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Author Topic: Secondary Peaks with PSM900  (Read 999 times)

Philip Roberts

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Secondary Peaks with PSM900
« on: October 12, 2022, 11:25:44 AM »

I'm working this week at a location where we have 4 channels of PSM900 being used as IFB's in the greater Baltimore area.

A colleague of mine noticed some unexpected RF level on one of our channels of ULXD's when a specific pair of PSM900's are on. A WWB6 coordination (even with "robust") didn't indicate any issues.

Using ULXD as scanners there seems to be a secondary RF peak generated by each PSM900 that are around 50 dB down from the main signal. It's exact location relative to the tuned frequency seems to vary. Putting the two transmit frequencies and one of the secondary frequencies into a generic intermod calculator results in a 3rd order hit at the frequency of the receiver where we noticed unexpected signal.

The PSM900's feed through a PA421B and then through around 75' of RG-8 that run's parallel to RG-8 runs that feed the ULXD's. Using UA8 antenna's for everything. Neither bypassing the PA421, switching to a UA8 right at the combiner removed this peak.

Is this secondary RF peak "normal"? If so how should it be worked into a RF coordination?

Thanks

Philip
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Henry Cohen

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Re: Secondary Peaks with PSM900
« Reply #1 on: October 12, 2022, 06:41:07 PM »

I'm working this week at a location where we have 4 channels of PSM900 being used as IFB's in the greater Baltimore area.

A colleague of mine noticed some unexpected RF level on one of our channels of ULXD's when a specific pair of PSM900's are on. A WWB6 coordination (even with "robust") didn't indicate any issues.

Using ULXD as scanners there seems to be a secondary RF peak generated by each PSM900 that are around 50 dB down from the main signal. It's exact location relative to the tuned frequency seems to vary. Putting the two transmit frequencies and one of the secondary frequencies into a generic intermod calculator results in a 3rd order hit at the frequency of the receiver where we noticed unexpected signal.

The PSM900's feed through a PA421B and then through around 75' of RG-8 that run's parallel to RG-8 runs that feed the ULXD's. Using UA8 antenna's for everything. Neither bypassing the PA421, switching to a UA8 right at the combiner removed this peak.

Is this secondary RF peak "normal"? If so how should it be worked into a RF coordination?

A secondary spur of that strength is not normal, or permitted by FCC technical specifications. It would appear the two PSM's plus a third, or even fourth, carrier (another PSM, a ULXD transmitter, another nearby transmitter not part of your kit) are mixing in one of the PSM's power amplifier stage and creating an intermod product.

That said, a few questions:

- You say "it's exact location relative to the tuned frequency seems to vary". Does this mean the spur varies frequency while all other carriers remain unchanged, or the spur varies when a PSM and/or ULXD frequency is changed?

- Where are your receive antennas relative to your transmit antenna?

- Where are your ULXD receivers (frames, not antennas) relative to the PSM antenna?

- Do you have another spectrum scanner or analyzer you can use to look at the spectrum?
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Henry Cohen

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Philip Roberts

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Re: Secondary Peaks with PSM900
« Reply #2 on: October 13, 2022, 09:33:53 AM »

A secondary spur of that strength is not normal, or permitted by FCC technical specifications. It would appear the two PSM's plus a third, or even fourth, carrier (another PSM, a ULXD transmitter, another nearby transmitter not part of your kit) are mixing in one of the PSM's power amplifier stage and creating an intermod product.

That said, a few questions:

- You say "it's exact location relative to the tuned frequency seems to vary". Does this mean the spur varies frequency while all other carriers remain unchanged, or the spur varies when a PSM and/or ULXD frequency is changed?

- Where are your receive antennas relative to your transmit antenna?

- Where are your ULXD receivers (frames, not antennas) relative to the PSM antenna?

- Do you have another spectrum scanner or analyzer you can use to look at the spectrum?

These are the kind of troubleshooting questions that I'd hoped to get by posting.

- The secondary spur's frequencies varies with the tuning frequency of the PSM900, I've tested a few different frequencies with the same PSM900 TX and found spurs from -8 MHz to + 1.75 MHz to the main carrier.
- The antennas are around 4-8' apart in a TV studio, in the attached picture the center antenna is the TX, near and far antennas are RX.
- The frames are either side of a CL5 in turret racks (see picture). The audio control room is across a hallway from the studio.
- I have an RF explorer and I've some times (but not always) seen the secondary spurs.

In the picture I posted yesterday only a single PSM was on and as far as I could tell no other near by transmitters were on.

Thanks

Philip
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Henry Cohen

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Re: Secondary Peaks with PSM900
« Reply #3 on: October 13, 2022, 09:07:40 PM »

These are the kind of troubleshooting questions that I'd hoped to get by posting.

- The secondary spur's frequencies varies with the tuning frequency of the PSM900, I've tested a few different frequencies with the same PSM900 TX and found spurs from -8 MHz to + 1.75 MHz to the main carrier.
- The antennas are around 4-8' apart in a TV studio, in the attached picture the center antenna is the TX, near and far antennas are RX.
- The frames are either side of a CL5 in turret racks (see picture). The audio control room is across a hallway from the studio.
- I have an RF explorer and I've some times (but not always) seen the secondary spurs.

In the picture I posted yesterday only a single PSM was on and as far as I could tell no other near by transmitters were on.

A couple of potential causes/contributors:
- A bad PSM or two (IM products due to leaky mixer stages);
- Bad measurement equipment. I would not trust a ULXD receiver or an RF Explorer ("I've some times (but not always) seen the secondary spurs") for anything more then showing active TV transmitters. Front end could be saturating and displaying ghost emissions.
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Henry Cohen

CP Communications    www.cpcomms.com
Radio Active Designs   www.radioactiverf.com

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Re: Secondary Peaks with PSM900
« Reply #3 on: October 13, 2022, 09:07:40 PM »


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