EMRFD Message Archive 3581

Message Date From Subject
3581 2009-09-27 22:37:35 soti_156 Poor man spectrum analyzer
Hi everyone, my name is Anthony: I'm writting from Venezuela. I come here because Bob (K3NHI) and others get me very good references of this Yahoo group. As well as some of you know, it's very dificult get dollars in my country, so I cant buy a lot of important parts in USA. But here I'm looking the way to make from 0 my SA.
I'm folowing the popular QST desing of Wes (W7ZOI) and Terry (K7TAU). Now, I'm wondering:
**What topology of oscillator is recommend to use? I'm thinking use a pair of 2N3866 biased at 0,7V@ 2mA to get a good view of weak signal. I'm planing use a ALC to keep the same level during the swing
**¿I could make the ALC changing the bias of base? (Of course taking the signal of the ouput of push pull amplifier)

**¿Can I use a 20Mhz cristal to get the 100Mhz 5th harmonic?

Thanks for your time
3589 2009-10-04 07:36:45 ep_mand Re: Poor man spectrum analyzer
Hello Anthony, are you going to replace POS-200 VCO? If you try, some tips may be given to you by the group members. - It is not usual to run common crystal at harmonics, as crystals are specifically built to oscillate best at a certain frequency only (overtone or fundamental). It can be tried, but the frequency will not by exactly harmonic. Remember to keep the RF current through the crystal at minimum (very low output power), as the crystal is forced to oscillate at abnormal mode. - A safe solution: just make an ordinary 20Mhz oscillator, clip its output to square wave and extract 100Mhz with a filter. (There are many examples in the EMRFD.) - EePee, OH2NFI.
3591 2009-10-04 12:29:25 Anthony Re: Poor man spectrum analyzer
Hi friend. yes, I'm looking the way to replace it with a hombrew VCO, I cant get if from Venezuela. It's a goog idea but whit a butler doesnt work?

Best regards
3799 2009-11-22 02:23:02 ep_mand Re: Poor man spectrum analyzer
Hello Anthony, it is difficult to replace POS-200, as it has quite linear frequency / control voltage relationship. Try to search internet with words: wide, vco, vhf. DL5UP has a replacement, but it is not so linear as POS-200. - EePee.