Hi,
Based on Nigel's post of Audioguru's opamp in the sticky of this forum.
I am trying to figure out how to calculate the values of C1-C4 and R1 (see attached)
My input is a 40 Khz transducer 400R160.
Also if anyone can comment on the values I have indicated and if I should add or remove caps or resistors.
R1 is the load impedance seen by your sensor. What does the sensor's data sheet say?
X(C1) should be small compared to R1 at the op. freq.
X(C2) should be small compared to R1 or 50K (which ever is smaller) at the op. freq.
X(c3) should be small compared to 4.7K at the op. freq
X(C4) should be small compared to the input impedance of the LM833 at the op. freq.
Terminating the receiver with only 3.9k causes its frequency to shift only 0.4% (160Hz) which is nothing but causes its valuable output level to drop one-third.
The resistor R1 should be 10k so that the output level is max and the frequency is changed only a tiny insignificant amount.
The capacitor C1 should be about 0.002uF (2nF) to pass 40kHz into 10k ohms very well.
I honestly be tempted to remove all resistors and caps but I know I would run into problems wouldn't I ?
I will defenitely try your suggestion. My first thaught is won't this make noize? from browsing I saw that the higher the resistors the more noize it makes but I will try and see the results on the scope.
Thanks
Mike,
Thank you, I am trying to digest and understand all this. The value 390000 is this from the 3.9K?
(3900Ω)?
Crap, I did it backwards! In order for the capacitor not to reduce the signal, make its impedance (1/(2ΠfC)) less than 1/10 of 3900Ω, so I should have used 3900/10 = 390Ω in the calculation.
390 =1/(2*3.14159*40e3*C)
or C=1/(2*3.14159*40000*390) = 1e-8 = 10e-9 = 10nF.
The datasheet for the LM833 shows that it works perfectly with a single supply as low as 9V to 10V. So when a 9V battery drops to 6V then it won't work.
I took everything apart and rebuilt again with same results.
I went back to 9V.
This is my drawing:
Values are:
C1,C2,C5 = .01uF
C3 = .1uF
C4,C6 = .047uF
R1 = 3.3k
R2,R3,R5,R7 = 100k
R4 = 4.7k
R6 = 2.2k
The transmitted signal from the Tx paired driven by a PIC is at steady 40.15KHz
I have built the Rx on a separate breadboard to isolate possible noize from the Tx circuit.
When an object (peace of wood 12 in sq.) is placed approx 2 feet away from the Rx/Tx (which are pointing towards the object) I see a change in the signal at pin 7 but within a few seconds the signal goes back to its original state (like before I added the object) without moving the object away.
On top of this the change in the signal is very small for beeing just 2 feet away.