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R-390A / SoftRock SDR AM Audio Test Results - WOW!!!




 
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Author Topic: R-390A / SoftRock SDR AM Audio Test Results - WOW!!!  (Read 14623 times)
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W1VD
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« on: November 19, 2008, 11:16:25 AM »

Finally got around to measuring the R-390A / SoftRock receiver setup I've been using for a while. A description of the system: R-390A i-f output > 15 dB attenuator > 455 kHz SoftRock > Delta 44 sound card > P4/2800 MHz/1G Ram > Delta 44 sound card > Hafler DH-110 preamplifier > Hafler P3000 amplifier.

The R-390A is run in the 16 kHz BW position and operated in manual gain control (MGC) mode with the rf  gain control set to 8 - 1/2. This level was previously determined to be 'optimum' for this particular receiver. Info on this is in a previous thread:

http://amfone.net/Amforum/index.php?topic=13686.80

Power SDR software was used for the test and set for 16 kHz BW, slow AGC and both standard AM and synchronous AM detectors were tested. On rare instances there was at most 1 dB difference between the two detectors - the vast majority of the time the measurements were identical.

Results are as follows:

AM audio S/N: 48 dB


AM Audio Frequency Response: in dB, referenced to 1 kHz measurement   

100 Hz  200 Hz  400 Hz  600 Hz  800 Hz  1 kHz  2 kHz  3 kHz  4 kHz  5 kHz  6 kHz
0   0   0   0   0   0   0   0   0   0   0


AM Audio Distortion:

Mod  100 Hz  200 Hz  400 Hz  600 Hz   800 Hz  1 kHz   2 kHz
30%  .63%  .63%  .63%  .63%  .63%  .63%  .63%
50%  .50%  .50%  .50%  .50%  .50%  .50%  .56%
70%  .63%  .63%  .63%  .63%  .63%  .63%  .71%
90%  .90%  .90%  .90%  .90%  .90%  .90%  .90%
100%  1.3%  1.4%  1.6%  1.6%  1.6%  1.6%  1.6%

Note: It's possible that the distortion numbers are even better as we're likely at the limit of the test equipment in use.

The test procedure and test results for conventional BA receivers, for comparison,  can be found here:

http://www.w1vd.com/BAreceivertest.html

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AB2EZ
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« Reply #1 on: November 19, 2008, 01:23:51 PM »

Jay

Again... what you are making available to the community is of great value and a real service to all of us. Thank you!

I would like to suggest one experiment... if you happen to find it convenient and quick to do...

Make a subset of the same measurements with only the I or Q output of your mixer connected to the sound card (feeding either the left channel or the right channel... but not both). [If you have a bandpass filter at the input to your mixer, you might want to bypass that, as well... but making that change would require a lot more work]

I have demonstrated that for an i.f. application, like this, one does not require anything more than a plain vanilla mixer. As a result, you can (if you wish) use any readily available mixer... possibly including inexpensive mixers that have better performance (e.g. more dynamic range) than the digitally-controlled analog switches used in the Softrock.

Stu
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« Reply #2 on: November 19, 2008, 03:21:42 PM »

Stu,
One thought a soft rock kit might be cheaper than a good mixer and oscillator.  I ran single input and it works fine as long as you get used to the close in images. A couple of my Racals have a 6.4 KHz. baseband output that I shot into a sound card. fc
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Mike/W8BAC
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« Reply #3 on: November 19, 2008, 05:00:27 PM »

Nice work Jay. I have been watching you and MOP with your SDR boat anchors and the light just came on for me. I have a nice old Racal and a spare computer with sound card I could use. My questions are a bit off topic but if you have time please fill me in. Who knows, maybe others are asking the same questions.

I assume you are looking at 455KHz (or whatever if frequency your receiver has) with the softRock/Power SDR and you tune the Boat anchor normally. That is you choose your band and frequency. I also assume you choose your mode and bandwidth with Power SDR. Do I have it right so far?

I'm just getting the picture here but I'm wondering why you used the R-390 filter for your testing? Doesn't the softRock/Power SDR system have filters? Seems to me the Collins filters ring a bit.

I'm not a critic. I really want to learn something here to see if I can use this on my RA-17 Thanks

Mike
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Steve - WB3HUZ
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« Reply #4 on: November 19, 2008, 05:06:46 PM »

The R-390 filter has to be used since the IF out feeds the SoftRock. If one were to get into the radio and tap the IF ahead of the filter, it could be avoided. Yes the mechanical filters do ring some, but from Jay's measurements they sure don't affect the distortion.
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W1VD
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« Reply #5 on: November 19, 2008, 07:32:06 PM »

Stu

I should be able to run the I or Q test over the next day or two. There's no additional bandpass filter other than the parallel/series tuned input circuit on the SoftRock board, which I'd prefer to leave intact. Agree that I and Q are not required if there isn't an i-f 'image' that needs to be removed. The SoftRock was already built and in use so it was used for this test.

Mike

Yes...the receiver is tuned as normal. The SoftRock + computer + external sound replaces the receiver i-f and audio stages. Yes...455 kHz is the i-f of the R-390A  but this technique could be applied equally well to most any i-f with a suitable change in the SoftRock frequency coverage.

One could tap off before the i-f filter, but in the case of the R-390A there is probably little value in doing so. With a 16 kHz filter available there won't be a need (at least with the modes I use on HF) for anything wider. Except for the rare occasion where the band is empty I'm normally receiving AM in a somewhat narrower bandwidth.  The distortion results show that there is little, if any, penalty for using the 16 kHz mechanical filter as Steve noted - so why not? Other receivers, with a more restrictive i-f filter, may be another matter, though.   
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« Reply #6 on: November 19, 2008, 07:36:02 PM »

A nice setup indeed!


* Nice Set Up.jpg (6.92 KB, 320x240 - viewed 1150 times.)
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« Reply #7 on: November 19, 2008, 08:51:34 PM »

Jay, presume you're measuring THD and not IMD, correct?

I've seen fairly horrendous IMD but pretty good THD in amps.... I gotta dig out my old Heathkit IMD meter one a these days.
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W1VD
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« Reply #8 on: November 20, 2008, 09:18:21 AM »

Stu

Ran some tests last night with just I (or was it Q) as the input to the sound card. S/N was identical and a spot check of about 1/3 of the audio frequency response and audio distortion data points showed no change...as expected.

John

Yes, those numbers are THD.

I started looking into possible AM detector IMD measurements a couple weeks back but got sidetracked with other projects.

Just thinking here...and after only one cup of coffee...

Wonder if it would be possible to two-tone AM modulate the HP8640B...characterize its two-tone IMD performance using a spectrum analyzer to verify a low level of internally generated IMD...and apply that to the receiver under test.

RF wise...since the average receiver has a noise floor of -139 dBm and a two-tone IMD DR of 85 dB the rf signal level for the two tones is about -54 dBm to generate IMD products just above the noise floor. Our AM audio tests so far have used an rf signal level of 1000 uV (-50 dBm) so IMD caused in the  rf stages by this 'AM audio two-tone test' should be down near the noise floor (except perhaps for the poorest of receivers tested).

If the rf stages don't contribute to the IMD then IMD we see should be from the detector and audio stages. Audio stages run at low output (as we've been doing for the previous tests) should have very low IMD levels so we're probably left with the detector as the main contributor.

May have time tonight to run a test or two. Thoughts and comments appreciated.

Now for another cup of coffee...   

     
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« Reply #9 on: November 20, 2008, 09:40:11 AM »



For testing IM and THD withing the bandlimit of modern sound cards (48khz or better), you can't beat a decent soundcard for doing these tests. You'll be able to easily get down to 0.001% and better with proper care and feeding and the use of a newer high sample rate and bit depth card, fyi.

                 _-_-bear
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Steve - WB3HUZ
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« Reply #10 on: November 20, 2008, 09:48:53 AM »

Quote
Audio stages run at low output (as we've been doing for the previous tests) should have very low IMD levels so we're probably left with the detector as the main contributor.

Or the AGC.
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