gating rf frequencies

[evengravy]

hi,

i am fairly new to all this so please excuse my lack of tecnical knowledge....

I am trying to design a device to gate the output of a function generator. The generator output is sine at 2vpp ac and needs to be varied between 3 and 5Mhz.

I have designed the 555 timer at the values: 1330hz at 25% duty cycle and i need to use this timing to gate the ac signal from the function generator (25% on 75% off).

i have been looking at using cmos chips to do this but these only gate dc signals right?

can you guys offer me some options/help in gating rf ac frequencies.

incedently the gated signal with be amplified to 15w using an rf amp (not yet designed) if this matters at all

thanks

View attachment 40625

 

[mvs sarma]

hi,

i am fairly new to all this so please excuse my lack of tecnical knowledge....

I am trying to design a device to gate the output of a function generator. The generator output is sine at 2vpp ac and needs to be varied between 3 and 5Mhz.

I have designed the 555 timer at the values: 1330hz at 25% duty cycle and i need to use this timing to gate the ac signal from the function generator (25% on 75% off).

i have been looking at using cmos chips to do this but these only gate dc signals right?

can you guys offer me some options/help in gating rf ac frequencies.

incedently the gated signal with be amplified to 15w using an rf amp (not yet designed) if this matters at all

thanks

You may consider 74HC4066. the datasheet can be studied for further details

 

[Hero999]

What about tuning the RF oscillator on and off?

 

[evengravy]

hi,

thanks for responses, i have been looking at the 74hc4006 since your reply, not sure if response time is fast enough yet untill i study data sheet in a bit more detail but it surely appears to have the bandwidth for my application, thanks.

am i correct in assuming that in order to use this bilateral switch that the gate trigger signal from my 555 timer should also swing bilaterally? (eg. -5v off to +5v on) as it swings dc to Vcc at present (or there abouts).

 

[Hero999]

The gate switch should never go above of below the negative rail. It should be connected to 0V to turn off and +V to turn on and should never be left floating.

An analogue switch is not a solid state relay. The signal being switched should always have a voltage within the supply voltage range and should be referenced to the IC's 0V rail.

The on resistance of the 74HC4066 also varies depending on the voltage of the signal being switched which can cause considerable distortion is the load impedance is low enough.

How fast do you need it to switch?

 

[Kevin Weddle]

An opamp at the function generator ouput will provide high impedance during switching. The PWM device can be used to control the opamp output.

 

[evengravy]

quote: The signal being switched should always have a voltage within the supply voltage range ----

Ok so maybe i should look at a solid state relay instead? the function generator output signal is sine and varies from -1v to +1v at 3-5mHz and i need to gate this signal at 25% duty (on) 75% off at a rate of 1330Hz. Any suggestions for solid state relays rather than a cmos type switch?

I have been looking at solid state relays: Sharp S202SE1, but these are rated at 8A 230V ac, total overkill? could you recommend a low voltage ssr at all?

I have also been toying with the idea of using n and p mosfet pair? any suggestions?

i dont mind the cost as long as it functions well. I also want to completely avoid regular "clicking" relays.


quote: An opamp at the function generator ouput will provide high impedance during switching. The PWM device can be used to control the opamp output.

I may look into this thanks, but i would prefer to use either a switch or a relay if i can.

thanks for responses guys

 

[Hero999]

quote: The signal being switched should always have a voltage within the supply voltage range ----

Ok so maybe i should look at a solid state relay instead? the function generator output signal is sine and varies from -1v to +1v at 3-5mHz and i need to gate this signal at 25% duty (on) 75% off at a rate of 1330Hz. Any suggestions for solid state relays rather than a cmos type switch?

There's no need for a solid state relay.

The signal is AC and the voltage swing is less than the power supply voltage so you can bias the signal at half the power supply voltage and AC couple the input and output via suitably sized capacitors.