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JimB

Breadboards - How bad are they at RF?

Over the years many threads have revolved around the poor operation of a circuit built on a breadbo

  1. JimB
    Over the years many threads have revolved around the poor operation of a circuit built on a breadboard, the type with lots of strips where components can be plugged in to make the connections.

    The usual problem is when the circuit is expected to operate at more than a few kilohertz, strange things start happening.

    When someone asks a question about this, all the experienced guys here will jump in and tell that the breadboard is totally unsuitable for use at "RF".
    What I have never seen is a comparison between a circuit build on breadboard and the same circuit built using a more robust RF technique.

    So, here is my attempt to demonstrate the scale of the problem.

    I knocked up a circuit of an amplifier using a BFY90 transistor which is intended for VHF/UHF working and has an Ft of 1000Mhz (yes 1 Ghz).
    The circuit is not exactly a masterpiece of RF design but it does work when build correctly.

    To test the circuit I used a spectrum analyser and tracking generator, this effectively draws a graph of the gain vs frequency.

    I first built the circuit on breadboard and got a bit of a surprise, something was very wrong, it would not amplify even at low frequencies, I then discovered that the thing was oscillating quite well at about 700Mhz, even with the emitter bypass capacitor removed reducing the gain considerably, it still oscillated.

    Still working on the breadboard, I replaced the BFY90 with a 2N3904, and re-installed the emitter bypass capacitor.
    The 700Mhz oscillation was gone and there was some useful gain at low frequencies, dropping to unity at about 5Mhz.
    But now there was something odd at low frequencies, the "amplifier" was also oscillating at about 200khz.

    I now built the BFY90 circuit on a piece of copper clad circuit board.
    No etching or anything like that, just plain "ugly construction", (see the photo) a very effective way of building RF prototypes.
    The support with the BNC connectors is a test jig which I use for things like this when I want to test a circuit or a component at RF, the BNCs make a convenient way of connecting the test equipment with low stray impedances.

    The amplifier was now stable, no spurious oscillations, the gain was good, rolling off to unity at about 500Mhz.

    Have a look at the pictures, I am sorry that some of them are not very sharp but I think that they show the analyser screen sufficient well enough to make my point.

    So, what can we learn from this?


    1 A circuit which works reasonably well when built with a good RF technique, will give problems on a breadboard.

    2 RF circuits don't work well on a breadboard.

    3 Even "low frequency" circuits can have high frequency spurious oscillations.

    4 If you dont have the test equipment, how can you know what the problem is?


    Edit: Lost pictures restored 6-1-14

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