RSP1A: wobbling spurs around 2.335MHz * n

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DK7OB
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Joined: Thu Mar 24, 2016 7:14 am

RSP1A: wobbling spurs around 2.335MHz * n

Postby DK7OB » Fri Dec 01, 2017 7:38 pm

I am a lucky RSP1 user but now wanted to try the RSP1A.

I swapped the RSP1 with the RSP1A in my installation. The RSP1A seemed to work fine.

But I detected a wobbly (unstable) spur in the 7MHz amateur band which is not present with the RSP1. Tuning through the HF band, I found the fundamental frequency at about 2.335 MHz. I can see the harmonics in the HF band, unfortunately some of them fall into amateur bands.

These spurs seem to be generated by the RSP1A, since I do not see them if I swap back to the RSP1 without changing anything else.

Questions:
- Is this a known issue?
- Is there a solution? (Moving the frequency out of the amateur band is acceptable for me)

Since I am using the RSP1(A) as receiver in my amateur radio station, this would be a show stopper.

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vk7jj
Posts: 72
Joined: Thu Mar 02, 2017 9:56 am

Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby vk7jj » Sat Dec 02, 2017 1:58 am

DK7OB wrote: I detected a wobbly (unstable) spur in the 7MHz amateur band which is not present with the RSP1. Tuning through the HF band, I found the fundamental frequency at about 2.335 MHz.


Thanks for that; yes.

RSP1A - from a cold start the fundamental here is at 2.27MHz and drifts down to settle close to 2.25, looks like other smaller spurs at 2.356 and 2.438.

RSP2 - none of the above, clean.

Not in the 40m band etc for me but "ouch" is the word.

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g1hbe
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Location: Cheshire

Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby g1hbe » Sat Dec 02, 2017 10:49 am

I think the tech team have said that these are products of the small switch-mode PSUs within the RSP1a itself. I don't think there's any cure, but they show up more at low gain settings and almost disappear into the noise as you turn the gain up.

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Andy

vk7jj
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Joined: Thu Mar 02, 2017 9:56 am

Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby vk7jj » Sun Dec 03, 2017 6:23 am

g1hbe wrote: they show up more at low gain settings and almost disappear into the noise as you turn the gain up.


That's not the case here, sadly; cooling the RSP1A down a little forced it's 3rd harmonic up into the 40m band.

You can see it wandering down past 7.120MHz as the RSP warms up.

It's quite intrusive, well above the general noise floor and in the pic below it's in the process of obliterating an otherwise armchair copy signal.

It's not acceptable where it or it's harmonics fall across something of interest.

Regards,

Phil

Image

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g1hbe
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Location: Cheshire

Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby g1hbe » Sun Dec 03, 2017 9:37 am

Oh dear, that's not good! A bit of an oversight by the usually thorough SDRPlay team? I'll stick with my RSP2.

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Andy

DK7OB
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Joined: Thu Mar 24, 2016 7:14 am

Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby DK7OB » Sun Dec 03, 2017 1:48 pm

I opened my new RSP1A and treated components with cold spray. I identified the circled components as the probable culprits for the 3 spurs (frequency of spur jumps up when component cooled) with number 1 being the worst (about 10dB stronger than the others and fundamental > 30dB above noise floor) and that's the one which is in the 40m band on my RSP1A. These components are marked SKY 3416 and are most probably of this type: SKY13416-485LF

With my RSP1a, the 40m spur settles at 7010 kHz with my room temperature, exactly where the DX sits :(

RSP1A.jpg
RSP1A.jpg (48.48 KiB) Viewed 2546 times


<edit>
I looked at the control signals of these ICs. Number 1 control signals change at the high frequency bands above 60MHz, number 2 at the low frequency bands below 60MHz (see the RSP1A technical description for the band limits).
</edit>

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vk7jj
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Joined: Thu Mar 02, 2017 9:56 am

Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby vk7jj » Sun Dec 03, 2017 10:24 pm

DK7OB wrote:These components are marked SKY 3416


Ironic, being an antenna switch, can't see what function three of them perform given their useful spec is well above HF. Well sleuthed, but one suspects that's the end of that, it's hard to see a user-based solution.

Otherwise I prefer the RSP1A to the RSP2 on the HF ham bands; for VLF it's hard to feed the RSP2's balanced input via anything other than balanced line, despite my strident support for balanced feeders, ferrites just don't go down there. The RSP1A band pass filtering is significantly better and the notch filtering is better thought out (ie. the lack of MW notch on HF with RSP2) which taken together result in many fewer imaging and MW broadcast overload problems.

What a pity.

Phil.

/edit
Here is a direct comparison between 1A and 2 near the spurious frequency. Made no difference whether the antenna inputs were terminated or not. Didn't check anywhere else as HF is my main use for SDR tech.

Image

Image

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DK7OB
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Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby DK7OB » Mon Dec 04, 2017 6:41 am

The switches have a shut down mode, but unfortunately the number 1 chip is always in the signal path, also in the HF band (I traced a 1 MHz signal), so that does not help.

I found the spur less prominent when I set front end gain to maximum and IF gain to minimum, but can't say how this influences large signal behaviour. Usually I try to use the smallest front end gain possible.

I'm not sure what to do. Keep the otherwise good RSP1A or send it back and go to the next higher performance (and price) level for HF?

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g1hbe
Posts: 302
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Location: Cheshire

Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby g1hbe » Mon Dec 04, 2017 9:31 am

Phil - you said ' for VLF it's hard to feed the RSP2's balanced input via anything other than balanced line, despite my strident support for balanced feeders, ferrites just don't go down there.'
They do if you get the right ferrite. I'm a low frequency enthusiast and I feed my Wellbrook 1530 loop to the High-z input via a homemade balun made from a ferrite binocular core of 'mix N30', RS part no 212-0617. This couples really well down to a few KHz (virtually lossless down as far as 20 KHz which is where the Wellbrook packs up).
Ordinary 43 mix ferrites don't work well at LF though.

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Andy

vk7jj
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Re: RSP1A: wobbling spurs around 2.335MHz * n

Postby vk7jj » Tue Dec 05, 2017 1:21 am

g1hbe wrote:They do if you get the right ferrite. I'm a low frequency enthusiast and I feed my Wellbrook 1530 loop to the High-z input via a homemade balun made from a ferrite binocular core of 'mix N30', RS part no 212-0617. This couples really well down to a few KHz (virtually lossless down as far as 20 KHz which is where the Wellbrook packs up).
Ordinary 43 mix ferrites don't work well at LF though.


Thanks for the info, I've never tried that mix, looking at the spec it certainly does go there doesn't it, I wonder how you find the rolloff as you go up in frequency? The spec seems to indicate it starts rolling at around 500KHz, two frequencies I sort of monitor are 0.135.700 and 0.472.500 and 160m.

Having the RSP1A as a second device allowed me to revisit openwebrx last night. Openwebrx's interface with rtl_sdr does not pass any antenna parameters (actually, looking at the source code, rtl_sdr doesn't support antenna switching) so in my previous trials with the RSP2 it defaulted to antenna A. Having the single antenna input on the RSP1A suits it better for HF.

*many thanks* to the tireless coders at SDRPlay for their Soapy support and keeping it updated.

It's running on a Raspberry Pi 3 here at the moment, I've restricted it to a low sampling rate to keep the Pi happy, here it is on 40m:

http://sdrplay.vk7jj.com:50054/

I won't leave my router open for more than today/tonight as I prefer to access my stuff remotely via a backyard VPN and I've restricted it to 4 users. Anyway the timing is bad, our 40m band is absolutely dead at the moment. Hope the Pi stays up, I've got to go out and kill weeds for the rest of the day.

/edit
The openwebrx author uses a browser detector to ensure you use the Chrome browser, but it also works in Brave (great browser) and Safari if you change browser IDs, it also works in Chrome on an iPad.

For anyone who manages to get my link to work (I only just set the DNS record), the web GUI is best used like this:

- if the waterfall isn't visible, click the
btn.png
btn.png (5 KiB) Viewed 2369 times
button once, or fine-tune the waterfall sliders.
- click the mouse near a signal of interest in the waterfall, as per most software
- then hover over the waterfall and use the mouse wheel to zoom in (and out) and drag the yellow marker to the precise frequency
- drag the waterfall sideways to see all the band while zoomed in
- drag the left edge of the yellow marker to change RX audio bandwidth, either edge in AM

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