Discuss and share your experience on homebrewing
I made some tests with my homebrew vfo.
You're absolutely rihgt.The voltage at the base is positive with LC disconnected, but while working it drops at a quite negative value. BUT: THE DRIFT IS EXTREMELY LOW. I coupled the LC circuit to tha base with 470 pf and the variable capacitor through a 390 pf capacitor to reduce the frequency span.I used a T 80-2 toroid core with 18 turns of 0,8 mm copper wire. I followed for the oscilator and buffer the PY2OHH circuit and the third one has the emmitter directly to ground,with the filter at the output.
Well, as soon as possible I'll write further observationa.
My best wishes and good time dear friend.
Hallo dear Sarma,
Good to see you again.
Please, I want to ask a question to you.
I have a KENWOOD TS-770E all mode dual band RTX. It works good but there is a problem in the display.
When I switch on the radio, I read the first number only of display. If I switch off and on the power switch, I can read the second number, and so on the third, the fourth etc..by turning off and on again the power switch.
Could please you tell me something about this that can help me in solving the problem?
Thanks very much dear friend and good work,good things!
Hallo again Sarma, how're you doing? Hope very good!
I tried only to clean some contacts to the kenwood ts-770, I had no results but I've to follow all your remaining tips.
But now I made another vfo following the schematic in this site,remember the discussion about positive or negative voltage of the oscillator transistor. I've assembled yesterday night the first and the second stage of this vfo. But this time I need a 9 Mhz vfo, because I'm going to make a 21 Mhz ssb rtx. I made the oscillating circuit oscillate at about 9.300 Mhz and surprisingly it's very stable,like the previous one! And think that I used two 1,500 pf styroflex capacitors instead of 1,000 pf, between the base and emitter and between emitter and ground! The circuit oscillates the same!! In the tank circuit, the one that determines the oscillating frequency, I used first a 1,000 pf styroflex capacitor and then I replaced it with a 910 pf styroflex capacitor. I am testing its stability,and it's very, very stable!! But I cannot calculate the values of the output filter for 9.3 Mhz. Please could you help me in calculating the values of the components for the filter for any frequency? I also wonder why the values of the capacitors are not the same at both ends of the filter (in the one of the article they are 470 pf and 3,900 pf), but I think that is because the out impedance is to be 50 ohm, do you think so? Maybe someone published some table to easily calculate these values but I didn't find any.
I'd like to thank you personally for your precious advices, but from here I give my best regards and say Thank you very much Sarma!
P.S. I didn't check the value of Vbe voltage, think it's unnecessary now.
73 from IK6AIZ.
MULTIPURPOSE VFO FOR YOUR RIGS, by N.S.Harisankar, VU3NSH. Is this what you have tried Please?
If so, i feel that the same values work. Generally we need to remove harmonics, Third harmonic in particular. Perhaps the filter designed by VU3NSH works for that purpose.
yes,it is the vfo that you described,and we talked about some time ago.So, if for instance you make it work on 4 mhz,the output filter cuts the third harmonic and not the second, if you make it work on frequencies above 5-6 mhz it cuts also the second.So, if I understand well, it's a low pass filter with cutoff on 9 mhz. If I'm right,that's great,because my last vfo works on 9.3 Mhz,like I told yesterday.I want to build a 21 Mhz rtx, I have some 12.288 mhz quartzs for the filter and bfo.I want to put in the ladder filter the same capacitors that I used for the 10 mhz filter,I think that there is no big difference. The rtx I want to build for the fourth time' is the famous bitx 20 in 15 meters version I hope it will work on this frequency!
Well,again thanks a lot, i'll tell you about my tests and results.It's always beautiful to learn more.
Excuse again Sarma,
the reasons for wich i disturbed you this time about this vfo include the fact that in the article there is not the value of inductance of the low pass filter,and I wondered how to calculate it for a certain cutoff frequency.I was forgetting to tell you this.
Thanks a lot again!
if your need is to be able to re design the filter to your needs, a free software at http://www.aade.com/filter.htm helps
please download and little effort around it , you should be able to make your own filter.
now a days these modern calculators do help. Incidentally, in the same file there is a link to take you to inductor design
otherwise you may also use mini ring core calculator software for toroid and aircore inductors to the arrived inductor value.
all the best
Thanks a lot. I'll try to calculate it by using this program. Maybe the author forgot to write the value of the inductor for the LPF, so I need to calculate it by putting the existing values of the condensers. So doing, it would be possible to obtain the value of inductor and all characteristics of the lpf.
Thanks again and best regards.
73 from IK6AIZ Ciro.
I'd like to update my experiences.
Today I tested the filter but..no output at the 3.9 Kpf side! I made the filter exactly as described.
I tried also to disconnect the 3.9 Kpf condenser, and a very small signal arose! Also the signal increased when I eliminated the ferrite nucleus of the coil. At the 470 pf side the signal is about 0,7 volts, at the opposite (with no 3.9 Kpf) the signal is about 0,2, and by eliminating the nucleus the signal is about 0,3.
Maybe I made some mistake, but I'm afraid that the filter isn't good for 9 mhz output from the vfo.
See you soon,and thanks again.
73 from IK6AIZ Ciro.
please eliminate the filter and measure .
I mean , remove all the three components and measure
using an rf probe
and let us see the measured value
it would be low igf the front end cap is of low value like 10pF , so please ensure that it is atlease 1kPF
Yesterday I made some measurements. Disconnecting the filter I see 1.3 volts on the collector of 3.rd transistor. In this situation, with the spectrum analyzer I see the fundamental frequency, the second harmonic down about,say, 20 db, and the third down about 20 db more. By re-connecting the filter, the signal is about 8 db down (it is 1,1 volts), but regulating the ferrite core the 2.nd and 3.rd harmonic disappear. I increased the voltage from 9 volts to 10, 11, 12 volts and the 2.nd and 3.rd harmonic appear, but at about 45-50 db down. I used 180 pf at the collector side of the filter and 330 pf at the 50 ohm side (i saw the data published by PY2OHH for his homebrew vfo based on Hamradioindia VFO).
My best regards dear Sarma,73 from IK6AIZ.
Last edited by IK6AIZ on Mon May 28, 2012 3:43 pm, edited 1 time in total.
The only thing that makes me not completely satisfied is that I made the modifications on the filter based on pure suppositions, even if I referred to PY2OHH's tables,and so because I'm not good with calculations. Certainly if I'd have time I'd study some more to be familiar with calculations, and this IS the aim of our splendid hobby,but I promise to myself(and also to you that are so available and kind) to find time to do it. However, I'll go on with construction of 21 mhz rtx but I'll try to understand and calculate what are the in-out impedances of the filter I have made. I'll certainly let you know.I think, however, that the best solution in this case is to transform first the collector impedance to 50 ohm through the 4:1 transfo, and then to easily calculate a proper filter by consulting the tables and whatever, knowing that you're finally working on 50 ohms. This is the right (and simplest) way, for me, according to my poor knowledge..
Thanks a lot and see you later.
Good things and 73, all the best.
P.S. Could you explain the position of 1Kpf cap? Maybe on the 50 ohm side opposite to collector? Do you mean this for front-end?If you think, I'll do these mods and try.Thank you!!
perhaps it calls for studying the complete network design at my 67th year of age
you would appreciate that it is more mathematical treatment and linked to amplifier basics like common emitter amplifier and its output impedance.
i would rather suggest you to do that exercise.
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