Mr. Clean - The new 4CX-350FJ driver/ IPA linear

[Steve - K4HX]

This is the per tone method of IMD measurement. All other things being equal, it will yield numbers 6 dB less (worse) than the PEP method.

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.

Below is a simplified spectrum (showing only 3rd-order products) of what you are likely to see on your SDR. The relative measurement shown is what is done for the per tone type measurement.

[KL7OF]

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...    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. This 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

Very cool Tom...Thanks for the explanation....You are getting results that you can compare and use to tell if things are better or worse....and you have been able to put some values/ (numbers)on them...and that is what I am looking to do..
   My first attempt didn't work out so good for me......Frank helped me some and encouraged me when I was having trouble as did a few others here on the Forum....When I get back to the shack, I'll be trying again ....It is all fun and interesting to me ...

[Steve - K4HX]

Below is an excerpt from "QST Product Reviews: A Look Behind the Scenes", QST, October 1994, pp 35-38. The article gives details on how the various performance measurements found in the product reviews are performed. Below is the section on IMD.

Two-Tone IMD

  Just like your receiver, your transmitter produces IMD distortion products. To measure IMD we use an audio generator. The generator produces two tones, one at 700 Hz and the other at 1900 Hz. We put the transmitter in the desired SSB mode and modulate it with the two-tone generator. The output is measured by the special PEP-reading watt-meter and observed on the spectrum analyzer.

  Figure 5 shows a typical IMD plot. Each vertical division is 10 dB; each horizontal division is 2 kHz. You can see the two desired signals in the center of the display. All other pips spreading out to the left and right are the odd-order distortion products. In the example shown, the worst-case third-order product is 35 dB below PEP, the worst-case fifth-order product is 39 dB down and the worst-case seventh-order product is down 45 dB.



  We test the transmitter in both upper and lower sideband; QST Product Reviews always shows the worst case. Poor IMD performance can result in unnecessary interference to others on the band and illegal out-of-band products. The greater this number, the better. Typical modern transmitters should have IMD performance better than 25 dB below PEP for the worst case low-order products. High-order products must be much less.


As you can see in Figure 5, the two test tones are 6 dB below the top of the spectrum display, or -6 dB since the top is zero. Yet the article says the worst case third-order product is 35 dB down (and it is 35 dB down from the top of the display). If you compare the level of the worst-case third-order product (the first pip to the right of the right hand test tone in Figure 5), to either of the two test tones (per-tone based measurement), you can see the difference is much less than 35 dB. It is 29 dB. This is because the PEP of the two tones together is 6 dB greater than either one alone. Since the ARRL references their IMD measurements to PEP, there will be a 6 dB difference between their numbers and any measurements done on a per tone basis.

[W3RSW]

Very illustrative Steve.
The closest ARRL comes to mentioning the center line or "phantom algebraically added PEP" between the two 6db down tones on the spectrum analyzer is:

The output is measured by the special PEP-reading watt-meter and observed on the spectrum analyzer.

Yes, too bad they didn't mention that "observed" was from the top of the grid at 0db.

[ke7trp]

Class AB mode graph. Not QST.

[KL7OF]

The output is measured by a "special  PEP wattmeter 

[ke7trp]

I am told Special means a NIST calibrated meter Steve. 

Tom,  How about his idea of testing the amp:

http://www.w0qe.com/Technical_Topics/imd_testing_of_amplifiers.html

I wonder what you use to test in your lab.

Attached is the Yaesu vs K3.  Another real world test thats interesting to compare to other published figures.

C

[K1JJ]

I am told Special means a NIST calibrated meter Steve.  

Tom,  How about his idea of testing the amp:

http://www.w0qe.com/Technical_Topics/imd_testing_of_amplifiers.html

I wonder what you use to test in your lab.

Attached is the Yaesu vs K3.  Another real world test thats interesting to compare to other published figures.

C

Hi Clark,

The method he is suggesting is the best way to generate a two-tone signal. We can use two RF signal generators or two CW carriers from two riceboxes and combine them using a combiner of some sort.  This will give a signal worthy of any test.

The Sherwood example is using white noise. It's difficult to compare this numerically to two-tone tests, but it does show that the MKV class A is cleaner than the K3.  I sometimes use my pulse pecker signal to get a rough idea of how an amplider bandwidth is doing too. It becomes more of a relative comparison thing, but works when making improvements within one test setup that doesn't change.

In my shack I use a standard two-tone audio signal generated from a Elecraft two-tone board.(The 2T)   When using my 3mW signal source and 1 watt ZHL-3A MiniCircuits lab amp, the HPSDR spec analyzer shows the source signal to be at least -60dB 3rd order or better. So this signal source is plenty good for my purposes, since I do not expect my amplifier chain to be any better than -45 to -50dB 3rd.   We want the signal source to be at least -10 dB cleaner than the amplifer we are testing, so this fits the bill.

If I were to do testing on high end receiver preamps, etc., I would need to look into dual signal generators, a combiner and better, isolated  sampling, which raises the price.

T

[ke7trp]

THanks for the information Tom.  The combiner looks easy enough to build.  The issue is that if one rice box does not get keyed up, then you could damage the receiver.  So you need to make sure you have mode, level and single PTT all working. 

I will look into the elecraft generator.

C

[K1JJ]

THanks for the information Tom.  The combiner looks easy enough to build.  The issue is that if one rice box does not get keyed up, then you could damage the receiver.  So you need to make sure you have mode, level and single PTT all working.  

I will look into the elecraft generator.

C

Damage to the unkeyed ricebox - that's a good point and I never thought of that.  And even if they are both keying properly, if one keys up 100 mS before the other, there could be a problem - damaged RX front end.  

The best way might be to get into each ricebox and disconnect the receiver at the internal T/R relay.  I did this on my FT-1000D to generate a second ant to my HPSDR receiver, but that is another story.

Remember, when using the two tone audio gen board, you still need ONE transmitter that is very clean to drive the amplifier.  Using just a two-tone with a standard ricebox may put you back into the -31dB category, which is unacceptable for testing.

You do not need the Elecraft 2-T gen when using two riceboxes and a combiner. They themselves create the two tone.

This is why they use two riceboxes to combine into one clean, high level signal.

In my case, I use clean drivers to generate the signal using the 2-T board.  The 2-T generator drives the 3mW into the ZHL-3A and into the 4CX-350FJ. This gives a 50 watt signal with about -50dB 3rd. This is barely clean enuff to test a big amplifier.

T

[ke7trp]

Yes. I understood the two rice boxes would generate the tones. I was thinking of the two tone gen for receiver testing. I am going to modify one of my Yaesu rigs here with a different roof filter and I would like to do the testing before and after and adjust/tweak the filter transformers for optimum performance.

I agree its dangerous to use two rice boxes like that. 
C

[Steve - K4HX]

Not worst case.

Class AB mode graph. Not QST.

You took that image from this link.

http://www.webalice.it/romano.cartoceti/yaesu_ft-2000.htm#PICTURES GALLERY


You failed to look at the image right next to it - the same rig tested on 20 meters. The IMD is almost 10 dB WORSE! This is why QST reports worst case, as stated in the article except I previously posted.



More from that page.

- g)   Tx is good.  I performed Two Tones IMD Test with a 50 Hz IF selective  and calibrated Marconi Spectrum Analyser and they are around  28 to 30 dBc down each of the two tones  at 100 w pep out (measured as above in to 50 OHm dummy load,  Mil-std-1131 Version A - test method 2204B; if you compare that number to the one referred to the PEP, the "commercial" EIA soft test method you add 6 dB to - 28 or 30 dB, taken as absolute value, so it is equal to - 34 or - 36 dB),  at a lower 50 w pep out the 5th, 7th and 9th order is even lower the figures measured at the higher level reference of 100 W PEP as expected. If quiescent current would have not set properly  this test usually reveals poorer numbers.

[ke7trp]

Nobody knows what you are talking about..   All you do is follow me around the net and post things nobody can decipher.

C

[Steve - K4HX]

You are delusional. Name one other site I've ever posted where you were involved.

Those who are technically competent understand. Come back when you have something intelligent to post.

[WD8BIL]

I have no problem following what Steve has presented. He's spot on. I do this almost everyday at 2.4 and 5 GHz.