[Lowfer] WSPR-15 tonite

[pbunn]

Garry,

Thanks for the information. Maybe I should use the K3 as the LF receiver. My comments were based on the fact that the R75 is seeing S4 max at the input when Bob is transmitting and when Bob was not transmitting, the other two signals did not indicate at all on the R75 S meter, so it seems that the R75 should not be overloading.

I have one more screen shot in which I decoded both signals with Bob not transmitting until the last 2-3 minutes. When he came on, there is a definite "notch" in the two WSPR 15 signals for his 2 minute duration.

I have a HP Selective Level Meter that I have not used much as well as the K3. Anyone using the SLM as a receiver and had good IMD performance?

Again, thanks Garry for the information and assistance.

Pat
N4LTA

-----Original Message-----
From: lowfer-bounces at mailman.qth.net [mailto:lowfer-bounces at mailman.qth.net] On Behalf Of Garry Hess
Sent: Wednesday, February 06, 2013 10:43 AM
To: lowfer list
Subject: Re: [Lowfer] WSPR-15 tonite

Pat,

The fact that you have a "strong" converter ahead of your R75 does not protect you from IM degradation. The reciprocal (coherent) system input 3rd order intercept (IIM3) equals the reciprocal of the first stage IIM3, plus the gain of the first stage divided by the second stage IIM3, plus ... The additional stages only degrade the overall IIM3 and if any stage isn't much better than the first stage when its IM3 is reflected back to the input the degradation can be substantial.

I believe you use a Clifton Labs e-probe like I do and it has a very good spec: output IM3=40 dBm and input IM3=46.5 dBm (I see conflicting numbers for the "gain" but Jack's testing method uses -6.5 dB). The rule of thumb for mixers is IIM3 equals the LO power + 10 dB. That means only about 27 dBm in your case and that's not nearly as good as the e-probe. 
Following up with a good Norton amplifier may be 50 dBm at its output, but at the input it's only about 40 dBm and going back through the lossy mixer and e-probe again gives an IIP3 less than the e-probe. The real killer may be the R-75. I can't find a clear IM3 number for it, but based on Eric's citation of 67 dB 3rd order dynamic range it may only be on the order of 0 dBm at its input and that's way worse than 40 dBm when reflected back to the e-probe input.

Having said the above I look at your capture and see 3rd order products in the waterfall that arean't much different than what I see with an SDR-IQ as the receiver. I'm not sure what the SDR-IQ IM3 number is either but I think it's in the area of 20 dBm. Much better than the R-75 but still lousy compared to the e-probe.

Your attachment shows WE2XGR/2 being decoded at about +2 dB SNR. The signal at 475.823 kHz is WG2XJM and while stronger, it doesn't decode. I think I have seen decode failures like that too on occasion, even with no really big signal either outside or inside the detection band like WE2XGR/6. But one must be careful to let the WSPR code "warm" up with a partial decode window before it really starts working. That can take nearly 30 minutes with WSPRX-15. I have also received an occasional false decode. More often when there are two strong in-band signals if either the upper or lower 3rd order product falls in band I get an unwanted decode for it.

To resolve your decode failure issue I'd suggest you save the .wav files and send them to Joe Taylor. I'm sure he'd be interested in exploring what's going wrong.
--
73, Garry, K3SIW, EN52ta, Elgin, IL
______________________________________________________________
Lowfer mailing list
Home: http://mailman.qth.net/mailman/listinfo/lowfer
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Lowfer at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html