Low Pass Filter for MW and LW

Low VHF receiver discussion
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daveB
Posts: 997
Joined: 07 Oct 2014 16:49

Low Pass Filter for MW and LW

Post by daveB » 30 Nov 2016 21:54

I have noticed interference from the 3.5 MHz - 4.0 MHz band on MW - in particular CW from the 80 m amateur band and strong AM from the 75 m band - tuning in reverse. I have hunted for a specific project for a low pass filter with a cut-off around 2 MHz - but the nearest I found was this one - which has a cut-off of 400 kHz. As I would be using standard components with 5-10% tolerance I didn't want to get too close to the top end of the MW band.

http://www.radiohobbyist.org/blog/?p=1894

However scaling to 2 MHz is simply a matter of dividing all values by 5. And here is the resulting filter - in a pre-used small diecast box.

https://app.box.com/s/klv0kkd5u9bu3ujnikd5oxqglgqfim9d

The filter design itself is a 9th order Chebyshev the LC design program is for one with a maximum 0.2 dB ripple and should in theory achieve around -50dB at 3.0 MHz and -80dB above 4 MHz

It's performance can be judged from the following set of screenshots. The receiver is an SDRPlay RSP1.

SDR receivers need good filtering because they will pick up signals at multiples of the sampling frequency due to under-sampling (look it up). That's how the Elad S2 gets Band 2 FM when the ADC only goes up to around 66 MHz.

Sample rate of SDR 8 mbps - MW and SW
https://app.box.com/s/zjxqd3jsvceseaqn1o6l4u0y6v0emenh
https://app.box.com/s/18ufok6czsaqtqt8t0lhg3bshc0zx7zh

Sample rate 8 mbps - FM Band II
https://app.box.com/s/k8hoc4hr2vpq9kxn0n5tergst9ypbu5e
https://app.box.com/s/rrby3l38hy8quu1w05atjzl5w3dpokyt

Sample rate of SDR 2 mbps
https://app.box.com/s/tgfy04thjqcie3y4x55psoti3ktx91fk
https://app.box.com/s/j08wgznn6pyls52rjx4l8nvheshtcsie

Notice the mess in the 300-400 KHZ segment of the band - some of which is coming from the 75 m band and some almost certainly from the 49 / 41 m bands
These results are from the OIRT dipole fed by a 15m length of 75 ohm coax - so hardly a good match - so some ripple in the pass band is to be expected but the modelling program showed that evena 500 ohm input impedance still gave more than acceptable performance.

The actual values used - 5 uH chokes at each end (2x10uH in parallel) - 10 uH in the middle. The four capacitors were all 2n2 polyester 5%. Although the middle two capacitors should be closer to 2n42 the filter modelling program showed the results to be more than acceptable - a slight ripple above 1.6MHz before plunging off. Component tolerance will have an effect, but by modelling a 2 MHz cut-off I avoided the risk of cutting of the top of the MW band.

The modelling progam can be found here: https://www-users.cs.york.ac.uk/~fisher/lcfilter/

It will come up with an ideal set of values - but if you substitute preferred values and click on the "Do Not Press" button it re-calculates it for you.

Now I don't have to worry whether a signal is genuine or an image from elsewhere - and it is cheap to build.
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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