signal generator - 2nd generation

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ed
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signal generator - 2nd generation

Post by ed » Thu Nov 19, 2020 10:56 am

This last week I are mostly doing......

Finally getting round to putting together the original signal generator replacement.

The original attempt: http://www.vitalstates.org/diy/amplifie ... siggen.htm

I’ve seen the light and abandoned the hair shirt methods of native atmel chips and assembler and embraced the luxury of arduino and it’s comprehensive C++ libraries.

I’m in the process of mapping out a pcb, but in the meantime here is the barebones without the opamp buffer stage:
1k sin
siggen3-1k.jpg
1k square
siggen3-1ksq.jpg
10k square
siggen3-10ksq.jpg
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Re: signal generator - 2nd generation

Post by ed » Thu Nov 19, 2020 10:59 am

and the distortion:

1k
siggen3-thd-1k.jpg
10k
siggen3-thd-10k.jpg
these are much as expected, not quite as good as the wien bridge, but I'm sure fit for purpose.
I will obviously do the measurements again with the buffer in circuit.
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Re: signal generator - 2nd generation

Post by ed » Thu Nov 19, 2020 11:10 am

the circuit so far....I've discovered with designspark there is no need to draw a schematic and then convert to pcb layout...it's possible just to draw the pcb layout to begin with:
siggen-pcb.jpg
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Re: signal generator - 2nd generation

Post by ed » Sun Nov 22, 2020 5:52 pm

an alternative. Uses an 18v 2a wall wart to dispense with the transformer, bridge and 2 x 9v voltage regs. All I have to do now is make up my mind which one. Any suggestions that might sway the decision?
siggen-alt.jpg
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Re: signal generator - 2nd generation

Post by Mike H » Mon Nov 23, 2020 11:36 am

Have you bread-boarded the new PSU idea to prove it works? With the loads it will 'see'. (Can be 'dummy loads', resistors.) Before getting too deep into PCB design. Guess how I know!

I looked up e.g. 3904 datasheet – hFE seems a mite low for my liking. But might be OK. Only one way to find out! Bear in mind hFE falls as Ic increases, in other words, the more you want it to turn on, the less gain it's got – and you should design for the minimum hFE it says on the datasheet – again, guess how I know ...

:)
 
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Re: signal generator - 2nd generation

Post by Nick » Mon Nov 23, 2020 12:21 pm

Resistance isn't futile it's V / I.

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Re: signal generator - 2nd generation

Post by ed » Mon Nov 23, 2020 12:47 pm

Mike H wrote:
Mon Nov 23, 2020 11:36 am
Have you bread-boarded the new PSU idea to prove it works? With the loads it will 'see'. (Can be 'dummy loads', resistors.) Before getting too deep into PCB design. Guess how I know!

I looked up e.g. 3904 datasheet – hFE seems a mite low for my liking. But might be OK. Only one way to find out! Bear in mind hFE falls as Ic increases, in other words, the more you want it to turn on, the less gain it's got – and you should design for the minimum hFE it says on the datasheet – again, guess how I know ...

:)
I haven't breadboarded the power side yet but I did do the spice thing down to 18R, i.e 0.5 amps without any drop showing. I started at 36R(0.25A) but thought I'd stretch it. If the whole thing takes more than 100ma I will be surprised.

I will deffo live test it before sending the pcb order off, but I don't have an 18v wall wart at the mo so it's on hold at the moment.

@Nick...thanks for the alternative. I must confess I don't fully understand how the circuit works in the first place(yet) so comparisons are beyond me at the moment. I must point out that I have zillions of 39xx and diodes in the box so it enticed me down this route.
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Re: signal generator - 2nd generation

Post by Mike H » Mon Nov 23, 2020 1:16 pm

I haven't breadboarded the power side yet but I did do the spice thing down to 18R, i.e 0.5 amps without any drop showing. I started at 36R(0.25A) but thought I'd stretch it. If the whole thing takes more than 100ma I will be surprised.
Fairy nuff! :D
 
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Re: signal generator - 2nd generation

Post by Nick » Mon Nov 23, 2020 1:20 pm

Mike H wrote:
Mon Nov 23, 2020 1:16 pm
I haven't breadboarded the power side yet but I did do the spice thing down to 18R, i.e 0.5 amps without any drop showing. I started at 36R(0.25A) but thought I'd stretch it. If the whole thing takes more than 100ma I will be surprised.
Fairy nuff! :D
Was thinking about the worst case, I guess that is when you need a DC offset on the output signal or a non symmetric wave.
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