114 kc/s notch filter in Stereo FM Tuners - UK Vintage Radio Repair and Restoration Discussion Forum

SteveCG · 26th Jan 2019, 12:25 pm

I have discovered that B&O (for example) included a 114 kc/s filter between the IF stage output and the stereo decoder in some of its designs. I wonder why this was?

114 is 3 times the stereo subcarrier frequency - but this filter is before stereo decoding. Were some FM transmitters radiating a low level third harmonic?

Nuvistor · 26th Jan 2019, 12:44 pm

I have not read all this but it gives some information , I will read it later, may help you.

http://ham-radio.com/k6sti/ibocfltr.htm

SteveCG · 26th Jan 2019, 1:15 pm

Thank you for the link Nuvistor - I had not even heard of HD Radio in that context!

By the way: the B&O unit is a B&O BeoCenter 2100 Type 2442, which is likely to have been made between 1985-88. So the use of this filter seems to predate the USA digital Radio efforts by quite a few years. I still wonder then why B&O included it?

G8HQP Dave · 26th Jan 2019, 5:44 pm

A switching decoder (i.e. almost all stereo decoders) will demodulate any signal in the vicinity of three times the subcarrier frequency. Many FM tuners have a low pass filter to stop this from happening. Perhaps B&O felt this was insufficient, or wanted something to put in their marketing?

Julesomega · 26th Jan 2019, 6:19 pm

Yes, audio spurii are likely to occur if there is another transmission 300kHz either side of the wanted. For this reason, the BBC national services from Crystal Palace are offset 4kHz from the nominal freq. which is 300kHz from Wrotham.
BBC Research Report

26th Jan 2019, 12:25 pm	#1
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	114 kc/s notch filter in Stereo FM Tuners I have discovered that B&O (for example) included a 114 kc/s filter between the IF stage output and the stereo decoder in some of its designs. I wonder why this was? 114 is 3 times the stereo subcarrier frequency - but this filter is before stereo decoding. Were some FM transmitters radiating a low level third harmonic?

26th Jan 2019, 12:44 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,431	Re: 114 kc/s notch filter in Stereo FM Tuners I have not read all this but it gives some information , I will read it later, may help you. http://ham-radio.com/k6sti/ibocfltr.htm __________________ Frank

26th Jan 2019, 1:15 pm	#3
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: 114 kc/s notch filter in Stereo FM Tuners Thank you for the link Nuvistor - I had not even heard of HD Radio in that context! By the way: the B&O unit is a B&O BeoCenter 2100 Type 2442, which is likely to have been made between 1985-88. So the use of this filter seems to predate the USA digital Radio efforts by quite a few years. I still wonder then why B&O included it?

26th Jan 2019, 5:44 pm	#4
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: 114 kc/s notch filter in Stereo FM Tuners A switching decoder (i.e. almost all stereo decoders) will demodulate any signal in the vicinity of three times the subcarrier frequency. Many FM tuners have a low pass filter to stop this from happening. Perhaps B&O felt this was insufficient, or wanted something to put in their marketing?

26th Jan 2019, 6:19 pm	#5
Julesomega Nonode Join Date: Dec 2012 Location: Stockport, Greater Manchester, UK. Posts: 2,102	Re: 114 kc/s notch filter in Stereo FM Tuners Yes, audio spurii are likely to occur if there is another transmission 300kHz either side of the wanted. For this reason, the BBC national services from Crystal Palace are offset 4kHz from the nominal freq. which is 300kHz from Wrotham. BBC Research Report __________________ - Julian It's good here