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Author Topic: An RX Spur In The Wrong Place?  (Read 700 times)
AC5UP
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Posts: 3927




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« on: September 27, 2000, 10:59:12 AM »

Here's one that has me scratching my head...

Let's see if anyone has been here and done this before:

I have a Harris RF-550 communications receiver (rack-mount commercial jobbie, early 80's vintage, digital tune by flip-switches .1 to 29.999 MHz AM/LSB/USB/ISB/CW, definitely not a cheapie) that I use for general RX and SWL work. Antenna is a random wire in the 60' range fed through a dual-core Guanella 4:1 balun against a ground rod at the feed point.

The antenna works -REAL- good above 800 kHz or so and the S-Meter in the Harris is calibrated in dB from -20 to +120 relative to 1uV. I have two local AM stations that come in at 100 dB over 1uV, and that's a very manly signal, probably better than 50 over 9 on a conventional S-Meter. A 'normal' signal in the AM band is between 50 and 70 dB over 1uV on this setup.

Here's the question:

I have a spur on 1040 kHz from the -two- strongest AM stations that happen to be on 1170 and 1300 kHz. A little mental math will tell you there's 130 kHz difference between 1170 and 1300, and if you subtract this from 1170 you get 1040, which is exactly where I hear the spur. Simple enough? I can't tell if there's another spur at 1300 plus 130, as there's a fairly strong local station on 1430.

If I understand my superheterodyne theory, when you mix two signals together you should hear their product at the "difference" between the two frequencies. Sure enough, if I tune 130 kHz there's a clean mix of the two stations. Tune the first or second harmonic of either, and they sound pretty good. Which tells me that if the RX is overloaded, it's not by much. Right? Tune the fundamental of either and they sound good, flip the filter switch to 'wide' (20 kHz) and they're (by AM standards) extremely Hi-Fi.

If I swap the balun for a 9:1 single core jobbie, it knocks 5 dB off everything and the spur at 1040 kHz goes away. But, the difference signal at 130 kHz remains. The spur on 1040 kHz doesn't bother me, (and my apologies to anyone at WHO in Des Moines for not hearing you at night) it just puzzles me as to why it's there.

On my main rig (an Icom IC-756 on a 136' Coaxial Windom) the "Spectrum Scope" shows a peak in the 'grass' on the bottom of the display with a ragged spike at 1040 kHz, but no audio is heard other than the 'scratch' from another local station on 1050 kHz. In other words, the 'spur' really is there on more than one receiver. (and the IC-756 will play the two-station mix on 130 kHz, same as the Harris RF-550)

Can anyone tell me why?

Note: I'm familiar with high side / low side injection in a mixer stage, but I still can't figure how mixing 1170 with 1300 kHz comes out at 1040 kHz with a 130 kHz differential. (?)

Your Turn.
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N2MG
Administrator

Posts: 0



« Reply #1 on: September 27, 2000, 09:13:36 PM »

I'm not sure what's causing the non-linearity (which you need for mixing to take place) but as far as your problem with 1040 kHz, you need only consider the second harmonic of one of the AM stations:

F1: 1170
F2: 1300

2F1-F2 = 2340-1300 = 1040

Does this help?

73 -Mike

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WB6BYU
Member

Posts: 13467




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« Reply #2 on: September 28, 2000, 07:38:03 PM »

Any non-linearity will cause multiple mixing products.  The 1040 is
a second order product, while the 130 kHz is first order.  If the signals
are strong enough, you could even detect signals of 3x +/- 2y, etc.

I suggest that the signal disappears on the other antenna either because
of the reduced input signals (reducing each of the fundamentals by 5dB
reduces the image by at least 10db, maybe much more if the input signals
were just pushing an amplifier into non-linearity.)  Simply listening to the
fidelty of the audio is not a good test of RF amplifier linearity.

Another possibility is that the original balun was non-linear at these
signal levels, though that doesn't seem likely.

Good luck!   - Dale WB6BYU
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