Does anyone know of a circuit that can trigger once a certain frequency is passed?

Specifically, I'm looking for something with an input of a small AC signal (50 - 250 mV peak with a 2.5V DC bias) that can activate a high pass filter once the 1kHz point is passed. I'm thinking of something that uses a BJT to switch in a resistor to make the filter but the frequency threshold detection for such a small signal is blocking my progress.

 

Hello,

There are chips for tone detection.
the NE567 is one example.
(see the attached datasheet for more info)

Bertus

 

Thank you Bertus, at first glance this looks perfect. Do you know of any discrete analog solutions though? I have a parts database which I'm supposed to use parts from and we don't have ripple counters listed. I might be able to get away with it since I'm just building a prototype. I had been looking at using ripple counters (yes, I know not analog, but cheap and easy to get) and the circuit found here: http://www.hanssummers.com/sfreq.html

The response time on that isn't good enough for my application though since the frequency of my signal accelerates too quickly for it to be useful.

 

Hello,

Wich frequencies do you want to detect?
Is there a range of frequencies?

Bertus

 

The signal goes up to 2 kHz. I'm looking to apply a filter once it gets to 1 kHz that heavily filters frequencies that are lower since they are probably noise. A frontend low pass in the circuit already deals with the higher frequencies

 

Hello,

Do you want to "gate" the signal between 1 and 2 kHz?

Bertus

 

I'm not sure I understand what you mean by "gate."

I want to look at the signal before it gets into my amplification stages and then apply a high pass filter based on whether the signal frequency is above the cut-off or not.

 

Hello,

There are analog switches, like the 4066.
These can pass a signal or block a signal dependend on a digital control signal.
If the control signal is high between 1 and 2 kHz, the signal will be passed.
Outside this range the signal will be blocked.
This is called gating a signal.

Bertus

 

Ah, I see. I'm not gating it then. I'm just filtering it once I know it has passed a threshold. It gets to its final frequency within about 20% of the runtime so after this I'm trying to filter out anything with a lower frequency.

The application is very noise sensitive which is why I'm trying to do this. Originally we were going to have a bandpass filter which tracks the signal as it gets faster, but this is hard/expensive to do without a micro and with a quick response time on the filter.

 

Why can't you just use a multi-pole, sharp roll-off HP filter with a 1kHz corner frequency and leave it continually connected? Why do you need to switch it in and out?

 

Hello,

The following PDF's might interest you:
sloa058 A Single-Supply Op-Amp Circuit Collection.pdf
Chapter 3 is about filters.

sloa065_filter_design_on_a_budget.pdf
sloa096_More_filter_design_on_a_budget.pdf

Bertus

 

Why can't you just use a multi-pole, sharp roll-off HP filter with a 1kHz corner frequency and leave it continually connected? Why do you need to switch it in and out?

The original idea was to track the signal around with a filter. Once it first comes on the frequency can be anywhere in the range so we are wide open. Once we detect it, we need to zoom in on it and filter everything else out as it accelerates.

This is just a pain in the butt since I don't get to use any micro for it. We had an adjustable bandpass but we had to count ripples to get a frequency to adjust the filter and this took too long considering the accelerating frequency of the signal.

 

Hello,

When you want to make a variable filter, you could have a look at switched capacitor filters.
The LMF100 is one and is used on this page:
http://www.swarthmore.edu/NatSci/echeeve1/Ref/FilterBkgrnd/SwitchedCap.html

Bertus

 

So your input is a swept frequency. Is it a single tone at any given instant? What is the wave shape and the amplitude?

 

Hello,

There are analog switches, like the 4066.
These can pass a signal or block a signal dependend on a digital control signal.
If the control signal is high between 1 and 2 kHz, the signal will be passed.
Outside this range the signal will be blocked.
This is called gating a signal.

Bertus

Hi bertus:
Do you have the practical application info about what you said with CD4066.
I always used CD4066 as the switches.

Is this similar what you said:
Input signal → The differential circuit, the integrator circuit → 74HC14 → CD4066 control pin.

Input signal → CD4066 Sw → output signal.

Thanks.

 

The signal goes up to 2 kHz. I'm looking to apply a filter once it gets to 1 kHz that heavily filters frequencies that are lower since they are probably noise. A frontend low pass in the circuit already deals with the higher frequencies

Maybe I'm missing something, but wouldn't it be better to run both filters permanently on, before the frequency detector?

ie; cut all low freq noise below 0.7kHz and cut all high freq noise above 1.5kHz etc, then put your 1kHz detector after that?

Why is it necessary to switchin a filter AFTER detecting the freq?

 

Hello,

Here is a block diagram of a circuit that will switch the signal between 1 and 2 kHz.

Bertus

 

Hello,

Here is a block diagram of a circuit that will switch the signal between 1 and 2 kHz.

Bertus

Thank you.