Strong images on SDRPlay

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wd4elg-2
Posts: 7
Joined: Thu Dec 24, 2015 12:15 am

Strong images on SDRPlay

Postby wd4elg-2 » Sun Dec 27, 2015 1:53 am

I am seeing strong images from off-frequency signals.

On all frequencies, I am seeing a strong carrier in the center of the display. This happens on both HDSDR and SDR Console. It is an AM broadcast of some sort, and I don't live near any AM stations.

On the 160 meter band, I see images of AM stations across the band.

On the 20 meter band, I observed an image from a station in Colombia, HK3C, who was transmitting on 14270, upper sideband. A mirror of the signal, on 14030, in lower sideband was mirrored on the opposite side of the center frequency of 14150.

I am trying different antennas, RF chokes, feedline chokes. Nothing seems to make a difference.

Looking for some suggestions to improve the situation. Thanks.

Mark Lunday WD4ELG
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CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: Strong images on SDRPlay

Postby CaptainNemo » Sun Dec 27, 2015 10:23 am

wd4elg-2 wrote:On all frequencies, I am seeing a strong carrier in the center of the display. ...

I'm afraid the line in the center is due to the sdr architecture. Some software have countermeasure to "suppress" it.
Anyway -- once you know it is always there in the center -- is not an big issue, just ignore it...

Other images are really an issue.
I'm an sdrplay user from more the 1 year, mainly in hf bands, and -- despite many sw releases (api, plugin etc.) in this timeframe -- images are always a problem.
I'm sure -- sooner or later -- sdrplay team will improve this area. (P.S. I dont know if Sdrplay v2.3 plugin for sdr# (coming soon, I hope) will help in this area.)

Also a good pre-selector will help.

Bye
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Harry
Posts: 35
Joined: Mon Nov 02, 2015 9:47 pm

Re: Strong images on SDRPlay

Postby Harry » Sun Dec 27, 2015 1:55 pm

I've experienced the same when I use the SDRplay as a panadapter to my HF rigs. Here is what I discovered:

(a) The strong images are often from AM broadcast stations (domestic and foreign)..I seldom have an issue with images from other amateur radio stations.

(b) The strong images are 99% present when I use my 80-10m OCF dipole and not when the HF beam is in operation.

(c) The center spike is the DC offset, I believe.

(d) My SDR-IQ and Afedri SDR-Net receivers suffered less from the above than the SDRplay.

With all that said there are some interim "fixes":

(a) The latest API release does an excellent job of reducing and/or eliminating the center spike when used with HDSDR. Simon has yet to incorporate the API into SDR-Radio Console. However, I've noticed that the size of the spike will reduce over time. In any event, it's not a big deal.

(b) Using the latest API and HDSDR I found the images can be reduced or eliminated by experimenting with the LNA (off and on) and the Gain Reduction Settings. For example, on 40m I have the LNA off and the Gain Reduction set to about 40. On other bands it will vary, but not by much. If I use SDR-Radio Console I find that a Gain Reduction of -60 usually works on all bands.

(c) While there are certainly SDR receivers out there with better specs and front-ends, the SDRplay offers considerable value for the money. This is especially true for my panadapter application where I can easily see signals that are S2 or S3. Yes, a QS1R or CloudIQ would be great, but I think it's overkill for how I use an SDR.

BTW, I had been using an RF Limiter from DXengineering (DXE-RG-5000) to protect my SDRs and rig from overload when transmitting. While not necessary, I employed it for extra protection. However, I discovered that this device created considerable receive overload while adding intermod products when I switched to the dipole for 40m, 20m, 17m and 15m. At first I thought it was an issue with the various SDRs. However, when removing the RG-5000 from the "circuit" the overload and intermod products disappeared. Initially, I suspected the device was defective. Yet a search led me to other hams and SWLs having the same experience when using this device. I believe this is an issue with various RF Limiters. I no longer use the DXE-RG-5000.

Harry
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DaveB
Posts: 129
Joined: Sun Feb 01, 2015 6:04 pm

Re: Strong images on SDRPlay

Postby DaveB » Sun Dec 27, 2015 5:48 pm

On the 20 meter band, I observed an image from a station in Colombia, HK3C, who was transmitting on 14270, upper sideband. A mirror of the signal, on 14030, in lower sideband was mirrored on the opposite side of the center frequency of 14150.


That is a result of and imbalance between the I and Q signals - the amplitude differences between the two channels needs to be as near to zero as possible - certainly les than 0.1 dB. The phase difference has to be 90 degrees +/- 0.01 for the image to be sufficiently reduced. The Mirics chips doesn't meet those specs - you'd need to pay a lot more or use a different architecture for the SDR.

SDRPlay released a new API on the 22nd Dec which has a software solution to bring the I/Q channels into balance. It certainly works - it might not be quite as good as the algorithm that was used in SDR# - but I can only hear a trace of the mirror image on a strong FM broadcast whereas previously it would be virtually noise free mono.

The API does work on SDR-console - it was the first thing I checked - but you have to copy it across from the Mirics folder to the SDR console folder replacing the old one.

As for images from other bands. Assuming it not mixer overload (turning off the LNA can help) then it could be that strong signals are being picked up due to under-sampling. Digital signal processing isn't as simple as it seems and provided a signal can make it through from the front then the SDR will happily sample and deliver a signal at x2, x3, x4 the sampling frequency. The only way round that is to have excellent filtering - and for £118 - that isn't going to happen.

You don't get it for £450 either as I've read an article about the benefits of adding a low pass filter (cut off below the MW band) to the Elad FM2 to eliminate MW breakthrough on LF reception. Elad sell an add-on preselector - for 169 euros with just one filter. That's similar to the price of the SDRPlay! Then 22.50 euros for additional boards. So fully populated for all ham bands its over $300. (BTW it is specific for control from the Elad).
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Harry
Posts: 35
Joined: Mon Nov 02, 2015 9:47 pm

Re: Strong images on SDRPlay

Postby Harry » Wed Dec 30, 2015 4:42 pm

DaveB wrote:

The API does work on SDR-console - it was the first thing I checked - but you have to copy it across from the Mirics folder to the SDR console folder replacing the old one.



Dave,

I've done that, but have not seen any changes.

Harry
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