GDO (Fet)

[boiss]

Hi All, While waiting for my transformer wire thought I'd knock up a GDO using a fet. Seems to work "But" the frequency range is pretty crumby. I wound 2 coils basically as per the instructions, 1 tunes 170KHz to 230KHz and the other 520KHz to 800KHz, a far cry from what was quoted. I'm using a variable cap from an old trans radio with the MW and FM sections paralelled up, the coils are 0.5 inch dia with 240 turns and 600 turns both centre tapped. Any suggestions please?
Mike

[Station X]

The coverage of a GIVEN coil depends solely on the tuning capacitor used with it. For higher frequencies you need less capacitance and for lower frequencies you need more capacitance.

For a GIVEN tuning capacitor used with ANY coil the max and min frequencies will always be in the same ratio.

I think you need to experiment with tuning capacitor arrangements.

[Biggles]

I presume that when you say you've paralleled up the MW and FM sections that you've also paralleled up the oscillator sections also. Some transistor radio tuning capacitors don't seem to have the same maximum capacity as the older valve types, for some reason, I've found. It's also important to keep "stray" capacitance down in the circuit, to ensure the higher frequencies aren't compromised.

Biggles.

[boiss]

Hi All, I changed the cap to a simpler 2 gang jobbie but still can get nothing like the range of the original design by SM0VPO. He used a 2x270pf in parallel with 2x20pf. with 300+300 turns, me 105KHz -170KHz, Him 0.3MHz-1.4MHz
with 120+120 turns, me 470KHz-756KHz, Him 1.0MHz-3.5MHz
My cap appears to be around 350pF per gang and the higher freq coil appears to be 670uH at least it appears to resonate at 159KHz with1500pF in parallel. wiring is short and of rats' nest type.
Design has centre tapped coil (virtual earth) and cap moving vanes are earthed ie caps are in series and in parallel with the coil.
Something odd about his or mine??
Mike

[Station X]

You need less capacitance and/or less inductance.

Were the coils specified as air cored?

I think I'm right in saying that a brass slug lowers the inductance of a coil, whereas an iron dust slug lowers it.

[boiss]

Hi Graham, Brass cores reduce inductance and magnetic materials raise the inductance. My problem is the poor range with each coil, on the lowest range coil the range is only 20% and on the next one it's about 70%. This is vastly different to the quoted design. the only difference I have made from the original design is a 120pf cap going to the gate instead of 100pf which I don't have.
Calculating the in circuit cap values gives a max value of 171pf and a min value of 66pf which appears somewhat on the high side.
Mike.

[GMB]

Assuming that you have checked that your variable capacitor doesn't contain integral trimmer capacitors that are limiting the range then I suggest you try replacing the variable capacitor with fixed ones so as to find out whether the unwanted capacitance is coming from the variable or from the rest of the circuit.

[boiss]

Hi GMB, the trimmer caps were set to minimum capacitance. I have changed the gate coupling cap to 68pf, this made a very slight change upwards in frequency. Taking your advice I disconnected the variable cap. On the low band the freq increased by 5KHz and on the higher band the increase was 35KHz. There is something like 55pf of capacitance lurking somewhere,the circuit itself would have very low capacitance by virtue of having short leads and hanging in the air. Coil plug and socket arrangement is a 5 pin DIN. Coil is pile wound and lacquered on a 0.5" dia plastic tube. This is the same arrangement as the original design and the same type of fet.
In the original design the range with this coil was 1 to 3.5MHz !
I have measured the coil + a parallel cap using the gdo and an Rx and the coil works out to be about 630uH. A quick calc tells me that resonance at 3.5MHz
with this coil requires " 3pF ". This would be entirely unrealistic, especially as in the original design there are 4 main variable caps in series /parallel + their 4trimmers.
Cheers,
Mike.

[GMB]

It seems to me that you have two problems - the tuning range and the coil size. I measured the inductance of the coil of my own dip oscillator for the same range and it was about 90uH which seems to be a more reasonable value. Could it be that there has been some confusion about the tapped coils resulting in a doubling of their size?

Once you have a coil that tunes the low end of the range on maximum capacitance you can then better asses the capacitor range.

[oldeurope]

Excuse me, I have a silly question. What does GDO (Fet) mean? I tried to get it from the text but I am not sure. Is it a griddipmeter using a field effect transistor?

Kind regards,
Darius

[GMB]

Yes.

[ppppenguin]

Originally Posted by oldeurope

Excuse me, I have a silly question. What does GDO (Fet) mean? I tried to get it from the text but I am not sure. Is it a griddipmeter using a field effect transistor

Gate Dip Oscillator. By analogy with Grid Dip Oscillator.

[oldeurope]

Thanks for explaining.
Darius

[Ed_Dinning]

Hi Mike, any chance of you posting the circuit?

Ed

[boiss]

Hi All, here is the article:-http://web.telia.com/~u85920178/use/gdo.htm
One difference is in the diameter of the coil former 1.4cm against 1cm and the plastic used, mine are the plastic tube inside cash register paper rolls and not childrens pens.(larger dia more inductance, 50% maybe?)
the only other difference is that my variable cap has one gang marked A (Antenna?) and the other O (Oscillator?) probably the gangs have different values, But the first cap that I tried was a similar type to the one used in the design only the C values were not known.
As for the number of turns, It is very clear in the design. Wire dia is around 0.09mm.
Mike.