Riddle Me This - A Question for you SDR Wizards

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  • Updated 1 year ago
Hello, Smart People!

I have a curious situation.

A neighbor of mine lives a block away and has started to use a local repeater, whose input frequency 147.680 MHz. His signal "bends the S meter" at 60+ over S9, with local harmonics on either side of his transmitting frequency. That's to be expected (the 10dB attenuator has no real effect).

The curious thing is that he is heard quite loudly by my 6700 on the output frequency of another repeater on 146.850MHz, which is 840KHz lower than his transmission. This even happens when he lowers his output to his minimum 5W level, and I don't seem to see any other odd received frequencies other than this one.

There is no spurious emissions noted on my Icom 7100 during these situations.

Is there something special about 840KHz when the 6700 is set to receive VHF signals (feel free to go all 'Tech' with Nyquist frequencies and fold over and such)?

Is there any simple solution for this type of interference?

Enquiring minds want to know <HIHI>.

A Merry Christmas and a Happy and Healthy New Year to all youz guyz (and galz)!

73,
Roy AC2GS
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Roy Laufer

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Posted 1 year ago

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Martin Ewing AA6E

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First thought: the two repeaters are linked! (Better to test on simplex frequencies.)

I doubt this has anything to to with your 6700, especially if adding attenuation does not help.

73 Martin AA6E
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Roy Laufer

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No, this is a very local phenomenon, and only when using my 6700. As I previously mentioned his signal is not seen on my Icom 7100 radio.
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Ken - NM9P, Elmer

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I am wondering if you have a strong nearby AM Broadcast station on 840 KHz that may be mixing his signal somewhere -- either in your rig, or in some other device or dissimilar metal junction?
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Roy Laufer

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No, my local AM stations are at 770KHz and 88KHz - nothing local at 840KHz.
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Rory - N6OIL

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Would it be coming through the transverter ports?
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Tim - W4TME, Customer Experience Manager

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There can be one of two issues.  Rectification (signal mixing and the resulting mixing products) is either happening external of the radio or it is happening inside the radio.  For the later, a defective ESD diode or the ADC when in the presence of strong signals can rectify regardless of the antenna port being used.  If the signal is rectified outside of the radio, then the radio is receiving real signals.
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Roy Laufer

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Any suggestions how to prove/disprove the different possibilities?

My 2M signal is going thru a first generation DEMI LDPA. My 70cm transverter is powered down. My IC-7100 is attached to another antenna and has no signal bleed over onto the other repeater's output frequency.

My assumption is that this is going on either in the LDPA or the 6700 - I thought the upper Nyquist frequency and unit-aliasing filters might be somehow sensitive at 840KHz down from an overloading signal???

I guess that I am assuming that it might be SDR tech related since the olde fashion analog 7100 just keeps rolling along without a problem like this.
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Martin Ewing AA6E

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Try a larger input attenuator -- maybe 30 or 40 dB.  If that reduces the spur, it is something in the Flex.  Also, you could bypass the LDPA for receive, to see if that's the problem.

Good luck!
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Roy Laufer

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Shutting off the LDPA leaves me with little of a signal to work with. If I attenuate my neighbor by 30 or 40dB, the other repeater's signal will be greatly attenuated as well.

If I turn my radio off, the problem is "solved", but I would prefer some other solution... 
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Martin Ewing AA6E

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If you have a receiver intermodulation problem, it's going to be very nonlinear.  That means that 10-20 dB of signal attenuation should decrease the intermod spur by much more than 10-20 dB. The spur should disappear well before your desired signal disappears.  If you don't see that behavior, the problem is not in your radio.  I would suspect the LDPA, as its preamplifier is going to be the most sensitive point in your system.  So put attenuation in the LDPA input (for receive tests, not transmit!)

73 Martin AA6E
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AA0KM

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I would be curious what radio your neighbor is using?


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Roy Laufer

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I don't recall, but it isn't anything special.

I don't believe this is something that isn't either happening at the LDPA or the 6700 since my 7100 is blissfully unaffected.
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Michael Coslo

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If you have a two meter radio, try it on the same frequency. I'd also check to see if your neighbor is using an early BaoFeng HT. Those things were very impressive for spraying merde all over the place. 

What happens to the 6700 if you transmit on the same frequency with the 7100?

What happens if you disconnect the antenna from the 6700?

You need to get with your neighboe and do a little troubleshooting. Eliminate eliminate, then bring things back one at a time.
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Ken - NM9P, Elmer

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Take the LDPA completely out of line and see if that changes anything. Its preamp may be either generating junk, or overdriving the 6700's input...

Check to see that the WNB and NB are off and not causing problems...

Another thought... what happens if you disconnect the antenna from the 7100 while the other station is transmitting? The mixing/intermod may be being generated inside the 7100.
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Roy Laufer

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When I transmit to the repeater he is using, with my IC-7100 on a separate antenna I get the same received waveform on the output frequency of another repeater that is 840KHz lower than the first repeater.

Upon further investigation very nearby VHF transmissions seems to consistently produce a signal received by a 840KHz offset lower frequency.

Whether this is taking place within the DEMI LDPA or the 6700 I do not know...
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Norm - W7CK

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You need to take the LDPA out of the system.  Just hook the antenna directly to the 6000 and see if the problem still exists....
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Roy Laufer

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Okay, here's a new little bit of information that you may find useful:

When I set my 6700 to the General Receiver band, switch the input to XVTR and disconnect the coax from the XVTR input on the back, I see a symmetric waveform at 841.600KHz -  it wiggles about +/- 100 Hz! (All of my other radio equipment is powered DOWN.)

So, I imagine it is this odd 840+KHz signal inside somewhere internal to the XVTR input that is mixing with a very very strong input signal and producing a -840KHz mixed spur!?!?!?



I find this signal on ALL my inputs, even when unplugged!

Any further thoughts? Ideas? Magic spells?
(Edited)
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Ken - NM9P, Elmer

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Are you using a switching power supply?
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Roy Laufer

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Yes, I am using a switching power supply.

But it would seem from G1XOW's helpful link that my 6700 also has an internal switching power supply, that just so happen to produce birdies at 840KHz...

Coincidence?

I think not <grin>.
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Steven G1XOW

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Roy,

A known issue and its an internal noise inside the F6k. See here my previous post on the subject..

https://community.flexradio.com/flexradio/topics/f6300-unstable-lf-carrier?topic-reply-list%5Bsettin...

73 de Steve G1XOW
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Roy Laufer

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Thanks for the link - this seems to be what I'm having a problem with.

It's just 'the perfect storm' of a neighbor overloading my 6700 at exactly an 840KHz offset of my favorite local repeater's output frequency!

My options seem to be:

Ask my neighbor 'pretty please' not to use that particular repeater.

Invest in a resonant cavity notch filter that will take up a significant part of my shack.

Use my IC-7100 for that particular repeater.

Move to somewhere else.

Take up model railroads, or bonsai trees, or something rather than VHF radio.


Anyone have any more ingenious ideas of abating this spurious signal???

(It would be nice if this internal 840KHz signal could be shifted a little up or down, out of the way of my 'perfect storm' <HINT HINT>.)
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Steven G1XOW

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Roy, Steve N5AC at Flex says this is under software control. So it should be possible to just put a click box somewhere tucked away in the setup screen to shift it +/- 50kHz I'd hope.

Good luck,
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Roy Laufer

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Yes, I've noticed that.

Perhaps I should offer a burnt offering to the great and glorious FRS programmers to write such a 'tweak' into SSDR?

Or some frankincense or myrrh? (I've already given them my gold.)

Ho Ho Ho!