmitters to faithfully reproduce a triangle wave, and if the wave isn't perfectly straight up and down, you can immediately see where in the waveform the nonlinearity occurs.

Hi Steve,

Within the Power SDR "Tests" tab there is an audio signal generator that is apparently capable of injecting a triangle wave to the input of the transmitter DSP. That being said, I have only used a scope for simple signal tracing and transmitter monitoring, so I would have no clue how to interpret what shows up. I may PM you a few scope pictures for some expert interpretation after I work out the SG settings! It would be interesting to see what other SDR type transmitters using Power SDR look like on a scope as they reach 100% positive while being fed with a pure sine wave.

I think the information I have gained from people in the thread has pretty much answered my original questions regarding reasonable levels for positive peaks that can be handled well by most receivers. Voice peaks reaching into the 110% - 120% region on the scope and enough compression to bring the audio density up so it sounds loud but still smooth seems to work fine. A few weeks ago when I was discussing a few things with someone from Flex, the guy laughed and said that the best thing about SDR is that the operator has control over so many settings, however, the worst thing about SDR is that the operator has control over so many settings. From what I have seen there's a lot of truth to that statement.

Rob

[KF1Z]

I was playing with this today Rob.

Used the triangle wave setting in the ""flex"" software.

Very nice trianlge out on the scope.

But as soon as ANY compression, or that silly "compandor" is in there ( setting of 1 ) ...
the result is disgusting...


Anyway...

On my rig I see "wavelets" when the positive peaks got near 100%.

So I tried to "swap I/Q" channels.
Sure enough, the wavelets went away, hit 100% positive, and just shy of 100% negative.

I went back through and carefully adjusted the Local Oscillator null, and adjusted the I/Q balance on the harware, and then again in software.

Now, the triangle wave looks perfectly symetrical, and with a sine wive, the positive and negative peaks are equal, negative just hits the baseline as the positive hits the 100% target.
AAnd, now, if I swap I/Q channels, the results are the same.


Not saying this would happen with the Flex ( would with the SDR-1000) , but not sure about the 1500-5000 models.

Also noted that at 6khz triangle wave frequncy, the triangle got mangled....
But ONLY At 6khz.
I assume this is because with these SDRs, the AM CARRIER IS GENERATED VIA A 6KHZ TONE!


Maybe later I'll take some scope pix

[WA1GFZ]

BTW, The Flex is a tayloe modulator so wonder if wavelets still appear.
HPSDR is a D/A so not sure how the firmware driving it reacts during data wrap around. Depends on how the code was written to handle overflow below zero.

[Steve - K4HX]

BTW, The Flex is a tayloe modulator so wonder if wavelets still appear.

RU sure. I would assume the AM is done in software. Why use Tayloe?

[KF1Z]

BTW, The Flex is a tayloe modulator so wonder if wavelets still appear.
HPSDR is a D/A so not sure how the firmware driving it reacts during data wrap around. Depends on how the code was written to handle overflow below zero.

My Genesis does Frank.

Same mixing scheme ( basically) as the Flex 1000.  But I haven't had my fingers in any of the other Flex's to know what they are doing.

As a matter of fact, I can lower the carrier level to the point where the "wavelets" are the exact same amplitude and shape as the "main"(?) modulation waves... (terminology sux)

The result is twice as many triangle waveforms on the scope, they all look identical!

How many % modulation is that? 400 maybe? I didn't actually figure that out.

[KF1Z]

BTW, The Flex is a tayloe modulator so wonder if wavelets still appear.

RU sure. I would assume the AM is done in software. Why use Tayloe?

Still have to mix the generated audio in Quadrature to RF .
I believe that's what Frank is reffering to.

There's a difference between QSE, and Tayloe quadrature mixers.
( I don't even pretend to remember the difference between the two, though I've had it explained to me a couple times     )

[Steve - K4HX]

So, is the AM not generated in SW on the computer?

[KF1Z]

So, is the AM not generated in SW on the computer?

Well, yes...

The modulating audio is derived through the software,

AND the 6khz tone for the carrier is derived in the software.

But in order for any of that to be translated into RF, it must go through the QSE, or Tayloe, mixer.

[K1JJ]

Phil, the VK working on the software spike problem I reported, said he may have it solved soon. After he's done, I'll axe him how difficult it would be to generate standard AM signal using a negative peak limiter in software. This is for Flex software as used on the HPSDR. Or the "KISS" version that he wrote.

T

[Steve - K4HX]

OK, so it's just up-conversion. That should not affect the modulation at all, unless the mixers are no good.

So, is the AM not generated in SW on the computer?

Well, yes...

The modulating audio is derived through the software,

AND the 6khz tone for the carrier is derived in the software.

But in order for any of that to be translated into RF, it must go through the QSE, or Tayloe, mixer.

[WA1GFZ]

Steve I have a schematic of the SDR1000 somewhere and they send audio or maybe baseband through the tayloe backwards.