80M


Posted on Feb 5, 2014

This 80M receiver design includes the necessary switching to allow the SSB filter and first I. F. amplifier to be used for transmitting. The receiver stages from R. F. input to speaker output will be described in detail below. This type of band pass filter gives very good attenuation at frequencies that are higher than the passband. This is desirabl


80M
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e because diode dbm`s are very prone to harmonic mixing. This filter should keep out strong VHF signals. The good H. F. rejection of this filter will also reduce the possibility of I. F. breakthrough. The 10 microHenry inductors were hand wound on TOKO 10K coil formers. I used 25 turns of thin copper wire. To calculate the inductance of a home brew coil on a toko 10K former, use the following formula. For best results, use a high-level DBM. Mixers that have very good strong signal performance require +17 to +23dBm of local oscillator drive. Unfortunately, such mixers are very expensive. It is not difficult to build a home made mixer with similar performance. The mixer above uses a ring of four 1N914 high speed switching diodes. The switching diodes can be replaced with proper Schottky mixer diodes like the BA481. With 3. 7MHz on the R. F. port, 11. 5MHz on the L. O. port and a 7. 8MHz I. F, the 1N914`s seem to be just as good. The transformers were wound on ferrite toroids. I used eleven turns, trifilar wound on ferrite toroids. See the receiver section of the SSB transceiver project rx. html for details. Local oscillator injection is +23dBm. After the 3dB attenuator pad, the local oscillator power is +20 dBm (100mW. ) To get the best performance from a dbm, all ports should be properly terminated. It is particularly important that the I. F. port is properly terminated. The diplexer between the mixer I. F. port and the post...




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