23 May, 2011

How to make a very cheap VHF receiver

What is the cheapest receiver you can make for VHF? Here is a candidate where all you need to do to modify a small FM headphone receiver is to desolder one end of two capacitors, and connect a short cable with an antenna connector. 
    One 22 pF capacitor lifted on the right-hand side near the headphone 
     connector for connection to the external antenna, and another one lifted on 
    the left-hand side, above the volume control for increasing the tuning range 
    (The receiver IC is on the foil side of the board).

  1. Find a simple pocket-size headphone FM receiver with tuning wheel (not push-button search). I found mine at a flea market for 10 NOK (less than 1 £) and it is designated "HS-822, UK  design," and runs from two 1.5 V AAA batteries. See picture of complete radio below.
  2. Open it and check the FM receiver chip. Mine has KA22429 which is equivalent to a TDA7021. It is a 16-pin surface mount device with an FM receiver with a 76 kHz intermediate frequency. Although the TDA7021 is specified for 1.5-110 MHz, don't let that scare you.
  3. The oscillator tuned circuit from pin 5 to Vcc (pin 4) consists of 56 nH in parallel with a fixed 22 pF capacitor + a tuning capacitor. Unsolder and lift the hot end of the 22 pF capacitor (the end connected to pin 5). 
  4. This receiver uses the headphone cable as the antenna. There is a coupling capacitor from the RF input on pin 12 which is connected to the headphone socket. Unsolder and lift the headphone side of this capacitor, and connect the RF input via the capacitor to a BNC antenna connector. Connect the BNC ground to ground (pin 3) or Vcc (pin 4), whatever is more convenient
  5. Performance:
    • The tuning range was 88-108 MHz. Now it is approximately 112-163 MHz. Mine receives airport communications (AM), amateur repeaters in the 2m band (144-146 MHz), and some public service transmissions in the 150-160 MHz range. If I connect my TV cable, channel S9 (sound 161.25 MHz) will be received at a setting of 108 MHz. 
    • It receives wideband FM, as well as AM and narrowband FM with somewhat reduced output level.
    • Don't except miracles in terms of signal handling. If there are two active repeaters in the 2m band, only the strongest will be received.
    • Compared to wideband FM, narrowband FM/AM requires more accurate tuning, and the receiver is somewhat sensitive to the placement of your hands.
I have not tried this with other chips, such as the SC1088 = TDA7088, or the TDA7000. Both the Philips chips are specified for 1.5-110 MHz, but who knows how high in frequency they will cover? I would be interested to hear from others who try to convert other single-chip FM receivers.


This article was originally published in SPRAT - the journal of the G-QRP Club - Winter 2004. I wrote in Sprat that I would be interested in hearing from others who try to convert other single-chip FM receivers. Ha-Jo, DJ1ZB then wrote and said that he had been successful in using the TDA7000 for a VHF receiver. Also Pete G1INF wrote about his experience in using the TDA7088 for a 40 m direct conversion receiver, so HF operation is also possible. I have now made such a receiver myself also with success. More details of this kind of conversion can be found on Hans Summers page about conversion of a Poundshop FM receiver to 40 meters, where I also contributed a small amount.

This blog post was taken from the SPRAT article in order to enable discussions relating to the circuit. The first comments are all based on emails I have received over the years with questions and comments, therefore you will see me as the author of these comments.



Added 16 April 2012:
Here's a picture of the "HS-822 FM headphone receiver" giving an impression of how it looks. The BNC connector to the lower right is the new antenna connector.

12 comments:

  1. Q: I read your article which describes how I can convert a std pocket FM receiver(88-108 Mhz) and boost it up to 2m band. I am not very good at reading datasheets (I have TDA7021's), so please could you tell me how I could build a 2m band FM receiver with it? As I'd like to gain a complete understanding of which capacitor/inductor in the circuit affects which aspect of the receiver.

    Or should I build a std FM receiver as given in the TDA's datasheet and then do your modifications??

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  2. A: If you have the chip, TDA7021, then refer to Fig. 8 in the datasheet. The only modification is to remove the 22 pF capacitor connected to pin 5. Then just connect the antenna to the 'aerial' input in the schematics. However, building a radio from Fig. 8 is not so easy if you haven't done these things before, as the circuit requires some care in PCB-layout (short leads etc).

    It is much simpler to get a cheap FM-radio and modify it than to build one yourself. The HS-822 designation is not important. The main thing is to get one with manual tuning, not one with push-buttons for up/down tuning, because they use a different chip and are not so easy to modify.

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  3. Q: I was able to find a readymade circuit which uses the SONY CXA1619BS. It seems to be a very basic ckt. It's mounted on a pre-made PCB and is abt 5cm by 4cm.

    This ckt with me, has just 1 input (manually turnable knob)(to tune into FM 88-108 Mhz), and doesn't support AM.

    The ckt seems to be very simple. Just just 7 resistors, 2 elecrolytic capacitors, 10 disc caps, 3 inductor coils, and 2 things that look like disc capacitors.

    ReplyDelete
  4. A: Yes, this circuit should be able to do the job, but there is no guarantee that it will work on 145 MHz like mine did, as it is beyond the specifications for either chip. You only need the FM-part of the circuit, i.e. without components connected to everything with AM designation on the pin (pins 5,10, and 16, + the switch connected to pin 15), as you seem to have.

    First test the FM-part of the circuit and verify that it actually receives local FM-stations. After a successful test, remove capacitors C2 (18pF) and C3 (22 pF) to push it up in frequency. Then see if it receives 145 MHz.

    You should definitely have someone local who understands the circuit to help you, otherwise your chances of success will be quite small.

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  5. Q: I love home brewing of circuits. I need the full circuit diagram with component and coil details to build this circuit or any Narrow band vhf receiver circuit to cover 144 to 146 MHz.

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  6. A: I did this modification for fun to see if it would work. I don't really consider this to be a serious receiver for use in the 2 m band. Therefore I would not recommend that you copy what I did unless you find a cheap similar FM-receiver to modify. I don't think the relatively poor performance is worth neither the component costs nor the effort required to build it from scratch.

    You would be much better off with for example a cheap Chinese-made handheld for VHF, like those from Puxing or Quansheng. I have a Quansheng myself.

    You ask about the schematic which I don't really have. The one which resembles mine the most can be found in the upper part of Fig. 8 in the TDA 7021 product specification, excluding everything except the TDA
    7021, ie. also excluding the BC549C transistors, the BAW62 diode and the two lower ICs.

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  7. I did with a pocket radio that has the SC1088... works like a charm... I connected my Cable TV to the radio antenna to see which frequencies had tuning, since Cable TV allows channels at 130-160 MHz band... works great, now I can even know some frequencies tuning "positions" at the radio.... great article, thanks for share with us...

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    Replies
    1. Good to hear Renê!

      (Sorry, due to some filter settings in blogger, this comment has taken a while for me to discover and publish)

      Delete
  8. Ranjit Fernando 4S7RZ22 December, 2013 03:58

    Some years ago I did this with a Phillips IC TEA5591A. This IC has a frequency locked loop. Therefore you can keep the set tuned for days and the frequency does not change. I think the ICs all you guys mention work the same.

    ReplyDelete
    Replies
    1. Hi Ranjit, that's a nice idea for an upgrade! Looking at the datasheet for the TEA5591A I can see the AFC (automatic frequency control) signal which must be the core of this function.

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  9. Hello, I am new to radio electronics, and I am wondering how it is possible to demodulate FM and AM transmittion using the same pin of the chip in this radio?

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    Replies
    1. Hi Emil. This only applies to narrow-band FM which shares some similarity with AM in that they both mainly have a single sideband on each side of the carrier. For AM this is exact, for narrow-band FM it is approximate. See more info here http://www.edaboard.com/thread64317.html

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