Noise on Spectrum Display- What's this?

AM Radio discussion. Directional arrays are FUN!
Post Reply
ChuckG
Posts: 890
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Noise on Spectrum Display- What's this?

Post by ChuckG » Tue Feb 09, 2016 3:07 pm

This was found while doing NRSC Measurements.
Solid state Omnitronix transmitter. (Yeah, I know....)
Carrier is on 1450. Noise from about 1550 to 1800 about 65 db below carrier, disappears when the carrier is turned off.
Looking back through past measurements, it's apparently been there for a while.
There is no corresponding noise below the carrier.
The only other RF on the tower is a wireless internet provider. Not sure of the frequency bands they use.
There are two cellular towers within 1/8 mile.
Transmitter readings, LV power supplies, final voltage, current, base current and VSWR haven't changed in years.

Any ideas what's causing it?

Image
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

Kelly
Posts: 476
Joined: Thu Aug 21, 2008 5:32 pm
Location: Washington D.C. Area

Re: Noise on Spectrum Display- What's this?

Post by Kelly » Tue Feb 09, 2016 5:45 pm

Interesting! A couple thoughts; can you hear anything if you try and demodulate the noise? If you kill modulation for a second or two, does the noise follow?
Skype:kellyalford Twitter: @KellyAlford

User avatar
Deep Thought
Posts: 3150
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: Noise on Spectrum Display- What's this?

Post by Deep Thought » Tue Feb 09, 2016 6:04 pm

What is the PWM frequency on that beast? The center is 225 KHz above your carrier freq., which just happens to be the third harmonic of 75 KHz, a popular PWM frequency.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

ChuckG
Posts: 890
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: Noise on Spectrum Display- What's this?

Post by ChuckG » Tue Feb 09, 2016 9:53 pm

Kelly wrote:Interesting! A couple thoughts; can you hear anything if you try and demodulate the noise?
Thanks, Kelly. No I can't hear a thing other than the usual static and noise.
If you kill modulation for a second or two, does the noise follow?
I didn't check that, I don't have the spec analyzer available to re-check. I may be able to borrow it & it's owner again this spring.
Last edited by ChuckG on Tue Feb 09, 2016 10:01 pm, edited 1 time in total.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

ChuckG
Posts: 890
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: Noise on Spectrum Display- What's this?

Post by ChuckG » Tue Feb 09, 2016 10:00 pm

Deep Thought wrote:What is the PWM frequency on that beast? The center is 225 KHz above your carrier freq., which just happens to be the third harmonic of 75 KHz, a popular PWM frequency.
Thanks Mark. I had that thought too. I don't know what the PWM frequency is and the manual doesn't say, but I recall it being in that range from past troubles with the box.
It's had spectral issues in the past when one of the PA modules went south (PWM filtering is done on each of the PA modules) but that left a spike both above and below carrier. The fact it's only above carrier here is tripping me up. It's the same filter section.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

User avatar
Deep Thought
Posts: 3150
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: Noise on Spectrum Display- What's this?

Post by Deep Thought » Tue Feb 09, 2016 11:05 pm

The reason I thought that is it resembles double sideband suppressed carrier modulated by noise. I wonder what the PDM waveform looks like.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

ChuckG
Posts: 890
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: Noise on Spectrum Display- What's this?

Post by ChuckG » Tue Feb 09, 2016 11:53 pm

I found the PDM Generator and PA module drawings. I attached them for S&G. Looks like it's 70Khz for the win.
Attachments
S-113-110 r3 300w PA module.pdf
(51.18 KiB) Downloaded 90 times
S-112-100 r5 PDM Exciter.pdf
(75.29 KiB) Downloaded 140 times
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

User avatar
Shane
Posts: 717
Joined: Fri Feb 01, 2008 12:08 am
Location: Omaha
Contact:

Re: Noise on Spectrum Display- What's this?

Post by Shane » Wed Feb 10, 2016 3:15 pm

I was thinking an FM mix product but since it requires 1450 to be on to exist, probably not. Plus you can't hear any semblance of audio in it.

I have a 1420 which for a time got quite noisy in a certain area of town where most of the FMs and TVs are located. I first just thought our signal was crappy there since it had been known to be weak in that direction.

But then I noticed two things. As I drove farther away from the transmitter (and also farther away from the farm), the 1420 signal seemed to get better with less noise.

Well here we have a couple of FMs on the same tower on 98.5 and 99.9. The math is obvious and I eventually could decipher the audio enough to identify one of the stations.

I believe once they were made aware of it, the noise disappeared fairly quickly. Never found out anything about the cure (never remembered to ask!) but we speculated that perhaps lightning took out part of the bandpass filters on one or both of the stations in the mix product.
Last edited by Shane on Wed Feb 10, 2016 3:18 pm, edited 1 time in total.
Mike Shane, CBRE
---Omaha---

User avatar
Shane
Posts: 717
Joined: Fri Feb 01, 2008 12:08 am
Location: Omaha
Contact:

Re: Noise on Spectrum Display- What's this?

Post by Shane » Wed Feb 10, 2016 3:16 pm

Sorry duplicate post
Mike Shane, CBRE
---Omaha---

User avatar
Deep Thought
Posts: 3150
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: Noise on Spectrum Display- What's this?

Post by Deep Thought » Wed Feb 10, 2016 7:01 pm

Hmmm...Mike triggered a latent brain cell. That could also pass for an FM signal modulated about +/- 100 KHz centered near 1690 KHz. Any FMs within a couple miles? If you do get the spec an back set it to 1690, switch it to FM demod and see if you can hear anything that way.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

ChuckG
Posts: 890
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: Noise on Spectrum Display- What's this?

Post by ChuckG » Wed Feb 10, 2016 8:45 pm

Deep Thought wrote: Any FMs within a couple miles? .
There's an FM farm with several 100kW FM's, but it's 21 miles away. The only other is a 6kW 12 miles away.
The only things close in are WIMax wireless and two cell sites. The WIMax is on the AM tower in question, the Cell sites are about 1/8 mile.
There is a 10 Watt 455 Mhz TRL also operating on that AM tower that I forgot about.

The engineer with the Spec An is not available for a while, my preliminary plan once it's available is to shut off each of the 4 PA amps in the Omni and see if the noise disappears with one of them muted.
All the filtering is done in the PA modules, after that there's only a combiner and wideband T-network.
I'm not 100% convinced it's in the filtering though. The 70 Khz artifacts above/below carrier are within NRSC specs and those Sprague "orange drop" caps used in the filter section just don't seem to fail often.
Fun, fun....
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

COMMENG
Posts: 80
Joined: Wed Dec 18, 2013 4:30 pm
Location: Midwest

Re: Noise on Spectrum Display- What's this?

Post by COMMENG » Thu Feb 11, 2016 9:49 pm

I will take a WAG here as I think DT is on to something.

However, I think it is residual Phase Modulation noise from the H-Bridge switchers.

Let's assume the QSIGNAL is falling to zero volts turning off Q3105 and Q3106.

About 0.345 microseconds later, the QBARSIG signal rises turning on the optocouplers which in turn activates the FET gate drivers and finally, Q3104 and Q3107 switches on, sending a current pulse through T3102.

Theoretically and with perfect switches, both Q3104 and Q3107 switch on (or off) at exactly the same time.

In real life, they do not switch on (or off) at exactly the same time, due to small variations in the Gate capacitances.

So what we see at the drains and sources of these switches is jitter, a "displacement" in time of the rising and falling edges of the switchng waveforms.

Now, any displacement in time w.r.t. the original signal is a "phase" difference. Combine that with the amplitude variations (modulation voltage), and the result is phase modulation.

Again, just a WAG.

COMMENG

Post Reply

Who is online

Users browsing this forum: No registered users and 2 guests