Dec 24, 2009

Upgrade the current VHF station III; MDS measurement

This follows the previous blogs on optimizing my Elecraft XV144 144/28MHz transverter + 28MHz IF transceiver

Purpose of MDS measurement

Find factual differences in 28MHz sensitivity of HF transceivers, using measuring equipment as well as ones own ears. Create repeatable and, as much as possible, reliable results.

Final decision criterion: Can I make that extra QSO Y/N ?

Prerequisites
Since my goal is to evaluate my local possible transverter+HF combo's, the actual measurements must reflect real life situations. There is plenty of good data available from ARRL, Sherwood, G3SJX etc.. But I need factual data on 28MHz. Every rig will be set-up for transverter use. If that is via other antenna entries, using extra relay routing, different internal circuitry etc... so be it. Every rig will be fine tuned for best performance on weak signal detection; being able to copy and read CW transmissions. That rules out filter settings which create ringing.

Getting a feel for measuring MDS

I have limited experience in doing this, I follow a different protocol and my equipment calibrations are outdated (2003). Below data is not scientific proof whatsoever. Nevertheless I have done my best to do make sure to deliver repeatable results. I tried a few different methods of determining MDS, just to get a feel and, more important, see how it relates to reality.

Test set-up



Headphones: David Clark Model 10/DC Stereo, with passive noise cancelling.
Picture shows a temporally test setup at a friends place.
The 'wires in the garden' is a K9AY low band receiving antenna.

Determining the MDS threshold reference level; method 1
This was determined by human ear recognition only:
Output RF generator: continuous carrier signal fixed set at -100dBm on 28.4 MHz and 144.4 MHz. It is capable of reducing its output down to -140dBm, but to rule out internal deviations and external possible load differences I used the step attenuator. A second identical step attenuator from a different brand was used from time to time for comparison and verification.
Generator connected via step attenuator to the 'Device Under Test', or to the Elecraft XV144 transverter, which in turn was connected to the DUT.
Output receiver: connected to headphones
Using the 10dB and 1dB switches of the step attenuator, a minimum threshold was found at which the RF signal can just be recognized and while turning the VFO the varying beat note can still be heard. This method delivers a more accurate and detectable threshold vs just a faint phantom signal xx dB buried in the noise.



The penultimate column shows the single receiver performance at 28.4MHz, the final column refers to the transverter combo result. Found 28MHz MDS values showed an average delta of 8dB compared to official published data which is 'promising'.


Determining the MDS threshold reference level; method 2
Taking it one step further I started doing the S/N measurement function on the Marconi communication analyser.


The second drawing shows the set-up with the 144/28 transverter 'combo'.
In both set-ups the audio is routed back to the analyser which determines the S/N figure.
At first I have done all tests using a 20dB S/N level as measuring reference, but since that is not resembling a true weak signal I cut it back to 10dB S/N. At that level all signals are already very near the noise level.
This test method quickly showed reliable results. I have repeated all measurements a minimum of 5 times at different days. To be sure I got the same repeatable results,
each time the modded K2 was used as measuring reference. I spent 3 weeks doing this almost every evening. There was one occasion where all test results shifted 1dB on a single day only. But apart from that everything stayed rock solid. I got enthusiastic and started dragging in other rigs as well.




So what do we have here?
Column D: extras which do not come standard with the product
Column E: selected crystal filter in the 1st IF being the standard or an optional filter
Column F: set DSP filter. IIR and PBT refer to receiver custom settings
Column G: set audio 'peaking' filter, only applicable for K2 and the IC746
Column H: measured Effective Noise Bandwidth using info from Owen at: http://vk1od.net/measurement/enb/MeasureIfBw.htm
Column I: calculated using the metric converter at http://vk1od.net/calc/RxSensitivityCalc.htm
Column J: published MDS results from ARRL at 14 MHz (for comparison only)
Column K: measured S/N using the Marconi generator/analyser at the 10dB S/N level reference
Column L: same
Column M: same but now using the 144/28 transverter combo
Column N: calculated delta on results found in column M
Column O: same as K but using human ear for MDS recognition
Column P: same as O but using the 144/28 transverter combo
Column Q: same as column P but now the RF generator is connected to a small antenna loop and the transverter is connected to the outdoors 144 MHz yagi. The results shows the delta to the found "best in class" receiver.
Column R: remarks will be discussed in detail in the next blog.

"I'm not happy with your results....."
What does this all mean? Why is X doing this and Y doing that? My Z-rig at home does better than your measured yours etc.......
Please note the whole measurement concentrates on reading the signal at 28MHz under weak signal circumstances.
That is a zillion light years away from performance during a contest, let alone on low bands.
The hearing measurements were done with two persons and done as serious as possible. It even turned out my friend was able to dig another dB or two in the noise but he created an extremely small brain filter which was already pre-synced on the signal :). Fun but not the agreed threshold level where we could both detect and read the CW transmission.

So ????????
Benchmark results are nice for comparison and I'm pleased with the found noise figure results.
This will allow me to do careful calculations using the mentioned VK3UM application, see my earlier blog.

I appreciate the MDS test results 'by human ear' the most.
In the end the QSO is made using that exact instrument, so by upgrading my whole 144Mhz set-up I want to use that 'measurement tool' as much as possible. I'm very reluctant it turned out to be a reliable tool.
The S/N test function on the Marconi generator produces a modulated signal which sounds like separated dots (e-e-e-e-e) at some 15WPM. That is the close enough to reality and certainly much better than any stable continuous carrier.

During the tests there were large differences in sound quality perceived. DSP filters sometimes sounded harsh and extremely small filters introduced ringing. Ringing does not need to be a problem if there is plenty of signal, or you need to filter out the adjacent station during contest etc. However at the weak signal level on a quiet band ringing is not making that extra QSO.

Conclusion
The modded K2 sounded best and proved the winner for decoding weak signals, followed closely by the Orion and K3. The Perseus not only shows a great picture but offers good readability too.

What's next?
In the next blog I will comment on all tested receivers individually and explain in detail how they were set up and modified for best results. Their measured filter responses will be shown as well.


2 comments:

DaMorseMan said...

HI Mark,

Interesting results ... !

BTW what is the mod on the K2 ?

HNY,

Enno, PF5X

Mark, PA5MW said...

Hello Enno,

I will show the mods in the next blog this week.
HNY to U2,

Mark, PA5MW

p.s. congrats with your 88 countries wkd on Magic Band in 2009 !!