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Author Topic: Mr. Clean - The new 4CX-350FJ driver/ IPA linear  (Read 33640 times)
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ke7trp
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« Reply #25 on: January 19, 2013, 05:07:08 PM »

Really cool amp and my hats off to you for trying to have a clean transmit signal.  Most simply do not care Tom.  That is a real problem.

I am on 20 meter SSB today searching for some DX and the band is crowded due to a contest.

Its filled with people overdriving amps, audio cranked. and splattering the entire band.  We are forced to build/buy better and better receivers to cope with idiots with cheap amplifiers and bad practices.

w9RE is on 14.224.4 right now taking out nearly 60 KC of band calling CQ contest on a voice keyer. His lookup shows a 781 with an alpha 87A.  Its so bad, I had to stop using the radio.

C
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« Reply #26 on: January 19, 2013, 08:02:57 PM »

Quote
You bery vely funny man, Jeff!

Yes, there are striking similarities, I must admit.  Unfortunately, National went out of biz about the time they adopted that slick color scheme...  


More like 25 years Tom!

Here is mine plus I also have 3 NCL-2000's on line. Note where the Mode switch is Grin



* NCX-1000.jpg (87.3 KB, 750x563 - viewed 488 times.)
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« Reply #27 on: January 19, 2013, 08:42:20 PM »

Rick,

You made some valid comments. Yes, the infrastructure to support the high level stages is huge if scaled up.

As far as QRO amplifiers go, I think -35db 3rd would be quite acceptable on the bands IF that was an average number. But I think it's probably in the high 20's these days. It's a bell curve with some very clean signals and some not so clean. The wild card is the human operating factor that can easily take a once clean amp and run it overdriven, etc.

There are times when we need a clean signal and times when it's not so important. In the daytime on 75M when no one's around, we can run about anything and no one will mind. But when the band opens up and DX starts rolling in, locals can be 40db over and the DX PW at  S5.  This is the time when it's easy to take out adjacent channels and get complaints.   The same holds true when QSOs get close, like the normal 3kc spacing for ssb or 5kc spacing for AM.  I hear it all the time... guys complaining about the next QSO up the band.  I'm just tired of worrying about it.

My goal is to get on and not be concerned about how close I am to the next QSO. If I am far enuff away to run a certain bandwidth of audio, then FB. I don't want to have to add in extra margin due to my own splatter, assuming it is excessive. Anything less than -30db 3rd, to me, will create problems when running QRO.

 I was talking with Frank again today and he's closing in on -40dB 3rd on his 300 watt SS amp. That's running it at 40 watts out, but it's getting better.

Carl:  When did National fold?  I figgered it was after the NCX-5, NCL-2000 and 500 stuff, but guess they contined on.

Clark:  OK on 20M.  I've noticed 20M has always been the roughest band aof all. I think it's the national competition, the ease of getting up a Yagi and especially the chance for RF to get into the audio more than the lower bands.  I think many of the severe splatter problems are related to RF in the audio, followed by being overdriven.

Well, doing some fine tuning mods and touches to the FJ amp today. I'm finding that C2 needs to be very big for best IMD. Q is important in the tank, but there appears to be a limit where higher Q degrades it.   The input Q appears to have little effect, though I am adding another cap to better match 50 ohms and allow the Q higher.

At this point I can run a pecker pulse thru the FJ amp chain and tune a receiver located about 200' away. The skirts of the signal appear very sharp with crud disappearing quickly off the sides.  In contrast, my FT-1000D at 200 watts cud be heard 5-6 kc up the band doing the same test.  The extra -15 dB makes quite a difference.

The downside is having an extra set of knobs to turn when QSYing...  Wink

T
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« Reply #28 on: January 20, 2013, 11:06:52 AM »

T.,
 I've been thinking about how to best right-size clean, two stage tetrode amps. 
Your 4x350 uses 300v screen at a measly 9ma and 1400v plate at say, 80ma. Those are minor compared to a second stage using the same screen voltage and, say, twice the plate voltage and much more current.

So a choke input HV supply with the choke in neg. rail could supply the final amp tube's HV (off the Positive post choke and filter cap. point) and the driver amp off a pick off point before the neg. rail choke.   Since your running considerable constant quiescent current at the no signal point, current drawn would almost eliminate any sort of high wattage bleeder.  Alternatively, the plate xfor could simply be full wave tapped at two or even three points for plate and screen voltages.
 
The screen supplies would both use the same transformer or same taps on HV xfor. with the driver pulling off a simple resistive divider if even needed for slightly lower voltage, say 400 down to 300.  Decent solid state control could be implemented for the driver screen's small current also if needed. 

Hi Q input and interstage bandswitched networks, not to mention a Pi-L output network (really big) would be somewhat bulky, but not a lot more so that all the relay switched networks needed for SS amp filtering and antenna tuning.

Bias supplies:, first stage -14v could be from a SS regulated supply taken from a small 24 volt @ 2 amp xfor. used for that and 4x350's fil. 24 volt relays, other stuff.  Second stage a common 120 vac. pri./sec. transformer.

All this should easily fit in a 32V-2 sized cabinet or a little smaller. After all, your shooting for 1000 to 1500 clean watts instead of 2500 "dirtys."   ...Aren't you?  Grin
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« Reply #29 on: January 20, 2013, 12:29:01 PM »

Rick,

What you've described is pretty much what I have now.  The screen definately needs electronic regulation to +- 100 mV. No more than 1/2 volt is possible. Must be a SHUNT regulator, not a pass.  A precision elec reg is what is needed to gets us into the -45dB 3rd area. Even a zener will drop 3-4 volts with only 8 mA draw.   The grid can be a simple zener shunt reg since there is no current drawn.  

I use a 47K in series with the grid for protection and isolation and stability swamping, since the bottom end is bypassed to ground.

The 4CX-350FJ plate idles at 80 ma but peaks are about 160ma.

The screen protection is important and covers other mistakes too.

I use the exact Eimac specs at 1400V as shown in that last link I gave you.  25 watts out is about 800 watts from the 8877, so it's still in the very clean area.  It will drive the 8877 easily to  2KW, but then the IMD specs of the 8877 are the bottleneck, as you know.  An 8877 will do -45dB 3rd when run below 600 watts.

BTW, from testing I found that the 8877, when running in the low power clean mode under 600 watts out, requires LOWER  plate loading for best IMD. IE, use MORE C2. This is opposite of what we usually do. For higher power closer to saturation, then running the tube more heavily loaded is cleaner. (less C2)  There is a cross over point probably when the stage starts drawing grid current at 700 watts or so.  These results are from experimental IMD tests with the FJ driving the 8877.    I was amazed yestersday how clean I could tune the whole system and how EASY it was to mess it up with bad IMD by improper tuning.

As you know, an IMD testing setup is mandatory when setting up these high performance systems.

Hope you build something up!

T
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« Reply #30 on: January 20, 2013, 12:35:38 PM »

BTW, we probably think this IMD stuff only applies to linears, but there's no reason why a standard plate modulated rig, class E rig, DX-100 or ANY AM transmitter cannot be tested for IMD. After all, we are mixing audio with a carrier and if not perfect will generate artifacts in the sidebands. Any AM rig could probably be tuned cleaner with adjustments in voltage, current, loading, ratios of modulation, running less power, different biasing in the final or modulators, etc.

I plan to test my 24 pill KW class E rig in the next day or so and will post the results. I've gotten good reports with it from the SDR guys, so shud be a reasonable example.

Someone correct me if I'm wrong, but on AM, I wud think a single tone for audio and viewed on the SDR spec analyzer is all that's needed.  But how do we compare it to ssb to get a similar standard? Is the main, single, audio  peak compared to the next peak up the band (3rd order) and subtracted in dB , just as on ssb?

T
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« Reply #31 on: January 20, 2013, 01:58:02 PM »

I'd start with just the carrier and look at (compare) the harmonics uo to say, the seventh.
Then inject single tone at slightly less than 100 p.c. Modulation.   Look at that.  Then go ahead and use your two tone gen.  It just represents constant amplitude speech so should be useful for spectrum analysis.  In each injection case you're comparing apples to apples, so decibel addition should apply.  I'd write more, but this darn iPad is not typing friendly.
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« Reply #32 on: January 20, 2013, 11:59:24 PM »

I'd start with just the carrier and look at (compare) the harmonics uo to say, the seventh.
Then inject single tone at slightly less than 100 p.c. Modulation.   Look at that.  Then go ahead and use your two tone gen.  It just represents constant amplitude speech so should be useful for spectrum analysis.  In each injection case you're comparing apples to apples, so decibel addition should apply.  I'd write more, but this darn iPad is not typing friendly.


Ya know, I've never checked the harmonics on any of my rigs.  With the HPSDR, I can easily dial up there and take a look.  Maybe I've been whirlwide on all bands all the time... :-)

I'll get a feel for AM IMD when I fire up the 1000D tmw and go from there on the class E rig IMD tests.

Today finished up a QRP SWR bridge KIT that is good for SWR down to 100mW and also reads power from 100mW to 8 watts.  Nice digital readout. I plan to use it with the 500 mW output to quickly PRETUNE the various ampifier input networks as I change bands.  

For only $50 it's a nice deal. Went together in about 2.5 hours and worked well.
http://www.4sqrp.com/QRPometer.php

I added grid current monitoring to all my GG amps today. For some reason I didn't fully understand how the shunt resistor in the negative lead worked. Everyone said to just stick a 25 ohm resistor there and hook up the grid meter. It didn't make any sense to me. Well, finally figured out that the shunt really is a shunt resistor and must be closely calculated and matched with the grid meter. I ended up with a 1 ohm shunt and a 1 mA meter movement for a 100ma full scale.. No wonder my grid current readings were always screwy.  


Here is an excellent paper about plate and grid meter monitoring for GG amps. Everything needed to do the whole job and do it safely:

http://www.w8ji.com/metering_amplifier.htm#Shunts_and_Multipliers_calculating_or_selecting_metering_resistance_values

T
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« Reply #33 on: January 22, 2013, 12:24:39 AM »

Update of testing:

I have come across an interesting combination of parameters that have brought the IMD performance of the 4CX-350FJ driver/ IPA up a notch.

By adding at least two times the C2 loading capacitance above the normal tank calculation for a given Q, I am able to get an even cleaner IMD number.  This also requires some more C1 and less L which increases Q, but the C2 seems a bit out of proportion.   This is the opposite of what we normally do to a GG amplifier that runs at full power. Both the tetrode and triode amps are showing this IMD improvement.

The downside is that the power output is reduced before flat topping occurs. Normally, by adding a lot of C2 (lightly loaded) and running at full power would create a nasty signal.  But we are looking for a super clean, low power signal from a driver stage. By running X2 C2 and pulling less power uncovers another higher level sweet spot.  Under these conditions, I can pull only about 50 watts from the 4CX-350FJ before flat topping occurs, but I estimate the IMD is approaching -50 dB 3rd.  This is the best I've seen so far.   The 8877 actually needs less than 25 watts drive for my purposes, so FB.

The same holds true for the 8877 when running about 500-750 watts out. It can tolerate much more C2, flat tops earlier, but gives better IMD at this lower power level. Never wuda thunk it.  

I am finding it best to tune each amplifier in the chain independently of one another. I make sure it is matched 50 ohms in and 50 ohms out, tune for the best IMD and then run them together.  Most of the time the amplifers need no further adjustments once cascaded in a chain for operation. Simpler and more standardized this way.

It appears we can get away with some things when demanding low power and running higher Q vs: running near sturation.


Current tank values:

C1 = 276 pf     L = 7 uh      C2 = 3100 pf.     Running at 25w output gives a plate load of about 7K.  The Q works out to be about 50!   Quite a bit of flywheel effect there.


Two-tone efficiency:   126 watts in /  25 watts out = 20%.   That's about normal for a quasi-class A1 stage.  It draws no grid current and when running 25 watts, the plate current barely moves. 

Take home:  IMD approaching -50 dB 3rd order.

T
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« Reply #34 on: January 23, 2013, 01:00:40 PM »

Officially they folded in 1991 Tom when the IRS had their auction. The NCL-2000 was in limited production till the end and lots of completed and partially completed NCL's were there along with the NCX-1000.

If I wanted -45 IMD Id run a 3-500 at 1500V and be done with it with less complications. A pair in a SB-220 with 160 added, run in the CW position and zero bias, would make a YC-156 very happy.
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« Reply #35 on: January 23, 2013, 09:13:32 PM »

Single band amps rule (or are an absolute necessity in this case).  Grin

Carl: What would you use to drive the 3-500?
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« Reply #36 on: January 24, 2013, 12:07:57 AM »

If I wanted -45 IMD Id run a 3-500 at 1500V and be done with it with less complications. A pair in a SB-220 with 160 added, run in the CW position and zero bias, would make a YC-156 very happy.

Yep, been there.  The 3-500Z's worked FB at 1500V, but as HUZ axed, what to use for a 50 watt driver? The bottleneck became the driver in the system. The FT-1000D is clean until 10 watts and then falls apart to normal ricebox standards.   No ham rigs rigs around will do -50dB third at 50 watts or so.  As I understand it even that FT-5000 DXX rig with the class A SS amp was measured by the ARRL at about -40dB 3rd.   Subtract their 6 dB IMD marketing factor and we are down to -34dB.

The 4CX-350FJ will be the liason disco duck diplomat  between the HPSDR 100 mW or the FT-1000D 3mW driver.   The 1000D driver and and HPSDR are both about -60dB 3rd.   So a clean 50w driver is important to complement the chain.  


Update:

Still playing around with the 8877, drivers, bells and whistles.  I'm trying hard to push the 8877 beyond -45dB 3rd. Presently I'm playing with a Q= 40 to 50 to see what effect it has on IMD.   This requires about 4,000 pf in C2 and only 5 uH for L1.  This is exciting stuff when good results are seen. I'm trying to apply some of the techniques I learned from the FJ project to the 8877.

Built up a universal grid current trip circuit for the GG amps in the shack. Since they feed off the same HV plate supply, I can use the same grid shunt resistor to trigger the same trip circuit and open the PTT circuit. No more drunken stuper mistakes with 200 watts into the grid of the 8877 or forgetting to turn on HV, etc.....  Shocked

I mounted a large grid current meter (8" X 6") to monitor grid current in all the GG amps. My goal is to run all of the GG amps without any or little grid current. It seems IMD starts to fall apart when it occurs. There are ways to set up the stage so that grid current is minimum for a given power output, but the tradeoff is less power output.  Stilll experimenting.


Getting there.


T


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« Reply #37 on: January 24, 2013, 12:36:08 AM »

Single band amps rule (or are an absolute necessity in this case).  Grin

You bring up a good point, Big Country.  Probably the major reason for single band is the tank coil optimization.  I have overcome that by using a roller inductor in both the grid and plate circuits of the FJ amp.  With a link input, the greatest gain can be had. Plus the roller lets me zero in the 50 ohm input.

The plate tank is also sensitive to optimum L, as shown how the IMD has a sweet spot when found.

I don't have a roller in the 8877 tank, though there is one in the cathode input.  If I'm gonna play the IMD game with the 8877, think I'll take a look around for a one KW roller inductor for the 8877.  Thanks for the thought, OM.

T
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« Reply #38 on: January 24, 2013, 09:28:31 AM »

When your running GG there has to be grid current.  Your feeding power through a power amplifier, not a voltage amplifier, and that will be added to the output...   Well, you know what I mean, some current has to be passed to make power.  So adding some driver power at low IMD will cumulatively help the output depending on how much of a fraction it is.  The distortion curves cross at some proportion point.

I admire trying to keep grid current very low but  this looks a very inefficient process with far more than one comparative stage to achieve low IMD.   Check the microwave, HDTV people to see how to amplify with very low IMD.

I'm awaiting the Clifton board (its size meets my needs better than the mini circuits) and will install it inside the qs1r cab with shielding and schrack 24012 relays to use it as both a rcv preamp and post qs1e amp.  This will get me to 1/4 very clean watts w/o any post filtering.
Then the 4cx350 or similar class A, little B,will yield some decent SSB (gasp).  Then  I'll feed my pair of 8875s GG and experiment with basing, load and drive.  The GM3SEK board will allow any degree of variable bias  desired.  I will be happy with real IMD's of 40 or so. My amp also has rotary coil on 80 and 40. Pix before 3SEK mod on QRZ.com.
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« Reply #39 on: January 24, 2013, 01:28:10 PM »

Sounds like a good plan with the Clifton amp and plans, Rick.  You should have everything you need to play around and get some outstanding results.

Let me know (or anyone else) if you have a one KW roller inductor for sale.  I need one for my 8877 amp.

The HDTV people - are you talking about feed forward systems and the like?  After reading about it, I've yet to find one real life working system that can be built by an average guy like me. There's a lot of ivory tower talk on the web. Way too complex and digital for me. What other way did you mean?

This extreme IMD game is like drag racing. The last improvements are more difficult and often seem like, "why bother?"   But that's where the fun is to me - trying to see how far I can push things. In the end, after a lot of work, we sometimes come up with a combination that woks surprisingly well.

As far as grid current... there are definately parameters that cause more or less grid current. They seem related to loading, Q and drive. The trend is the lower the grid current, the lower the IMD. For example, with the 8877, I started out with about 30ma of grid current for 600 watts out. By changing parameters, I was able to get the same output power, 10ma of grid current and better IMD.  But remember that I am axing for only about 1/3 of the 8877's output power.  All of this IMD stuff will certainly mean an extra stage of amplification in the end. It is not meant to be an efficient game at all, at least in the lower level stages.

BTW, I was listening to 75 AM the other night and heard a 30 minute squabble between some southern and northeastern AM stations concerning splatter.   This bandwidth issue is still with us, AM or SSB.  It's  a worthy goal to run our own rigs through IMD tests to be sure they're reasonably clean.  This is something that cannot be seen on an  o'scope alone.   An SDR spec scope or even tuning a good receiver with a 6" antenna stub can tell the story of our own signals.


T
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« Reply #40 on: January 24, 2013, 10:57:25 PM »

Nice work tom.  Keep the updates comming.

SH*t in and SH*t out.   Have you seen the Elecraft and Flex radio IMD numbers?  27 and 22DB?  Garbage. So tired of hearing the Flex's trash the band. Run that into a two or three holer and its buckshot city. The 2000D has a real world 41DB 3rd order test by the way. Not sure if the 5000 has the exact same output section or not.  At any rate. The Class A option is nice for low drive amps.

Keep up the good work
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« Reply #41 on: January 24, 2013, 11:15:41 PM »

That -41 number is in Class A and is the ARRL measurement, so subtract 6 dB to compare to Tom's numbers. In other words, it's -35. Not exceptional but much better than most of the new crap out these days. The FT5000 supposedly beat the 2000 by 2 dB. I wonder how many people run either of these in the Class A setting?

The Flex 5000 was measured by QST at -35 (29 for comparison with Tom's numbers), better than many of the newer radios but not as good at the class A output radios. I've heard some claim at reduced power output the IMD number improves on the Flex 5000.
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« Reply #42 on: January 24, 2013, 11:44:06 PM »

To add to what Steve said, here's a few postings from the HPSDR refector about how IMD is measured, the ARRL way and the Mil spec way. These guys know much more than I do on the subject so I will defer to them:



"Hi All,
ARRL method of measuring IMD covers up the poor quality of rice boxes on the market.
I'm here to get away from that over priced junk. The mil method accounts for PEP by running at
a power level that puts the combined waveform peak at the amplifier power rating. The spectrum analyzer
displays the carriers and IMD with the same demodulation so how is it valid to add 6 dB to the carriers and
not the distortion.  It is all about high performance as in HPSDR.
Frank WA1GFZ"


"When you look at a signal on Hermes and Mercury and see IMD distortion it is X dB down. What you see is the real number.
I don't agree with the commercial method claiming another 6dB for PEP. The distortion should also get another 6dB in my view so you end up with the same number. The Mil Method checks IMD at 6dB below max power (3dB on an average power meter) Then  the IMD is the difference between IMD and carrier peak on the display.
Funky specs are for rice boxes.....Frank WA1GFZ"


"Hi Frank,
 
completely agreed.  What you mention is not only a Mil Method, it’s  a commercial standard in IMD evaluation e.g. for base stations in mobile networks where linearity is even more important than in our ssb application. The 6 dB PEP bonus is not more than an advertising trap.
73, Helmut"      [A Talented German engineer]




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« Reply #43 on: January 24, 2013, 11:53:10 PM »

Here's two pics of the new grid current monitor and trip circuit for the 8877 GG amplifier. It also works for the other amps in the shack since they all use the same HV supply and grid shunt resistor.

Whenever I make a mistake to overdrive an amplifier, run without HV, use the wrong antenna, etc., the grid current will hit a predetermined point and open the PTT.  Works pretty well.

The BIG meters -  I bought them back in the 80's at Deerfield thinking maybe I'd use them someday. They measure  6" X 8".   Used for HV and grid current.  The top scale of each meter is accurate - 0 to 12KV  HV,  and 0 to 120 mA grid current.  [That's in SonoRays, of course]

Also notice the temperature gauge that can be preset and trigger an alarm if the tube exit temp gets too high.  I've done some dumb things in the past like forget to turn on a blower or have a ducting fall out - ruined tube.  It presently says that the exit temp is 63F and the alarm goes off at 110F.

Notice Mr. Clean in the third pic sports a new C2 padder. This is for the higher Q experiments.


Thanks for the comments, Clark.  I think we're all learning some new things here.

T


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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
ke7trp
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« Reply #44 on: January 25, 2013, 01:37:17 PM »

Nice.  I respond, The post is up for few minutes and then is removed.  Had lots of good data in that post too.  I will email you a copy Tom. You would have enjoyed it before I got sensored.

c
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K1JJ
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« Reply #45 on: January 25, 2013, 02:02:38 PM »

Nice.  I respond, The post is up for few minutes and then is removed.  Had lots of good data in that post too.  I will email you a copy Tom. You would have enjoyed it before I got sensored.

c



Clark,

Yep, losing a post can be frustrating...

Sometimes I type up a reply and when sent, the little icon goes round and round and never updates.  This is why I always copy the message just in case. It's easy then to paste it and try again.

Post it again and it will probably go thru OK.


T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
ke7trp
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« Reply #46 on: January 25, 2013, 02:27:36 PM »

It was up on the site and removed.  It listed out private party testing of rigs not ARRL and it was showing Rob sherwoods tests of the Flex and elecraft and I think that got a Flex owner upset to see his rig is dirty and the post was deleted. The 6DB thing came from Rob sherwood. He used that in a power point presentation. You cannot just take 6DB off ARRL listings. It was a blanket statement used to prove a point.  Oh well The power point is on the web for transmitting signals if anyone wants to read it.  Most radios where tested private party in format once his gripe with the ARRL came out. People wanted to know what the real figures are with the Sherwoods suggested meathod.

In short, The Class A radios Mk5, 2kd, 5kd, 9k ect are the cleanest by far and the BS figure listed here is just that, BS.  The real world figures are in the low to mid 40's.  It has merit when using a Low Drive amplifier like a two holer or three holer that needs just 15 to 25 watts drive for full legal limit.

It thought it was pertinent info for this thread but I guess not.. Sorry.
C
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Steve - K4HX
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« Reply #47 on: January 26, 2013, 11:22:10 AM »

Nonsense Clark. I just checked the logs. No post of yours or anyone else was deleted in the past three days. Your Flex conspiracy is silly and completely out of line. It tells me you really don't know what you are talking about relative to IMD measurements.

The 6 dB thing did not come from Rob Sherwood. It comes from specifying IMD on a per tone basis versus a PEP basis. Using PEP gives a 6 dB higher number when compared to the per tone basis. It's simple math. No conspiracies, no bashing and no scarey Flex boogie men involved. It doesn't mean one method is right and one is wrong. It just means you have to pay attention to how the measurements were made when doing comparisons. So, you can't compare measurements made by the ARRL using the PEP approach to measurements made with the per tone approach without applying a 6 dB correction factor to the ARRL measurements.

All this has been covered in professional journals, magazines, T&E app notes. It has nothing to do with amateur radio per se. IMD measurements are hardly unique to the ham radio realm. Let me quote from an Analog devices app note:

Quote
Therefore, any distortion specification is relatively meaningless unless the exact test conditions are specified.

In other words, your "real figure" and "real world figures" statements are meaningless.

Then there is error. To measure -41 dB IMD, the sources must have IMD much less than -41. If the sources were 10 db cleaner (-51dB) there could still be as much as a +2.4 dB and a -3.3 dB error. Once again, exact info on how the measurements were done is important.

Further, no one in their right mind is going to claim two-tone IMD measurements (regardless of how they are made) represent real world dynamic conditions. This is why three-tone (and multi-tone) and pulsed tests are done (although apparently not in the amateur radio world).

All the rest of your rant makes even less sense than the Flex conspiracy part. No one ever claimed that the Class A radios weren't the cleanest on the ham market at the moment. Please stop with the nonsense so we can discuss things here like adults.
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KL7OF
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« Reply #48 on: January 26, 2013, 12:26:52 PM »

Tom...Can you post some pictures of the HPSDR display?   I am curious about how you feel on the accuracy of the measurements you are taking from the HPSDR...Is it easy to read?  Does the display compare to a spec analyzer presentation...?..I'm trying to get a softrock Ensemble ll going  and I'm wondering if I will be able to measure my rigs like you are....I have NO experience with SDR to speak of...I tried to get something going last winter and gave up after I couldn't master the software and wound up messing u p the hardware as well....I have a new "tony built" Ensemble ll now and am ready to start over....I'm on travel in AZ right now so I'm away from the shack..This is all very interesting to me... Keep up the good work..It is inspiring...Steve
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K1JJ
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« Reply #49 on: January 26, 2013, 01:12:21 PM »

Tom...Can you post some pictures of the HPSDR display?   I am curious about how you feel on the accuracy of the measurements you are taking from the HPSDR...Is it easy to read?  Does the display compare to a spec analyzer presentation...?..I'm trying to get a softrock Ensemble ll going  and I'm wondering if I will be able to measure my rigs like you are....I have NO experience with SDR to speak of...I tried to get something going last winter and gave up after I couldn't master the software and wound up messing u p the hardware as well....I have a new "tony built" Ensemble ll now and am ready to start over....I'm on travel in AZ right now so I'm away from the shack..This is all very interesting to me... Keep up the good work..It is inspiring...Steve

Hi Steve,

The HPSDR or any SDR system will open up a new whirl for you. I understand that VE7KHZ just got his HPSDR system working in software the last few days. The HPSDR guys are there to help and won't let you fail.

Well, first let me say that I don't do this for a living like some of the engineer guys here. I was trained as a technician and don't have a BSEE so I am a TECHNICIAN... Grin   I axe a lot of questions of my mentors when I get stuck  - K1KW, WA1GFZ and a few others.  I do what I must to get by to complete a new project and no more. So you will not find a lot of theoretical stuff from me - mostly practical stuff I have "discovered" which many already know....:-)

I like to work in relative comparisons, not absolutes. For example, I have trained myself to recognize what a clean signal looks and sounds like on an S-meter and on the HPSDR. I have learned what a -31dB 3rd IMD FT-1000D looks and sounds like in a relative way. When I hear a signal that is -45dB 3rd, it jumps out at me. And a dirty -22dB 3rd signal is obvious too.

Once these relative standards are in place, I can then work on what I have and make incremental improvements by comparison.  Notice I try not to become absolute. Absolute precision requires an excellent test setup using a solid sampler, pads, and repeatability. The HPSDR screen is accurate enough, but the stray crap and pickups, poor sources and different environmental conditions can change readings. I eventually want to get a stable setup, and realize the limitations of what I presently have.

So, if you want to be able to give out absolute numbers compared to the real whirl, then focus on good and accurate accessories to support the HPSDR.  Look at what Clifton Labs uses for his IMD tests. He uses two separate transmitters (or sig gens) sources that are mixed into a combiner. This can give a source -60dB 3rd or better.  In contrast, I use my own trasmitter with a two-tone audio signal that is only as good as the 2-tone and transmitter itself.  Go to his website and see how much $ he has invested in his.

BUT, if you go with a modest setup like I did, you can still make meaningful measurements and improve on your own equipment. Just do it in a relative manner and don't expect to pecker match with the real whirl.

You can also get a good idea of whats going on on the air.  In fact, Frank sent out some 2-tone audio on the air the other day and I was able to tell him (good guess) within about 3dB what his 3rd order IMD was by looking at my HPSDR.  He has a very nice and accurate lab setup there.

I will take some pics later, once I get everything finalized, but it's just a matter of using the curser to take a reading of your peak of the 2-tone and comparing it against the next significant (and lower) peak up the band. (3rd order)     The following lower peak will be your 5th, etc.  The difference of these peaks in dB is a rough idea of your 3rd, 5th, etc IMD figures.

Keep reading, test some of your own stuff and you'll catch on quickly.   And keep posting the results you are finding with your rigs.

Hope this helps.

T

Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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