It does give a TC of around 1 Second and I always err on the high side with RC networks.
It is subjective though.
It does work better than expected however.
Working on auto switching in _real-time_ at the moment and putting this baby in a box with input attenuators for 1V an 10V per division.
I will put the auto switching in a separate box too so that everything is modular and can be used for something else in the future.
Gimme time to draw the circuit(s) as doing it in text mode is NOT easy.
The only thing I have to be careful of is crosstalk from the earphone output(s) to the mic input in simultaneous soundcard access mode, but, I do have a workaround if it is required.
The O/P CR is a differentiator and I put the C in to _isolate_ the input from the output.
Also if the input resistance, (note resistance like in my vertical amp circuit), of any other home brew vertical amp, (it doesn't have to be mine), is low to medium then the high value C will aid in reducing any differentiation and isolate the two.
Although at about 2KHz clock speed I probably do not need to worry.
Differentiation is precisely what you don't want a chopper to do. When the chopper's on, you want the signal grounded out, when it's not, you want raw, undistorted signal.
A capacitor certainly doesn't isolate it, in any case. It has to be charged up by the input signal, the larger the value, the harder it sucks on it. You're putting your signal, input and output, through a fairly brutal low pass filter.
If the chopper is distorting the input, your input resistance of 1K may be too low.
Back to your reply:-
In the above cct the C 0.1uF in that case is acting as a DC blocker exactly the same as mine.
Mine has 100K to ground on the input and 1M to ground on the output, there is a reason, - why?
The TC, (time constant), on the O/P when not connected to anything at all is 1 Second.
This approximately gives me a turnover real audio frequency of 1Hz at 6dBV down. The upper limit depends on CCT strays. This TC is OTT for a 2 KHz chopper clock signal but I tend to go OTT at times.
I could have used 0.01uF and this would still work because the TC is 100Hz before turnover occurs.
The C is purely for AC coupling and is technically in fact a high pass filter not low pass. High-pass filter - Wikipedia, the free encyclopedia Low-pass filter - Wikipedia, the free encyclopedia
I want the 2 KHz to go through as it is AM at any random DC, in other words it is effectively a carrier of 2 KHz modulated at any random DC level.
It goes without saying that with a little jiggery pokery an AC signal modulating this carrier could be used to extend the BW of the MBPs microphone audio from DC - 20KHz fairly flat.
It works and if you want real AC coupled oscilloscope displays then I will create some when the box is finished.
And finally the WHOLE system is AC coupled so I don't really need the 1uF and 1M at all.
But this is modular project which leads to what I was taught......
NEVER put DC into any random black box system without some kind of blocking capacitor and in my case a 1M shunt resistor.
EDIT:
A simple CCT change that generates a 2KHz carrier at 1V P-P, where a maximum of 2V P-P signal, centred at 1V instead of ground, can modulate it from DC to say 200Hz.
Code:
R3 = 1M
O/P O-----------------*----/\/\/\----+
| __|__
| /////
=== C1 = 1uF
|
LF AC OR R1 = 1K |
DC I/P O----/\/\/\----*--*--+
| |
\ o C Q1 = BC549
/ \ b
R2 = 100K \ \|__o----/\/\/\----O Square wave in from approx 2KHz multivib.
/ /| R4 = 22K
\ |/_
/ o e
| |
_GND_ O----*-------)-----*--------------------O
__|__ |
///// |
|
+1V REF O------------+
Last edited by wisecracker; 01-31-2015 at 06:31 AM..
Reason: Add simple modified circuit.
This User Gave Thanks to wisecracker For This Post:
Another variant using an extra transistor as an input offset bias from + or - 0.5V.
This generates the regulated 5V rail for the clock and creates a virtual earth, IF REQUIRED, for the input from a non regulated negative rail from a single PP3 battery only. The -ve BIAS is taken from the volts drop across a(n) LED through a PNP transistor. For all intents and purposes under normal conditions the LED is a regulator of around 1.8 to 2.1V. This is not critical...
I now have the second CCT but with contol of any offset carrier if needed.
Decoupling may be required from A to _GND_ because of the 2KHz clock, we will see... ;o)
Bazza...
Code:
R3 = 1M SUPPLY FOR CLOCK AND BIAS TRANSISTOR
O/P O-----------------*----/\/\/\----+ +5V O----*----------*----------+
| __|__ | | |
| ///// | ^ |
=== C1 = 1uF | FROM R6 |
| | + + |
LF AC OR R1 = 1K | --+--+ __|__
DC I/P O----/\/\/\----*--*--+ / \ | ZD1 ===
| | 2KHz TTL '___' 5.1V |
\ o C Q1 = BC549 +-+ +- | BY1 |
/ \ b -+ +-+ | |
R2 = 100K \ \|__o----/\/\/\----O | __|__
/ /| R4 = 22K CLOCK IN | ===
\ |/_ O | |
/ o e | | LED1 |
| | | | |\|>>> R5 = 120R |
_GND_ O----*-------)-----)--------------------*------*-*---| +---*---/\/\/\--*
__|__ | | | |/| + | |
///// A +-----*--*--/\/\/\---> +5V | | |
| R6 = 3K9 +---/\/\/\=-+ RV1 = 2K2 |
"A" SET BIAS, -0.5V TO +0.5V | BIAS -+- |
AS REQUIRED | e c | |
+-o o-----------------------)----------------+
\ / |
b _\|/ Q2 = BC557 |
--+-- |
| |
o--------------------------+
Last edited by wisecracker; 02-01-2015 at 05:49 PM..
Reason: Add line at the end...
This User Gave Thanks to wisecracker For This Post:
I really don't get it. A chopper is supposed to make a squarewave of nearly the same amplitude. Putting a high frequency chopper through a brutal low-pass like that would turn it into a tiny triangle wave, no?
Hi all...
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01-31-2015
