Message-ID: <4FC00C2E.6070101@iup.uni-heidelberg.de>
Date: Sat, 26 May 2012 00:48:14 +0200
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: LF: 475 kHz exciter
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Hi John, MF,

Sounds good! How many steps left until i see you on the band? :-)

These days i'm building a PA for the coming 630m band. It already works 
but i need a suitable VFO to drive the FETs. I've been playing with a 
frequency generator based on a CD4093 and a 4013. With a RC circuit on 
the NAND gate i'm generating a 950 kHz signal which is then divided by 2 
to obtain 475 kHz. It works but the frequency is drifting several 100 Hz 
in a minute, so actually it does not work anyway ;-)

I've been thinking about using the 2nd D Flipflop of the 4013 and build 
a divide by 4 circuit. With this exciter one could easily use a HF TRX 
like the IC-706 and drive the exciter by transmitting QRP in the 160m 
band. 1900 kHz will then result in 475 kHz and the frequency accuracy 
should be fine. In Split-mode one could transmit on 160m and receive 
directly on MF. Using 3x 150 Ohm / 2W in parallel and another 1 kOhm 
resistor in series with 2 antiserial Z-diodes should be OK for driving 
the input of the 4013 and getting a perfect SWR for the TRX... Maybe 
this is how i will do it.

But a SI570 based VFO which is part of the PA circuit would be very 
nice. I like it ultra-compact so a tiny LCD showing the frequency and a 
direct key-input should be fine. Also some up / down buttons should 
work. I simply have to learn how to work with the PIC/ATMEL stuff...

Once the PA is complete i will publish the circuit here. It runs on 12V 
DC, class-D, 2x IRFZ48N, FT140-77, a 7667 driver, 4 stage LPF using 
resonant circuits are used, 6.8 nF + 50 turns @ T80-2 and a switch for 2 
power levels 10W/100W is planned... Dimensions of the complete PA are 
0.1m x 0.15m x 0.03m

73, Stefan/DK7FC




Am 25.05.2012 20:56, schrieb John Rabson:
> LF,
>
> This is now working! I used the QRP 2000 synthesiser (completed with assistance from G8DQX) and have added a divide-by-100 unit based on the design by G3KAU with some modifications:
>
> - the original design assumed 10 to 50 W sine wave input. I left out the attenuator because the synthesiser delivers 2.6 V peak to peak square wave, which is enough to drive the 4017,
>
> - I was not intending to use anything other than 137 kHz and the power amplifier requires square wave drive, so I left out the post-divider filters,
>
> - the output from the second divide-by-10 stage is sufficient to drive my Class E Power Amplifier so I didn't bother with a buffer,
>
> - there are several versions of the 4017 but with different capabilities and voltage ratings. I used the CD74HC4017,
>
> - and provided a 6 V rail rather than 13.
>
> The unit is now in a die-cast box and I shall shortly be testing it with the PA into a dummy load.
>
> After that, I need to look at software for driving the synthesiser.  For test purposes I have been using the SI570 programmes referred to in the documentation but they clearly are not appropriate for transmission.
>
> Many thanks for all your help and encouragement.
>
> 73
> John F5VLF
>