120-ish MHz Low Pass filter (DAB killer)

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daveB
Posts: 1170
Joined: 07 Oct 2014 16:49

120-ish MHz Low Pass filter (DAB killer)

Post by daveB » 21 Mar 2018 22:16

I have just cobbled together a 7th order Low Pass filter to remove images of DAB signals from the OIRT band.

I ended up winding my own inductors and finally ended up with 6 turns of 22 swg of 6 mm diameter and I was able to adjust the coils to pass the top end of the FM band without attenuation. My best estimate as it stands at the moment is 0 dB at 107 MHz is -2dB at 108 MHz, - 4dB at 112 MHz, -6 dB at 112 MHz and - 10 dB at 120 MHz. It offers around 20 dB of attenuation at around 135 MHz greatly attenuating the upper airband, but only a few dB at 120 MHz. Hopefully there is just enough attenuation to stop any aircraft overhead causing intermodulation.

Building this prototype answered two key questions 1. could it be done on veroboard without getting signal leakage and 2. would standard capacitors be good enough? It didn't completely answer the question as to whether standard axial inductors would work for a VHF LPF - but it sure works well for a MW/HF LPF.

Circuit and component values are attached together with a picture of the finished prototype. It's the same box I used for the wideband pre-amp.Before anyone asks - why not just buy an FM bandpass filter?, or even just one leg of a diplexer - the answer is simply - it would filter out Band I / OIRT. When I get everything together I will have a low noise pre-amp/filter combination for 45 - 108 MHz, as I intend to use a standard mast head power unit which does a good job of filtering out below 45 MHz.
120 MHz LPF.jpg
Attachments
7th order Low Pass Filter.jpg
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

KPC850
Posts: 94
Joined: 20 Aug 2016 15:49

Re: 120-ish MHz Low Pass filter (DAB killer)

Post by KPC850 » 22 Mar 2018 10:08

Does your OIRT receiver use a 10.7MHz I.F. with a Local Oscillator that's 10.7MHz above the received frequency?

I assume the images are due to 3rd harmonic of the Local oscillator and that being then 10.7MHz away from a DAB MUX.

(just figuring out why it is happening)

the BBC ‎12B (225.648 MHz ) MUX (doing a bit of math) would as an example, if the above is the case, have an image centred on 68.08 MHz (and spreading from 67.83 to 68.34 MHz)

(a DAB MUX image on an analogue receiver will be an increase in noise with a rapid tapping or pulsing effect also audible (this is due to interruption of the DAB signal every 96 milliseconds between frames- an example seen here: https://signals.narda-sts.com/en/Bilder ... gScope.png)

I have experienced strong DAB images in the regular FM band on poor quality receivers with a 10.7 IF near DAB transmitter (in this case mixing with the 2nd harmonic of the local oscillator ) doing a bit of math, as an example 12B MUX (225.648 MHz) would have an image spreading from 107.1 to 107.85 MHz

daveB
Posts: 1170
Joined: 07 Oct 2014 16:49

Re: 120-ish MHz Low Pass filter (DAB killer)

Post by daveB » 22 Mar 2018 16:41

It is a Software Defined Radio - the SDRPlay RSP. So there are all sorts of potential issues including the fact that it uses a down-convertor architecture with a 120 MHz IF.

For everyone reading this - I have an RSP - the very first version - which has an f-connector. One of the selling points of the RSP1A is that it has better bandpass filtering.

So for the sake of fairness, so as not put anyone off, here is a comparison between the published characteristics of the 60-120 MHz band pass filters - top the old RSP and bottom - the new RSP1A. Effectively there is no filtering of the DAB band on the RSP, but on the RSP1A they have put the deepest part of the curve slap bang into the DAB band.
60 - 120 MHz bandpass filter SSRPlay RSP1-1A.jpg
Note that I have had to stretch the original jpg image from the RSP to show them at the same horizontal scale.

Given that the RSP1A is around £90 it begs the question whether the time effort and cost of building filters to improve the RSP performance is worth it or whether the money is better spent on an RSP1A

In my case, at the moment, I enjoy the challenge of looking things up and building stuff.
David
SDRPlay, AirSpyMini, RTL dongles, SB920s, Yamaha TX930. 5 ele compact yagi on rotator at 18ft agl. FM5 facing east, FM5 facing south, FM3 facing NE, OIRT dipole, 3-ele Moxon vertical facing east.

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