Broadband transistor amplifier limiter schematic needed
I need to limit the RF output of a broadband oscillator (20KHz-30MHz) so as its amplitude is constant throughout the band.
If you use a limiter, you will be peak clipping and so generating harmonics. Is this what you want? For broadband use, when harmonic filtering can not be used a better method would be to use a voltage controlled attenuator (for this frequency ~ 6 hot carrier diodes), driven by a RF detector and amplifier.
Frank
No I just need to attenuate the waveform is it is more than a predefined level. Some kind of detection and variable amplification is needed, but I have not seen any such schematic to flatten the output amplitude.
In audio technique, the term limiter is referring to a VGA with level detector and control loop that keeps the peak level below a defined maximum to prevent peak clipping.
For oscillator amplitude stabilization, an average level detection and control loop is sufficient. Preverably, you would want to manipulate the oscillator gain instead of connecting a VGA behind the oscillator. I wonder if the wide band oscillator doesn't already incorporate a variable gain element for amplitude control?
Yes it does. It is the lower transistor in the oscillator section. The amplitude is dead stable from 65KHz up to 16MHz, then it is twice as much up to 22MHz and then it drops too much up to 30mhz. I am trying to figure out why this is happening in order to correct it.
I would have thought that its a freq Vs gain response of the transistor. every device is going to have a given response curve
and I would suggest that few of them are flat over a broad bandwidth
Dave
That's in fact a different problem. You already have a variable gain element and want to achieve a better level constancy.
In this case, you are asking for frequency independent level detector operation, the right node to measure the actual output level and control loop accuracy.
By the way, you splitted your questions related to this circuit into at least 4 different threads. Why?
I thought this way would be easier for the searchers of the forum to find the specific information after a long time, based on the topic. If you suggest me not to do it, I won't. Thanks.
Now about the circuit, would it worth it to try different transistors than the bf494 so see if I get a flatter response above 16mhz?
It is not a problem of the discrete mmic amplifiers as I have tested the response at the output of the second bf256.
Frequency mixers requiring +7dbm LO, can still mix at half power with increased loss and at more power maybe, so that is not much of a problem, but It would me nice to achieve a flat response also on the upper bands.
Also, I am not sure if the response has to do with the coil types or not...
By the way, I have also tried to switch different coils electronically. The addition of a diode in the before or after the coil really distorts much the signal. I have thought of another approach, which I need to try. Use a PNP transistor between vcc and each coil and control their base electronically. Clean and stable DC will pass through the transistor if it is enabled, to the coil. I do not know if this will cause any distortion to the waveform, but it behaves like another psu regulator (like the BD436), so I do not see why this should cause any problem there.
I have also tried to replace the capacitor with a bb212 with no success. Much distorted waveform again. Something to try next...
The present thread seemed to start with a different topic but finally referred to a schematic posted in the other thread without explicitely linking it. And you found it suitable to mention the capacitance diode results which have their own thread. At this point, the discussion gets a bit confused, I think. On the other hand, threads should be theme-centered to a certain extend.
Technically, I stay with my "better level detector" suggestion.
Ok, I apologize for any inconvenience. I will open another topic asking about the electronic band switching.
Bach on the topic, a potentiometer at the output could provide attenuation and at the same time not affect the sinusoidal but it wont be at 50R output