HF preamplifier with threshold?

[neazoi]

Hi,
I am looking for a HF preamplifier circuit, that has a kind of threshold, which can be set by the user.
That is, the amplifier should only amplify signals above a preset threshold (say S9+40) and ignore any lower level signals (say S9).

If there such a circuit? Can you drive me to the right path?

Thanks

 

[BigBoss]

You cannot do that directly but it's possible to realize indirectly.
First, you should measure the signal strength. It's not easy at RF frequencies but it's pretty easy to implement at IF stages if there is.
"Squelch" circuits are widely used in decades to suppress the noise during non communication time slot and it turns off the audio stage to prevent the operator from noisy sound. But this is done at IF stages then audio.
You can implement a similar system for your target. You measure the signal strength by RSSI signal ( some IF circuits have it ) then you limit/turn off the Gain of the LNA below of a certain limit. Some IF circuits are equipped with RSSI feature.
For instance this one works up to 500MHz

AD8307ARZ Analog Devices Inc. | Integrated Circuits (ICs) | DigiKey

Order today, ships today. AD8307ARZ – Logarithmic Amplifier IC Receiver Signal Strength Indication (RSSI) 8-SOIC from Analog Devices Inc.. Pricing and Availability on millions of electronic components from Digi-Key Electronics.

www.digikey.fr

There are possibly some cheaper ones. It's just an example.

 

[neazoi]

You cannot do that directly but it's possible to realize indirectly.
First, you should measure the signal strength. It's not easy at RF frequencies but it's pretty easy to implement at IF stages if there is.
"Squelch" circuits are widely used in decades to suppress the noise during non communication time slot and it turns off the audio stage to prevent the operator from noisy sound. But this is done at IF stages then audio.
You can implement a similar system for your target. You measure the signal strength by RSSI signal ( some IF circuits have it ) then you limit/turn off the Gain of the LNA below of a certain limit. Some IF circuits are equipped with RSSI feature.
For instance this one works up to 500MHz

AD8307ARZ Analog Devices Inc. | Integrated Circuits (ICs) | DigiKey

Order today, ships today. AD8307ARZ – Logarithmic Amplifier IC Receiver Signal Strength Indication (RSSI) 8-SOIC from Analog Devices Inc.. Pricing and Availability on millions of electronic components from Digi-Key Electronics.

www.digikey.fr

There are possibly some cheaper ones. It's just an example.

Thanks. I re-thought of my idea and I think is not correct. When the amplifier amplifies higher level signals, it does not ignore the lower level ones. The amplifier sees voltages at it's input and amplifies anything. So yes, it can be "triggered" by the higher level signals, but along with them, as long as it is switched ON, it amplifies the lower level signals too. So you cannot somehow differentiate higher level signals from lower level ones. I think...

 

[BigBoss]

Since the input signal strength is very low at LNA stage, it's not possible to distinguish whether the signal is very low or high. Because measurement o low signal levels is very difficult. For instance, let say a FM receiver. The signal strength is around -80dBuV up to -30dBuV. It's really tough to measure this level. Only specialized equipment may measure. Whereas if the signal is amplified and observed, this will be feasible to detect the signal's threshold level and decide where the LNA will be turned on or off. This is the main issue.