The Direct Conversion receiver with VFO and XO
Here is a basic direct conversion receiver. the construction is rather simple. you will need to wind L1 and L2 on a 1 1/2" to a 2" form. wrap 24 turns of #22 gauge magnet wire tap the 12th turn. L1 acts as an antenna tuner. L2 tunes the band. this receiver will cover 80 and 40 meter short wave bands. it will receive AM stations, if you want to receive ssb or cw you will have to build a vfo or xo. the xo will only let the radio tune to the xtals fundamental frequency. this is good if you want to receive limited frequency.
Receiver
- L1 - 22 turns #22 gauge magnet wire taped at 12
- L2 - 22 turns #22 gauge magnet wire taped at 12
- C1 - 10-365pF Air variable capacitor
- C2 - 10-365pF Air variable capacitor
- C3 - 5pF
- C4 - .043uf
- C5 - .1uf
- C6 - 10uf
- C7 - .043uf
- C8 - 100 uf
- C18 - .001uf
- R1 - 100K/270K ohm you can try diffrent values till you find the right one for you. i use 100K
- R2 - 4.7K
- R3 - 10K audio pot. thats your volume control!
- U1 - lm386
- D1 - Germanium diode
- Q5 - any small signal transistor should do i use 2n3904 or 2n2222
- AS _ antenna switch
The vfo is a variable frequency oscillator. this will let you tune 50khz (+ - 25khz) from center frequency. The frequency the oscillator is the set by the combination of the inductor and capacitor LC and by making the inductor variable you can tune the oscillator to the exact frequency Frequency (F) = 1 / (2 * Pi * Sqrt(LC)). in this version i use adjustable inductors and fixed capacitors. L3: 4.6-8.5 uH adjustable RF coil and C9 25pf will get you in the 80 meter band, L4: 2.4-4.1 uH adjustable RF coil and 130pf with will get you in the 40 meter band
you could use adjustable capacitors also for more tuning freedom. in my next receiver I'm going to use air variable capacitor for C9 and C10.
The vfo should not be any way connected to the receiver except for the power supply. you will need a well regulated supply to keep the oscillator from drifting off frequency. i always use the lm78xx regulators and the voltage should be around 6v dc. and you always want to supply the regulator with at least 2 volts more than the rated output of the regulator.
alignment procedure
you will need one of the following,
- frequency counter,
- set R6 to center position
- adjust Lx to desired frequency
- short wave receiver
- set R6 to center position
- tune the short wave receiver to the desired frequency
- tune Lx till you hear it whistle then tune slowly to you hear nothing
- you should hear the sound when the slug is turned a little left and a little right, but in the center null sound or zero beat.
- crystal oscillator
- set R6 to center position
- power up the receiver and the XO
- tune the receiver till you hear a hissing sound.
- tune Lx till you hear it whistle then tune slowly to you hear nothing
- you should hear the sound when the slug is turned a little left and a little right, but in the center null sound or zero beat.
- your vfo will be tuned to what ever frequency the crystal is, but now you wil have 25khz swing up and down.
- turn off the xo
now you need to tune the receiver till you hear a hissing sound. now your close to the vfo frequency. tune the vfo with R6 slowly if the band is busy you will hear cw or ssb. try turning C1 or C2 a little also, before you know it you will be a pro at it.
VFO
- Q1 - 2N3053
- Q2 - 2N3053
- Q3 - MPS2907
- L3-C9 L4-C10 (F in hz) = 1 / (2 * Pi * Sqrt(LC))
- C11 - 50PF
- C12 - 380PF
- C13 - .01UF
- C14 - .01UF
- C15 - .01UF
- R4 - 220k
- R5 - 2.2k
- R6 - 100k
- R7 - 47k
- R8 - 22k
- S1 - SPDT
the xo will only let the radio tune to the xtals fundamental frequency. this is good if you want to receive limited frequency. i built one for my set i use it as a backup oscillator and for receiver alignment. the xo is rather stable and could be used for the beginning of a cw transmitter.
XO
- C16 100-pF, 15-WVDC ceramic-disc
- C17 100-pF, 15-WVDC ceramic-disc
- X1
- X2
- Q4 - NPN small signal high frequency, 2n3904 or 2n2222
- R9 - 470k
- R10 - 4.7k
back to My CRYSTAL RADIO projects
Formula: Frequency (F) = 1 / (2 * Pi * Sqrt(LC))
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