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Re: LF: Today's soldering

from [Andy Talbot] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: Today's soldering
From: Andy Talbot <[email protected]>
Date: Wed, 15 Jul 2015 14:00:33 +0100
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Assume you've tried putting an R||C in the source to bias the BF245 back a bit and still ensure oscillation? 
Does it make any difference to current consumption?

Or run off a lower voltage.  Try a 74HC4013 instead of an HEF4013 and run that off 3.3V as well as the FET;  may work.

I remember seeing the circuit of an old Phillips or Motorola personal radio of the 1970's era.  All the RF stages were separately floating wrt. ground for both +ve and -ve rails, properly decoupled with capacitors.   Then several stages were connected in series at DC to run off the high voltage rail that the battery gave  without wasting power dropping the volts needlessly.  

Of course, each stage is forced to consume the same current, and biasing has to be arranged to drop the correct voltages across each stage,  but it was a neat way to conserve power in the front end and IF amplifier stages. Can't remember, but I suspect it may have used J-FETS 
Constant current operation is a good way to bias J-FETs.  They're notoriously difficult things to get biased exactly where you want them, so just force the current you want and let Vg sort itself out.

Just downloaded the data sheet for the BF245 to see if it was anything special, then noticed it's the wire ended equivalent of the BF545 I already have the data sheet for.

Andy  G4JNT


On 15 July 2015 at 13:16, DK7FC <[email protected]> wrote:
:-)
Yes, there's a JFET on the other side. So far a BF245A.
It works well. But the circuit consumes 3.5 mA. I thought it could be less. It is running at 5V DC and the xtal frequency is 1.8432 MHz.
I looked for 74HC40.. devices but they  seem to consume even a bit more.
Circuit attached.
For a home operated RX i would not care about the power consumption in certain ranges but here, for the solar operated system i want to save any mA. But i think i have to be happy with the result.

So far, the complete RX, i.e. 2 mixers, LO and a LM2936 consumes 8 mA! I hope that the gain of the SA612 will suffer...

73, Stefan

Am 15.07.2015 07:37, schrieb Andy Talbot:
OK, I'm all confused now.

The [HEF] 4013 is a dual Flip-Flop;  how have you made it oscill with a crystal ?

(Or have you hidden something under the board)

Andy  G4JNT

On 14 July 2015 at 22:16, DK7FC <[email protected]> wrote:
Hi all,

Just for fun i like to share large images showing compact circuits ;-)
Today, the LO for the stereo MF RX
https://dl.dropboxusercontent.com/u/19882028/MF/20150714_231144.jpg

73, Stefan

PS: Filling the summer hole :-)



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