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Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Wed Jul 05, 2017 3:25 pm
by OH2BUA
I hope that you are also getting a more educated answer - but the truth is, that if you connect a decent antenna into RSP2, during nighttime / good condix in MW band you will also be hearing a lot of signals which actually don't reside in MW band, but are from somewhere higher.

73, Jukka

Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Wed Jul 05, 2017 6:25 pm
by F1BJB
oh1nd wrote:It looks like the IQ mixers are not analog multipliers like in old Motorola MC1496 circuit but instead IQ mixers are working in switched mode. So the LO is like a square wave. The spectrum of that includes odd harmonics and the only way to get rid of spurious conversion products is to filter input signals at 3rd harmonic with a low pass filter.

Hi
May be not the only way. Look here if you have the time:
https://patentimages.storage.googleapis ... 048480.pdf
Difference with MC1496 may be small as in receivers LO tends to drive it into saturation to avoid other problems.

Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Wed Jul 05, 2017 6:43 pm
by F1BJB
OH2BUA wrote:but the truth is, that if you connect a decent antenna into RSP2, during nighttime / good condix in MW band you will also be hearing a lot of signals which actually don't reside in MW band, but are from somewhere higher.

I totally agree with you .
The nice thing is that playing with LO frequency and IF offset you can generally get rid of interference with an SDR.

Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Sun Jul 09, 2017 3:58 pm
by OH2BUA
Ok, here it is, in the middle. 2MHz low-pass filter. Plug-in version. Cute as a small pig, as we tend to say hereby.
2M_filter_verysmall.jpg
2M_filter_verysmall.jpg (196.59 KiB) Viewed 1831 times

73, Jukka

Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Fri Jul 21, 2017 4:26 pm
by Glennfin
g1hbe wrote:Jukka - if you want to use off-the-shelf filters, you will need to transform to 50 ohms on the input side and then back up to 1k to feed the RSP. Not ideal. A better solution would be to download an RF package (something like RFSim99) and use it to design your own filter for a 1000 ohm system. All you do is enter the filter type (Chebyshev or Butterworth) HP or LP, roll-off frequency and impedance and press GO. It'll give you a circuit diagram and will also simulate the response for you. Even I can use it!
I've just thought - your system is balanced, so either you'll need to do a balanced filter or transform to unbalanced on the input to the filter (use a 1:1 balun) and back to balanced on the output (another balun).
RFSim is quite old, but it still works, even on my W8 laptop.

Just one more thought re the spurious responses. If you think about a conventional receiver, you don't know about all the unwanted signals until you happen to tune across one. So you tend to think the receiver is virtually free of such things.
When you can see a wide chunk of spectrum, all the nasty stuff is laid out before you and you think the receiver is a sprog box! Good luck with it.


Why is using a 1k to 50 ohm balun at the rsp input "not ideal" ?... this would allow you use off the shelf 50ohm unbalanced devices like filters, attenuators, pre-selectors, etc... I understand the reason for the wideband front end due to the 10mhz wide spectrum display but couldn't the mfg include some additional filters and make them software switch selectable?

Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Fri Jul 21, 2017 11:38 pm
by g1hbe
Hi Glennfin. It's not ideal because it's just more 'stuff' in the signal path. And assuming it's a ferrite based device, there's always the possibility of increased loss at certain frequencies. Many ferrites used in popular low-cost transformers and baluns have poor performance at frequencies below 500 KHz. This may not be a problem for most Amateurs or SWL's, but it's a shock to LF listeners to see the LF beacons and clock signals disappear into the noise when they add the device.
I've got some surplus binocular ferrites that just won't play below the bottom end of MW. They work great up to about 70 MHz, but at 100 Khz they may as well be made of wood!

Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Sun Jul 23, 2017 7:27 am
by DF1KZ
Hello Andy

Give a BN 73-202 a try. For me it works down to 10 kHz as a matching transformer for the HiZ-input of the RSP2.

73, Wolfgang (DF1KZ)

Re: RSP2 LO 3rd harmonic spurious response only -20dBC

Posted: Sun Jul 23, 2017 9:31 am
by g1hbe
Thanks Wolfgang. These cores (picked up for almost nothing at a rally) measure very much like mix 43. I see Mouser sell the 73 equivalent, which should be much better. Thanks.