Dec 30, 2009

Upgrade the current VHF station IV; MDS measurement EXPLANATION

General remarks
All rigs have been tested at 14 and 28.4Mhz and other adjacent frequencies to rule out bandfilter anomalies.
Pre-amps,RF gain, AGC and other possible settings which would affect the results have been optimised every time. During each measurement a verification was done with the reference K2 to rule out potential errors as much as possible. Many measurements, especially the hearing tests, were done by two persons.
The found ENB (Effective Noise Bandwidth, column H) allows for comparing apples to apples when talking filter bandwidth.
The filter curves were taken from the audio output using Spectrogram. The Elecraft Wide Band Noise Generator module was used to create a constant signal of about S2-S4.

Performance evaluation

Both column N and R are key in the evaluation of the measured results and hearing results on 144MHz.
Note:the measurement in column Q is totally different from the rest, where both the RF generator and transverter are using in- and outdoor-antennas:

As such there is a true radio spectrum in between as media. A real life performance test so to speak. Column Q represents the set RF output of the generator at which the MDS recognition threshold by human ear was found.
As mentioned before; I'm testing 28Mhz weak signal performance on a quiet band using a transverter, that is totally different from lowband and/or contest use

What really matters is about making that extra QSO yes/no.

Kenwood TS570SG

Frankly this rig was only tested during the preliminary test 'Method 1'; it is currently in use as an NCDXF HF beacon tracking receiver. The 570SG has the Inrad 400Hz CW filter and a temperature stabiliser for the LO crystal. Its internal DSP is an early 16 bit version; OK at 200Hz and above, but heavy ringing below.

28Mhz sensitivity is, next to the icom 7400, tops. However, for serious weak signal reception connected to a VHF transverter, this rig (and another 570DG) showed its weaknesses; the sound is rough, noisy and it just does not bring out the real weak ones. Is this due to its published high phase noise? Can't tell cause I have nothing to prove that. The

ICOM 746
This rig also has a 400Hz CW filter from Inrad. It's owner installed Individual Inrad filters for both IF stages, but previous experiences showed there is too much attenuation caused by bad internal gain distribution. Only the 400Hz in the 2nd IF was used as well as the original 2400Hz in the 3rd 455KHz IF stage. The additional AF CW peaking filters do work very well, but so does the free extra AF background noise. On 144 MHz its own internal VHF front end was used. In total it lacked some 5 dB in performance. When owning such a rig I would use an external preamp and bypass the internal.

ICOM 7400
The '746 Pro version' showed the best sensitivity on 28Mhz. Using its internal VHF front end, it does hold its own listening to carriers down in the noise (test method 1). But when trying to copy real CW signals the DSP offers a very rough sound and is ringing already from 200Hz and lower. I tried getting a better S/N result by adjusting its internal IF gain setting (input to the DSP unit), but apart from the gain difference in AF volume it did not offer any better result in the measurements, nor during the hearing results.

DSP filter curves at 300Hz and 200Hz settings
Please notice there 'brickstone' sharp shapes.
I have had another 7400 several years ago and used it intensively on 144MHz SSB and CW. The sound was very harsh and annoying over time. Then came the first K2 and a DEM 144/28 transverter; what a sonic relief!
Next, from the AB4OJ's Icom website I tried the following trick: choose a filter setting >500Hz and then reduce the bandwidth by offsetting the twin PBT controls. The final result is a 50Hz filter with a very bad shape factor, totally failing in selectivity but the sound is next to noisy much more mellow.

In practise this 50Hz 'low quality filter' offered the best results for the IC7400. Only feasible on a very quiet band. Maybe this will help me on my 50MHz set-up next season.

Microtelecom Perseus
This is not your typical rig where you turn the VFO knob and quickly use some dials & knobs to make a QSO. It is more a tool to monitor single frequencies or a whole frequency band and make that visual. But it does this very well. This high potential receiver has many capabilities (visual and audio) and has phase noise at such a low level(-140dBc @ 2KHz, -150dBc@10KHz), only seen at scientific equipment.
It offered the lowest measured result at 28MHz sensitivity. Due to its internal latency, a true S/N measurement using the Marconi analyser was not possible.
Interesting fact is that despite lacking 6dB at the bench test(column P), it does perform well during a real life hearing test, using an outdoor antenna (column Q). Might this be related to its extremely low phase noise?

Perseus at 416Hz and 202Hz settings

From this 141Hz setting and lower the ringing becomes worse; I mean ok for monitor carriers but incapable of decoding weak signal CW transmissions.

Elecraft K3 serial:173
We're in the upper class of this test now and the left differences are only minor. The K3 is third best at the bench test in column N. The final column R shows no result as the K3 was not available at that time. An 8-pole 400Hz roofing filter was installed. Interestingly, using the 250Hz roofer and tightening the DSP filter down from 250 to 50Hz the K3 showed a reverse result (worse S/N ratio) on the Marconi. Several actions have been tried: internal IF gain setting, AGC and finally switching from FIR to IIR at 100Hz and 50HZ. The final one was the only setting to show another single dB improvement, but the ringing is awful and makes a weak signal QSO impossible.
The K3 starts with slight ringing from 150Hz and down. From there it becomes worse whatever the settings.

K3 400Hz and 200Hz FIR filters

K3 100Hz FIR and 100Hz IIR, please note that the IIR filter actually is wider.
This was also noted at the chapter FIR versus IIR filters at Clifton Laboratories.

50Hz FIR and 50Hz IIR; being a better 'peaking filter' the latter one did show minor improved measurement results.
Officially, any result difference within 3 dB should be considered "measurement deviations". But the differences are clearly there every day and remain when doing the test at a different QTH. When switching from the Marconi analyser to the hearing test, these last few dB's are still very apparent. Even more the perceived sound quality. This is where the K3 differs from the last 3 below; it sounds to harsh for weak signal. I have put out some questions on mailing lists and contacted different K3 owners; no special setting(s) for weak signal listening on VHF and up were found. Both the original K3 owner and another 6M enthusiast reported the same perceived 'harshness' compared to what they are used to at analogue rigs.
I'm not satisfied yet and convinced that we were not able, to let the K3 show its full merits here. Based on its published specs and current results at major HF contest stations,
I trust the K3 will be a killer during 6m/VHF contests. Elecraft is continuously offering substantial firmware upgrades and the K3 shows increasing potential. I still fancy one..

Elecraft K2 serial:3323 original
This is a B version. It is a K2 in original shape and aligned according the written procedure. The internal crystal filters have been set-up for 1000, 700, 400 and 200Hz bandwidth.
It also has the additional KAF2 audio peaking filter (270Hz and 100Hz). This reduces the out of band noise:

The second picture shows the wideband performance curve using both the 200Hz crystal filter as well as the AF2 100Hz audio filter switched in. Unfortunately this AF2 100Hz filter introduces slight audible ringing.

The measured reduced sensitivity at 28MHz compared to 14Mhz is a normal behaviour.
The K2 does have a relatively low phase noise but at 7Mhz. But like most contenders, not particular that good at higher frequencies like 28Mhz. Nevertheless its performance is superb; hey we're in analogue heaven now! The noise-free AF section of the K2 makes the volume control even act like an additional RF gain.
The sound is sweet and offers plenty of details in the quiet background.

I have often used this rig for performance comparisons during the last 4 years and it has always been a close draw with the finalist below. The ORION had been modified by exchanging the 1000Hz crystal (roofing)filter for an Inrad 4-pole 600Hz. This special filter #762 was initiated on request of Bill Tippet, W4ZV
as the original 500 and 300Hz filter upgrades created a worse IMD spec due to a distributed gain issue. This was solved in the later ORION II model.
During the sensitivity measurements it became very apparent that any RF gain setting above 92 was creating extra noise and upsetting the S/N measurement. I believe this is the reason why so many new users confirm so called noisy receiver behaviour. Setting the RF gain to an appropriate level is outside the general consumer perception. The manual does explain proper use of the RF gain, but for weak signal you are used to "dialing it up to the max". Thus I got results which are several dB's better than published elsewhere. But the verdict is in the hearing tests and the perceived audio performance. The ORION does very well in this area, being the best DSP rig in the test.

The above 200Hz and 100Hz DSP filter settings are 227Hz and 155Hz in reality.
Both filter curves represent a more analogue shape.
Not the slightest ringing noted. Even the 100Hz DSP filter sounds as clean as possible. Perhaps this is due to different chosen DSP filter algorithms by Ten-Tec designers. It does offer the most analogue sound and shows great receiving performance; signals could still be detected deep down in its calm noise floor. What a relief for your ears.
Do not forget the PLL design offers an extremely low close-in phase noise, not matched by any other but the Perseus.
So far the ORION has offered me the best overall results in 22 years of Hamradio.

Elecraft K2 serial:2036 modified
In terms of options it is similar to the other K2. Several small upgrades have been done for boosting general performance. However the following modifications have been done over the years for optimisation on 28Mhz transverter use:

1. Pre-amp and AGC
The original preamp is a compromise in terms of system noise figure vs IMD performance. Changing its gain boosts its total system noise figure. More preamp gain involves a lower intercept point however.
See the Elecraft application note

2. 28MHz Band Filter
This is now peaked at 28.2 MHz

3. AF filter

The AF2 filter is changed from 100 to 140Hz. This small change reduces ringing completely.
The audio gain was slightly optimised by changing R9 to 12K.
Spectran was used to do the adjustment on the KAF2 board, which now peaked a few dB higher.

400Hz crystal +270 Hz audio filter. And 200Hz crystal +140Hz audio filter.

This modified K2 has proven its superiority during these tests. It also functioned as the reference rig during the tests. Switching back from any set-up to this little analogue wonder always immediately showed its advantage.

The verdict
Best reception: modified K2
Best overall performer: ORION
Best alternative 'second receiver': Perseus (visual performance)
Great future potential: K3

Analogue beats digital, but only just.
Razor sharp DSP filters offer bad audio for VHF weak signal CW message detection.
K2 and ORION offer sonic details like you are diving into the abyss of the RF noise floor.
What is the true 'supporting performance' of low phase noise?

Mission completed?
I wanted to measure the noise figure of my K2 and, in the process decided to do some more testing..
Got a bit carried away.... But then again, I never got to measure known VHF performers like the TS850, FT1000MP etc.. And then there's that superb Javornik transverter. And Down East Microwave is (re-)designing a new transverter.
Oh well... lot's of promises for the future ;)

Wish you all a Happy New Year !!!

73 Mark, PA5MW


Anto IK4PMB said...

Hello Mark, you did a good job.
I don't know if K3 is the best rig for 144MHz IF available right now, even if I'm very hapy with mine.
Probably the best combination for very weak signal is not the best for contest use with band crowed of strong signal.
During last MMC my K3 did a very good job, but I'm not sure on weak signals it's the best, I've heard about old valves RIG that are less noisy.
Another thing is X-Verter features, I'm sure you read about DF9IC measures, I agree with him about " Nevertheless they are still far off what could be realized".

Anonymous said...

Hi Anto,

Thanks, and yes there's still room left for improvements at VHF set-ups. TX wise; see what measurement results DF9IC and SM5BSZ show to on their websites. But RX-wise there has been so little developments over the last 10 years. How come?
73 Mark

Anonymous said...

Hi Mark,
I read your notes, and they are quite interesting.
If your objective is to compare two (or more) receivers, the best test for your own scenario is to A/B switch between them.
Standard tests of receiver sensitivity or noise figure are somewhat artificial as the receiver is not exposed to the out of band interfering signals that would be the case if it were attached to an antenna.
In my experience, it is quite possible that a receiver that seems a few dB better on a SSG could be worse on an antenna.


PS: I dislike the term MDS (meaning signal level for 0dB S/N) popularised by the ARRL, I prefer the term Noise Floor as it doesn't imply that lesser signals cannot be detected.

Mark, PA5MW said...

Hi Owen,

Yes , I fully agee with you.
Although column Q represents a live on-air situation (using an outdoor antenna),the best comparison indeed is A/B on air. And the positive difference includes being able to decode the CW/SSB mesage from the other end. Not just notice the signal is 'just there'. For every possible upgrade to my station I ask myself this: "can I make that extra QSO yes/no?".
Doing a lot of A/B comparison on the air (50Mhz 144Mhz) this summer season, I noticed that clean analogue receivers outperform DSP rigs on weak signals. Both on a quiet band as well as on a band full of noise and signals. But it needs more to drop an opinion here.

Anonymous said...

Another measurement fact that can cloud the results is the inband audio IMD. It would be good seeing how the different radios compare with different AGC settings. The best sounding radios always have the lowest inband audio RX IMD. Most reports of harshness on various radios is a result of bad RX inband IMD. I always find that the best weak signal CW radio is one which has the lowest RX IMD. Besides who can argue against less IMD, hiss and distortion?

Mark, PA5MW said...

Fully agree with above comment.
As I have access to an R&S Audio Analyser (UPL) I would like to know how to proper measure this AF in-band IMD. Do you have a suggestion?

Marijan Miletic said...

"Analogue beats digital, but only just." People learn by time! Perseus is 1st generation DDC SDR while analog has reached it's peak with low noise VFO after a century of development. Mobile phones are sexy :-)

73 de Mario, S56A, N1YU