Spurs ?

General discussion and support for the Peaberry SDR V2.

Spurs ?

Postby W4MMP » Fri Mar 13, 2015 4:53 pm

Hi,
Has anyone been reading a particular thread on the Softrock40 forum called "RXTX Leakage and Transmit Spurs" ? It appears the issue is with over driving the transmit output mixer (U8, in the case of the Peaberry). I monitor my transmitted signal with a Ham It UP clone (Ranverter). As hard as I try, I can not see the issue. This is on 15M with a 30/20/17/15 rig and this is with an audio signal that is as high as HDSDR will drive the Peaberry. Anyone that monitors their Peaberry output signal please take look at the thread and then take a look at your output.
(Cut and paste from one of the posts):
Modulator
-We have a modulator that is producing a significant modulated spur that reliably occurs in both level and frequency at 3 times the LO offset and on the opposite side of the LO.
-The level of this spur is unaffected by IQ balance.
-The level of this spur is disproportionately affected by input IQ levels
-The level of the spur is affected by the LO frequency range. It tends to be reduced in level as the LO frequency increases.
-The spur tracks the LO offset at 3X Offset and only disappears when the radio is adjusted for zero offset. There is a large chance that, in a zero offset mode, the spur results in unacceptable levels of IMD3 but that has not been tested because zero IF is not an objective of this exercise.

Please let us know what you find.

P.S. I would post on the Softrock40 forum, but I don't really feel like dealing with the crap I will receive because the Peaberry is a "simply a Softrock RXTX clone" and certain folks don't like that idea.
73
Ron / W4MMP
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Re: Spurs ?

Postby AE9RB » Sat Mar 14, 2015 7:04 am

It's just one guy on the SoftRock40 forum who has a problem with Peaberry. The same guy who brought up this issue. The same guy who ported your firmware into hardware for the SoftRock. Rather than acknowledge the synergy of these projects, he prefers to post libel at every opportunity. Sorry about that. Both Tony and myself have tried to put a stop to it but trolls will be trolls. Like you, I got tired of being attacked and stopped posting there.

The core issue is whether or not LO mixing from the transmitter is considered IMD or spurious emissions under FCC part 97. The Peaberry and SoftRock RXTX both use the same 2N2222+BS170 amplifier. So, for the most part, whatever comes of this will be the same for both devices.

A SoftRock RXTX is difficult to definitively test since the DAC is a variable (depends on sound card). I assume this is why you've never seen a review in QST. However, the ARRL did test a Peaberry V2 and published the results in November 2013 QST. The editors contacted me beforehand and were quite clear that a review couldn't be published unless it met both FCC and ARRL requirements. I concede to wisdom of the ARRL, who has lawyers that regularly work with the FCC on rules and regulations.

From the ARRL tests of a 30/20//17/15 Peaberry V2:
Spurious-signal and harmonic suppression: 47 dB worst case (30 meters); 58 dB typical. Meets FCC requirements.
Third-order intermodulation distortion (IMD): 2 W PEP, 3rd/5th/7th/9th order, –28/–42/–51/–56 dB (20 meters).
73 David AE9RB
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