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Friday, 4 June 2010

IF/AF Strip

I have been building a few things lately all using a 612/xtal/612/lm386 IF/AF strip so I made a PCB layout for it as I ran out of manhatten pads and the expresspcb software was easy to use. I just reworked it and this was done using Wine under Mint Linux (see W8DIZ latest series of QQ articles). If you really need it ask and I can send you the .pcb file, but I never drew designed or made a schematic file and I tend to use pretty wide tracks and spacings as much of the use is with older salvaged components and I found it easier to solder some things that way.

I have made using single and double sided PCB with blue press n peel film I don't actually send off to Express PCB to make them up.

The layout is for variable band width xtal filter using diodes as variable capacitors (no identification on the layout) but you could omit them and the resistors and an external pot/voltage control and just use fixed value caps instead. I have used MV209, MVAM109 varicaps, 330pf and 560pf caps for the filter. I seem to use 100pF or 220pf for the xtal filter in/out. Xtals are cheap HC49 types I have bags of and I sort of match them crudely (colpitts oscillator) including the BFO. If don't want to use 4 crystals just bridge the ones you do not want to use. Note the BFO one of the capacitors is a trimmer (don't have a component for it sbut I know for me a normal cap pin out works. The L on one leg of the BFO crytsal can be removed, it allows it to be pulled a bit.

The L on the input can be bridged.

I have included dual voltage regulators providing 8V and 6V from a typical 12V source but a 9V source will do just as well. In that case apply 9V to the 12V location, omit the first regulator (right of 12V in) and the electrolytic but use the regulator upper hole and place a 0.047uF or 0.1uF in it to (ground) centre regulator pin hole. I made up with TO220 but the holes also suit TO92 as well (check the component layout for the TO92!).

After fun and games with a QRP ARCI 72 part challenge entry I never managed to submit in the end I altered the PCB and it now includes RX mute using almost any FET (2n7000, BS170 are what I use) across the LM386 input (watch the FET pins, G goes to the RX mute, S to pin 2 D to pin 3 of the 602/612! Note that the component layout does not match the body of all FET). The 6V RX signal activates a simple NPN transistor (2N2222A, 2N4124 almost anything with Hfe about 150-300 seems to work) to apply a couple dB amplification to the RX signal. I can 'see' a signal on an oscilloscope at about 0.25uV without the transistor and c0.17uV with it.

Any SA612/SA602/NE602 et al can be used all pins are the same. The electrolytics are all 100uF 16V and might not be optimal for every situation, the R and C values I change and suit what I am building. Check your circuit and sure it will provide the values that will work. I do not provide what I have used.

The LM386 I added no volume control as I control volume on the speaker/headphones. Usually you would do this on the input to the LM386 but I grabbed the idea of this from the K1 which does this and uses a pot of the output into an LM380. This set up works for me.

The LO input can be almost anything, xtal, VFO, VXO, PTO or DDS as long as the signal is large enough and it is buffered (J310 FET is ideal) I use anything from a 100pf to a 1000pf cap between the LO and the 602/612.

Obviously all filtering of harmonics of the LO and the RF input need to be handled external to the PCB and I use twin TOKO can (3333,3334,3335) or T37-6 BPF (see GQRP for examples) on the RF input.

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