interesting...

[N2DTS]

About modern rig crud on the bands...

http://www.sm5bsz.com/dynrange/dubus313.pdf

[KA2DZT]

From reading the whole report,  I guess my HB xmtr doesn't have any of these problems.  Something to be said about analog tube design.  The only thing digital in my station is the freq counter.

Fred

[N2DTS]

Yes, the flex stuff (and the kx3) are the worst.
Old and homebrew rigs will not have this problem.

The sdr receivers are great, the transmitters seem to be very poor.

The K3 does very well, and I thought about getting one, but most report the rx audio is very bad, very harsh.
I wonder what drives that....

[wa2dtw]

The K3 does very well, and I thought about getting one, but most report the rx audio is very bad, very harsh.
I wonder what drives that....

They say that if you set the AGC properly, the harsh noisy audio will improve dramatically.  So far, I haven't been able to remedy this problem.  The K2 doesn't have the same noisy audio issue.

[N2DTS]

Yes, I have heard them sound quite good on AM, limited bandwidth iis not a big issue with me, as long as its not 200 to 2800 Hz.

But there are many reports of the RX audio being hard to listen to.
I agree, the K2 was nice, amazing rig for a kit. I built two just because it was so much fun.

I have been thinking about an all mode all band rig, the flex 3000 did not do it for me, Powersdr bugs me, the KX3 has very limited bandwidth, is low power, has a dirty TX, the K3 has the audio problem, something like the old icom 756 pro 3 has a very bad receiver, and most other modern rigs look like plastic VCR's with a lot of crappy displays and no real meters.

I wish Elecraft would make a full size rig with more buttons and less menu's, and good clean audio.

[W1ITT]

Transmitter cleanliness is about to take a quantum step forward on the amateur bands.  The HPSDR group has beta software in the field, and they promise regular release in a couple weeks, that will facilitate IMD figures in excess of 50 db down.  Most of the "linear" amplifiers we use today are doing well to achieve minus 30 db IMD in a tailwind.  The technique, which is used in the HPSDR "Hermes" as well as the Apache Labs "Anan", uses predistortion and a low level output sample fed back into the receiver (very carefully!) to create a running correction of the input to an amplifier or chain of amplifiers.  The beta test lads have reported astounding success with both solid state and vacuum tube amps in real on-the-air conditions.
Tom, K1JJ, has reported IMD in this region using carefully adjusted Class A linear amps, but the HPSDR solution looks much easier to implement.  So far, it's only available for the above mentioned exciter, but the Penny/Mercury boards will probably be programmed later.  There is other info on the openhpsdr.org archives site, and on the weekly TeamSpeak mp3's. 
It will be beyond most of our lifetimes before -50 db IMD rigs outnumber the current stuff, but the option is there.  The HPSDR group has some amazing talent, all working smart and hard for free, and they've blown Flex out of the water on this.  And yes, the Hermes generates some very nice full carrier AM at about a half watt.  How you choose to amplify that is left as an exercise for the student.

[N2DTS]

But in the link I posted, IMD was not the issue.

It was phase, amplitude and other noise garbage getting out.

[W1ITT]

n2dts...
I read and understood the article.  The HPSDR stuff skins the same cat, just using methodology that is not now common in amateur radio.  Anything to clean up the bands at this level is a step in the right direction.
Norm

[W3RSW]

Illuminating article and stuff I never compared before.

Regarding IMD Brett, I was about to make the same observation relative to the topic of phase noise.
Reduction in xmit phase noise is all well and good for cleaning up signals in our service, but IMD reduction serves other services might be in order too.

Until we get lousy 3rd, 5th,.. IMD down we're just adding to the general composit mayhem of the whole band's noise floor.  Granted on 75-nighttime nobody including nature cares or can even hear...

Being reasonably competent hams perhaps we can cajole manufacturers in the direction of cleaning  up signals without a FCC mandate.

A good start would be for ARRL to measure IMD based on peak signal of one of the two tones instead of the "phantom" 6db higher central peak.

Also On my K3 I haven't noticed bad audio, it's rather pleasant and a nice change at times from that of my QS1R which is very crisp.   EQ settings can be made on both receive and xmit.  My older ears are in no shape to make critical comparisons though.  I'll have to see what I can find on net concerning AGC settings mentioned.  Both thes receivers are way better than stock audio on any analog receiver in the shack.
For hiss you can't beat my TR7.  Diode pickoff on the 75a2a is fine through outboard amp, but AM only, of course.

[KJ4OLL]

Very interesting article, - I wonder if FCC Part-15 specs for non-commercial transmitters even regulates these types of values.
OR
If Part-15 does address, are the numbers not stringent enough for amateur transmitters so as to give manufacturers any incentive to comply?

[N2DTS]

I SEE a lot of what looks like poor oposite sideband suppression, but it might be something else.

Some signals are as clean as can be (it seems), and others show a lot of garbage.

I would think old VFO's and PTO's would be quite good at not generating digital garbage and other leakage signals, and the sdr receivers are as good as any old receiver as far as fidelity goes, and much better at everything else.

Its kind of odd the TX side would be so poor, but I suppose no one pays any attention to that unless its really bad, and then its someone elses problem anyway...

There are a lot of complaints about the K3 RX audio. I have no idea if it would bother me with my goldfish hearing response, but it always seems to be a very low priority for the people who design modern rigs.
And it is likely very easy to use some low noise, low distortion, hi fidelity audio chip instead of the 1 watt at 15% distortion chips they use now.

In the case of the K3, I think the defect is in the dsp or whatever they use to demodulate signals.

I wonder if anyone has ever done any distortion measurements on the K3 or other radios audio output.
They rate the audio amps, but not the detectors...

My home brew RX sounds good to me, the detector is a low distortion design.
The design had info about the distortion which maxed out at 1.5% I think (worst case) which I think is very good for an AM detector using tubes.

A good sdr receiver can sound very good, but any dsp based setup seems very bad.

 

[K1JJ]

Transmitter cleanliness is about to take a quantum step forward on the amateur bands.  The HPSDR group has beta software in the field, and they promise regular release in a couple weeks, that will facilitate IMD figures in excess of 50 db down.  Most of the "linear" amplifiers we use today are doing well to achieve minus 30 db IMD in a tailwind.  The technique, which is used in the HPSDR "Hermes" as well as the Apache Labs "Anan", uses predistortion and a low level output sample fed back into the receiver (very carefully!) to create a running correction of the input to an amplifier or chain of amplifiers.  

Vely interesting Norm!

I also have an HPSDR rig and even tried the pre-distortion software about one year ago. This was pre-beta and I found it difficult to implement and was also concerned about QSY ability using it.  But I understand the lads have improved it since. I'm looking forward to trying it again.

So this software will permit a sample from ANY linear amplifier, correct? IE, It's not limited to just the PennyLane stuff - but someone running a homebrew 8877 linear should be able to sample some RF from the big amp and feed it back to the HPSDR low level, yes?


I still use my FT-1000D as a main exciter for the class A / AB1 linear system, so would have to finally switch over to full HPSDR use.  I can just imagine using my -55dB 3rd IMD system and then adding in the predistortion software. If like most things, starting with a clean system can only get better.

I agree that a -30dB 3rd order amplifier system is the bare minimum and even this is not easy to achieve with modern transceivers and amplifers. Many of the bigger solid state amps are more like -25dB 3rd. Running a marginal rig like this on 6M with its low noise floor - or at sunset on 75M can tear up the band.  Predistorton or any similar improvements will open up a new whirl, something like going from arc gap to CW. (Well not quite that dramatic, but well worth it. It's about time.  :-))

BTW, Brett, you mentioned opposite sideband suppression...  One of the easiest and quickest ways to check an SSB signal for bad IMD is to switch the receiver to the opposite sideband and note the s-meter reading of the transmitter.  Often, the difference between the two sidebands is the amount of 3rd order suppression.  IE, if a signal drops only -20 dB when looking at the opposite side, he is probably tearing up the band in both directions.  A clean signal for IMD will have crud that is hardly discernible on the opposite sideband.  (This assumes good sideband suppression on both the transmitter and receiver in the first place)

T

[N2DTS]

Well, I was looking at the sdr display.
Some signals show nothing on the other sideband, some a little, and some a LOT.
I see some old tube rigs (phazing?) that seem to be a mess, but many modern rigs run into amps show a LOT of garbage on the other sideband.

[wa3dsp]

Well besides the fact that some are intent on running 3.8khz (or more) wide SSB when most of those receiving have 2.4khz filters meaning probably 30% of the energy is not even heard there are those that have to get every last drop out and overdrive amplifiers. In most cases you can't run a full 100 watts into an amp. unless you have a properly aligned ALC connected.

[Steve - K4HX]

Not true. Most amps (maybe all) with two 3-500Zs in grounded-grid will take 100 watts of drive without ALC and will not be over driven. Using ALC is likely to cause more distortion, not less.

[wa3dsp]

True, I was thinking of multiple 572/single 3-500 designs of 1200 watts or less.

[Steve - K4HX]

For sure. Also, I have to wonder how clean many 100 watt PEP transceivers are when being pushed right up to 100 watts to get max power out of the following linear. I have a 200 watt transceiver and I drive a pair of 3-500s. On SSB I turn back the power until the ALC meter doesn't move or just barely moves on the biggest peaks. I can still get full peak power out of the amp and this set up is clean (or as clean as it can get).

[wa3dsp]

On SSB I run my SB200 without ALC and about 60 watts drive. That gives me about 600 watts output. On AM I set the carrier level to 150 watts maximum which is about 15 watts input. This is a modified SB200 with a different filament choke so YMMV.

On a rare occasion I have forgotten to reduce the power from the 100 watt level and although meter wise (unless you are looking at grid current) it looks normal it causes a bizarre amount of splatter.

[WD5JKO]

   This is an interesting thread for sure.

One issue raised in the article linked is the choice to lower the RF drive, or to use an attenuator. He clearly shows with the Flex-1500 that using a 20DB attenuator would have been a far better choice then simply cutting back on the RF level in software.

Cutting back the RF drive will leave certain "crud" such that the desired signal is poking 20 db less above the "crud" then before. This is similar to those using one of those Italian made SS amplifiers that have a 12W PEP drive requirement, and then driving them with a 100 W PEP rig. Perhaps in this case, backing off the rig to 50 watts  PEP, and then using a power rated 6 DB pad would be a better choice. The Transceiver IMD should be better at 1/2 power then at rated power.

Some in this thread have tossed some modern gear under the bus. Sure there are issues with all gear, old, and new. Any VFO that runs the oscillator in class C will have some phase noise...as do many DDS VFO's. The old tube stuff then uses frequency multiplication which will spread out the trash as the frequency is multiplied. The vintage (stock) plate modulators then if driven to achieve a high average modulation will distribute trash up and down the band as the modulation exceeds 100% negative several times a second as the OM talks.

My point is with any vintage equipment, or circuit topology, we can have clean rigs, and dirty ones. A clean rig operated improperly is also a dirty rig. Those that depend on ALC usually have a dirty rig since there will be momentary overloads during the ALC attack time.

I have been a major player in the 20M AM Monday Night Gatherings on 14330. This gathering is temporally in limbo until the days get longer. We see several categories of AM transmitters on 20M AM:

1>) SDR Stuff

2>) Converted Broadcast Stuff

3>) Vintage AM Ham rigs, mostly Johnson, but a few others.

4>) Rice Boxes driving an amplifer

In category 4 we often experience overload from the amplifier either being over driven, or not loaded to the proper PEP power. A guy running a 30L1 amplifier cannot do 300 watts carrier on AM...but they try!

In category three We have had VFO's that drift 5KHZ or more in 60 seconds, others that jump 20 Khz and back, over modulation splatter, etc.

It is the SDR guys that remain on frequency, and clean.

The one or two BC transmitters are also doing well.

I run a Gonset G-76 (tweaked), and I am for the most part dead on frequency and not tearing up the band. It took a lot of effort on my part to get there. Still, if I don't tune it right, it will be trash too.

There were nights on 20M AM that sounded like 75M when I was a kid. This is when the group was dominated by Johnsons. It reminded me of the 50 year old recording made on 75M AM when Otis, K5SWK opened up "the Box" on the air. In the following 58 minute recording, fast forward to 1:45 minutes:

https://www.youtube.com/watch?v=Hzq72P6EX8k

My point is, when you have a "Big Johnson", you just cannot put it anywhere.  

Jim
Wd5JKO

[Steve - K4HX]

Low IMD would not be new for commercially made amateur radio equipment .

I didn't know that Palstar made transceivers. Just came across this thing today. They claim 3rd order IMD of -48 dB PEP or -42 dBc.

http://www.palstar.com/en/tr30/


The ADAT transceiver has been around for about 4 years. The rig uses adaptive predistortion. They claim < - 45dBc 3rd order IMD.

http://www.adat.ch/p4e_specification.html

I also have an HPSDR rig and even tried the pre-distortion software about one year ago. This was pre-beta and I found it difficult to implement and was also concerned about QSY ability using it.  But I understand the lads have improved it since. I'm looking forward to trying it again.

So this software will permit a sample from ANY linear amplifier, correct? IE, It's not limited to just the PennyLane stuff - but someone running a homebrew 8877 linear should be able to sample some RF from the big amp and feed it back to the HPSDR low level, yes?


I still use my FT-1000D as a main exciter for the class A / AB1 linear system, so would have to finally switch over to full HPSDR use.  I can just imagine using my -55dB 3rd IMD system and then adding in the predistortion software. If like most things, starting with a clean system can only get better.

I agree that a -30dB 3rd order amplifier system is the bare minimum and even this is not easy to achieve with modern transceivers and amplifers. Many of the bigger solid state amps are more like -25dB 3rd. Running a marginal rig like this on 6M with its low noise floor - or at sunset on 75M can tear up the band.  Predistorton or any similar improvements will open up a new whirl, something like going from arc gap to CW. (Well not quite that dramatic, but well worth it. It's about time.  :-))

[W1ITT]

The HPSDR  "Hermes" board is specified at -50dbc for third order IMD, at 400 mw output.  Amplification is up to the user.  The HPSDR  "Pennylane" amplifier will maintain this level at up to about 5 watts output with no special techniques.  It will be interesting to see how far predistortion will improve run of the mill tube amplifiers. 

http://tapr.org/kits_hermes.html

TAPR did a production run, and now Apache Labs, run by Abhi who was principal designer on Hermes, manufactures them in India, along with the Anan rigs which are similar.
The price, both from TAPR and Apache is $900, considerably less than Adat, for better IMD performance.  I wonder where we'll be five years from now. 

[Steve - K4HX]

The ADAT is pricy. I wonder how many have been sold?

One would hope that this is the start of a trend.

[K1JJ]

Yep, if someone wanted to start all over from stratch with their gear, the Hermes and PennyLane at 5 watts, driving a solid state amplifer chain WITH the predistortion would be the way to go.  I would take that over a Flex 5000 any day. Add on a big tube amp like a pair of 8877s and that would be the whole shack, 160-10M.

Not all that efficient for AM use, but by biasing the big amp into class C for AM (see previous thread about this) it would be a good compromise.  When will PDM linears become available?  A couple of friends have threatened to build one, but nothing so far.

I look around at my homebrew shack and feel like the guy who had a wall of kludge IBM 1960's computers when the first PC came out in the 70's... :-)

Steve, I meant the stand-alone solid state amplifiers available out there that seem to have poor IMD. Something like the MFJ (Ameritron) 600 watt SS amp ? is rated at about -25dB 3rd.   But even that could be band-aided with the new pre-distortion software.

Norm said that they may make the software work with the old HPSDR transmitter like I have, so maybe I'll wait on that.  I don't remember the name of that TX - is it "Achmed the dead terrorist" or something like that?  :-)

T

[AB2EZ]

Tom

I wonder if predistortion (in this context) and class C biasing of the RF amplifier would be a viable combination

Stu

Yep, if someone wanted to start all over from stratch with their gear, the Hermes and PennyLane at 5 watts, driving a solid state amplifer chain WITH the predistortion would be the way to go.  I would take that over a Flex 5000 any day. Add on a big tube amp like a pair of 8877s and that would be the whole shack, 160-10M.

Not all that efficient for AM use, but by biasing the big amp into class C for AM (see previous thread about this) it would be a good

T

[Steve - K4HX]

The ADAT must be running something other than Class AB as they claim 70% efficiency.

EER could be used with a Class D/E final and PDM for very high efficiency. There are some commercial systems/amps that use this method (although I'm not aware of any for HF).