FM RFI in Console

[museic]

I'm hoping someone might take an interest in this strange problem I'm having.

Basically I get a strong FM signal in my recording console. Its a Toft ATB-16. It is FM 94x in Duluth MN. I understand that this seems to be a pretty rare occurrence since this is an FM signal. I tried to get some help with this from toft but they weren't too interested in helping me out.

As far as the strength of the signal, I can basically listen to the radio if I crank the monitor level and the master bus with no other channels. For some reason the solo function reduces the loudness of the signal, although it is still present.

I've done quite a bit of reading, but I really can't find anyone else who has had this problem. Possible solutions? Isolation transformer? FM notch filter? UBS?

My crown xls also picks up the same frequency into my bass cab.

Anyway thats the basics of it. Hoping someone more electronically inclined here could lead me towards a solution!

 

[JohnRoberts]

Yes decoding 94 Mhz is not very common (too high frequency).

I suspect you have an unusually high RF field  "and" some susceptible circuits. 

You need to divide and conquer to see exactly where the RF signal is being decoded. It may not be rectified, it could be another oscillating circuit beating with the FM (or not, just a guess).

You can make a signal sniffer, kind of like a scope probe but a cap coupled audio input that you can probe around inside you mixer listening for where exactly the RFI signal first reveals itself as audible audio signals. Note: where you ground this probe should be local to the circuitry you are sniffing, and preferably different than the product ground you are troubleshooting. A small battery powered headphone amp can be perfect for this.

Buses can be susceptible to AM rectification because the high noise gain from summing multiple stems means RF voltage at the input gets a lot of voltage gain that the circuitry can't keep up with.

I fixed one noisy bus, inside a small mixer by replacing the sum amp with a bifet opamp. The high Vthj (input transconductance range of the LTP) was a few volts, vs the tens of mV for bipolar opamps, so the RF was harmlessly LPF. You might try modern very low noise  bifets for sum bus amps.. I didn't have these options back in the day. 

Caveat, the vast majority if my experience is chasing AM radio our of circuits.

JR

PS: Power amps can also pick up RF on the speaker wires and this snakes back into the NF network of the amp and causes rectification in the amp gain stage. 

 

[museic]

Ok thanks for the reply. Any link for an example of a signal sniffer? I'm not technical enough to diy this without a plan.

Whats strange is that I'm about 15 miles from the tower, so its not like I'm right under the tower or anything.

 

[moamps]

Is there any another FM radio on the tower?
What's distance between you and FM studio location?

 

[museic]

I just did some searching for info on this tower. Turns out its less than a mile from my house! I guess thats part of my problem!

 

[museic]

Here is the info..

http://www.radio-locator.com/info/KZIO-FM

I'm guessing I'm actually picking it up on the 104.3 Mhz, as thats the tower close to me.

 

[JohnRoberts]

Ok thanks for the reply. Any link for an example of a signal sniffer? I'm not technical enough to diy this without a plan.

Whats strange is that I'm about 15 miles from the tower, so its not like I'm right under the tower or anything.

Typing "signal sniffer" into the search box at the top of the page returns two pages of hits.

The search function on a site like this generally pretty useful.

If you can't roll your own sniffer, disregard my post about opmap Vth and other design esoterica... sniff out the RFI and smack it with a hammer until it stops. 

JR

PS In the bad RF environments I've been in an inch of bare wire in a scope probe stuck into the air, allowed me to ID the station frequency.

 

[museic]

I've been close a few times to smacking it with a hammer. Particularly when I'm tracking something quiet and Metallica comes on. If it was a classical station it wouldn't bother me so much. Anyway I get the idea about the sniffer. Found this little explanation..

http://www.el34world.com/Hoffman/tools.htm

 

[moamps]

Here is the info..
http://www.radio-locator.com/info/KZIO-FM
I'm guessing I'm actually picking it up on the 104.3 Mhz, as thats the tower close to me.

There is another small transmitter in the town.
http://www.radio-locator.com/info/K231BI-FX
What's your distance from that position?

 

[museic]

I'm something like 13 miles from there.

 

[bruce0]

I typed signal sniffer into the search box, and I got two pages of references but none that found a signal sniffer schematic.

Here is a small headphone amp at a great little site.  You don't even need to build the whole amp as you don't need stereo.  It is cap coupled.

http://www.tangentsoft.net/audio/cmoy-tutorial/misc/cmoy-tangent-sch.pdf

 

[museic]

I typed signal sniffer into the search box, and I got two pages of references but none that found a signal sniffer schematic.

Here is a small headphone amp at a great little site.  You don't even need to build the whole amp as you don't need stereo.  It is cap coupled.

http://www.tangentsoft.net/audio/cmoy-tutorial/misc/cmoy-tangent-sch.pdf

Cool, I like how it fits in an altoids tin! I'll try that one. Thanks

 

[abbey road d enfer]

I'm hoping someone might take an interest in this strange problem I'm having.

Basically I get a strong FM signal in my recording console. Its a Toft ATB-16. It is FM 94x in Duluth MN. I understand that this seems to be a pretty rare occurrence since this is an FM signal. I tried to get some help with this from toft but they weren't too interested in helping me out.

As far as the strength of the signal, I can basically listen to the radio if I crank the monitor level and the master bus with no other channels. For some reason the solo function reduces the loudness of the signal, although it is still present.

I've done quite a bit of reading, but I really can't find anyone else who has had this problem. Possible solutions? Isolation transformer? FM notch filter? UBS?

My crown xls also picks up the same frequency into my bass cab.

Anyway thats the basics of it. Hoping someone more electronically inclined here could lead me towards a solution!

You should first determine where the demodulation happens.
Do you hear the radio with just the stereo bus on, but all inputs muted, or does it follow the channel fader level, and maybe the input gain?
Maybe you have the schemo of the LOFT console? That would allow us to indicate some possible fixes.
On mic input, it is generally not too difficult. Applying a pair of 0.01uF ceramic capacitors between each leg and chassis ground is worth a try.
If its demodulated in the summing amp, it's more complex; I would first try inserting a 100uH inductor right at the input of the summing amp.

 

[JohnRoberts]

I'm hoping someone might take an interest in this strange problem I'm having.

Basically I get a strong FM signal in my recording console. Its a Toft ATB-16. It is FM 94x in Duluth MN. I understand that this seems to be a pretty rare occurrence since this is an FM signal. I tried to get some help with this from toft but they weren't too interested in helping me out.

As far as the strength of the signal, I can basically listen to the radio if I crank the monitor level and the master bus with no other channels. For some reason the solo function reduces the loudness of the signal, although it is still present.

I've done quite a bit of reading, but I really can't find anyone else who has had this problem. Possible solutions? Isolation transformer? FM notch filter? UBS?

My crown xls also picks up the same frequency into my bass cab.

Anyway thats the basics of it. Hoping someone more electronically inclined here could lead me towards a solution!

You should first determine where the demodulation happens.
Do you hear the radio with just the stereo bus on, but all inputs muted, or does it follow the channel fader level, and maybe the input gain?
Maybe you have the schemo of the LOFT console? That would allow us to indicate some possible fixes.
On mic input, it is generally not too difficult. Applying a pair of 0.01uF ceramic capacitors between each leg and chassis ground is worth a try.
If its demodulated in the summing amp, it's more complex; I would first try inserting a 100uH inductor right at the input of the summing amp.

TOFT not LOFT... I designed the LOFT consoles (seems like a hundred years ago) and they only receive AM radio**...  8)

JR

** 960kHz to be precise.. less than a mile from the main lobe of an AM tower in VA... Every piece of electronic gear in that studio received that station. 

 

[PRR]

> decoding 94 Mhz is not very common (too high frequency).

The frequency is no big deal.

But the usual un-intended radio reception is a diode or diode-like action.

AM through a diode directly gives the modulated speech/music.

FM through a diode gives a steady DC voltage. NOT the audio.

So there is some other factor. The worst sort of FM radios use slope detection. Have a circuit that falls-off from 93MHz to 95MHz. Now the "DC" off the diode varies with the speech/music on the FM. Recovery level is typically low, unless slope is high (off-tuned circuit, not simple roll-off).

But your signal is low. Good to check all connections, grounding, covers, and any place contacts may be tarnished.

However. It can happen that the tower guy-wires get tarnished contacts. Since they are in a HIGH signal zone, "low" recovery can lead to quite large AM radiation on the 94MHz frequency. In the old days, radio stations had on-site engineers who would look into such things. Now, no daily attendence is needed, annual proof-tests are contracted to a company in Iowa or Georgia, so good-luck getting anything done.

> its less than a mile from my house!

Hate to say it. MOVE!

I grew-up a mile from a 50KW AM station. We had that signal on EVERYTHING. The phone company went around the neighborhood every few years trying different protectors, capacitors, and cleaning junctions. I could pick-up the signal on ANYTHING "like" a crystal radio, and I could not pick up ANYthing else without going to four tuned-circuits (so I never made that leap, and know little about radio).

 

[pucho812]

You know there was a SSL here in town that acted like  a radio antenna and for the life the board belong to a famous owner it picked up all kinds of things.

 

[museic]

I'm hoping someone might take an interest in this strange problem I'm having.

Basically I get a strong FM signal in my recording console. Its a Toft ATB-16. It is FM 94x in Duluth MN. I understand that this seems to be a pretty rare occurrence since this is an FM signal. I tried to get some help with this from toft but they weren't too interested in helping me out.

As far as the strength of the signal, I can basically listen to the radio if I crank the monitor level and the master bus with no other channels. For some reason the solo function reduces the loudness of the signal, although it is still present.

I've done quite a bit of reading, but I really can't find anyone else who has had this problem. Possible solutions? Isolation transformer? FM notch filter? UBS?

My crown xls also picks up the same frequency into my bass cab.

Anyway thats the basics of it. Hoping someone more electronically inclined here could lead me towards a solution!

You should first determine where the demodulation happens.
Do you hear the radio with just the stereo bus on, but all inputs muted, or does it follow the channel fader level, and maybe the input gain?
Maybe you have the schemo of the LOFT console? That would allow us to indicate some possible fixes.
On mic input, it is generally not too difficult. Applying a pair of 0.01uF ceramic capacitors between each leg and chassis ground is worth a try.
If its demodulated in the summing amp, it's more complex; I would first try inserting a 100uH inductor right at the input of the summing amp.

Yes I hear the radio with just the stereo bus around 0db and the monitor turned up. This is with every channel muted. So im afraid it is demodulated in the stereo bus somewhere. If I solo every channel on the board and monitor through the solo monitor the interference is much less.

 

[pucho812]

I would look for loose connection or contact

 

[abbey road d enfer]

TOFT not LOFT... I designed the LOFT consoles (seems like a hundred years ago) and they only receive AM radio**...  8)

Sorry about it! Chalk it up to Poe's Angel of Odd...

** 960kHz to be precise.. less than a mile from the main lobe of an AM tower in VA... Every piece of electronic gear in that studio received that station.

I've had this problem a number of times. One day a customer gave me the fix: "Put a variable capacitor so I can tune between stations" 

 

[moamps]

All FM transmitters also generate an AM parasitic signal. Usually that's not a problem because it's very small in relative ratio, but you are so close to the very high power transmitter (50kW), so that can be your problem. Also, sometimes the AM parasitic level is higher if the transmitter's modulation circuitry is overloaded with the modulation signal  (deviation is more than 75kHz), or the transmitter doesn't work correctly at all (for example, the band pass filter is too narrow, or the antenna isn't properly matched to the transmitter so SWR is poor). There might be also IM interference related to the link type between transmitter and studio, so IMO it will be a good idea to call inspection to check the transmitter. Especially if you receive the signal in the console and bass cab individually.