my new DDS vfo

[PA0NVD]

For my Phillips AM transmitter SFZ 395, I had a tube VFO, but unfortunately, the customs did find it interesting and stole it. So I had to build an other one this time around one of those cheap Chinese DDS vfo's
As an amplifier I did build the general purpose RF amp that I did post a while ago, but a little more power full with a BFQ43 in the final. There are 2 switched resonance circuits to transform the voltage to approx. 80 - 100 Vpp. open circuit. That should be enough to drive most tube transmitters. Beside transforming the voltage, the resonance circuits clean up the signal and attenuate birdies.
One resonance  circuit does 1,8 - 7.5 MHz, the other one 7 - 30 MHz. The tuning cap is a foil cap for transistor MW radios, 2 x 300 pF with the sections parallel
Still to do is adding the band switch to switch the resonance circuits and add a relay to switch the power to the amplifier on when transmitting and make a cover for the VFO

[KK4YY]

Well, now THAT'S ham radio! Finding a way to do something when the odds are against you.

Your construction method is a fine example of simplicity. The fact that you overcame adversity to build it, an fine example of a man with great determination. Well done.

[PA0NVD]

Thanks for the flowers..
Just enjoy designing and making things

[K1JJ]

Very nice, Nico!

I like your tuned circuits. They do help.



Great minds think alike....  Funny coincidence, but I built one up too, about 3 years ago. I use 11N90s as the finals to give me 40 watts or so to drive my 4-1000A class C plate modulated rig.   I use it on 160-40M as a power VFO.  I found the DDS signal got cleaned up a lot once it went thru the 4X1 tuned grid circuit.

I like the DDS accuracy and stability, broadband, no tuning -  and once amplified, for the price, ya can't go wrong.

T

[PA0NVD]

Indeed Tom, you are right. For that price you can't go wrong
I have a question, When receiving, I switch the amp after the VFO off. But I am worried a little that the VFO signal still can be heard at the receiver. How is your experience in that aspect?
Regards

[K1JJ]

Nico,

Yes, there is a small DDS residual signal as is.  I see I had the same problem and put a relay in the RF output. I think by disconnecting the output helps a lot - and maybe experiment with a 0.1 across the DDS output to ground would help, I dunno.   In the end, tight shielding may be the only effective solution.


The designer should have included a mute input. Turning the power amp vcc on and off is no solution.  

Also, I'll bet if you knew which lead in the DDS to key/bypass, it could be muted, but it would be easy to blow it up experimenting... :-)


T


 

[WD5JKO]

Nice VFO Nico,

Here is another amplifier that could have many uses such as a VFO amplifier:

https://qrp-labs.com/linear.html

26 DB gain, 10 watts, all for $26.00 in Kit form.

Jim
Wd5JKO

[PA0NVD]

That's a cute little amp Jim. Thanks for the info. But I find more pleasure in designing and building, and I had everything laying around, so it did cost nothing. In addition, when I have it imported into Costa Rica, it will cost me at least twice as much. I did request Fedex and DHL to sent a second hand bread maker of 30 $ from the USA  to Costa Rica. They did calculate 280 $ for the transport  So, no-go

[PA0NVD]

Ok Tom, thanks for the info.
Lets start with the tight shielding and switching the amp off.  Shielding is easy, the case is PCB material.
If I still receive the VFO, I will look further for a solution using a relay that shorts the output via a cap or something like that

[KK4YY]

If I still receive the VFO, I will look further for a solution using a relay that shorts the output via a cap or something like that

Maybe have a relay toggle a 'bit'. It will still oscillate, but on a different frequency.

Don

[PA0NVD]

I did measure, and sure enough there was quite some signal left when the amp had no power. I measured more than 200 mV.
So I did add a reed relay (low inductance) to short the VFO output via a 0.1 uF multi-layer ceramic cap. That lowered the voltage to 20 mV tt, not enough. The impedance at the vfo output seems to be quite low. So I did add an other reed relay to short the resonance circuit. Now I can't see any y RF anymore, in the uV range. So lets hope that the level is low enough.
Thanks for making me aware that indeed there was a problem.

[PA0NVD]

I did measure the output impedance. The vfo generates 80 Vtt open and 70Vtt over a 10 kOhm load. Quite sufficient for most tube transmitters and a nice clean signal due to the resonance circuit at the output. Happy with the results

[PA0NVD]

The schematic diagram attached