Author Archives: Mehdi Asgari

FM Notch Filter for SDRPlay RSP1

RF filters are used (as the name implies) to filter/remove the frequencies you are not interested in and/or let frequencies you want pass . They come in lots of types. For example a band-pass filter lets the signals in a frequency range to pass through it and rejects/attenuates other frequencies. The opposite of band-pass filter is a band-reject or band-stop filter (also called a notch filter) which rejects/attenuates signals in a specific range and lets other frequencies get through the filter. Lots of different filters are used in SDRs and traditional radios. For example AM low-pass filters (only let frequencies lower than 1.7MHZ or so pass) or band-pass filters for various ham radio bands.

One of the popular use cases for a notch filter is in the FM broadcast range (88-108 MHZ in most parts of the world)

When you live near a powerful transmitter, it can affect the operation of your receiver in other near frequencies (or overload your receiver’s front-end), but I didn’t want the notch filter for this reason. I’ve got a SDRPlay RSP1 (among many other SDRs) which due to its architecture, has some images of FM band in the UHF range (for example in 330-350 MHZ). In fact they’re the images of the product of LO harmonics and FM frequencies.

You can temporarily move/shift the frequency by changing the LO frequency which does not remove them, but moves them around.

Another method to remove these images is using a band-stop filter.

This is the filter I’m using (Thanks to my friend Amirhosein Hasanpur who designed and built it):

Here you can see the effect of using a FM notch filter on my SDRPlay RSP1:

FM, without filter:

FM, with filter:

UHF (images) without filter:

UHF (images) with filter:

Here’s a link to a Zip file containing the PCB (in Protel), schematics (pdf) and S Parameters (pdf):

Note: Like any other SDR test/review, the results depend on lots of different parameters (various gain values, LNA, antenna, software, etc). These pictures are captured with the same conditions just to show the effectiveness of this filter and your milage will definitely vary, but expect a similar outcome. If you live close to a powerful transmitter or use LNAs, you will receive some signals, even when using the filter.

Final note: this issue is solved in the newer version of SDRPlay (RSP2) : it has software-selectable notch filters for FM and MW broadcast frequencies.

Mehdi Asgari, the author of this post, is a regular contributor to the SWLing Post. Mehdi lives in Tehran and is an active member of the EP2C amateur radio club.

First Iranian ham radio contest


To celebrate 37th anniversary of Iranian Islamic revolution, the first Iranian ham contest is going to be held on February 1st, 2016 and will last for 10 days.

Objective: To encourage and increase contacts (especially DX ones) with Iranian radio amateurs.


Contest Period: 10 days (Feb 01, 00:00 UTC – Feb 11, 23:59 UTC)

All modes (CW, SSB, RTTY) can be used on this period.


40, 20, 15 and 10 meters

Note: On 20m, the upper limit is 14.250 MHZ

Contact information:

For contest information and any sort of inquiries contact


Certificates will be awarded to:

1- Top single operator (at least 30 contacts on all modes)

2- Top CW operator

3- Top SSB operator

4- Top RTTY operator

5- Any operator who contacts at least 3 Iranian hams

Getting QSL card:

If you need the printed certificate, you should send 5USD (or equivalent in your          currency) to this address: “P.O. Box 14185-736 , Tehran, Iran”

Please use registered mail to make sure it delivers without problems.

WebMoney and Bitcoin are also accepted. (Contact us for more information)


Iranian amateurs will use “/contest” at the end of their call sign for this contest.

The deadline for contacting us for certificate is 3 months after the contest.

List of Iranian participants:

  1. EP2FM  : Abdollah Sajjadian
  2. EP3RB : Reza Batebi
  3. EP3AG  : Ali Ghanbari
  4. EP2FA   : Farman Aghdasi
  5. EP2CM : Jamshid Mansoori
  6. EP4HR  : Hamid-Reza Rahimi
  7. EP3MIR : Mohammad Mobini
  8. EP3CQ : Ali Solh-Joo
  9. EP2LMA : Mohammad Azimi
  10. EP2LSH: Saeed Shokrollahi
  11. EP2HZM : Hassan Zohoorian
  12. EP2HEK : Hekmatollah Rahimi
  13. EP5MKN : Majid Kiani Nejad
  14. EP5ABD : Bagher Mir-Abdolhagh
  15. EP7AHN : Hamed Nahrir
  16. EP4KHA : Amin Kharadmehr
  17. EP4MMM : Mohammad Mirab
  18. EP2MA : Mohammad Ameli
  19. EP2LSD : Sepehr Dalir
  20. EP3EEE: Ehsan Esteki

Mehdi Asgari, the author of this post, is a regular contributor to the SWLing Post. Mehdi lives in Tehran and is an active member of the EP2C amateur radio club.

Icom IC-R72: a repair story

Icom-IC-R72-Front-Mehdi-AsgariIn this post I’m going to tell you how I repaired my Icom IC-R72 receiver. Although it’s about a specific device, the logic and methodology applies to all radios.

I bought an Icom R72 from a friend for about $200. At first, I just checked 7.0MHZ (40 meter band) and 21.0MHZ (15 meter band) and it was OK. After some time, I tried to listen to some ham radio on 20 meters (14.0MHZ) and it was deaf! I checked everything: antenna connectors, balun…everything.

With some trial and error, I found out that it was deaf from 11-15 MHz. With the help of the members of “Icom R72” Yahoo Group, I found one of the usual suspects: bandpass filters’ switching diodes.

I took a look at the service manual and apparently this radio uses multiple bandpass filters  for different  frequency ranges.


As you see in the above picture (grabbed from service manual), one of the bandpass filters is for 11-15 MHz range–that’s the range where my radio was deaf.

Note that there are multiple ways to test that a radio is deaf at a frequency. One of the simplest ways: connect an antenna or even a long wire to the antenna socket of radio. The noise level should increase; if not, there’s a problem.


After testing diodes with a multimeter, I found out that D31 is faulty. Almost all multimeters have a diode-test functionality.

I replaced it. The original diode was 1SS53, but I used a 1N4148 which is very common and found everywhere. Now I have a working radio! 🙂

If you have a radio that’s deaf at a frequency range, there is probably a problem in bandpass filters.

Here are some internal pictures of my Icom IC-R72:

IC-R72-Internal-IF-Stage-Mehdi IC-R72-Board-1 IC-R72-Board-2 Icom-IC-R72-Open

I should thank my friend and electronics mentor, Saeed (EP2LSH) who always helps me in my electronics adventures.

Mehdi Asgari, the author of this post, is a regular contributor to the SWLing Post. Mehdi lives in Tehran and is an active member of the EP2C amateur radio club.

How to use the SDRPlay RSP as a panadapter


In this post, I will show you how to use an SDR as a panadapter for a commercial communications receiver. I’m using an Icom R72 and a SDRPlay RSP, but you can do it with nearly all receivers and SDRs.

The Icom R72 is a double conversion HF (0-30 MHz) communications receiver.

A panadapter lets you see the spectrum of your receiver (it gives you a broader, higher-level picture of what’s around your tuned frequency). After using it, you’ll wonder how you’ve been using your receiver without a spectrum display! 🙂

Almost all the radios we use are of superhetrodyne type. To connect a SDR to the radio, you should first find its 1st IF mixer output (if it has more than one Intermediate Frequency stage).

To find it, the best way is to consult the service manual. The first IF of Icom R72 is 70.4500 MHz, so we can use RTL-SDR dongles too.

Here’s a part of R72’s schematics, extracted from its service manual:


Look at the R82 resistor; it’s located at the output of first IF stage, so I soldered a connector to its pins (its other pin is connected to the ground)

Between the R82 resistor and the output, we’ve added a series 470-ohm resistor (to prevent loading the IF stage)


Click to enlarge.

The other side of this connector is connected to the antenna input of my SDRPlay RSP.

Now I can view my spectrum on a PC.

Here’s a screenshot of CubicSDR on Mac OS X (you can use other apps like HDSDR, SDR Console, etc.):


Although this post was about the SDRPlay RSP and Icom R72, this procedure can be done for almost any combination of receivers and SDRs provided that your SDR covers the frequency of first IF stage.

Also, don’t forget that you should tune the SDR to the frequency of IF–in this case: 70.4500 MHz.

Mehdi Asgari, the author of this post, is a regular contributor to the SWLing Post. Mehdi lives in Tehran and is an active member of the EP2C amateur radio club.